JP5994748B2 - High-strength press parts and manufacturing method thereof - Google Patents
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- 238000004519 manufacturing process Methods 0.000 title claims description 24
- 229910000831 Steel Inorganic materials 0.000 claims description 79
- 239000010959 steel Substances 0.000 claims description 79
- 239000010410 layer Substances 0.000 claims description 44
- 239000002344 surface layer Substances 0.000 claims description 35
- 238000000034 method Methods 0.000 claims description 27
- 238000010791 quenching Methods 0.000 claims description 21
- 230000000171 quenching effect Effects 0.000 claims description 16
- 238000005496 tempering Methods 0.000 claims description 16
- 229910052799 carbon Inorganic materials 0.000 claims description 15
- 239000000203 mixture Substances 0.000 claims description 11
- 230000009466 transformation Effects 0.000 claims description 11
- 239000012535 impurity Substances 0.000 claims description 8
- 239000003792 electrolyte Substances 0.000 claims description 2
- 235000019589 hardness Nutrition 0.000 description 33
- 239000000463 material Substances 0.000 description 12
- 238000010438 heat treatment Methods 0.000 description 10
- 229910000734 martensite Inorganic materials 0.000 description 10
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 9
- 238000000137 annealing Methods 0.000 description 9
- 230000000694 effects Effects 0.000 description 9
- 238000005336 cracking Methods 0.000 description 8
- 238000010521 absorption reaction Methods 0.000 description 7
- 229910001566 austenite Inorganic materials 0.000 description 7
- 239000010960 cold rolled steel Substances 0.000 description 6
- 238000000465 moulding Methods 0.000 description 6
- 229910000859 α-Fe Inorganic materials 0.000 description 6
- 239000011358 absorbing material Substances 0.000 description 4
- 238000005098 hot rolling Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- 238000004458 analytical method Methods 0.000 description 2
- 229910001563 bainite Inorganic materials 0.000 description 2
- 238000005452 bending Methods 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 238000009749 continuous casting Methods 0.000 description 2
- 238000001816 cooling Methods 0.000 description 2
- 238000004453 electron probe microanalysis Methods 0.000 description 2
- 229910001562 pearlite Inorganic materials 0.000 description 2
- 239000002994 raw material Substances 0.000 description 2
- 238000005096 rolling process Methods 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 239000006096 absorbing agent Substances 0.000 description 1
- 230000033228 biological regulation Effects 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
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- 239000003245 coal Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
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- 238000005260 corrosion Methods 0.000 description 1
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- 238000005520 cutting process Methods 0.000 description 1
- 238000005261 decarburization Methods 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000007731 hot pressing Methods 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
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Description
本発明は、主に自動車産業分野で使用される高強度プレス部品であって、特に引張強さ（TS）が1300MPa以上となる高強度プレス部品およびその製造方法に関するものである。 The present invention relates to a high-strength press part mainly used in the automobile industry field, and particularly to a high-strength press part having a tensile strength (TS) of 1300 MPa or more and a method for manufacturing the same.
近年、地球環境の保全の観点から、自動車排ガス規制が強化されている。このような状況下、自動車の燃費向上が重要な課題となっており、自動車部品の高強度・薄肉化が要求されている。自動車部品の高強度・薄肉化を図る手段としては、自動車部品の素材として鋼板を用い、プレス焼入れにより鋼板を所望の部品形状に成形する手段が知られている。プレス焼入れでは、オーステナイト単相域（Ac3変態点以上の温度域）まで加熱したブランク（鋼板）を、ダイとパンチからなる金型を用いて所望の形状に熱間プレス成形しつつ、金型内で抜熱して焼入れを行う。 In recent years, automobile exhaust gas regulations have been strengthened from the viewpoint of conservation of the global environment. Under such circumstances, improvement in fuel efficiency of automobiles is an important issue, and high strength and thinning of automobile parts are required. As means for increasing the strength and thickness of automobile parts, there is known a means for forming a steel sheet into a desired part shape by press hardening using a steel sheet as a material for the automobile part. In press quenching, a blank (steel plate) heated to an austenite single-phase region (temperature range above the Ac 3 transformation point) is hot-pressed into a desired shape using a die and die mold, Heat is removed from the inside and quenched.
以上のように、プレス焼入れでは、高温域に加熱した鋼板、すなわち軟質化して加工し易い状態にある鋼板をプレス成形するため、鋼板を複雑な部品形状に成形することができる。また、鋼板を所望の部品形状に成形しつつ焼入れを行うため、成形後には引張強さ（TS）が1500MPaを超えるような強度の極めて高いホットプレス部品が得られる。更に、金型内で焼入れを行うため、熱処理ひずみの抑制が可能であり、寸法精度に優れたホットプレス部品が得られる。 As described above, in press hardening, a steel plate heated to a high temperature region, that is, a steel plate that is softened and easily processed is press-formed, so that the steel plate can be formed into a complicated part shape. In addition, since the steel sheet is quenched while being formed into a desired part shape, a hot-pressed part having an extremely high strength such that the tensile strength (TS) exceeds 1500 MPa is obtained after forming. Furthermore, since quenching is performed in the mold, it is possible to suppress heat treatment distortion and to obtain a hot-pressed part with excellent dimensional accuracy.
しかしながら、プレス焼入れにより製造された従来のホットプレス部品は、板厚全域に亘り焼入れ組織（主にマルテンサイト組織）が形成されているため、延性が低い。このように延性の低い部品を、自動車のエネルギー吸収材などの衝突変形時に大きな変形を受ける部位に使用すると、衝突時に割れが生じて、耐衝突特性が低下する問題がある。ゆえに、従来、プレス焼入れにより製造されたホットプレス部品は、自動車部品に適用する際、適用部位が限定されていた。 However, conventional hot-pressed parts manufactured by press quenching have a low ductility because a quenched structure (mainly martensite structure) is formed over the entire plate thickness. When such a part having low ductility is used in a portion that undergoes a large deformation during a collision deformation such as an energy absorbing material of an automobile, there is a problem that a crack occurs during the collision and the collision resistance is deteriorated. Therefore, hitherto, hot-pressed parts manufactured by press hardening have limited application sites when applied to automobile parts.
上記の問題に対し、ホットプレス部品の延性を高めて割れを抑制する技術が提案されている。例えば特許文献１には、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板を、加熱速度1〜100℃/秒の加熱速度にて加熱し、700〜850℃の温度域で10〜6000秒の保持を行い、550〜700℃の温度域にてプレス成形を行うことで、成形後の鋼板の組織を、主相として冷却後面積率で40〜90％のフェライトを含有し、第二相として10〜60％のマルテンサイトを含有し、残部組織がベイナイトから成る組織とする技術が提案されている。そして、特許文献１で提案された技術によると、鋼板をフェライトとオーステナイトの２相域に加熱して熱間プレスを行い、熱間プレス成形後の組織をフェライトとマルテンサイトの２相組織とすることで、プレス成形後の鋼板において780N/mm2以上の引張強さが得られるとともに、良好な延性が確保できるとされている。 In order to solve the above problem, a technique for suppressing cracking by increasing the ductility of a hot-pressed component has been proposed. For example, in Patent Document 1, a steel sheet having a microstructure of ferrite and pearlite, or ferrite, cementite and pearlite is heated at a heating rate of 1 to 100 ° C./second, and 10 to 10 in a temperature range of 700 to 850 ° C. Holds for 6000 seconds, and press forming in a temperature range of 550 to 700 ° C., the structure of the steel sheet after forming, containing 40 to 90% ferrite in the area ratio after cooling as the main phase, A technique has been proposed in which 10 to 60% martensite is contained as the second phase, and the remaining structure is composed of bainite. According to the technique proposed in Patent Document 1, the steel sheet is heated to a two-phase region of ferrite and austenite to perform hot pressing, and the structure after hot press forming is a two-phase structure of ferrite and martensite. Thus, it is said that a steel sheet after press forming can have a tensile strength of 780 N / mm 2 or more and can ensure good ductility.
また、特許文献２には、鋼板を、750℃以上1000℃以下の温度に加熱し、5〜1000秒間保持したのち、350℃以上900℃以下の温度域で熱間プレスを行い、ついで50℃以上350℃以下の温度まで冷却した後、塩浴炉を用いて350℃以上490℃以下の温度域に昇温し、該温度域に5秒以上1000秒以下保持することにより、マルテンサイトと残留オーステナイトとベイニティックフェライトを含むベイナイトを有し、マルテンサイトのうちの25％以上が焼戻しマルテンサイトである組織を有する高強度プレス部材とする技術が提案されている。そして、特許文献２で提案された技術によると、焼入れ後に焼戻し処理を行い、プレス部材の組織を焼戻しマルテンサイトを含む所望の複合組織とすることで、強度と延性に優れ、かつ引張強さが980MPa以上の高強度プレス部材が得られるとされている。 In Patent Document 2, a steel sheet is heated to a temperature of 750 ° C. or higher and 1000 ° C. or lower, held for 5 to 1000 seconds, hot-pressed in a temperature range of 350 ° C. or higher and 900 ° C. or lower, and then 50 ° C. After cooling to a temperature of 350 ° C. or lower, the temperature is raised to 350 ° C. or higher and 490 ° C. or lower using a salt bath furnace. There has been proposed a technique for forming a high-strength press member having bainite containing austenite and bainitic ferrite and having a structure in which 25% or more of martensite is tempered martensite. And according to the technique proposed by patent document 2, it is excellent in intensity | strength and ductility, and tensile strength by performing the tempering process after quenching, and making the structure | tissue of a press member into the desired composite structure containing a tempered martensite. It is said that a high-strength press member of 980 MPa or more can be obtained.
しかしながら、特許文献１で提案された技術により得られるホットプレス部品は、その強度が引張強さで高々1270MPa程度である。また、延性に関しても不十分であり、自動車のエネルギー吸収材などで問題となる自動車衝突時の割れを抑制するには更なる改善が必要となる。 However, the hot-pressed part obtained by the technique proposed in Patent Document 1 has a tensile strength of about 1270 MPa at most. Further, the ductility is also insufficient, and further improvement is required to suppress cracking at the time of automobile collision, which is a problem with automobile energy absorbers and the like.
一方、特許文献２で提案された技術によれば、十分な強度と延性を兼ね備えたプレス部材を製造し得る。しかしながら、特許文献２で提案された技術では、プレス焼入れ後に再加熱・保持して板厚全体に焼戻し処理を施すことを必須としていることから（特許文献２の段落００６６に「鋼板組織全体」と記載あり）、そのための加熱炉が必要となり、コスト的に不利である。また、このように焼戻し処理を必須とするため、生産性が低いという問題もある。 On the other hand, according to the technique proposed in Patent Document 2, a press member having both sufficient strength and ductility can be manufactured. However, in the technique proposed in Patent Document 2, it is essential to perform re-heating and holding after press quenching and subjecting the entire sheet thickness to tempering treatment (in paragraph 0066 of Patent Document 2, "the entire steel sheet structure" There is a description), which requires a heating furnace, which is disadvantageous in terms of cost. In addition, since the tempering process is essential, there is a problem that productivity is low.
本発明は、上記の従来技術が抱える問題を有利に解決し、引張強さ（TS）1300MPa以上の強度を有するとともに、自動車のエネルギー吸収材などに適用しても衝突変形時に割れが生じるおそれのない十分な衝撃吸収特性を備えた高強度プレス部品を提供すること、および、このように優れた特性を有する高強度プレス鋼板を安価且つ効率的に製造する方法を提供することを目的とする。 The present invention advantageously solves the above-mentioned problems of the prior art, has a tensile strength (TS) of 1300 MPa or more, and may be cracked at the time of collision deformation even when applied to an energy absorbing material of an automobile. It is an object of the present invention to provide a high-strength press part having a sufficient impact absorption characteristic and to provide a method for inexpensively and efficiently producing a high-strength press steel sheet having such excellent characteristics.
上記課題を解決すべく、本発明者らは、プレス焼入れにより成形される高強度プレス部品に関し、1300MPa以上の引張強さを確保しつつ、衝撃吸収特性の向上を図る手段について鋭意検討した。 In order to solve the above-mentioned problems, the present inventors diligently studied a means for improving impact absorption characteristics while securing a tensile strength of 1300 MPa or more with respect to a high-strength press part formed by press quenching.
自動車部品のうち、例えばエネルギー吸収材などは、衝突時、座屈変形することにより衝撃エネルギーを吸収する。そして、これらの部品における衝突変形時の割れは、主に座屈部の曲げ変形により生じる。座屈部に曲げ変形が生じると、部品の座屈部表層に最も大きな歪みが入り、表層を起点に割れが進展し、最終的に破断に至る。 Among automobile parts, for example, an energy absorbing material absorbs impact energy by buckling deformation at the time of collision. And the crack at the time of the impact deformation in these parts arises mainly by the bending deformation of a buckling part. When bending deformation occurs in the buckled portion, the largest strain is applied to the surface layer of the buckled portion of the component, cracks start from the surface layer, and eventually break.
そこで、本発明者らは、割れの起点となる部品表層を軟質化し、割れの発生を抑制することに思い至った。そして、更に検討を進めた結果、ホットプレス部品の板厚中央の硬さをHv400以上とする一方、表層に厚さ20μm以上200μm以下であり硬さHv300以下の軟質層を形成することにより、引張強さ1300MPa以上の強度を確保しつつ、自動車衝突時の割れを抑制することが可能な衝撃吸収特性を備えた高強度プレス部品が得られることを知見した。 Therefore, the present inventors have come up with the idea of softening the component surface layer, which is the starting point of cracking, to suppress cracking. As a result of further investigation, while the hardness at the center of the thickness of the hot-pressed part is set to Hv 400 or more, the surface layer is formed with a soft layer having a thickness of 20 μm to 200 μm and a hardness of Hv 300 or less. It was discovered that a high-strength press part with impact-absorbing properties capable of suppressing cracking during a car crash while securing a strength of 1300 MPa or more was obtained.
また、本発明者らは、表層に脱炭層を備えた所定の組成を有する鋼板にプレス焼入れを施すことにより、上記の如く板厚中央と表層において所望の硬さを有する高強度プレス部品を、焼戻し処理を施すことなく、安価かつ高効率に製造できることを知見した。更に、所定の組成を有する鋼板にプレス焼入れを施したのち、表層のみに焼戻し処理を施すことによっても、上記の如く板厚中央と表層において所望の硬さを有する高強度プレス部品が得られることを知見した。このように焼戻し処理を施す場合においても、焼戻し処理の対象部が表層のみであるため、部品の板厚全体に焼戻し処理を施す場合よりも安価且つ高効率に高強度プレス部品を製造することができる。 In addition, the present inventors, by applying press quenching to a steel sheet having a predetermined composition having a decarburized layer on the surface layer, a high-strength press part having a desired hardness at the center of the plate thickness and the surface layer as described above, It has been found that low-cost and high-efficiency production can be achieved without tempering. Furthermore, after press-quenching a steel sheet having a predetermined composition, a high-strength press part having a desired hardness at the center of the plate thickness and the surface layer as described above can also be obtained by tempering only the surface layer. I found out. Even in the case where the tempering process is performed in this way, the target part of the tempering process is only the surface layer, and therefore, it is possible to manufacture a high-strength press part at a lower cost and higher efficiency than in the case of performing the tempering process on the entire plate thickness of the part. it can.
本発明は上記の知見に基づき完成されたものであり、その要旨は次のとおりである。
［１］ 鋼板にプレス焼入れを施してなるプレス部品において、前記鋼板を、質量％で、C：0.2％以上0.6％以下、Si：0.001％以上3.0％以下、Mn：0.5％以上3.0％以下、P：0.1％以下、S：0.07％以下およびAl：0.01％以上0.1％以下を含有し、残部がFeおよび不可避的不純物からなる組成を有する鋼板とし、プレス部品の板厚中央の硬さがHv400以上であり、プレス部品の表層が硬さHv300以下でありかつ焼戻し組織を有する軟質層を有し、該軟質層の厚さが20μm以上200μm以下であることを特徴とする引張強さが1300Mpa以上の高強度プレス部品。
The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] In a pressed part formed by press-quenching a steel plate, the steel plate is, in mass%, C: 0.2% to 0.6%, Si: 0.001% to 3.0%, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, and Al: 0.01% or more and 0.1% or less, with the balance being composed of Fe and inevitable impurities, the hardness at the center of the thickness of the pressed part is Hv400 or more, has a soft layer having a surface layer is not more than the hardness Hv300 and tempered structure of the press parts, or more 1300Mpa tensile strength, wherein the thickness of the soft electrolyte layer is 20μm or more 200μm or less high-strength press parts.
［２］ 質量％で、C：0.2％以上0.6％以下、Si：0.001％以上3.0％以下、Mn：0.5％以上3.0％以下、P：0.1％以下、S：0.07％以下およびAl：0.01％以上0.1％以下を含有し、残部がFeおよび不可避的不純物からなる組成を有する鋼板を、Ac3変態点以上に加熱した後、金型に挿入してプレス成形し、金型下死点位置で5秒以上保持するプレス焼入れ処理を施してプレス焼入れ部品とし、次いで、該プレス焼入れ部品の表層に焼戻し処理を施し、表層が硬さHv300以下でありかつ焼戻し組織を有する軟質層を厚さ20μm以上200μm以下の範囲で形成するとともに、板厚中央部の硬さをHV400以上とすることを特徴とする引張強さが1300MPa以上の高強度プレス部品の製造方法。 [ 2 ] By mass%, C: 0.2% to 0.6%, Si: 0.001% to 3.0%, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, and Al: 0.01% A steel plate having a composition containing 0.1% or less and the balance consisting of Fe and unavoidable impurities is heated to the Ac 3 transformation point or higher, then inserted into a mold and press-molded, at the bottom dead center position of the mold. and 5 seconds or longer press quenching alms press quenching parts that, then, the press quenching component surface subjected to tempering treatment in the surface layer is not more than the hardness Hv300 and tempered structure thickness 20μm or more soft layer having a A method for producing a high-strength press part having a tensile strength of 1300 MPa or more, characterized in that it is formed in a range of 200 μm or less and the hardness at the center of the plate thickness is HV400 or more .
本発明によると、引張強さが1300MPa以上であり、しかも自動車衝突時の変形による割れを抑制することが可能な高い衝撃吸収特性を備えた高強度プレス部品が得られる。したがって、本発明によると、自動車のエネルギー吸収材など、衝突時に大きな変形を受ける部位においてもホットプレス部品を適用することが可能となり、産業上格段の効果を奏する。 According to the present invention, it is possible to obtain a high-strength press part having a high impact absorption characteristic that has a tensile strength of 1300 MPa or more and can suppress cracking due to deformation at the time of automobile collision. Therefore, according to the present invention, it is possible to apply a hot-pressed part even to a part that undergoes a large deformation at the time of a collision, such as an automobile energy absorbing material, and has a remarkable industrial effect.
以下、本発明について具体的に説明する。
まず、本発明の高強度プレス部品について説明する。本発明の高強度プレス部品は、鋼板にプレス焼入れを施してなるプレス部品である。すなわち、本発明は、Ac3変態点以上の温度域まで加熱した鋼板を、ダイとパンチからなる金型を用いて所望の形状に熱間プレス成形しつつ、金型内で抜熱して焼入れを行うことにより得られるホットプレス部品を対象とする。したがって、冷間プレス成形や温間プレス成形により得られるプレス部品や、熱間プレス成形後に金型内で焼入れ処理を施さずに離型して得られるプレス成形は、本発明の高強度プレス部品に含まれない。素材としてクラッド鋼を用いたプレス部品も、本発明の高強度プレス部品に含まれない。
Hereinafter, the present invention will be specifically described.
First, the high-strength press part of the present invention will be described. The high-strength press part of the present invention is a press part formed by subjecting a steel sheet to press hardening. That is, the present invention is a method in which a steel sheet heated to a temperature range above the Ac 3 transformation point is hot-pressed into a desired shape using a die and a die, and is heat-extracted and quenched in the die. It is intended for hot-pressed parts obtained by performing. Therefore, a press part obtained by cold press molding or warm press molding, or a press molding obtained by releasing from a mold without performing quenching after hot press molding is a high-strength press part of the present invention. Not included. A press part using clad steel as a material is not included in the high-strength press part of the present invention.
本発明の高強度プレス部品の素材となる鋼板は、質量％で、C：0.2％以上0.6％以下、Si：0.001％以上3.0％以下、Mn：0.5％以上3.0％以下、P：0.1％以下、S：0.07％以下およびAl：0.01％以上0.1％以下を含有し、残部がFeおよび不可避的不純物からなる組成を有する。成分組成の限定理由は以下のとおりである。なお、以下の成分組成を表す％は、特に断らない限り質量％を意味するものとする。 The steel sheet used as the material of the high strength pressed part of the present invention is in mass%, C: 0.2% to 0.6%, Si: 0.001% to 3.0%, Mn: 0.5% to 3.0%, P: 0.1% or less. , S: 0.07% or less and Al: 0.01% or more and 0.1% or less, with the balance being composed of Fe and inevitable impurities. The reasons for limiting the component composition are as follows. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C ：0.2％以上0.6％以下
Cは、鋼の強度向上に寄与する元素である。また、焼入れ組織（主にマルテンサイト組織）の生成を促進する元素でもある。C含有量が0.2％未満であると、プレス部品の板厚中央の組織を所望の硬さ（Hv400以上）の焼入れ組織とすることができず、引張強さを1300MPa以上とすることが困難となる。但し、C含有量が0.6％を超えると、プレス部品の溶接性や靭性が劣化する。したがって、C含有量は0.2％以上0.6％以下とする。好ましくは0.25％以上0.4％以下である。
C: 0.2% or more and 0.6% or less
C is an element that contributes to improving the strength of steel. It is also an element that promotes the formation of a quenched structure (mainly martensite structure). If the C content is less than 0.2%, the structure at the center of the thickness of the pressed part cannot be a hardened structure with the desired hardness (Hv400 or higher), and it is difficult to set the tensile strength to 1300 MPa or higher. Become. However, if the C content exceeds 0.6%, the weldability and toughness of the pressed parts deteriorate. Therefore, the C content is 0.2% or more and 0.6% or less. Preferably they are 0.25% or more and 0.4% or less.
Si：0.001％以上3.0％以下
Siは、固溶強化により鋼の強度向上に寄与する元素であり、延性の低下を招来することなく鋼の高強度化が可能な元素でもある。このような効果を発現させるために、Si含有量を0.001％以上とする。一方、Si含有量が3.0％を超えると、プレス部品の表面性状が著しく劣化し、化成処理性や耐食性の低下を招く。したがって、Si含有量は0.001％以上3.0％以下とする。好ましくは0.1％以上2.0％以下である。
Si: 0.001% to 3.0%
Si is an element that contributes to improving the strength of the steel by solid solution strengthening, and is also an element that can increase the strength of the steel without causing a decrease in ductility. In order to express such an effect, the Si content is set to 0.001% or more. On the other hand, when the Si content exceeds 3.0%, the surface properties of the pressed parts are remarkably deteriorated, and chemical conversion treatment properties and corrosion resistance are lowered. Therefore, Si content shall be 0.001% or more and 3.0% or less. Preferably they are 0.1% or more and 2.0% or less.
Mn：0.5％以上3.0％以下
Mnは、固溶強化により鋼の強度向上に寄与する元素である。また、Mnは、鋼の焼入れ性向上を介して焼入れ組織（主にマルテンサイト組織）の生成を促進する元素でもある。このような効果を発現させるために、Mn含有量を0.5％以上とする。一方、Mn含有量が3.0％を超えると、上記の効果が飽和する。したがって、Mn含有量は0.5％以上3.0％以下とする。好ましくは0.75％以上2.5％以下である。
Mn: 0.5% to 3.0%
Mn is an element that contributes to improving the strength of steel by solid solution strengthening. Mn is also an element that promotes the formation of a hardened structure (mainly martensitic structure) through improvement of the hardenability of steel. In order to express such an effect, the Mn content is 0.5% or more. On the other hand, when the Mn content exceeds 3.0%, the above effect is saturated. Therefore, the Mn content is 0.5% or more and 3.0% or less. Preferably they are 0.75% or more and 2.5% or less.
P ：0.1％以下
Pは、固溶強化により鋼の強度向上に寄与する元素であるが、粒界に偏析して低温靭性や延性の低下を招く元素でもある。したがって、本発明では、P含有量を0.1％以下に抑制する。好ましくは0.01％以下である。
P: 0.1% or less
P is an element that contributes to improving the strength of the steel by solid solution strengthening, but is also an element that segregates at the grain boundary and causes a decrease in low-temperature toughness and ductility. Therefore, in the present invention, the P content is suppressed to 0.1% or less. Preferably it is 0.01% or less.
S ：0.07％以下
Sは、Mnと結合して粗大な硫化物を形成し、鋼の延性低下を招く元素である。そのため、S含有量は極力低減することが好ましいが、0.07％程度までの含有は許容できる。したがって、S含有量は0.07％以下とする。好ましくは0.01％以下である。
S: 0.07% or less
S is an element that combines with Mn to form coarse sulfides and causes a reduction in the ductility of steel. Therefore, it is preferable to reduce the S content as much as possible, but the content up to about 0.07% is acceptable. Therefore, the S content is 0.07% or less. Preferably it is 0.01% or less.
Al：0.01％以上0.1％以下
Alは、脱酸剤として作用し、鋼の清浄度を向上させるのに有効な元素である。このような効果を得るために、Al含有量を0.01％以上とする。一方、Al含有量が0.1％を超えて過剰になると、酸化物系介在物の増加を招き、鋼の延性を低下させる要因となる。したがって、Al含有量は0.01％以上0.1％以下とする。好ましくは0.01％以上0.07％以下である。
Al: 0.01% or more and 0.1% or less
Al acts as a deoxidizer and is an effective element for improving the cleanliness of steel. In order to obtain such an effect, the Al content is set to 0.01% or more. On the other hand, when the Al content exceeds 0.1% and becomes excessive, an increase in oxide inclusions is caused, which becomes a factor of reducing the ductility of the steel. Therefore, the Al content is 0.01% or more and 0.1% or less. Preferably they are 0.01% or more and 0.07% or less.
本発明において、上記以外の成分は、Feおよび不可避的不純物である。不可避的不純物としては例えばNが挙げられ、N含有量は0.008％以下に低減することが好ましい。より好ましくは0.005％以下である。また、不可避的不純物としては、Cu等も挙げられ、Cuの含有量は0.01％以下であれば許容できる。 In the present invention, components other than those described above are Fe and inevitable impurities. Inevitable impurities include, for example, N, and the N content is preferably reduced to 0.008% or less. More preferably, it is 0.005% or less. Moreover, Cu etc. are mentioned as an unavoidable impurity, and if Cu content is 0.01% or less, it will be accept | permitted.
また、本発明の高強度プレス部品は、表裏面からそれぞれ厚さ方向深さ20μm以上200μm以下までの表層と該表層以外の領域とでは硬さが異なり、板厚中央の硬さはHv400以上である。一方、上記表層は、硬さHv300以下の軟質層である。 In addition, the high-strength press parts of the present invention have different hardnesses in the surface layer from the front and back surfaces to a depth direction in the thickness direction of 20 μm or more and 200 μm or less and areas other than the surface layer, and the hardness at the center of the plate thickness is Hv400 or more. is there. On the other hand, the surface layer is a soft layer having a hardness of Hv300 or less.
プレス部品の板厚中央の硬さ（ビッカース硬さ）がHv400未満になると、強度が不十分となり、引張強さが1300MPa以上の高強度プレス部品が得られない。したがって、プレス部品の板厚中央の硬さはHv400以上とする。好ましくはHv450以上である。但し、プレス部品の板厚中央の硬さが過剰に高くなると、遅れ破壊が生じ易くなることが懸念されるため、Hv600以下とすることが好ましい。 If the hardness (Vickers hardness) at the center of the thickness of the pressed part is less than Hv400, the strength becomes insufficient, and a high-strength pressed part with a tensile strength of 1300 MPa or more cannot be obtained. Therefore, the hardness at the center of the thickness of the pressed part is set to Hv400 or more. Preferably it is Hv450 or more. However, if the hardness at the center of the thickness of the pressed part is excessively high, there is a concern that delayed fracture is likely to occur.
なお、プレス部品に所望の強度（引張強さ1300MPa以上）を付与するうえでは、前記表層以外の領域における平均硬さをHv400以上600以下とすることが好ましい。 In addition, when giving desired strength (tensile strength of 1300 MPa or more) to the pressed part, it is preferable that the average hardness in a region other than the surface layer is Hv 400 or more and 600 or less.
本発明の高強度プレス部品は、上記の如く板厚中央の硬さを高めて所望の強度を確保する一方、表層を軟質化することにより衝突変形時の座屈部での割れの発生を抑制する。
表層の軟質層の硬さがHv300を超える場合、衝突変形時の座屈部での割れの発生を抑制する効果が十分でなく、本発明の効果が得られない。したがって、表層の軟質層の硬さはHv300以下とする。好ましくはHv250以下である。
The high-strength press part of the present invention increases the hardness at the center of the plate thickness as described above to ensure the desired strength, while softening the surface layer to suppress the occurrence of cracks in the buckling part during impact deformation. To do.
When the hardness of the surface soft layer exceeds Hv300, the effect of suppressing the occurrence of cracks at the buckled portion during impact deformation is not sufficient, and the effect of the present invention cannot be obtained. Therefore, the hardness of the soft layer of the surface layer is set to Hv300 or less. Preferably it is Hv250 or less.
また、表層の軟質層の厚さが20μm未満である場合も、やはり、衝突変形時の座屈部での割れの発生を抑制する効果が十分でなく、本発明の効果が得られない。一方、表層の軟質層の厚さが200μmを超えると、プレス部品の強度が低下し、所望の引張強さ（1300MPa以上）を有する高強度プレス部品が得られない。したがって、表層の軟質層の厚さは20μm以上200μm以下とする。好ましくは20μm以上100μm以下である。 In addition, even when the thickness of the surface soft layer is less than 20 μm, the effect of suppressing the occurrence of cracks at the buckling portion at the time of impact deformation is not sufficient, and the effect of the present invention cannot be obtained. On the other hand, when the thickness of the surface soft layer exceeds 200 μm, the strength of the pressed part is lowered, and a high-strength pressed part having a desired tensile strength (1300 MPa or more) cannot be obtained. Therefore, the thickness of the surface soft layer is set to 20 μm or more and 200 μm or less. Preferably they are 20 micrometers or more and 100 micrometers or less.
上記軟質層は、硬さがHv300以下である焼戻し組織を有する層とする。 The soft layer hardness is a layer having a Der Ru tempering tissue Hv300 or less.
また、本発明において、高強度プレス部品の素材となる鋼板は、熱延鋼板、冷延鋼板のいずれでもよく、これらに焼鈍処理を施した鋼板でもよい。
なお、上記鋼板の板厚も特に問わないが、剛性を確保する観点からは、板厚を0.8mm以上とすることが好ましく、1.0mm以上とすることがより好ましい。一方、プレス焼入れにより板厚中央に焼入れ組織を形成するには、板厚を4.0mm以下とすることが好ましく、3.0mm以下とすることがより好ましい。
In the present invention, the steel plate used as the material for the high-strength press part may be either a hot-rolled steel plate or a cold-rolled steel plate, or a steel plate obtained by annealing them.
The plate thickness of the steel plate is not particularly limited, but from the viewpoint of ensuring rigidity, the plate thickness is preferably 0.8 mm or more, and more preferably 1.0 mm or more. On the other hand, in order to form a quenched structure in the center of the plate thickness by press hardening, the plate thickness is preferably 4.0 mm or less, and more preferably 3.0 mm or less.
次に、本発明の高強度プレス部品の製造方法について説明する。
本発明は、鋼板素材にプレス焼入れを施すことにより高強度プレス部品を製造する方法、または、鋼板素材にプレス焼入れを施したのち、表層のみに焼戻し処理を施すことにより高強度プレス部品を製造する方法である。以後、前者の製造方法を第１の製造方法と称し、後者の製造方法を第２の製造方法と称する。
Next, the manufacturing method of the high strength press part of this invention is demonstrated.
The present invention is a method for producing a high-strength press part by press-quenching a steel sheet material, or a high-strength press part by subjecting only a surface layer to tempering after press-quenching the steel sheet material. Is the method. Hereinafter, the former manufacturing method is referred to as a first manufacturing method, and the latter manufacturing method is referred to as a second manufacturing method.
先ず、第１の製造方法について説明する。
第１の製造方法では、上記組成を有し、表層に厚さ20μm以上200μm以下の脱炭層を有する鋼板を、鋼板素材として用いる。ここで、脱炭層とは、炭素濃度が0.1％以下にまで低減された層を意味する。このような脱炭層（厚さ20μm以上200μm以下、炭素濃度0.1％以下）は、例えば上記組成を有する鋼板を、炉内雰囲気の露点を−20〜＋10℃程度に調整した連続焼鈍炉で750〜950℃程度の温度域に加熱し、該温度域に30〜1000秒程度保持する焼鈍処理を施すことにより形成することができる。また、鋼板素材を製造する際の熱間圧延工程において、巻き取り温度を650℃以上の高温とすることによっても、脱炭層を形成することができる。
First, the first manufacturing method will be described.
In the first production method, a steel plate having the above composition and having a decarburized layer having a thickness of 20 μm or more and 200 μm or less on the surface layer is used as a steel plate material. Here, the decarburized layer means a layer in which the carbon concentration is reduced to 0.1% or less. Such a decarburized layer (
なお、脱炭層を形成する前の鋼板は、従来公知の方法にしたがい製造することができる。例えば、連続鋳造法や、造塊−分塊圧延法、薄スラブ連鋳法等、公知の鋳造方法により鋳造されたスラブに、熱間圧延を施して熱延鋼板（板厚：約1.4mm以上4.0mm以下）とし、該熱延鋼板に脱炭層を形成してもよい。また、上記熱延鋼板に、必要に応じて酸洗、焼鈍を施したのち、冷間圧延を施して冷延鋼板（板厚：約0.8mm以上2.3mm以下）とし、該冷延鋼板に脱炭層を形成してもよい。また、上記熱延鋼板や冷延鋼板に焼鈍処理を施した後、脱炭層を形成してもよい。 In addition, the steel plate before forming a decarburization layer can be manufactured according to a conventionally well-known method. For example, a hot-rolled steel sheet (sheet thickness: about 1.4 mm or more) is subjected to hot rolling on a slab cast by a known casting method such as a continuous casting method, an ingot-bundling rolling method, or a thin slab continuous casting method. And a decarburized layer may be formed on the hot-rolled steel sheet. In addition, the hot-rolled steel sheet is pickled and annealed as necessary, and then cold-rolled to obtain a cold-rolled steel sheet (sheet thickness: about 0.8 mm to 2.3 mm). A coal bed may be formed. In addition, a decarburized layer may be formed after annealing the hot-rolled steel sheet or the cold-rolled steel sheet.
第１の製造方法では、上記の如く表層に脱炭素を有する鋼板を、Ac3変態点以上、すなわちオーステナイト域に加熱した後、金型に挿入してプレス成形する。プレス成形時の鋼板温度は、Ar3変態点以上の温度とし、オーステナイト組織の状態にある鋼板にプレス成形を施すことが好ましい。なお、鋼板の加熱方法は特に限定されない。 In the first manufacturing method, the steel sheet having decarbonized surface as described above is heated to the Ac 3 transformation point or higher, that is, the austenite region, and then inserted into a mold and press-molded. The steel plate temperature during press forming is preferably set to a temperature equal to or higher than the Ar 3 transformation point, and press forming is performed on the steel plate in an austenite structure state. In addition, the heating method of a steel plate is not specifically limited.
次いで、プレス成形後、直ちに離型せず、金型下死点位置で5秒以上保持する。金型下死点位置で5秒以上保持することにより、プレス成形後の鋼板が、金型内で抜熱され、板厚中央の温度がMs点以下の温度域になるまで冷却される。したがって、鋼板の脱炭層以外の領域の組織は、焼入れ組織（マルテンサイト組織）となり、板厚中央の硬さがHv400以上となる。また、鋼板の脱炭層以外の領域の硬さは、平均でHv400以上600以下程度となる。 Next, after press molding, the mold is not released immediately and is held at the bottom dead center position of the mold for 5 seconds or more. By holding at the bottom dead center position of the mold for 5 seconds or longer, the press-formed steel sheet is removed from the heat in the mold and cooled until the temperature at the center of the sheet thickness falls below the Ms point. Therefore, the structure of the region other than the decarburized layer of the steel sheet becomes a quenched structure (martensite structure), and the hardness at the center of the sheet thickness is Hv400 or more. In addition, the hardness of the region other than the decarburized layer of the steel plate is about Hv400 or more and 600 or less on average.
一方、鋼板の表層は、厚さ20μm以上200μm以下の脱炭層であるため、焼入れされ難い状態となっている。そのため、プレス成形後の鋼板を金型内で抜熱しても、表層では焼入れ組織の割合が低くなる。その結果、プレス部品の表層には、厚さが20μm以上200μm以下であり、硬さがHv300以下である軟質層が形成される。 On the other hand, the surface layer of the steel sheet is a decarburized layer having a thickness of 20 μm or more and 200 μm or less, and thus is hard to be quenched. For this reason, even if the heat-treated steel sheet after press forming is removed in the mold, the ratio of the hardened structure is reduced in the surface layer. As a result, a soft layer having a thickness of 20 μm or more and 200 μm or less and a hardness of Hv300 or less is formed on the surface layer of the pressed part.
続いて、第２の製造方法について説明する。
第２の製造方法においては、前記の組成を有し脱炭層が形成されていない鋼板を素材として用いる。なお、この鋼板も、先に例示したような従来公知の方法にしたがい製造することができる。そして、第１の製造方法と同様に、鋼板を、Ac3変態点以上、すなわちオーステナイト域に加熱した後、金型に挿入してプレス成形する。プレス成形時の鋼板温度は、Ar3変態点以上の温度とし、オーステナイト組織の状態にある鋼板にプレス成形を施すことが好ましい。
Next, the second manufacturing method will be described.
In a 2nd manufacturing method, the steel plate which has the said composition and the decarburized layer is not formed is used as a raw material. In addition, this steel plate can also be manufactured according to a conventionally known method as exemplified above. Then, as in the first manufacturing method, the steel sheet, Ac 3 transformation point or more, that is, after heating to the austenite region, press-molded and inserted into a mold. The steel plate temperature during press forming is preferably set to a temperature equal to or higher than the Ar 3 transformation point, and press forming is performed on the steel plate in an austenite structure state.
また、第１の製造方法と同様に、プレス成形後、直ちに離型せず、金型下死点位置で5秒以上保持する。金型下死点位置で5秒以上保持することにより、プレス成形後の鋼板が、金型内で抜熱され、板厚中央の温度がMs点以下の温度域になるまで冷却される。したがって、鋼板の組織は焼入れ組織（マルテンサイト組織）となり、板厚中央の硬さがHv400以上となる。また、鋼板の板厚方向全体の平均硬さはHv400以上600以下程度となる。なお、第２の製造方法では、脱炭層を備えていない鋼板を素材として用いている。そのため、第２の製造方法では、金型下死点位置で5秒以上保持することにより、板厚全域に亘り焼入れ組織を有する焼入れプレス部品が得られる。 Further, in the same manner as in the first manufacturing method, after press molding, the mold is not released immediately, but is held at the bottom dead center position of the mold for 5 seconds or more. By holding at the bottom dead center position of the mold for 5 seconds or longer, the press-formed steel sheet is removed from the heat in the mold and cooled until the temperature at the center of the sheet thickness falls below the Ms point. Accordingly, the steel sheet has a hardened structure (martensite structure), and the hardness at the center of the plate thickness is Hv400 or more. Moreover, the average hardness of the whole steel plate thickness direction is about Hv400 or more and 600 or less. In the second manufacturing method, a steel plate that does not have a decarburized layer is used as a material. Therefore, in the second manufacturing method, a quenched press part having a quenched structure over the entire plate thickness is obtained by holding the mold at the bottom dead center position for 5 seconds or more.
第２の製造方法では、得られた焼入れプレス部品の表層のみに、焼戻し処理を施す。このように、焼入れプレス部品の表層のみに焼戻し処理を施すことにより、表層以外の領域の組織を焼入れ後の組織に維持したままで、表層のみを焼戻し組織とすることができる。その結果、板厚中央の硬さをHv400以上に維持したまま、表層に厚さ20μm以上200μm以下の軟質層であって、硬さHv300以下の軟質層を形成することができる。なお、上記焼戻し処理後の表層以外の領域の平均硬さはHv400以上600以下程度となる。 In the second manufacturing method, only the surface layer of the obtained quenched press part is tempered. In this way, by performing the tempering process only on the surface layer of the quenched press part, only the surface layer can be made a tempered structure while maintaining the structure of the region other than the surface layer in the structure after quenching. As a result, it is possible to form a soft layer having a thickness of 20 μm or more and 200 μm or less and having a hardness of Hv 300 or less on the surface layer while maintaining the hardness at the center of the plate thickness at Hv 400 or more. In addition, the average hardness of the region other than the surface layer after the tempering process is about Hv400 or more and 600 or less.
第２の製造方法において、焼入れプレス部品の表層のみに焼戻し処理を施す手段としては、焼入れプレス部品の表層にレーザを照射して、焼入れプレス部品の表裏面から深さ方向に20μm以上200μm以下までの領域を、500℃以上800℃以下程度の温度域に加熱する手段等を例示することができるが、勿論、この手段に限定されない。 In the second manufacturing method, as a means of tempering only the surface layer of the quenched press part, the surface layer of the quenched press part is irradiated with a laser, and from 20 to 200 μm in depth direction from the front and back surfaces of the quenched press part. A means for heating the region to a temperature range of about 500 ° C. to 800 ° C. can be exemplified, but it is needless to say that the means is not limited thereto.
表１に示す化学成分を有する鋼を溶製して鋳片とし、該鋳片を、1250℃に加熱し、900℃の仕上げ圧延終了温度で熱間圧延した後、表２に示す巻取り温度で巻き取り熱延鋼板とし、該熱延鋼板を、酸洗後、60％の圧下率で冷間圧延し、板厚1.4mmの冷延鋼板とした。 A steel having the chemical composition shown in Table 1 is melted to form a slab. The slab is heated to 1250 ° C. and hot-rolled at a finish rolling finish temperature of 900 ° C., and then the coiling temperature shown in Table 2 The hot-rolled steel sheet was rolled up and then cold-rolled at a reduction rate of 60% after pickling to obtain a cold-rolled steel sheet having a thickness of 1.4 mm.
なお、表１に記載のAc3変態点は、以下の(1)式より算出した（William C.Leslie著、幸田成康監訳、熊井浩、野田龍彦訳、「レスリー鉄鋼材料学」、丸善株式会社、1985年、p.273参照）。 (1)式において、[C]、[Si]、[Mn]、[P]、[Al]は、各元素（C、Si、Mn、P、Al）の含有量（質量％）である。
Ac3（℃）＝910−203√[C]＋44.7×[Si]−30×[Mn]＋700×[P]＋400×[Al]… (1)
The Ac 3 transformation points listed in Table 1 were calculated from the following formula (1) (William C. Leslie, translated by Kouda Shigeyasu, Kumai Hiroshi, Noda Tatsuhiko, Leslie Steel Materials Science, Maruzen Co., Ltd.) 1985, p.273). In the formula (1), [C], [Si], [Mn], [P], and [Al] are contents (mass%) of each element (C, Si, Mn, P, Al).
Ac 3 (° C.) = 910−203√ [C] + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] (1)
上記冷延鋼板に、焼鈍処理を施すことにより、プレス部品の素材鋼板を得た。焼鈍処理は、表２に示すように、冷延鋼板を炉内雰囲気の露点を−45〜＋10℃の温度に調整した連続焼鈍炉で750〜950℃の温度域に加熱し、該温度域に30〜1000秒保持することにより実施した。 By subjecting the cold-rolled steel sheet to an annealing treatment, a material steel sheet for a pressed part was obtained. As shown in Table 2, the annealing treatment is performed by heating a cold-rolled steel sheet to a temperature range of 750 to 950 ° C. in a continuous annealing furnace in which the dew point of the furnace atmosphere is adjusted to a temperature of −45 to + 10 ° C. It was carried out by holding for 30 to 1000 seconds.
素材鋼板の、脱炭層の厚さおよび脱炭層の炭素濃度を測定した結果を、表２に示す。脱炭層の厚さと、脱炭層の炭素濃度は、EPMA分析により求めた。具体的には、EPMA分析で素材鋼板の板厚方向の炭素濃度分布を測定し、表裏面で炭素濃度が0.1％以下となる領域の厚さの平均値を、脱炭層の厚さとした。また、上記領域（表裏面で炭素濃度が0.1％以下となる領域）の平均の炭素濃度を、脱炭層の炭素濃度とした。 Table 2 shows the results of measuring the thickness of the decarburized layer and the carbon concentration of the decarburized layer of the raw steel plate. The thickness of the decarburized layer and the carbon concentration of the decarburized layer were obtained by EPMA analysis. Specifically, the carbon concentration distribution in the thickness direction of the material steel plate was measured by EPMA analysis, and the average value of the thickness of the region where the carbon concentration was 0.1% or less on the front and back surfaces was defined as the thickness of the decarburized layer. The average carbon concentration in the above region (region where the carbon concentration is 0.1% or less on the front and back surfaces) was defined as the carbon concentration of the decarburized layer.
なお、本実施例においては、熱間圧延工程の巻取り温度を制御すること、または焼鈍処理で主に炉内雰囲気の露点を制御することにより、素材鋼板の脱炭層の厚さを調整した。表２に示すように、本発明の発明例では、熱間圧延工程における巻取り温度を650℃以上とする（部品No.9,10,12）、或いは焼鈍処理の炉内雰囲気の露点を−20〜＋10℃の間とした（部品No.6〜8,11）。 In this example, the thickness of the decarburized layer of the raw steel sheet was adjusted by controlling the coiling temperature in the hot rolling process or by controlling the dew point of the furnace atmosphere mainly by annealing. As shown in Table 2, in the inventive example of the present invention, the coiling temperature in the hot rolling process is set to 650 ° C. or higher (part Nos. 9, 10, 12), or the dew point of the annealing furnace atmosphere is − The temperature range was 20 to + 10 ° C (part Nos. 6 to 8 and 11).
次いで、上記により得られた素材鋼板に、プレス焼入れを施してプレス部品（部品No.1〜12）とした。また、一部の素材鋼板については、プレス焼入れを施したのち、表層のみに焼戻し処理を施してプレス部品（部品No.13,14）とした。 Next, the material steel plate obtained as described above was subjected to press quenching to obtain pressed parts (part Nos. 1 to 12). Further, some of the raw steel plates were subjected to press quenching and then tempered only on the surface layer to obtain pressed parts (part Nos. 13 and 14).
プレス焼入れは、素材鋼板を、電気炉にて加熱した後、プレス機に搬送し、プレス機の金型（SKD61製）に挿入してプレス成形を行い、金型下死点位置で所定時間保持することにより実施した。電気炉での鋼板加熱温度、プレス成形時の鋼板温度、金型下死点位置での保持時間は、表２に示すとおりである。焼戻し処理は、プレス焼入れにより得られた部品の表層に、レーザ照射することにより実施した。 In press hardening, the steel sheet is heated in an electric furnace, then transported to the press machine, inserted into the press die (SKD61), press-formed, and held at the bottom dead center position for a predetermined time. It was carried out by doing. Table 2 shows the steel plate heating temperature in the electric furnace, the steel plate temperature during press forming, and the holding time at the bottom dead center of the mold. The tempering treatment was performed by irradiating the surface layer of the part obtained by press hardening with laser.
得られたプレス部品の形状を図１に示す。
得られたプレス部品について、表層の硬さを測定し、Hv300以下である軟質層の厚さを求めた。Hv300以下である軟質層の厚さは、プレス部品を切断して断面を研磨したサンプルを用いて、マイクロビッカース硬度計にて表裏面から深さ方向（厚さ方向）に2.5μmピッチで測定し、Hv300以下である領域の厚さを表裏面について求め、その平均値を算出することにより求めた。
また、得られたプレス部品を切断して断面を研磨したサンプルを用いて、マイクロビッカース硬度計にてプレス部品の板厚中央の硬さを求めた。
これらの結果を、表３に示す。
The shape of the obtained pressed part is shown in FIG.
About the obtained press part, the hardness of the surface layer was measured and the thickness of the soft layer which is Hv300 or less was calculated | required. The thickness of the soft layer of Hv300 or less was measured at a 2.5μm pitch from the front and back to the depth direction (thickness direction) with a micro Vickers hardness tester using a sample whose cut surface was cut by cutting a pressed part. , The thickness of the region of Hv300 or less was determined for the front and back surfaces, and the average value was calculated.
Moreover, the hardness of the plate | board thickness center of a press part was calculated | required with the micro Vickers hardness meter using the sample which cut | disconnected the obtained press part and grind | polished the cross section.
These results are shown in Table 3.
また、得られたプレス部品について、以下の圧潰試験を実施することにより、衝撃吸収特性を評価した。
得られたプレス部品を、図２に示すように２枚あわせて、フランジ部を30mmピッチでスポット溶接し、圧潰試験用のサンプルを作製した。圧潰試験は、図３に示すように、サンプルを縦に置き、片側の開口部側から、サンプルが10m/秒の一定速度で変形するような外力を負荷し、ストローク120mmまでサンプルを変形させることにより実施した。
Moreover, the impact absorption characteristic was evaluated by implementing the following crush tests about the obtained press part.
As shown in FIG. 2, two of the obtained pressed parts were put together and the flanges were spot-welded at a pitch of 30 mm to prepare a sample for a crush test. In the crushing test, as shown in Fig. 3, the sample is placed vertically and an external force is applied from one side of the opening to deform the sample at a constant speed of 10m / sec. It carried out by.
上記変形時の吸収エネルギー、および圧潰試験後のサンプルの割れの有無を確認することにより、プレス部品の衝撃吸収特性を評価した。
吸収エネルギーは、ストローク・荷重曲線をストローク：0〜120mmの範囲で積分することにより測定した。また、圧潰試験後のサンプルの割れの有無は、圧潰試験後のサンプルを目視観察することにより確認した。これらの結果を、表３に示す。また、表３中の部品No.1〜14について、先に求めた軟質層の厚さと、上記変形時の吸収エネルギーおよび圧潰試験後のサンプルの割れの有無との関係を表すグラフを、図４に示す。
The impact absorption characteristics of the pressed parts were evaluated by confirming the absorbed energy during the deformation and the presence or absence of cracks in the sample after the crush test.
Absorbed energy was measured by integrating the stroke / load curve in the range of stroke: 0 to 120 mm. Moreover, the presence or absence of the crack of the sample after a crush test was confirmed by observing the sample after a crush test visually. These results are shown in Table 3. Further, for parts Nos. 1 to 14 in Table 3, a graph showing the relationship between the thickness of the soft layer previously obtained, the absorbed energy at the time of deformation and the presence or absence of cracking of the sample after the crush test is shown in FIG. Shown in
更に、得られたプレス部品の上面（図１（ａ）参照）から、引張方向がプレス部品の長手方向になるようにJIS 5号試験片を採取し、JIS Z 2241(2011)の規定に準拠した引張試験（歪み速度：10mm/min）を行い、引張強さTSを測定した。この結果を、表３に示す。 Furthermore, from the top surface of the resulting pressed part (see Fig. 1 (a)), JIS No. 5 test specimens are collected so that the tensile direction is the longitudinal direction of the pressed part, and conforms to the provisions of JIS Z 2241 (2011). The tensile test (strain rate: 10 mm / min) was performed, and the tensile strength TS was measured. The results are shown in Table 3.
表３に示すように、部品No.1〜14のいずれのプレス部品においても、引張強さは1420MPa以上であって、1300MPa以上の強度を有していたが、Hv300以下である軟質層の厚さが20μm未満の部品No.1〜5（比較例）のプレス部品では、圧潰試験で割れが生じ、吸収エネルギーが低くなる。これに対し、Hv300以下である軟質層の厚さが20μm以上の部品No.6〜14（発明例）のプレス部品では、割れが発生せず、高い吸収エネルギーが得られた。 As shown in Table 3, in any of the pressed parts of parts Nos. 1 to 14, the tensile strength was 1420 MPa or more and the strength was 1300 MPa or more, but the soft layer thickness was Hv 300 or less. In the pressed parts of parts Nos. 1 to 5 (comparative examples) with a length of less than 20 μm, cracking occurs in the crushing test, and the absorbed energy decreases. On the other hand, in the pressed parts of parts Nos. 6 to 14 (invention examples) in which the thickness of the soft layer having Hv of 300 or less is 20 μm or more, cracks did not occur and high absorbed energy was obtained.
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