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JP4673558B2 - Hot press molding method and automotive member excellent in productivity - Google Patents

Hot press molding method and automotive member excellent in productivity Download PDF

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JP4673558B2
JP4673558B2 JP2004017193A JP2004017193A JP4673558B2 JP 4673558 B2 JP4673558 B2 JP 4673558B2 JP 2004017193 A JP2004017193 A JP 2004017193A JP 2004017193 A JP2004017193 A JP 2004017193A JP 4673558 B2 JP4673558 B2 JP 4673558B2
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steel sheet
press molding
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steel
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JP2005205477A (en
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昌史 東
重人 竹林
聡 赤松
寛哲 佐藤
武秀 瀬沼
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Nippon Steel Corp
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Description

本発明は、衝突安全用補強部材等の高強度を要求される部品を製造するための鋼板の成形方法及びそれを使用した自動車用部材を提供するものである。   The present invention provides a method for forming a steel plate for producing a part requiring high strength such as a collision safety reinforcing member and an automobile member using the same.

自動車のドアガードバーやサイドメンバー等の部材は、近年の燃費軽量化の動向に対応すべく軽量化が検討されており、材料面では、薄肉化しても強度および衝突安全性が確保されるという観点から鋼板の高強度化が進められている。しかしながら、材料の成形性は強度が上昇するのに伴って劣化するので、上記部材の軽量化を実現するには、成形性と高強度の両方を満足する鋼板を製造する必要がある。高強度と同時に高成形性を得る手法としては特開平1−230715号公報(特許文献1)や特開平2−217425号公報(特許文献2)に記載されている残留オーステナイトのマルテンサイト変態を利用したTRIP(TRansformation Induced Plasticity)鋼があり、近年用途が拡大しつつある。   Automotive door guard bars and side members are being studied for weight reduction in response to recent trends in fuel economy and weight reduction, and in terms of materials, the strength and collision safety can be ensured even if the thickness is reduced. Since then, the strength of steel sheets has been increasing. However, since the formability of the material deteriorates as the strength increases, it is necessary to manufacture a steel sheet that satisfies both the formability and high strength in order to reduce the weight of the member. As a method for obtaining high strength and high formability, the martensitic transformation of retained austenite described in JP-A-1-230715 (Patent Document 1) and JP-A-2-217425 (Patent Document 2) is used. TRIP (Transformation Induced Plasticity) steel has been used in recent years.

しかし、この鋼により、成形性の優れた980MPa級の高強度鋼板を製造することは可能であるが、更に、高強度、例えば1480MPaというような超高強度鋼、かつ高成形性を有する鋼板の製造を考えた場合、CやMn等の合金元素の大量添加は不可欠であり、それに伴った製造コストの増大と製造性の劣化という問題が生じる。また、TRIP鋼を成形する場合、部材加工時に残留オーステナイトがマルテンサイトに変態することによる遅れ破壊特性の劣化が懸念される。加えて、高強度鋼板をプレス成形する場合、強度が高すぎるため、形状凍結性が悪いという問題を有している。更には、強度が高いため金型による成形や切断機による切断を行う際の損傷が激しいという問題も有している。   However, with this steel, it is possible to produce a high-strength steel sheet of 980 MPa class with excellent formability. Furthermore, a high-strength steel sheet having a high strength, for example, 1480 MPa, and a steel sheet having high formability. When manufacturing is considered, it is indispensable to add a large amount of alloying elements such as C and Mn, which causes problems of increase in manufacturing cost and deterioration in manufacturability. Further, when forming TRIP steel, there is a concern about the deterioration of delayed fracture characteristics due to the transformation of retained austenite into martensite during member processing. In addition, when press-molding a high-strength steel plate, the strength is too high, and thus there is a problem that shape freezing property is poor. Furthermore, since the strength is high, there is also a problem that damage when performing molding with a mold or cutting with a cutting machine is severe.

一方、成形性に劣る高強度鋼板を形状凍結性良く成形する手法としては、特開2002−143935号公報(特許文献3)や特開2003−154413号公報(特許文献4)に記載されている温間プレスと呼ばれる手法が存在する。これは、鋼板強度が低下する200から500℃程度温度にて成形を行う手法であるが、1180MPa以上の高強度鋼板の成形を考えた場合、成形温度を上昇させたとしても、依然として鋼板強度が高く成形しがたい、あるいは、加熱により成形後の鋼板強度が低下してしまい所定の強度が得られないという問題を有することから、1180MPa以上の鋼板の成形には適さない。   On the other hand, methods for forming a high-strength steel sheet having poor formability with good shape freezing properties are described in Japanese Patent Application Laid-Open No. 2002-143935 (Patent Document 3) and Japanese Patent Application Laid-Open No. 2003-154413 (Patent Document 4). There is a technique called warm press. This is a method of forming at a temperature of about 200 to 500 ° C. at which the strength of the steel sheet decreases, but when forming a high-strength steel sheet of 1180 MPa or higher, even if the forming temperature is increased, the steel sheet strength is still high. This is not suitable for forming a steel plate of 1180 MPa or more because it is difficult to form high, or the strength of the steel plate after forming is reduced by heating and a predetermined strength cannot be obtained.

これら問題を解決する手法として、軟質な鋼板を所定のサイズに切断後、鋼板を700℃以上のオーステナイト単相域まで過熱した後、特開2002−18531号公報(特許文献5)に開示されているようなオーステナイト単相域でプレス成形を行い、その後焼き入れを行う熱間プレスと呼ばれる手法が存在する。これは鋼板強度が低下する高温で加工を行い、その後、焼き入れを行うことで高強度化が可能である。その結果、強度が980MPa以上で、かつ、形状凍結性に優れた部材を製造可能である。この手法によりCやMnのみを含有する単純な組成の鋼においても、980MPa以上の形状凍結性に優れた高強度部材の製造が可能となった。   As a method for solving these problems, after a soft steel plate is cut into a predetermined size, the steel plate is overheated to an austenite single phase region of 700 ° C. or higher, and then disclosed in Japanese Patent Application Laid-Open No. 2002-18531 (Patent Document 5). There is a technique called hot pressing in which press forming is performed in the austenite single-phase region, followed by quenching. It is possible to increase the strength by processing at a high temperature at which the strength of the steel sheet decreases and then performing quenching. As a result, a member having a strength of 980 MPa or more and excellent shape freezing property can be manufactured. With this method, even a steel having a simple composition containing only C and Mn can be manufactured with a high strength member having a shape freezing property of 980 MPa or more.

特開平1−230715号公報Japanese Patent Laid-Open No. 1-2230715 特開平2−217425号公報JP-A-2-217425 特開2002−143935号公報JP 2002-143935 A 特開2003−154413号公報JP 2003-154413 A 特開2002−18531号公報Japanese Patent Laid-Open No. 2002-18531

特許文献5に開示されている手法は、焼入れの際のオーステナイトからのマルテンサイト変態を利用した高強度化を行っているため、加熱炉より取り出した鋼板を700℃以上の高温で加工することが必須となる。しかしながら、加熱炉より取り出された鋼板は、すぐさま温度低下が引き起こされる、あるいは、成形前に鋼板の一部が金型との接触することで、冷却されてしまい700℃以上での加工が困難であることから、鋼板の加熱温度としては900℃超での高温加熱を余儀なくされる。さらには、高温まで鋼板を加熱するため、加熱炉中での在炉時間を長くする、もしくは、加熱炉の温度を上げる等の対策が必要となることから、生産性及び経済的に好ましくないという問題を孕んでいる。   Since the technique disclosed in Patent Document 5 uses a martensite transformation from austenite during quenching, the steel sheet taken out from the heating furnace can be processed at a high temperature of 700 ° C. or higher. Required. However, the temperature of the steel sheet taken out of the heating furnace is immediately reduced, or a part of the steel sheet comes into contact with the mold before forming and is difficult to process at 700 ° C. or higher. Therefore, the heating temperature of the steel sheet is inevitably high-temperature heating exceeding 900 ° C. Furthermore, because the steel sheet is heated to a high temperature, it is not preferable in terms of productivity and economy because measures such as increasing the in-furnace time in the heating furnace or increasing the temperature of the heating furnace are necessary. I have a problem.

あるいは、実際の熱間プレス成形を考えた場合、金型による成形前に、金型との接触により鋼板の一部が700℃未満となり、この部位にて十分な焼きが入らず、同一部材内においても材質がばらつくという問題を有する場合がある。同時に、900℃超の温度域に加熱された鋼板強度は非常に柔らかいため、加熱炉内での加熱中、あるいは、取り出した薄い鋼板を搬送する際に、撓んでしまうため搬送しがたい、あるいは、撓んだ鋼板を成形した場合、しわ等の成形部材の形状不良を引き起こすという問題がある。   Alternatively, when considering actual hot press forming, a part of the steel sheet becomes less than 700 ° C. due to contact with the mold before forming with the mold, and sufficient baking does not occur at this part, and the same member In some cases, the material may vary. At the same time, since the strength of the steel sheet heated to a temperature range exceeding 900 ° C. is very soft, it is difficult to transport because it is bent during heating in the heating furnace or when transporting the thin steel sheet taken out, or In the case of forming a bent steel plate, there is a problem of causing a shape defect of a forming member such as wrinkles.

さらには、900℃超の高温加熱を行った場合、鋼板の加熱中に結晶粒径が粗大化してしまい、同一の鋼を800℃にて加熱を行い、その後熱間プレス成形を行った部材に比べ靭性に劣るという問題を有している。
本発明は上記課題を解決し、生産性に優れた熱間プレス用鋼板の成形方法を提供することを目的とする。
Furthermore, when high-temperature heating exceeding 900 ° C. is performed, the crystal grain size becomes coarse during the heating of the steel sheet, and the same steel is heated at 800 ° C. and then subjected to hot press forming. It has the problem that it is inferior to toughness.
An object of the present invention is to solve the above-mentioned problems and to provide a method for forming a hot-press steel sheet having excellent productivity.

本発明者等は、鋼板成分と熱間プレス成形方法の関係について種々の検討を行った結果、鋼板中に含まれるMn量を2%超とし、TiおよびBを複合添加した鋼板を、400℃から700℃の温度域にて成形したとしても、十分な焼きが入り、かつ、均一な強度を有する部材が成形可能なことを見出した。この結果、従来の熱間プレスの問題点であった高温加熱に要する加熱時間の増大や鋼板強度低下に伴う生産性の低下が解決できる。加えて、得られる部材は内部での材質変動が少なく、部材形状も良好で均一性に優れた自動車用部材の製造が可能である。この効果は、鋼中のMn量を2%超とし、Ti量を0.01以上、B量を0.0002%以上含有する鋼を、750℃から900℃で加熱し、400℃から700℃の温度域にて成形することで発揮される。   As a result of various studies on the relationship between the steel plate components and the hot press forming method, the present inventors have determined that a steel plate in which the amount of Mn contained in the steel plate exceeds 2% and Ti and B are added in combination is 400 ° C. It was found that a member having sufficient baking and uniform strength can be formed even if it is molded at a temperature range of from 700 to 700 ° C. As a result, an increase in heating time required for high-temperature heating and a decrease in productivity due to a reduction in steel sheet strength, which are problems of conventional hot pressing, can be solved. In addition, the obtained member is less susceptible to internal material variation, and the member shape is good, and it is possible to produce a member for an automobile having excellent uniformity. This effect is achieved by heating a steel containing more than 2% of Mn in steel, Ti content of 0.01 or more and B content of 0.0002% or more from 750 ° C to 900 ° C, and from 400 ° C to 700 ° C. It is demonstrated by molding in the temperature range.

(1)鋼板が、質量%で、C:0.1〜0.40%、Si:0.5%以下、Mn:2.8〜4.0%、B:0.0002〜0.1%、Ti:0.001〜3.0%、P:0.1%以下、S:0.05%以下、Al:0.005〜0.1%、N:0.01%以下を含有し、残部がFeおよび不可避的不純物からなり、ミクロ組織が、フェライト及びパーライト、又はフェライト、セメンタイト及びパーライトより成る鋼を加熱速度1〜100℃/秒の加熱速度にて加熱し、750〜900℃の温度域で10〜6000秒の保持を行った後、400〜700℃の温度域にてプレス成形を行い、前記プレス成形中、金型パンチを下死点にて1〜60秒保持し、ダイとパンチを用いて1〜500℃/秒の範囲内の冷却速度にて鋼板を冷却し、冷却後面積率で95%以上のマルテンサイト組織とすることを特徴とする生産性に優れた熱間プレス成形方法。 (1) The steel sheet is in mass%, C: 0.1 to 0.40%, Si: 0.5% or less, Mn: 2.8 to 4.0%, B: 0.0002 to 0.1% Ti: 0.001 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less, The balance consists of Fe and inevitable impurities, and the microstructure consists of ferrite and pearlite, or steel consisting of ferrite, cementite and pearlite, heated at a heating rate of 1 to 100 ° C./second, and a temperature of 750 to 900 ° C. After holding in the region for 10 to 6000 seconds, press molding is performed at a temperature range of 400 to 700 ° C. During the press molding, the die punch is held at bottom dead center for 1 to 60 seconds, After cooling the steel plate using a punch at a cooling rate in the range of 1 to 500 ° C./sec. Excellent hot press forming method in productivity, which comprises 95% or more martensitic structure at moment.

(2)前記鋼板が、質量%で、さらに、Mo:0.01〜3.0%を含有することを特徴とする(1)に記載の生産性に優れた熱間プレス成形方法。
)前記鋼板が、質量%で、さらに、Nb:0.01〜3.0%、V:0.001〜3.0%、W:0.005〜3.0%、の1種または2種以上を含有することを特徴とする(1)または(2)に記載の生産性に優れた熱間プレス成形方法。
(2) The hot-press forming method with excellent productivity according to (1), wherein the steel sheet further contains Mo: 0.01 to 3.0% in terms of mass%.
( 3 ) The said steel plate is 1 mass%, and also Nb: 0.01-3.0%, V: 0.001-3.0%, W: 0.005-3.0%, or 1 type or The hot press molding method excellent in productivity as described in (1) or (2) , comprising two or more kinds.

)前記鋼板が、質量%で、さらに、REM:0.0005〜0.01%、Y:0.0005〜0.01%、Ca:0.0005〜0.01%、Mg:0.0005〜0.01%の1種または2種以上を含有することを特徴とする(1)〜()の何れか1項に記載の生産性に優れた熱間プレス成形方法。
プレス成形後、金型パンチを下死点にて保持することなく、下死点まで移動させ、鋼板を金型より取り出し、液体又は気体にて冷却することを特徴とする(1)〜()の何れか1項に記載の生産性に優れた熱間プレス成形方法。
( 4 ) The said steel plate is the mass%, and also REM: 0.0005-0.01%, Y: 0.0005-0.01%, Ca: 0.0005-0.01%, Mg: 0.00. The hot press molding method with excellent productivity according to any one of (1) to ( 3 ), comprising one or more of 0005 to 0.01%.
( 5 ) After press molding, the mold punch is moved to the bottom dead center without being held at the bottom dead center, and the steel plate is taken out from the mold and cooled with liquid or gas (1). The hot press molding method excellent in productivity as described in any one of ( 4 ).

)鋼板が、その表面にAlを主体とする金属皮膜を有することを特徴とする(1)〜()の何れか1項に記載の生産性に優れた熱間プレス成形方法。
)部品の一部又は全部が(1)〜()の何れか1項に記載の方法にて成形した鋼板からなることを特徴とする自動車用部材にある。
なお、本発明にて、熱間プレス(あるいは、ホットプレスとも言う。)とは、Ac3 以上の温度域に加熱した鋼板をプレス成形する際に、金型を用いて焼き入れることで鋼板を強化する成形方法である。
( 6 ) The hot press forming method with excellent productivity according to any one of (1) to ( 5 ), wherein the steel sheet has a metal film mainly composed of Al on the surface thereof.
( 7 ) It exists in the member for motor vehicles characterized by consisting of the steel plate shape | molded by the method of any one of (1)-( 6 ).
In the present invention, hot pressing (also referred to as hot pressing) refers to a steel plate that is quenched by using a mold when a steel plate heated to a temperature range of Ac 3 or higher is press-formed. This is a strengthening molding method.

本発明により、加熱温度を900℃以下、成形温度を700℃以下という低温域で成形すること、及び成形時の下死点保持の短縮を行うことが可能となり、大幅な生産性の向上をもたらすことができ工業的に大きな価値がある。加えて、成形された部材の材質は均一であり、高温加熱を行った部材に比べ靭性の観点でも優れる。   According to the present invention, it is possible to perform molding in a low temperature range of 900 ° C. or less and a molding temperature of 700 ° C. or less, and to shorten the bottom dead center retention at the time of molding, resulting in significant productivity improvement. Can be industrially of great value. In addition, the material of the molded member is uniform and is excellent in terms of toughness as compared with a member subjected to high temperature heating.

以下に本発明を更に詳細に説明する。
まず、本発明における鋼の化学成分の限定理由について説明する。
C:Cは、鋼板の強度を上昇できる元素である。また、CはAc3 点を下げることから、鋼板の加熱温度を下げることから、その下限を0.1質量%とした。一方では、Cが0.4質量%を超えると溶接が困難となる事から、その上限を0.4質量%とした。
Si:Siは、強化元素であり、鋼板の強度を上昇させることに有効である。しかしながら、SiはAc3 点を上昇させるため、加熱温度の増加を招くため、0.5%を上限とする。下限は特に定めないが、極低下は製造コストの高騰を招くことから、0.005%以上の添加とすることが望ましい。
The present invention is described in further detail below.
First, the reasons for limiting the chemical components of steel in the present invention will be described.
C: C is an element that can increase the strength of the steel sheet. C lowers the Ac 3 point and lowers the heating temperature of the steel sheet, so the lower limit was made 0.1 mass%. On the other hand, if C exceeds 0.4 mass%, welding becomes difficult, so the upper limit was made 0.4 mass%.
Si: Si is a strengthening element and is effective in increasing the strength of the steel sheet. However, since Si raises the Ac 3 point and causes an increase in heating temperature, the upper limit is 0.5%. Although the lower limit is not particularly defined, it is desirable to add 0.005% or more because the extreme decrease leads to an increase in manufacturing cost.

Mn:Mnは、フェライト変態を抑制することから、焼入れ性確保に非常に重要な元素である。加えて、Ac3 点の低下を引き起こすことから、加熱温度の低減も可能となる。この効果は添加量が2.8%以上となることで顕著と成ることから下限値を2.8%としたしかし、2.8%未満ではこの効果が得られないので、下限値を2.8%とした。逆に多いとP、Sとの共偏析を助長するだけでなく、製造時および熱延時の製造性に悪影響を及ぼすため4%以下、好ましくは3.0%を上限値とする。 Mn: Mn is a very important element for ensuring hardenability because it suppresses ferrite transformation. In addition, since the Ac 3 point is lowered, the heating temperature can be reduced. Since this effect becomes significant when the addition amount is 2.8 % or more, the lower limit is set to 2.8 % . However, this effect cannot be obtained at less than 2.8 %, so the lower limit was set at 2.8 %. On the other hand, if the amount is too large, it not only promotes co-segregation with P and S, but also adversely affects manufacturability during production and hot rolling, so the upper limit is made 4% or less, preferably 3.0%.

B:Bは、鋼板の強度上昇に有効な元素である。しかし、0.0002%未満ではこれらの効果が得られないため、下限値を0.0002%とした。逆に、0.1%超含有すると熱間加工性が劣化するため、上限値を0.1%とした。
Ti:Tiは、Nと化合物を作り、鋼板中の固溶N量を低減するため、Bを用いて鋼板の焼入れ性を向上させる場合には、添加することが望ましい。そこで下限値を0.001%とした。逆に、これらの元素を3%超含有すると、炭窒化物の析出が多くなり加工性および耐遅れ破壊性低下が生じるため、上限値を3%とした。また、Tiは、鋼板を再加熱する際、結晶粒の粒成長を抑制し、粒径を小さくする効果も有することから、靭性向上の観点からもその添加は望ましい。また、Tiを含有する析出物および晶出物は水素トラップサイトとなるため、耐水素脆化の観点からも重要である。
B: B is an element effective for increasing the strength of the steel sheet. However, since these effects cannot be obtained if the content is less than 0.0002%, the lower limit is set to 0.0002%. On the other hand, if the content exceeds 0.1%, the hot workability deteriorates, so the upper limit was made 0.1%.
Ti: Ti makes a compound with N and reduces the amount of solute N in the steel sheet. Therefore, when using B to improve the hardenability of the steel sheet, it is desirable to add Ti. Therefore, the lower limit is set to 0.001%. On the other hand, if these elements are contained in excess of 3%, the precipitation of carbonitrides increases and the workability and delayed fracture resistance decrease, so the upper limit was made 3%. Ti also has the effect of suppressing grain growth and reducing the grain size when reheating the steel sheet, so addition of Ti is also desirable from the viewpoint of improving toughness. In addition, since precipitates and crystallized substances containing Ti serve as hydrogen trap sites, they are also important from the viewpoint of hydrogen embrittlement resistance.

P:Pは、0.1質量%を超える量の添加では、溶接性ならびに製造時および熱延時の製造性に悪影響を及ぼす。このことから上限値を、0.1質量%とした。Pの下限値は特に定めないが、0.0001質量%未満とすることは、経済的に不利であることからこの値を下限値とすることが好ましい。
S:Sは、溶接性ならびに製造時および熱延時の製造性に悪影響を及ぼす。このことから、その上限値を0.05質量%以下とした。
Al:Alは、脱酸材として用いられるために0.005%以上を添加するが、Ac3 点の増加を引き起こし、高温加熱を余儀なくされるため0.1%を上限とする。
When P: P is added in an amount exceeding 0.1% by mass, it adversely affects weldability and manufacturability during production and hot rolling. Therefore, the upper limit value was set to 0.1% by mass. Although the lower limit value of P is not particularly defined, it is preferable to set this value as the lower limit value because it is economically disadvantageous to set it to less than 0.0001% by mass.
S: S adversely affects weldability and manufacturability during production and hot rolling. Therefore, the upper limit value is set to 0.05% by mass or less.
Al: Al is added as 0.005% or more in order to be used as a deoxidizing material, but causes an increase in Ac 3 point and requires high temperature heating, so 0.1% is made the upper limit.

N:Nは、加工性劣化や溶接時のブローホール発生にも寄与するため少ない方が良い。0.01%を越えると加工性が劣化してくるので、0.01%を上限とする。
Mo:Moは、鋼板の強度上昇、粒径の微細化及び焼入れ性向上に有効である。これらの効果は、添加量が0.01%未満ではこれらの効果が得られないため、下限値を0.01%とした。逆に、これらの元素を3%超含有すると、製造時および熱延時の製造性に悪影響を及ぼすため、上限値を3%とした。また、Moは、加熱炉を用いて鋼板を再加熱する際、結晶粒の粒成長を抑制し、粒径を小さくする効果も有することから、靭性向上の観点からもその添加は望ましい。
N: Since N contributes also to workability deterioration and blowhole generation at the time of welding, it is better to reduce N. If it exceeds 0.01%, workability deteriorates, so 0.01% is made the upper limit.
Mo: Mo is effective for increasing the strength of the steel sheet, reducing the grain size, and improving the hardenability. Since these effects cannot be obtained if the addition amount is less than 0.01%, the lower limit is set to 0.01%. On the other hand, if these elements are contained in excess of 3%, the manufacturability at the time of production and hot rolling is adversely affected, so the upper limit was made 3%. Moreover, Mo has the effect of suppressing the grain growth of the crystal grains and reducing the grain size when the steel sheet is reheated using a heating furnace, so addition of Mo is also desirable from the viewpoint of improving toughness.

Ni:Niは、強化元素であるとともに焼入れ性の向上に重要である。加えて、Ac3 点の低下を引き起こすことから、加熱温度の低減も可能となる。しかし、0.005%未満ではこれらの効果が得られないため下限値を0.005%とした。逆に、5%超では製造時および熱延時の製造性に悪影響を及ぼすため、上限値を5%とした。また、Ni硫化物が水素侵入を抑制し遅れ破壊特性を向上させる効果や、鋼板の焼入れ性を高めることにより鋼板の強度を確保する効果があることから鋼板への添加は有効である。 Ni: Ni is a strengthening element and is important for improving hardenability. In addition, since the Ac 3 point is lowered, the heating temperature can be reduced. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Conversely, if it exceeds 5%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was set to 5%. Further, since Ni sulfide has the effect of suppressing hydrogen intrusion and improving delayed fracture characteristics, and the effect of ensuring the strength of the steel sheet by enhancing the hardenability of the steel sheet, addition to the steel sheet is effective.

Cu:Cuは、強化に有効である上、強化元素であるとともに焼入れ性の向上に重要である。加えて、Ac3 点の低下を引き起こすことから、加熱温度の低減も可能となる。しかし、0.005%未満ではこれらの効果が得られないため下限値を0.005%とした。また、過剰添加は製造時および熱延時の製造性に悪影響を及ぼすため、上限を5.0%とした。 Cu: Cu is effective for strengthening and is an element for strengthening and important for improving hardenability. In addition, since the Ac 3 point is lowered, the heating temperature can be reduced. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. Further, excessive addition has an adverse effect on manufacturability during production and hot rolling, so the upper limit was made 5.0%.

Nb:Nbは、鋼板の強度上昇、粒径の微細化及び焼入れ性向上に有効である。これらの効果は、添加量が0.01%未満ではこれらの効果が得られないため、下限値を0.01%とした。逆に、これらの元素を3%超含有すると、炭窒化物の析出が多くなり加工性および耐遅れ破壊性低下が生じるため、上限値を3%とした。また、Nbは、鋼板を再加熱する際、結晶粒の粒成長を抑制し、粒径を小さくする効果も有することから、靭性向上の観点からもその添加は望ましい。   Nb: Nb is effective for increasing the strength of the steel sheet, reducing the grain size, and improving the hardenability. Since these effects cannot be obtained if the addition amount is less than 0.01%, the lower limit is set to 0.01%. On the other hand, if these elements are contained in excess of 3%, the precipitation of carbonitrides increases and the workability and delayed fracture resistance decrease, so the upper limit was made 3%. Nb also has the effect of suppressing grain growth and reducing the grain size when the steel sheet is reheated, so addition of Nb is also desirable from the viewpoint of improving toughness.

V:Vは、鋼板の強度上昇及び粒径の微細化に有効である上、Vを含有する析出物および晶出物は水素トラップサイトとなるため非常に重要な元素である。しかし、0.001%未満ではこの効果が得られないために、下限値を0.001%とした。逆に、これらの元素の1種又は2種以上の合計として3%超含有すると炭窒化物の析出が顕著になり、延性低下が著しくなる。このため上限値を3%とした。   V: V is an extremely important element because it is effective for increasing the strength of the steel sheet and making the grain size finer, and precipitates and crystallized substances containing V become hydrogen trap sites. However, since this effect cannot be obtained at less than 0.001%, the lower limit is set to 0.001%. On the other hand, if the total content of one or more of these elements exceeds 3%, the precipitation of carbonitrides becomes prominent and the ductility deteriorates remarkably. For this reason, the upper limit is set to 3%.

W:Wは、鋼板の強度上昇に有効である上、Wを含有する析出物および晶出物は水素トラップサイトとなるため非常に重要な元素である。しかし、0.005%未満ではこれらの効果が得られないため、下限値を0.005%とした。逆に、3%超含有すると加工性低下が生じるため、上限値を3%とした。また、Wは、鋼板を再加熱する際、結晶粒の粒成長を抑制し、粒径を小さくする効果も有することから、靭性向上の観点からもその添加は望ましい。   W: W is an extremely important element because it is effective for increasing the strength of the steel sheet and precipitates and crystallized substances containing W become hydrogen trap sites. However, since these effects cannot be obtained at less than 0.005%, the lower limit is set to 0.005%. On the contrary, if the content exceeds 3%, the workability deteriorates, so the upper limit was made 3%. Further, W has an effect of suppressing the grain growth of the crystal grains and reducing the grain size when the steel sheet is reheated, so addition of W is also desirable from the viewpoint of improving toughness.

REM、Ca、Yは、介在物の形態制御に有効で、耐遅れ破壊性に寄与することから、それぞれ0.0005%以上の添加とした。一方、過剰添加は熱間加工性を劣化させるため、それぞれ0.01%以下の添加とした。ここで、REMはRare Earth Metalの略でLaから始まるランタノイド系元素の総称である。
Mg:Mgは、自身の化合物が耐遅れ破壊向上に効果的なだけでなく、他元素との複合析出物または複合昇出物を生成させ、かつそれらの形態を耐遅れ破壊性向上に寄与するよう制御するために必要な元素であることから、0.0005%以上とした。しかし、0.01%超では粗大酸化物および硫化物を生成して、形態制御に効果的でなくなる上、薄鋼板の基本的要求特性である加工性を低下させるため、上限を0.01%とした。
REM, Ca, and Y are effective for controlling the shape of inclusions and contribute to delayed fracture resistance. On the other hand, since excessive addition deteriorates hot workability, the addition was made 0.01% or less. Here, REM is an abbreviation for Rare Earth Metal and is a general term for lanthanoid elements starting from La.
Mg: Mg is not only effective in improving delayed fracture resistance of its own compound, but also forms composite precipitates or composite ascendants with other elements, and contributes to improving the delayed fracture resistance of those forms. Since it is an element necessary for such control, the content was made 0.0005% or more. However, if it exceeds 0.01%, coarse oxides and sulfides are produced, which is not effective for shape control, and lowers the workability, which is a basic required characteristic of a thin steel sheet, so the upper limit is 0.01%. It was.

次に、熱間プレス用鋼板の組織限定理由について述べる。ミクロ組織をフェライト及びパーライト、又はフェライト、セメンタイト及びパーライトとしたのは、熱間プレス用鋼板の機械切断性を向上させるためである。本発明鋼は、多量のCを含有しているため、熱間プレス用鋼板としての強度が高くなる傾向にある。加えて、その組織をベイナイトやマルテンサイトを含む組織とすると、鋼板の強度が高くなり過ぎてしまい機械切断の際に、切断機を傷めてしまう恐れがある。そこで、鋼板のミクロ組織をフェライト及びパーライト、又はフェライト、セメンタイト及びパーライトとすることで、機械切断を行いやすい熱間プレス用鋼板とした。しかしながら、ガスやレーザーを用いて鋼板の切断を行う場合、鋼板組織をマルテンサイトやベイナイトを多く含む組織としたとしても、材質及び成形上何ら問題を生じず、本発明の熱間プレス成形に用いる鋼板の製造は可能である。   Next, the reason for limiting the structure of the steel sheet for hot pressing will be described. The reason why the microstructure is ferrite and pearlite, or ferrite, cementite and pearlite is to improve the mechanical cutability of the steel sheet for hot pressing. Since the steel according to the present invention contains a large amount of C, the strength as a steel sheet for hot pressing tends to increase. In addition, if the structure is a structure containing bainite or martensite, the strength of the steel sheet becomes too high, and the machine may be damaged during mechanical cutting. Then, it was set as the steel plate for hot press which is easy to perform a mechanical cutting by making the microstructure of a steel plate into ferrite and pearlite, or ferrite, cementite, and pearlite. However, when cutting a steel sheet using a gas or a laser, even if the steel sheet structure is a structure containing a lot of martensite and bainite, no problem is caused in terms of material and forming, and it is used for the hot press forming of the present invention. Steel sheets can be manufactured.

熱間プレス後の部材の組織限定理由について述べる。
面積率で95%以上をマルテンサイトとする理由は、980MPa以上、好ましくは1180MPa以上の引張強度を得るためであり、そのためには、硬質相であるマルテンサイトを面積率で100%とすることが好ましい。その他の組織として、フェライト、ベイナイト、残留オーステナイトの1種又は2種以上を面積率で5%以下含有しても良い。ただし、ここで言う面積率100%とは、当然鋼材中には不可避的不純物、炭化物及び介在物が存在し、厳密には100%とならないが、光学顕微鏡での観察ではこれらの不可避的不純物や介在物が認識できないレベルの大きさで存在することから、100%であるとした。
The reason for limiting the structure of the member after hot pressing will be described.
The reason why 95% or more of the area ratio is martensite is to obtain a tensile strength of 980 MPa or more, preferably 1180 MPa or more. For this purpose, the martensite that is the hard phase is made 100% by area ratio. preferable. As other structures, one or more of ferrite, bainite, and retained austenite may be contained in an area ratio of 5% or less. However, the area ratio of 100% here naturally means that inevitable impurities, carbides and inclusions are present in the steel material, and strictly speaking, it is not 100%. However, these inevitable impurities and Since inclusions exist at a level that cannot be recognized, it was assumed to be 100%.

次に熱間プレス成形方法について述べる。
熱間プレス成形の条件は、1〜100℃/秒で昇温して750から900℃に加熱して、10から6000秒保持後、プレスダイスの上に置きプレスを行う。昇温速度を1〜100℃/秒の範囲としたのは、昇温速度が1℃/秒より遅いと生産性が低下することから好ましくない。一方、100℃/秒より速くする事は通常の炉の昇温では不可能である。しかしながら、高周波加熱等によって、100℃/秒を上回る昇温速度で加熱したとしても、本発明の効果は得ることができる。
Next, the hot press molding method will be described.
The conditions for hot press molding are 1 to 100 ° C./second, heated to 750 to 900 ° C., held for 10 to 6000 seconds, placed on a press die and pressed. The temperature rising rate in the range of 1 to 100 ° C./second is not preferable when the temperature rising rate is slower than 1 ° C./second because productivity decreases. On the other hand, it is impossible to increase the temperature faster than 100 ° C./second with a normal furnace temperature rise. However, the effect of the present invention can be obtained even if heating is performed at a temperature rising rate exceeding 100 ° C./second by high-frequency heating or the like.

加熱最高温度を750から900℃としたのは、加熱温度が750℃より低いと、焼き入れ前の組織がオーステナイト単相とならず、フェライト及びオーステナイトの2相からの焼きいれとなり、所定の強度が得られないからであり、900℃より高いと本発明の効果である優れた生産性が発揮されない。しかしながら、900℃超の温度域に加熱し成形を行ったとしても、材質変動のない自動車用部材の生産は可能である。
同時に、加熱温度の低下により、オーステナイト粒の粗大化が抑制され、靭性の向上が図られることから、自動車用部材の特性向上の観点からも好ましい。
The maximum heating temperature was set to 750 to 900 ° C. When the heating temperature was lower than 750 ° C., the structure before quenching did not become an austenite single phase, but tempered from two phases of ferrite and austenite. Is not obtained, and when it is higher than 900 ° C., the excellent productivity that is the effect of the present invention is not exhibited. However, even if molding is performed by heating in a temperature range exceeding 900 ° C., it is possible to produce automobile members without material fluctuation.
At the same time, the reduction of the heating temperature suppresses the coarsening of the austenite grains and improves the toughness, which is preferable from the viewpoint of improving the characteristics of the automotive member.

また、最高到達温度での保持時間を10から6000秒としたのは、保持時間が10秒未満であると鋼板内部では所定の温度となっておらず、鋼板内部では十分な量のオーステナイトが得られておらず、焼き入れ後に所定の強度が得られないため10秒以上とする。また、6000秒より長時間保持するとオーステナイト粒が粗大化し、焼入れ後に所定の強度が得られないため6000秒以下とする。   In addition, the holding time at the highest temperature reached 10 to 6000 seconds. If the holding time is less than 10 seconds, the steel plate does not reach a predetermined temperature, and a sufficient amount of austenite is obtained inside the steel plate. Since a predetermined strength cannot be obtained after quenching, it is set to 10 seconds or more. Further, if held for a longer time than 6000 seconds, the austenite grains become coarse and a predetermined strength cannot be obtained after quenching, so the time is set to 6000 seconds or less.

プレス時の加工温度は400から700℃の温度範囲とする。下限値を400℃としたのは、温度低下に伴い鋼板強度が向上し、金型を傷つける恐れがあるためである。しかしながら、鋼板温度が400℃未満にて加工されたとしても、同様の機械特性が発揮される。一方、上限の成形温度を700℃としたのは、900℃に過熱された鋼板を取り出し、すぐさま成形を行ったとしても、鋼板の一部が金型との接触により700℃以下となり、実質700℃超での成形が困難なためである。しかしながら、700℃超の温度域で成形したとしても、材質の均一な部材の成形は可能である。   The processing temperature during pressing is in the temperature range of 400 to 700 ° C. The lower limit value is set to 400 ° C. because the steel plate strength is improved as the temperature is lowered, and the mold may be damaged. However, even if the steel plate temperature is processed at less than 400 ° C., the same mechanical properties are exhibited. On the other hand, the upper limit forming temperature is set to 700 ° C. Even when the steel sheet heated to 900 ° C. is taken out and formed immediately, a part of the steel plate becomes 700 ° C. or less due to contact with the mold, which is substantially 700 ° C. This is because it is difficult to mold at a temperature higher than ° C. However, even if molding is performed in a temperature range exceeding 700 ° C., it is possible to mold a member having a uniform material.

プレス成形時の冷却速度は1〜500℃/秒とすることが好ましい。ここでプレス成形時の冷却速度を1℃/秒未満とすると、冷却に過度の時間を要するため経済的に好ましくないためである。一方、500℃/秒より早くする事は製造上困難であるためである。この際の冷却手法は、金型を用いた冷却を基本とするが、成形中に水をはじめとする液体やガスを用いた冷却を行ったとしても何ら問題は生じない。   The cooling rate during press molding is preferably 1 to 500 ° C./second. This is because if the cooling rate during press molding is less than 1 ° C./second, it takes an excessive amount of time for cooling, which is economically undesirable. On the other hand, it is difficult to make the speed faster than 500 ° C./sec. The cooling method at this time is based on cooling using a mold, but no problem occurs even if cooling is performed using liquid or gas including water during molding.

また、鋼板を成形する際、十分な焼入れ性を確保するため、パンチとダイスを下死点にて1から60秒の下死点保持を行うことが望ましい。下死点における保持時間が1秒以上であれば十分に焼きが入り、面積率で95%以上のマルテンサイト組織となり、980MPa以上の強度が確保される。下死点での保持を行わなかったとしても取り出した鋼板は大気により冷却されることから焼きが入り、高強度の自動車用部材の生産は可能である。一方、保持時間の上限を60秒としたのは、過度の保持時間の増大は、生産性の低下をもたらし経済的に好ましくないためである。しかしながら、60秒を超える保持を行ったとしても、材質変動のない自動車用部材の生産は可能である。   Further, when forming the steel sheet, it is desirable to hold the bottom dead center for 1 to 60 seconds at the bottom dead center in order to ensure sufficient hardenability. If the holding time at the bottom dead center is 1 second or more, the steel is sufficiently baked to obtain a martensite structure with an area ratio of 95% or more, and a strength of 980 MPa or more is ensured. Even if the holding at the bottom dead center is not carried out, the steel sheet taken out is cooled by the atmosphere, so that it becomes baked and production of a high-strength automobile member is possible. On the other hand, the upper limit of the holding time is set to 60 seconds because an excessive increase in the holding time causes a decrease in productivity and is not economically preferable. However, even if holding is performed for more than 60 seconds, it is possible to produce automobile parts that do not vary in material quality.

さらには、金型にての成形後、直ちに鋼板を金型より取り出し、空冷、窒素ガス等の気体、水や有機溶媒等の液体を用いた冷却を行うと、面積率で95%以上のマルテンサイト組織となり、980MPa以上の強度が確保されるので好ましい。
また、本発明の熱間プレス用鋼板の素材は、通常行われている熱延鋼板、冷延鋼板及びAlを主体としためっき鋼板製造設備の内いずれの設備を用いても製造可能である。Alを主体とする金属皮膜とは、質量%で、Si:1〜15%、Mg:0.5〜10%を含有するAl系めっきであり、めっきにはZnを1〜60%の範囲内で添加してもよい。
Furthermore, immediately after forming in the mold, the steel sheet is taken out of the mold and cooled with air, a gas such as nitrogen gas, or a liquid such as water or an organic solvent. It becomes a site structure and is preferable because strength of 980 MPa or more is secured.
Moreover, the raw material of the steel sheet for hot pressing of the present invention can be manufactured using any of the commonly used hot-rolled steel sheet, cold-rolled steel sheet, and plated steel sheet manufacturing equipment mainly composed of Al. The metal film mainly composed of Al is Al-based plating containing, by mass%, Si: 1 to 15% and Mg: 0.5 to 10%, and the plating contains Zn in the range of 1 to 60%. May be added.

次に、実施例で本発明をより詳細に説明する。表1に示す成分の鋼を溶製し、常法に従い連続鋳造でスラブとした。符号A〜Kが本発明に従った成分の鋼で、符号LとMは成分が逸脱するものである。これらの鋼片を加熱炉中で1160℃〜1250℃の温度で加熱し、仕上げ温度を850℃とし620℃まで4〜8℃/秒で空冷後巻き取った。この熱延板を50%冷延し、それぞれ冷延鋼板及びAlメッキ鋼板とした。冷延鋼板は最高加熱温度750℃で90秒保持後、10℃/秒で450℃まで冷却後200秒保持したのち空冷した。また、Alメッキ鋼板は、最高加熱750℃で90秒保持後、500℃のAl浴中に浸漬し、ガスワイピングでめっき付着量を片面あたり25μmに調節した。このときのめっき組成は、主成分のAl以外に残部成分としてSi:9.5%、Cr:0.05%、それに2%のFeが含まれていたが、Feは浴中の機器やストリップから供給される不可避のものである。   Next, an Example demonstrates this invention in detail. Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. Symbols A to K are steels of the components according to the present invention, and symbols L and M are components that deviate. These steel pieces were heated in a heating furnace at a temperature of 1160 ° C. to 1250 ° C., the finishing temperature was set to 850 ° C., air-cooled to 620 ° C. at 4 to 8 ° C./second, and wound up. This hot-rolled sheet was 50% cold-rolled to obtain a cold-rolled steel sheet and an Al-plated steel sheet, respectively. The cold-rolled steel sheet was held at a maximum heating temperature of 750 ° C. for 90 seconds, cooled to 450 ° C. at 10 ° C./second, held for 200 seconds, and then air-cooled. The Al-plated steel sheet was maintained at a maximum heating of 750 ° C. for 90 seconds, then immersed in an Al bath at 500 ° C., and the amount of plating adhered was adjusted to 25 μm per side by gas wiping. The plating composition at this time contained Fe: Si: 9.5%, Cr: 0.05%, and 2% as the remaining components in addition to the main component Al. Inevitable that is supplied from.

こうして製造した冷延鋼板、およびめっき鋼板を1〜100℃/秒で昇温して800〜1000℃に加熱した後、300秒間この温度で保持を行った後、成形直前の板温を500〜900℃と変化させ、常温の金型でプレスを行い、下死点にて30秒保持を行った。また、一部の部材に関しては、下死点での保持を行わず直ちに取り出し空冷を施した。その後、成形した部材に関してその特性調査を行った。材質調査はプレス成形の際に冷却された部位より、JIS Z 2201 5号試験片を加工し、同2241記載の試験方法に従って行った。本発明鋼AからKは、本発明条件で製造したものは、加熱温度及び成形時の温度が低い場合においても、熱間プレス後の高強度が確保されており、かつ、成形温度に材質が依存せず、機械特性の変動の少ない鋼板が得られた。その結果を表2〜4に示す。   The cold-rolled steel sheet and the plated steel sheet thus manufactured were heated at 1 to 100 ° C./second and heated to 800 to 1000 ° C., then held at this temperature for 300 seconds, and then the sheet temperature immediately before forming was changed to 500 to 500 ° C. The temperature was changed to 900 ° C., pressing was performed with a normal temperature mold, and held at the bottom dead center for 30 seconds. Some members were immediately taken out and air-cooled without being held at the bottom dead center. Thereafter, the characteristics of the molded member were investigated. The material investigation was performed according to the test method described in 2241 by processing a JIS Z 2201 No. 5 test piece from the part cooled during press molding. Steels A to K of the present invention are manufactured under the conditions of the present invention, and high strength after hot pressing is ensured even when the heating temperature and the temperature at the time of molding are low, and the material at the molding temperature is A steel sheet with little variation in mechanical properties was obtained without depending on it. The results are shown in Tables 2-4.

Figure 0004673558
Figure 0004673558

Figure 0004673558
Figure 0004673558

Figure 0004673558
Figure 0004673558

Figure 0004673558
Figure 0004673558

Claims (7)

鋼板が、質量%で、C:0.1〜0.40%、Si:0.5%以下、Mn:2.8〜4.0%、B:0.0002〜0.1%、Ti:0.001〜3.0%、P:0.1%以下、S:0.05%以下、Al:0.005〜0.1%、N:0.01%以下を含有し、残部がFeおよび不可避的不純物からなり、ミクロ組織が、フェライト及びパーライト、又はフェライト、セメンタイト及びパーライトより成る鋼を加熱速度1〜100℃/秒の加熱速度にて加熱し、750〜900℃の温度域で10〜6000秒の保持を行った後、400〜700℃の温度域にてプレス成形を行い、前記プレス成形中、金型パンチを下死点にて1〜60秒保持し、ダイとパンチを用いて1〜500℃/秒の範囲内の冷却速度にて鋼板を冷却し、冷却後面積率で95%以上のマルテンサイト組織とすることを特徴とする生産性に優れた熱間プレス成形方法。 Steel sheet, in mass%, C: 0.1~0.40%, Si : 0.5% or less, Mn: 2.8 ~4.0%, B : 0.0002~0.1%, Ti: 0.001 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less, with the balance being Fe And steel composed of ferrite and pearlite, or ferrite, cementite and pearlite at a heating rate of 1 to 100 ° C./second and having a microstructure of 10 in a temperature range of 750 to 900 ° C. After holding for ˜6000 seconds, press molding is performed at a temperature range of 400 to 700 ° C. During the press molding, the die punch is held at the bottom dead center for 1 to 60 seconds, and a die and a punch are used. The steel sheet is cooled at a cooling rate in the range of 1 to 500 ° C./second, and the area ratio after cooling Excellent hot press forming method in productivity, which comprises 95% or more of martensite. 前記鋼板が、質量%で、さらに、Mo:0.01〜3.0%を含有することを特徴とする請求項1に記載の生産性に優れた熱間プレス成形方法。 The hot-press forming method with excellent productivity according to claim 1, wherein the steel sheet further contains Mo: 0.01 to 3.0% by mass%. 前記鋼板が、質量%で、さらに、
Nb:0.01〜3.0%、
V:0.001〜3.0%、
W:0.005〜3.0%、
の1種または2種以上を含有することを特徴とする請求項1または2に記載の生産性に優れた熱間プレス成形方法。
The steel sheet is in mass%, and
Nb: 0.01-3.0%
V: 0.001 to 3.0%,
W: 0.005 to 3.0%,
The hot press molding method excellent in productivity according to claim 1 or 2 , characterized by containing one or more of the following.
前記鋼板が、質量%で、さらに、
REM:0.0005〜0.01%、
Y:0.0005〜0.01%、
Ca:0.0005〜0.01%、
Mg:0.0005〜0.01%、
の1種または2種以上を含有することを特徴とする請求項1〜の何れか1項に記載の生産性に優れた熱間プレス成形方法。
The steel sheet is in mass%, and
REM: 0.0005 to 0.01%,
Y: 0.0005 to 0.01%
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%,
The hot press molding method excellent in productivity according to any one of claims 1 to 3 , wherein one or more of these are contained.
プレス成形後、金型パンチを下死点にて保持することなく、下死点まで移動させ、鋼板を金型より取り出し、液体又は気体にて冷却することを特徴とする請求項1〜の何れか1項に記載の生産性に優れた熱間プレス成形方法。 After press molding, without holding the die punch at the bottom dead center is moved to the bottom dead center, the steel plate was taken out from the mold, according to claim 1-4, characterized in that cooling in liquid or gaseous The hot press molding method excellent in productivity described in any one of the items. 鋼板が、その表面にAlを主体とする金属皮膜を有することを特徴とする請求項1〜の何れか1項に記載の生産性に優れた熱間プレス成形方法。
法。
The hot press forming method with excellent productivity according to any one of claims 1 to 5 , wherein the steel sheet has a metal film mainly composed of Al on a surface thereof.
Law.
部品の一部又は全部が請求項1〜の何れか1項に記載の方法にて成形した鋼板からなることを特徴とする自動車用部材。 A part for the whole consists of the steel plate shape | molded by the method of any one of Claims 1-6 , The member for motor vehicles characterized by the above-mentioned.
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