JP4456581B2 - High-strength automotive parts with excellent post-painting corrosion resistance of molded parts and hot pressing methods thereof - Google Patents

Description

    The present invention relates to a high-strength automobile member excellent in post-coating corrosion resistance of molded parts such as automobile pillars, door impact beams, and bumper beams using an Al-based plated steel sheet and a hot press method for producing a strength member.

  In recent years, there has been a strong demand for higher strength steel sheets for automobiles due to the trend toward lower fuel consumption due to global environmental problems. However, in general, increasing the strength is accompanied by a decrease in workability and formability, and a steel sheet that achieves both high strength and high formability is desired.

    One of the countermeasures is TRIP (TRansformation Induced Placiticity) steel using martensitic transformation of retained austenite, and its use is expanding in recent years. With this steel, it is possible to produce 1000MPa-class high-strength steel sheets with excellent formability, but it is difficult to ensure formability with ultra-high-strength steels with higher strength, for example, 1500MPa or more. .

    Therefore, hot press (also called hot press, die quench, press quench, etc.) has recently attracted attention as another form that achieves both high strength and high formability. This eliminates the problem of formability of a high-strength steel sheet by forming it hot after heating it in an austenite region at 800 ° C. or higher, and obtains a desired material by baking after cooling after forming. .

Although this method is promising as a method for forming ultra-high strength members, it usually has a step of heating a steel plate in the atmosphere, and an oxide (scale) is generated on the surface. It was necessary to remove in a subsequent process such as pickling. However, with shot blasting, it was difficult to completely eliminate scale, and there was a possibility of deformation due to shots. Pickling also requires wastewater treatment and the like, and it may be necessary to take countermeasures from the viewpoint of environmental impact, which may lead to increased manufacturing costs. As a technique for improving this, there is known a technique for suppressing oxidation during heating using a sample obtained by plating Al on a steel sheet containing 0.15-0.5% carbon. For example, Japanese Patent Application Laid-Open No. 2003-181549, This is disclosed in Japanese Utility Model Publication No. 2004-244704.
Japanese Patent Laid-Open No. 2003-181549 JP 2004-244704 A

    The techniques disclosed in these publications are effective in efficiently producing a high-strength molded part having excellent corrosion resistance after coating, but still have the following problems.

    When manufactured using this technique, the Al plating layer is heated to change to an intermetallic compound by mutual diffusion between Al and Fe of the steel sheet. This intermetallic compound layer (also called an alloy layer) is very hard and brittle and cannot follow during press molding. Further, the surface is very stable, and chemical conversion treatment (phosphate treatment) usually used for improving paint adhesion and post-coating corrosion resistance in the automobile manufacturing process does not adhere to the surface. In recent years, Al materials are increasingly used as automotive materials, and chemical conversion treatment chemicals that can treat Al have been developed, but even if chemical conversion treatment materials for Al are used, this intermetallic compound layer can be applied. Conversion treatment was impossible.

    At this time, there was a problem that the Al plating layer alloyed in the hot pressing step was partially lost and the post-coating corrosion resistance of the portion subjected to severe molding was lowered because the chemical conversion treatment was not attached.

    In order to overcome the above-mentioned problems, the inventors of the present invention focused on the heating process and the hot pressing process, and as a result of various studies on appropriate manufacturing conditions and the state of the intermetallic compound layer, the following findings were obtained.

    In general, chemical conversion treatment is said to have the effect of improving the paint adhesion and the post-coating corrosion resistance of cold-rolled steel sheet, galvanized steel sheet or Al sheet, but the Al-based steel sheet is heated to an intermetallic compound. It has the same effect on the layer. Here, the Al-based plated steel sheet is a general term for Al-plated steel sheets and Al alloy-plated steel sheets. Specifically, Al, Al—Si, Al—Zn, Al—Mg, Al—Si—Mg, Al—Ti, and the like correspond to this. However, as described above, the chemical conversion treatment does not adhere to the surface that has been hot-pressed under normal conditions, and there is a concern about post-coating corrosion resistance for severely formed parts. In the case of a surface-treated steel sheet, it is clear that the corrosion resistance of the part where plating damage has occurred in the formed part is the most inferior, and the concern about the reduction of the plate thickness due to corrosion and the decrease in strength is greatest in such a part.

  However, it is possible to attach the chemical conversion treatment to the molded part by adjusting the part shape or pressing conditions. It has been found that the reason why the chemical conversion treatment does not adhere to the intermetallic compound layer is due to the extremely stable oxide film formed on the surface of the intermetallic compound layer. This oxide film is generated in a heating furnace, and the chemical conversion processability can be improved by rubbing the surface in the pressing step. In order to rub the surface strongly, it is necessary to have a thickened portion, and the chemical conversion treatment property tends to be better when the difference in the plate thickness of the pressed product is larger. Moreover, the chemical conversion property of a forming part is greatly improved by lowering the start temperature of press molding. When the molding start temperature is high, even if the oxide film on the surface is torn by press molding, an oxide film is formed again at that location, whereas when the molding start temperature is low, an oxide film is difficult to form at the molded part, resulting in that part. It is thought that the chemical conversion processability of is improved. Thus, extremely excellent post-coating corrosion resistance can be obtained by attaching the chemical conversion film.

  At this time, a chemical conversion treatment film is formed in the molded part and the surface rubbing part, and the amount thereof is about 2 to 3 g / m 2 (one side), which is the same as the amount of the normal chemical conversion treatment film, or slightly less. The film shape is the same as that of a normal one. Since almost no chemical conversion treatment film is formed on the surface where the oxide film is not scratched, it is necessary to select a site when measuring the amount of adhesion. A uniformly shaped portion such as a vertical wall is suitable for obtaining the most accurate value. In addition, it is a molded part where stress in the tensile direction is applied, typically the outer surface side of the bent part, etc., but local cracks of the intermetallic compound layer occur at such sites, The chemical conversion film is also formed at the bottom.

For this reason, the present invention defines a high-strength member having excellent post-coating corrosion resistance and a method for producing the same. The gist is as follows .
(1) By mass%, C: 0.05 to 0.7%, Si: 0.1 to 1%, Mn: 0.7 to 2%, P: 0.003 to 0.1%, S: 0.003 to 0.1%, and its hardness ( on the surface of the steel hv) of 400 or more, FeAl2, Fe2Al5, FeAl3, FeAl , have a coating layer composed of one or more Al solid solution alpha-Fe, the total thickness of the steel and the coating layer most difference thick portion and the thinnest portion is at least 10 percent of the total thickness, it more to the molding portion having a single-sided per 0.5 g / m @ 2 or more phosphate film on the interface between the coating layer and the coating film of A high-strength automobile member with excellent corrosion resistance after painting of the molded part.
(2) The hot pressing method of a high-strength automobile member having excellent post-coating corrosion resistance of the molded part according to (1), wherein a press start temperature in the pressing step is 600 to 690 ° C.

    According to the present invention, a high-strength member can be produced by using a hot-press method using an Al-based plated steel sheet with little plating peeling, and its industrial significance is extremely great.

The alloy layer structure when the Al-based plated steel sheet is heated will be described. FIG. 1 shows a typical cross-sectional structure after alloying. In general, the Fe—Al-based coating layer after alloying often has a five-layer structure. In FIG. 1, this is represented by 1 to 5 layers. Al concentration in the 1st and 3rd layers is about 50%, Al concentration in the 2nd layer is about 30%, 4th and 5th layers are 15-30% and 1-15% respectively. It becomes. Formation of voids may be observed near the interface between the fourth layer and the fifth layer. The structure below the fifth layer is a steel base, and is a hardened structure mainly composed of martensite. Fig. 3 shows a binary phase diagram of Fe-Al. The first and third layers are mainly composed of FeAl2, and the fourth and fifth layers correspond to FeAl and αFe, respectively. The two layers are Si-containing layers that cannot be explained from the Fe-Al binary phase diagram, and the details are not yet clear, but it is estimated that FeAl2 and Fe-Al-Si compounds were mixed finely. is doing.
Next, the reason for limitation of the present invention will be described.

  In the present invention, the Al-based plated steel sheet is heated in a heating furnace to be changed to an intermetallic compound layer mainly composed of Al and Fe, and a chemical conversion treatment film (phosphate film) is provided on the surface by applying a chemical conversion treatment step after forming. The amount of adhesion should be 0.5 g / m2 or more per side. The type of the phosphate film is not particularly limited, but is usually a zinc phosphate or iron phosphate-based film. The chemical conversion treatment liquid is not limited to the Al treatment, and a chemical conversion treatment liquid for a normal cold-rolled steel sheet may be used. The adhesion amount is 0.5g / m2 or more, and the effect on the corrosion resistance after painting is recognized. Usually, the surface is covered with a coating film such as electrodeposition coating or intermediate coating or top coating, but these coatings are peeled off using commercially available paint removal chemicals such as Neo River from Sansai Chemical Co., Ltd. be able to.

  Adhesion of the chemical conversion coating as described above may vary depending on whether measures where because it depends on the site, but typically hot pressed parts often have vertical walls, it is preferable to measure at such sites. X-ray fluorescence is the simplest measurement method, but in the case of pressed parts, there are many cases where there is no flat part, so if measurement with fluorescent X-rays is difficult, the chemical conversion treatment film is chemically peeled off before and after peeling. It is desirable to measure from the change in weight of

    In order to produce a chemical conversion coating satisfactorily, the difference between the thickest part and the thinnest part as the part shape should be 10% or more of the total thickness. This is because, as described above, the thickened portion in the pressing process is in strong contact with the mold to eliminate the oxide film on the surface. Molded parts such as vertical walls and curved parts are free from oxide films. The thinned portion, which is one of the molded portions, is often subjected to tensile processing, and plating cracks occur, so that the chemical conversion treatment film easily adheres to such a portion.

  As the molding conditions, it is necessary that the press start temperature is 600 to 690 ° C. In hot pressing, a steel sheet is quenched by quenching with a mold to obtain high strength. However, if the pressing start temperature is too low, sufficient strength after quenching cannot be obtained. On the other hand, when the press start temperature is 690 ° C. or higher, an oxide film is generated in the molded part and it is difficult to attach the chemical conversion film to the molded part.

  Other conditions in the heating and pressing steps are not particularly limited. Heating methods include radiant heating using a furnace such as an electric furnace, radiant heating using near infrared rays, high frequency induction heating, current heating, etc. Currently, the most hot press uses a furnace Radiant heating. The heating atmosphere at this time may be air, combustion gas, nitrogen, or the like, but the heating atmosphere is not particularly limited. Although the heating temperature is not particularly limited, it is usually heated to 850 ° C. or higher in order to transform the steel sheet into austenite.

  The idea of controlling the atmosphere around the press machine to suppress the formation of oxide film after press forming is not without the idea, but controlling the atmosphere of the entire press is a large facility, Al is extremely oxygen In practice, it is almost impossible because a high concentration of hydrogen is required to suppress the oxidation of Al.

  Next, the intermetallic compound layer to be refined on the steel sheet surface will be described. In the present invention, an Al-based plated steel sheet is used, and the steel sheet is heated to generate an intermetallic compound up to the surface. If metal Al that does not become an intermetallic compound remains, it is not desirable from the viewpoint of corrosion resistance after coating. This can be easily determined by analyzing the presence or absence of metal Al from the surface by a technique such as X-ray diffraction. The intermetallic compound layer is often five layers as shown in FIG. 1, but the structure is not particularly limited. The interface between the intermetallic compound layer and the steel sheet can be determined by performing 2 to 3 vol% nital etching. The thicker the layer, the better the corrosion resistance, but it tends to be lost during molding. Usually, this thickness is 5-50 μm. In addition, as for the coating adhesion amount of the Al type plated steel plate used for this invention, 60-200 g / m <2> is desirable. If it is less than this, the alloy layer of the part after hot pressing is not sufficiently formed, and the corrosion resistance cannot be ensured. If it is too much, the plating layer will not be sufficiently alloyed during hot pressing, and the plating metal will adhere to the mold in powder form.

  Next, the reasons for limiting the steel components will be described. The present invention is characterized in that pressing and quenching with a mold are performed simultaneously, and the steel sheet needs to be a component that is easily quenched. For the purpose of improving hardenability, the C content in the steel is desirably 0.05% or more, and if the C content is too high, the toughness of the steel sheet is remarkably lowered, so 0.7% or less is desirable. Regarding other steel components, it is desirable that Si: 0.1 to 1%, Mn: 0.7 to 2%, P: 0.003 to 0.1%, S: 0.003 to 0.1%. Mn is an element contributing to hardenability, and it is effective to add 0.7% or more. On the other hand, it is not preferable to exceed 2% from the viewpoint of toughness after quenching. When Si is added in excess of 1%, the Al plating property is reduced, and when P and S are added in excess of 0.1%, the toughness after quenching is reduced. Setting Si, P, and S to 0.1, 0.003, and 0.003% or less is contrary to economic rationality in the steelmaking process. In addition, addition of Cr, B and Ti is preferable from the viewpoint of improving hardenability. When adding, it is desirable to set it as the range of Cr: 0.05-1%, Ti: 0.01-0.1%, B: 0.001-0.01%.

As other elements, Al, N, Mo, Nb, Ni, Cu, V, Sn, Sb and the like can be added. The preferred range of addition is A l: 0. 1% or less, N: 0. 0 1% or less, M o: 0. 5% or less, T i: 0. 5% or less, N b: 0. 1% or less, Ni: 1% or less, Cu: 1% or less, V: 0. 1% or less, Sn, Sb: 0. 1% or less.
The pressed parts become products through welding, chemical conversion treatment, electrodeposition coating, and the like. Usually, cationic electrodeposition coating is often used, and the film thickness is about 1 to 30 μm. After electrodeposition coating, coating such as intermediate coating and top coating may be applied.

  The method of Al-based plating on the steel sheet is not particularly limited, and electroplating, vacuum deposition, cladding, and the like including hot dipping are possible. Currently, the most popular industrially is the hot dipping method, and usually Al-10% Si is often used as a plating bath, which contains inevitable impurities such as Fe. Other additive elements may include Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, Misch metal, etc. As long as the plating layer is mainly composed of A l, it is applicable. The addition of Zn and Mg is effective in the sense that red rust is hardly generated, but excessive addition of these elements having a high vapor pressure causes the generation of fumes of Zn and Mg, and the addition of Zn and Mg to the surface. The resulting powdery substance is generated, and addition of Zn: 60% or more and Mg: 10% or more is not desirable.

  In the present invention, the pre-treatment and post-treatment of A 1 plating are not particularly limited. Ni, Cu, Cr, Fe pre-plating, and the like may be used as the plating pretreatment, but this is also applicable. Further, post-plating treatment may include chromate treatment, resin coating treatment, etc. for the purpose of primary rust prevention and lubricity. In consideration of the recent hexavalent chromium regulation, the chromate treatment is preferably a trivalent treatment film such as electrolytic chromate. In addition, post-treatment other than inorganic chromate is also applicable. Alumina, silica, MoS2 or the like can be pretreated for the purpose of lubricity. Although it is possible to perform a phosphate-based treatment before heating, it is desirable that a chemical conversion treatment film be provided after the heating step because the phosphate is altered in the heating step and loses its anticorrosive action.

(Example 1)
Hot Al-plating was performed using cold-rolled steel sheets (thickness 1.4 mm) having steel components as shown in Table 1 that had undergone normal hot rolling and cold rolling processes. For molten Al plating, a non-oxidation furnace-reduction furnace type line was used. After plating, the amount of plating adhered was adjusted by a gas wiping method, followed by cooling. The plating bath composition at this time was Al-9% Si-2% Fe. Fe in the bath is inevitable supplied from plating equipment or strips in the bath. The plating appearance was good with no plating. The steel sheet was heated to 900 ° C. in an atmospheric furnace, cooled to the predetermined temperature in the air, and then press-formed. The cooling rate between the molds was about 50 to 200 ° C./second. The molding shape at this time is shown in FIG. This molded product has a flat vertical wall of about 50 mm. After press molding, chemical conversion treatment was carried out with Nippon Parkerizing Co., Ltd. chemical conversion treatment solution PB-3081M, the amount of chemical conversion treatment of the vertical wall portion was measured with fluorescent X-rays, and the formation state of the chemical conversion film was observed with SEM. Furthermore, a part of the coating was applied with a cationic electrodeposition paint Powernics 110 made by Nippon Paint Co., Ltd. to a thickness of about 20 μm, and was used for evaluation of corrosion resistance after painting. Molding was also performed under conditions in which the thickness reduction rate of the molded product was adjusted by changing the shoulder r and the pressing force of the die. The ratio of the difference between the maximum and minimum plate thicknesses at this time to the plate thickness was calculated.

塗装後耐食性の評価に当たっては、その後、カッターで塗膜にクロスカットを入れ、自動車技術会で定めた複合腐食試験（ＪＡＳＯ−６１０Ｍ）を１５０サイクル（５０日）行ない、クロスカットからの膨れ幅（両側最大膨れ幅） を測定した。このときの腐食の判定基準を下に示す。
〔膨れ幅〕
○ ： ６ ｍ ｍ 以下
△ ： ６ ｍ ｍ 超〜 ９ ｍ ｍ
× ： ９ ｍ ｍ 超
また成形後に縦壁部の一部を切出して焼入れ後の母材硬度を測定した。測定はビッカース硬度で荷重は10kgfとした。このときの硬度の測定結果も表中に示す。
〔断面硬度〕
○：Hv430以上
△：Hv400超430未満
×：Hv400未満

In evaluating the corrosion resistance after painting, a crosscut was applied to the coating film with a cutter, and a composite corrosion test (JASO-610M) determined by the Automotive Engineering Society was performed for 150 cycles (50 days). The maximum swollen width on both sides was measured. Criteria for corrosion at this time are shown below.
[Bulge width]
○: 6 mm or less △: Over 6 mm to 9 mm
X: More than 9 mm Further, after molding, a part of the vertical wall was cut out and the hardness of the base material after quenching was measured. The measurement was Vickers hardness and the load was 10 kgf. The hardness measurement results at this time are also shown in the table.
(Cross section hardness)
○: More than Hv430 △: More than Hv400 and less than 430 ×: Less than Hv400

表２に成形開始温度と最大-最小板厚差の板厚に対する比率、燐酸塩付着量、断面硬度並びに塗装後耐食性の評価結果を示す。成形開始温度が高い場合(番号1)、あるいは最大-最小板厚差の板厚に対する比率が低い場合(番号8)には燐酸塩が付着せず、塗装後耐食性は不良であるが、成形開始温度を下げる、あるいは板厚比率を高くすることで良好な縦壁部の塗装後耐食性が得られている。また成形開始温度が低すぎると(番号5)母材硬度が低下している。番号6、7のように付着量が少ない場合でも良好な塗装後耐食性が得られた。
（ 実施例２ ）

Table 2 shows the evaluation results of the ratio of the molding start temperature and the maximum-minimum sheet thickness difference to the sheet thickness, the phosphate adhesion amount, the cross-sectional hardness, and the corrosion resistance after coating. When the molding start temperature is high (No. 1), or when the ratio of the maximum-minimum thickness difference to the plate thickness is low (No. 8), phosphate does not adhere and the corrosion resistance after coating is poor, but molding starts. Good corrosion resistance after painting of the vertical wall is obtained by lowering the temperature or increasing the thickness ratio. If the molding start temperature is too low (No. 5), the base material hardness is lowered. Good post-coating corrosion resistance was obtained even when the adhesion amount was small as in Nos. 6 and 7.
(Example 2)

表３に示した様々な鋼成分を持つ冷延鋼板（板厚１．２ｍｍ）に実施例１ と同じ要領で溶融Ａｌめっきを施した。めっき付着量は両面160g/m2とした。これらのＡｌめっき鋼板を、大気炉加熱により900℃に加熱し、650℃まで空冷後1対の平板状金型に挟んで焼入れた。焼入後の硬度（ビッカース硬度、荷重１0ｋｇ）を測定した結果も表３に示しているが、鋼中C量が低いと焼入後の硬度が低下するため、C量として0.05%以上あることが好ましい。

A cold-rolled steel sheet having a variety of steel components shown in Table 3 (thickness 1.2 mm) was subjected to hot-dip Al plating in the same manner as in Example 1. The amount of plating was 160 g / m2 on both sides. These Al-plated steel sheets were heated to 900 ° C. by atmospheric furnace heating, air-cooled to 650 ° C., and then quenched between a pair of flat plate molds. The results of measuring the hardness after quenching (Vickers hardness, load 10 kg) are also shown in Table 3. However, if the C content in steel is low, the hardness after quenching decreases, so the C content must be 0.05% or more. Is preferred.

It is a figure which shows the optical microscope cross-sectional structure | tissue after the heating of the Al plating layer by a prior art. It is a figure which shows the corrosion condition by the optical microscope after the corrosion test of the Al plating layer by a prior art. It is a figure which shows a Fe-Al binary system phase diagram.

Claims (2)

In mass%, C: 0.05-0.7%, Si: 0.1-1%, Mn: 0.7-2%, P: 0.003-0.1%, S: 0.003-0.1%, and its hardness (Hv) is on the surface of the steel is 400 or more, FeAl2, Fe2Al5, FeAl3, FeAl , it has a coating layer composed of one or more Al solid solution alpha-Fe, the thickest of the total thickness of the steel and the coating layer and the difference between the thinnest portion and sites with more than 10% of the total thickness, and wherein the further having an interface on one side per 0.5 g / m @ 2 or more phosphate coating with the coating layer and the coating film to the molded part A high-strength automotive component with excellent post-painting corrosion resistance. The hot pressing method for a high-strength automobile member having excellent post-coating corrosion resistance of the molded part according to claim 1, wherein a press start temperature in the pressing step is 600 to 690 ° C.

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