JP4022063B2 - High-strength aluminum-plated steel sheet and high-strength automotive parts with excellent workability and corrosion resistance - Google Patents

Description

【００１７】
【表２】

【００１８】
Ｎｏ．８のようにＣが低めであると、強度が低めである。ビッカース硬度は一般に３倍するとその材料の引張強度（ＭＰａ）に近い値となるが、この場合だと、６００ＭＰａクラスの強度である。Ｎｏ．１〜７については、強度、加工性の双方とも良好な結果が得られた。このときのＦｅ−Ａｌ被覆中のＡｌ量をＥＰＭＡ定量分析で分析したところ、１５％前後の値を示した。なお、この分析は焼入れ後の試料の断面の表面から１０μｍ以内を５点分析し、その値を平均したものである。
【００１９】
（実施例２）
実施例１の表１中のＮｏ．１の鋼板を使用して、Ａｌ−１０％Ｓｉ−２％Ｆｅをベースとしてめっきを行い、付着量は両面で６０〜２００ｇ／ｍ² まで変化させた。これらの試料を大気雰囲気、９５０℃での保定時間を変えて加熱し、インパクト試験で加工性を評価し、実施例１の方法でＦｅ−Ａｌ被覆中のＡｌ量を測定した。表３に示すように、加工性はＦｅ−Ａｌ被覆中のＡｌ量に依存し、この量が３５％以下で良好な加工性を有する。また、Ｆｅ−Ａｌ系被覆層のＡｌ量は付着量と保定時間に依存し、付着量が少ない程、また、保定時間が長い程拡散が進行してＦｅ−Ａｌ被覆中のＡｌ量が減少していくことが判る。
【００２０】
【表３】

【００２１】
【発明の効果】
以上述べたように、本発明は、加工性、特に加工部耐食性に優れたホットプレス用アルミめっき鋼板を提供する。従って、本発明は、今後の自動車軽量化に大きく寄与するものと思われ、産業上の寄与は大きい。
【図面の簡単な説明】
【図１】９５０℃におけるＦｅ−Ａｌ−Ｓｉ系三元状態図である。[0001]
BACKGROUND OF THE INVENTION
The present invention provides a steel material for producing a part requiring high strength such as an underbody of an automobile.
[0002]
[Prior art]
In recent years, there has been a strong demand for higher strength for steel sheets for automobiles due to the trend toward lower fuel consumption due to global environmental problems. However, generally, the increase in strength is accompanied by a decrease in workability and formability, and there is a demand for a steel sheet that achieves both high strength and high formability. One of the corresponding measures is TRIP (Transformation Induced Practice) steel using martensitic transformation of retained austenite, and its use is expanding in recent years. However, with this steel, it is possible to produce a 1000 MPa class high strength steel plate with excellent formability, but it is difficult to ensure formability with ultra high strength steel such as a higher strength, for example 1500 MPa. is there.
[0003]
Therefore, hot press is recently attracting 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 the steel sheet at a high temperature of 800 ° C. or higher, and obtains a desired material by cooling after forming. However, since it involves heating in the atmosphere, an oxide is generated on the surface and needs to be removed in a later step. In order to improve this, Japanese Patent Laid-Open No. 2000-38640 discloses a technique for suppressing oxidation during heating by aluminizing a steel sheet containing 0.15 to 0.5% carbon.
[0004]
[Problems to be solved by the invention]
The above-described invention is effective in efficiently producing a high-strength molded part, but the aluminum plating layer may cause cracks in the plating during processing depending on the heating conditions. This is because the plating layer changes to a brittle intermetallic compound layer by heating, and when such a crack occurs, the plating may peel off in some cases, which is not so severe In addition, there was a concern that the corrosion resistance after painting at that location would be lowered.
[0005]
[Means for Solving the Problems]
In order to overcome the above-mentioned problems, the present inventors have made various studies on the workability of the aluminum-plated steel sheet after heating. The knowledge of suppressing the occurrence of plating cracks during processing has been obtained. That is, the hot-dip aluminum-plated steel sheet usually has a plating bath composition of Al-10% Si, and when the aluminum-plated steel sheet is heated, it is alloyed to the surface and becomes a 1-5 phase structure depending on the heating conditions, but the heating temperature is high. It was also found that when the heating time is long, the alloy layer has one or two phases. At this time, the brittleness is relaxed and the alloy layer is not cracked even if processed.
[0006]
The gist of the invention is that
(1) By mass% as a steel component, C: 0.05 to 0.7%, Si: 1% or less, Mn: 0.5 to 3%, P: 0.1% or less, Al: 0.1% Hereinafter, N: 0.01% or less , Ti: 0.5% or less, Cr: 2% or less , the steel plate surface of steel consisting of the balance Fe and inevitable impurities, Fe-Al coating, A high-strength aluminum-plated steel sheet excellent in workability and processed part corrosion resistance, wherein the Al concentration of the Fe-Al coating is 35% or less by mass.
(2) In addition to the steel component described in (1) above, the processability and processing further include one or two of Mo: 0.5% or less and B: 0.05% or less. High-strength aluminum-plated steel sheet with excellent corrosion resistance.
[0007]
( 3 ) Fe-Al coating further contains 15% or less of Si by mass%, and has high workability and excellent corrosion resistance as described in (1) or (2 ) above Aluminum-based plated steel sheet.
( 4 ) High-strength aluminum-based plating excellent in workability and processed portion corrosion resistance according to any one of (1) to (3), wherein the coating adhesion amount is 120 g / m ² or less on both sides. steel sheet.
( 5 ) A high-strength automotive part obtained by press-molding the steel according to any one of (1) to ( 4 ).
[0008]
Hereinafter, the present invention will be described in detail.
FIG. 1 shows an Fe—Al—Si ternary phase diagram at 950 ° C., and this one- and two-phase state is the lower left region in this phase diagram, and this region is Fe (bcc) and Fe ₃ Al. (Bcc) is mixed. The Al content in this region is at most 35%. When such an alloy layer is formed, the workability is improved and the occurrence of cracks can be suppressed. As a matter of course, the smaller the adhesion amount of the plating, the easier it is to change to this region. Therefore, the knowledge that the lower adhesion amount is excellent in workability and post-processing corrosion resistance was also obtained.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, the reason for limitation of the present invention will be described.
Alloy layer by heating the aluminum plating, i.e. a Fe-A l be covered layer, it is the main point of the present invention that the Al amount is 35% or less. In street this reason described above, by making the Fe-A l to be Kutsugaeso Al, Si is solid-dissolved Fe + Fe ₃ Al, much better processability than those other phases were mixed, working unit Corrosion resistance is obtained.
C: 0.05-0.7%
C has a high strength of 1000 MPa or more after molding, and is rapidly cooled after hot pressing to transform into a structure mainly composed of martensite. For this purpose, C is preferably 0.05% or more. On the other hand, even if the amount of C is increased to more than 0.7%, the upper limit is set to 0.7% in order to cause saturation of the strength and to easily cause weld cracks.
[0010]
In the present invention, the steel component is not particularly defined except for C, but elements such as Si, Mn, P, S, Al, N, Cr, Mo, Ti, Nb, B, Ni, Cu, V, Sn, Sb, etc. May be added elements or unavoidable impurities. These elements can be added as necessary. For example, Mn and B are effective for hardenability, and Cr, Ti, and Mo are effective for heat resistance of the aluminum plating layer. Ni and Cu are effective in corrosion resistance. Desirably, the addition range is Mn: 0.5 to 3%, Si: 1% or less, P: 0.1% or less, S: 0.1% or less, Al: 0.1% or less, N: 0.01 %: Cr: 2% or less, Mo: 0.5% or less, Ti: 0.5% or less, Nb: 0.1% or less, B: 0.05% or less, Ni: 1% or less, Cu: 1 % Or less, V: 0.1% or less, Sn, Sb: 0.1% or less.
[0011]
As described above, the smaller the amount of aluminum plating attached, the easier it is to be in the Fe + Fe ₃ Al region. In this sense, the amount of adhesion is preferably 120 g / m ² or less on both sides. The method of aluminum plating is not particularly limited, and electroplating, vacuum deposition, cladding, and the like including hot dipping are possible. Currently, the most popular in the industry is the hot dipping method, and Al-10% Si is often used as a normal plating bath, which contains inevitable impurities such as Fe.
[0012]
At this time, Si enters the alloy layer after heating, and the amount of Si may vary depending on the phase structure. However, in the Fe + Fe ₃ Al region, Si is 15% or less, and Si has an upper limit. As other additive elements, there may be Mn, Cr, Mg, Ti, Zr, Sb, Sn, Cu, Ni, Co, In, Bi, Misch metal, etc., but this is applicable as long as the plating layer is mainly composed of Al. Is possible. Addition of Zn and Mg is effective in terms of making red rust unlikely to occur, but excessive addition of these elements having a high vapor pressure causes generation of fumes of Zn and Mg, and powdery substances derived from Zn and Mg on the surface. The addition of Zn: 60% or more and Mg: 10% or more is not desirable.
[0013]
In the present invention, the pre-treatment and post-treatment of aluminum plating are not particularly limited. Ni, Cu, Cr, Fe pre-plating, etc. may be used as the plating pretreatment, but this is also applicable. Further, as post-plating treatment, there may be chromate treatment, resin coating treatment, etc. for the purpose of primary rust prevention and lubricity, but the organic resin disappears when heated, which is not preferable. 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, MoS _{2 and the} like can be pretreated for the purpose of lubricity. The steel sheet is processed at 800 ° C. or higher to become an automobile part. In that case, it is desirable to provide a coating film having a thickness of about 1 to 200 μm on the surface.
[0014]
【Example】
Next, the present invention will be described more specifically with reference to examples.
Example 1
Hot-aluminum plating is performed using pickled steel sheets (thickness 1.8 mm) and cold-rolled steel sheets (1.2 mm thick) as shown in Table 1, which have undergone normal hot rolling and cold rolling processes. It was. No. in Table 1 1, 3, 5 and 7 are cold-rolled steel sheets, and the rest are hot-rolled steel sheets. For hot-dip aluminum plating, a non-oxidation furnace-reduction furnace type line was used. After plating, the amount of plating adhered was adjusted to 80 g / m ² on both sides by a gas wiping method, then cooled and subjected to zero spangle treatment. The plating bath composition at this time was Al-10% 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 characteristics at this time are shown in Table 2.
[0015]
The hardenability and workability of the hot-dip aluminized steel sheet thus manufactured were evaluated. Workability is determined by impact testing after heating an aluminum-plated steel sheet at 950 ° C for 10 minutes in the air, and then cooling it between steel dies (cooling rate is about 30 ° C / second). The workability of plating was evaluated. Moreover, the cross-section Vickers hardness of the steel plate was measured at a load of 100 g.
[Evaluation criteria for workability]
○: No peeling Δ: Cracking ×: Powdery peeling occurrence [0016]
[Table 1]

[0017]
[Table 2]

[0018]
No. When C is low as in 8, the strength is low. When the Vickers hardness is generally tripled, it becomes a value close to the tensile strength (MPa) of the material. In this case, the Vickers hardness is 600 MPa class strength. No. For 1 to 7, good results were obtained in both strength and workability. When the amount of Al in the Fe—Al coating at this time was analyzed by EPMA quantitative analysis, a value of around 15% was shown. In this analysis, five points are analyzed within 10 μm from the surface of the cross section of the sample after quenching, and the values are averaged.
[0019]
(Example 2)
No. 1 in Table 1 of Example 1 No. 1 steel plate was used for plating based on Al-10% Si-2% Fe, and the adhesion amount was changed from 60 to 200 g / m ^{2 on} both sides. These samples were heated by changing the holding time at 950 ° C. in the air atmosphere, the workability was evaluated by an impact test, and the amount of Al in the Fe—Al coating was measured by the method of Example 1. As shown in Table 3, the workability depends on the amount of Al in the Fe-Al coating, and this amount is 35% or less and good workability is obtained. In addition, the amount of Al in the Fe—Al-based coating layer depends on the amount of adhesion and the holding time. The smaller the amount of adhesion and the longer the holding time, the more the diffusion proceeds and the amount of Al in the Fe—Al coating decreases. You can see that
[0020]
[Table 3]

[0021]
【The invention's effect】
As described above, the present invention provides an aluminized steel sheet for hot pressing that is excellent in workability, in particular, corrosion resistance of a processed part. Therefore, the present invention is considered to greatly contribute to the future weight reduction of automobiles, and the industrial contribution is great.
[Brief description of the drawings]
FIG. 1 is a Fe—Al—Si ternary phase diagram at 950 ° C. FIG.

Claims (5)

As a steel component in mass%,
C: 0.05-0.7%
Si: 1% or less,
Mn: 0.5-3%,
P: 0.1% or less,
Al: 0.1% or less,
N: 0.01% or less
Ti: 0.5% or less,
Cr: 2% or less,
Workability characterized by having a Fe—Al coating on the steel plate surface of steel comprising the balance Fe and unavoidable impurities, and the Al concentration of the Fe—Al coating being 35% or less by mass% High-strength aluminum-plated steel sheet with excellent corrosion resistance in the processed area. In addition to the steel component of claim 1,
Mo: 0.5% or less,
B: 0.05% or less,
A high-strength aluminum-plated steel sheet excellent in workability and processed portion corrosion resistance, characterized by containing one or two of the following. The Fe-Al coating further contains, by mass%, processability, high strength aluminum-based plated steel sheet excellent in processability portion corrosion resistance according to claim 1 or 2, characterized in that it contains 15% or less of Si. The high-strength aluminum-plated steel sheet excellent in workability and processed part corrosion resistance according to any one of claims 1 to 3, wherein the coating adhesion amount is 120 g / m ² or less on both sides. A high-strength automobile part formed by press-molding the steel according to any one of claims 1 to 4 .

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