• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.1
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• pp.89-94
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• 2015

This study was conducted to compare the laser weldability of boron steel and hot-stamped steel. In general, boron steel is used in the hot-stamping process. Hot-stamping is a method for simultaneously forming and cooling boron steel in a press die after heating it to the austenitizing temperature. Hot-stamped steel has a strength of 1500 MPa or more. Thus, in this study, the laser weldability of boron steel and thet of hot-stamped steel were investigated and compared. A continuous wave disk laser was used to produce butt and lap joints. In the butt welding, the critical cooling speed at which full penetration was obtained in the hot-stamped steel was lower than that of boron steel. In the lap welding, the joint widths were similar regardless of the welding speed when full penetration was obtained.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.29-29
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• 2010

레이저 용접은 고밀도 에너지빔을 이용하는 용접방법으로 아크용접에 비해 빠른 용접과 깊은 용입이 가능하며, 낮은 열이력을 가지는 장점이 있다. 때문에 열에 의해 연화되는 고강도강의 용접에 큰 이점을 가지고 있다. 차체경량화 추세와 더불어 차량에 고강도강의 적용이 늘어나고 있는데 충돌시 차량 구조를 유지시켜주는 범퍼나 B-필러와 같은 부품에 적용되는 무도금 보론 합금강과 알루미늄 코팅 보론 합금강은 핫스템핑(Hot Stamping) 기술에 의해 제조된 소재로 약 1.5GPa의 인장강도를 가진다. 알루미늄 코팅 보론 합금강의 경우 제조공정과 이송 중 소재 표면산화에 의한 산화철발생 또는 표면 탈탄 현상을 방지하기 위해 알루미늄 코팅 처리를 하는데 이러한 코팅층이 용접시 용접부의 물성을 저하시키는 역할을 한다고 보고되어 있다. 본 연구에서는 1.5GPa급 무도금 보론 합금강과 알루미늄 코팅 보론 합금강을 대상으로 레이저 용접을 적용하여 용접부 특성을 파악하고자 하였다. 실험은 겹치기 형상으로 Fiber Laser, Disk Laser를 적용하여 진행하였으며 빔Size, 용접속도, Gap등을 변경하며 해당조건에서의 용입특성, 파단모드, 기계적특성 등을 알아보았다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.4
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• pp.383-391
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• 2011

The hot-stamping technique is used to manufacture high-strength parts by press forming by heating at a temperature above the Austenite transformation temperature and then rapid cooling. Boron steel, which contains a very small amount of boron, is one of the materials used for hot stamping. The purpose of this study is to show the microstructures and to investigate the mechanical properties under different heat-treatment conditions. The heat treatment of water quenching was conducted at the various temperatures and different elapsed times. These can be practical data useful when boron steels are used for hot stamping. Furthermore, the microstructures and mechanical properties of the spot-welded specimen with coatings and counterpart materials (SPRC 340, SPRC 590) is investigated in order to determine the welding characteristics of boron steel at different welding condition.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.3
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• pp.319-324
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• 2012

The hot-stamping technique is a forming method for manufacturing high-strength parts, in which the part is cooled rapidly after press forming above the austenite transformation temperature. Boron steel, which contains a very small amount of boron, is one of the materials used for hot stamping. The purpose of this study is to investigate the microstructure and mechanical properties according to the heat-treatment conditions. Die-quenching from various temperatures was conducted for different elapsed heat-treatment times. Laser-welded boron steel after quenching has a tensile strength of 1454 MPa and an elongation of 6 %. It has 94 % of the tensile strength of the base metal (1522 MPa). These properties can provide practical information for the use of boron steels for hot stamping.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.10
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• pp.1221-1226
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• 2012

The hot-stamping technique is a forming method used for manufacturing high-strength parts, in which a part is cooled rapidly after press forming above the austenite transformation temperature. Boron steel, which contains a very small amount of boron, is one of the materials used for hot stamping. The purpose of this study is to investigate the mechanical properties of boron steel according to the heat-treatment conditions and the formability by using an Erichsen cupping test. Die quenching from various temperatures was conducted for different elapsed heat-treatment times. Laser-welded boron steel after quenching at 1173 K-0 s has a tensile strength of 1203 MPa. This is 79% of the tensile strength of the base metal (1522 MPa). The formability of boron steel was not significantly different from that at the mold temperature. However, it decreased with increasing forming speed. These properties provide practical information for the use of boron steels for hot stamping.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1373-1377
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• 2014

Hot-stamping is a method of obtaining ultrahigh-strength steel by simultaneously forming and cooling boron steel in a press die after it has been heated at $900^{\circ}C$ or above. After heat treatment, boron steel has a strength of 1500 MPa or more. This material ensures a high level of quality because it overcomes the spring-back phenomenon, which is a problem associated with high-strength steel materials, and the degree of dimensional precision is improved by 90 or more because of the good formability compared with existing types of steel. In this study, the welding characteristics were identified through the butt and lap welding of hot-stamped steel using a disk laser. Full penetration was obtained at a faster speed with butt welding compared to lap welding, and a white band was observed in every specimen.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.185-189
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• 2009

Recently, ultra high strength products can be manufactured by the hot press forming process of Boron steel in automotive and electronics industries. In order to get high strength, the hot press forming should be accompanied by quenching process inducing phase transformation. There are several types of the hot press forming processes according to the quenching method, water quenching and die quenching, etc. In the study, the process was numerically and physically simulated to compare the two types of quenching processes, and then the strength, hardness and dimensions of the products were compared with try-outs.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1367-1372
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• 2014

As the awareness of the environmental crisis has recently increased around the world, numerous studies in the transport industry have been conducted to solve this problem through lightweight car bodies. The hot-stamping process has been presented as solution to achieve a light weight. Hot-stamping is a method that is used to obtain ultra-high strength steel (1,500 MPa or greater) by simultaneously forming and cooling boron steel in a press die after heating it to a temperature of $900^{\circ}C$ or above. This study involved a, fundamental examination of laser parameters to investigate the laser weldability of boron steel. As a result, the following optimum parameters for the shielding gas were found: Q = 20 l/min, ${\alpha}=40^{\circ}$, d = 20mm, and l = 0 mm. The hardness of butt weldment increasesed sharply as a result of martensite formation at the fusion zone.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2001.04a
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• pp.802-806
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• 2001

Chrome-molybden steel or chrome steel for machine structural use been shown to excellent hardenability adding boron of a small amount at low carbon steel. In the country boron steel has been used widely high strength volt and wear resistant components of construction equipment. SEM results showed classical fatigue fractures, consistent with surface crack initiation. The speciments were cycled using under load controlled rotary bending fatigue tests. In this study, the fatigue crack initiation as well as the fatigue crack growth behavior and the fracture mechanism were investigated through observations of fracture surface boron steel(AISI 51B20).

• Journal of the Korean Society for Precision Engineering
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• v.18 no.11
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• pp.174-178
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• 2001

Chrome-molybden steel or chrome steel for machine structures has been shown to excellent hardenability adding boron of a small amount at low carbon steel. These days, boron steel has been used high strength bolt and wear resistant components of construction equipment. SEM results showed classical fatigue fractures, consist with surface crack initiation. The specimens were tested repeatedly(9 times) under controlled load rotary bending fatigue tests. In the study, the fatigue crack initiation as well as fatigue crack growth behavior and the fracture mechanism were investigated through observations of fracture of boron steel surface(AISI 51B20).