• Transactions of Materials Processing
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• v.31 no.1
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• pp.17-22
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• 2022

Hot stamping is widely used to manufacture structural parts to satisfy requirements of eco-friendly vehicles. Recently, hot forming technology using uncoated steel sheet is being studied to reduce cost and solve patent problems. In particular, research is focused on process technology capable of suppressing the generation of an oxide layer. To evaluate the oxide layer in the hot stamping process, Gleeble testing machine can be used to evaluate the oxide layer by controlling the temperature history and the atmosphere condition. At this time, since cooling by gas injection is impossible to protect the oxide layer on the surface of a specimen, research on a method for securing a quenching speed through natural cooling is required. This paper proposes a specimen shape design method to secure a target quenching speed through natural cooling when evaluating the oxide layer of an un-coated boron steel sheet by Gleeble test. For the evaluation of the oxide layer of the un-coated steel sheet through the Gleeble test, dog-bone and rectangular type specimens were used. In consideration of the hot stamping process, the temperature control conditions for the Gleeble test were set and the quenching speed according to the specimen shape design was measured. Finally, the quenching speed sensitivity according to shape parameter was analyzed through regression analysis. A quenching speed prediction equation was then constructed according to the shape of the specimen. The constructed quenching speed prediction equation can be used as a specimen design guideline to secure a target quenching speed when evaluating the oxide layer of an un-coated boron steel sheet by the Gleeble test.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.18 no.6
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• pp.33-38
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• 2019

Researchers have recently begun to study hot stamping processes to shorten the mold cooling time and improve productivity. These publications explain that the mold cooling time can be reduced by using a curved cooling channel, where the mold surface is processed to a uniform depth, instead of a straight cooling channel that uses the conventional gun drilling machine. This study investigates the characteristics of the front pillar of an automobile after using a mold with a curved cooling channel. To analyze the change in properties, we used a 1.6 mm boron steel blank and heated the prototype at $930^{\circ}C$ for 5 minutes. Next, we formed the prototype with a load of about 500 tons while varying the mold cooling time between 1 and 10 seconds. We subjected each prototype specimen to a tensile strength test, a hardness test, and a tissue surface observation.

• Transactions of Materials Processing
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• v.26 no.2
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• pp.121-126
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• 2017

The hot stamping process is an innovative forming method that could prevent the cracking of high strength steel sheets. The formability test of boron steel sheet using forming limit diagrams at elevated temperature is very complicated and time consuming job. In this paper, an alternative test method to evaluate the formability of boron steel in hot stamping has proposed. It measured the FLD0 instead of whole strain combinations of FLD with the tensile test machine and specially designed test rig. Test results shows that the proposed test method can simulate the plain strain condition fracture and can make the FLD of boron steel sheet at elevated temperature with less effort.

• Design & Manufacturing
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• v.11 no.2
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• pp.37-41
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• 2017

With light vehicle weight gradually becoming ever more importance due to tightened exhaust gas regulations, hot-stamping processing using boron alloyed steel is being applied more and more by major automobile OEMs since process assures both moldability and a high strength of 1.5 GPa. Although laser trimming is generally applied to the post-processing of the hot-stamped process with high strength, there have been many studies of in-die hot trimming using shear dies during the quenching of material in order to shorten processing times. As such, this study investigated the effects of the Shear rate and Shear mechanism on shear processes during the quenching process of hot-stamping material. In case of pad variable, padding force is very weak compared with shear force, so it does not affect the shear surface. In case of shear rate, the higher the shear at high temperatures and the higher the friction effect. As a result the rollover and the fracture distribution decreased, and the burnish distribution increased. Therefore, it is considered that the shear quality is guaranteed when high shear rate is applied in high temperature shear process.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.20 no.10
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• pp.44-49
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• 2021

Seats in autonomous vehicles must be able to rotate to fully utilize the interior space. Generally, ultra-high strength steel is used for the rotation module because it should have high strength and high rigidity. In addition, the rotating parts are difficult to form because they have complex shapes. In this study, the upper plate of the rotating module, whose complex shape makes it difficult to form, was formed by applying the hot stamping method. The drawing method and the form-drawing method, which are generally used to form components of complex shapes, were compared. We showed that the form-drawing method increased the degree of freedom of the material flow to improve the formability, thus enabling the forming of the plate. In addition, the die and blank shapes were found to be important factors in determining the success of the hot stamping. The validity of the analysis results was confirmed through forming analysis and experiments.

• Proceedings of the Materials Research Society of Korea Conference
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• 2010.05a
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• pp.39.2-39.2
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• 2010

Recently, hot stamping process has been paid attention greatly by automobile makers in accordance with the fuel efficiency and environmental issues as well as crash safety issue. The hot-stamped parts, however, demand extreme mechanical properties such as tensile strength of over 1470 MPa or equivalently Vickers hardness of around 450. In this work, to meet the demand efficiently, a method to predict mechanical property of hot-stamped parts based on numerical phase transformation scheme has been proposed associated with the thermo-mechanical coupled finite element analysis. This work deals with various phase transformation equations and validates them to select appropriate model for 0.2C-0.1Si-1.4Mn-0.5Cr-0.01Mo-0.002B steel sheet. The authors show that an efficient method saving time and cost to develop hot-stamped automobile parts ensuring suitable mechanical properties such as Vickers hardness and strength.

• Transactions of Materials Processing
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• v.20 no.6
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• pp.427-431
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• 2011

In general, hot stamped component is trimmed by costly and time consuming laser cutting when the material strength is over 1,500MPa. The aim of this work was to demonstrate that the trimming die life is improved and the trimming load is decreased by lowering the strength of the region to be trimmed. The model employed in this study was a hat shape, similar to the cross section of many hot stamped products. FE-analysis of hot stamping process was performed to evaluate the effect of tool shape on cooling rate at the area to be trimmed. The best tool shape was thus identified, which created slower cooling and lower hardness at the region to be trimmed. The wear at the cutting tool edge was also reduced.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.375-378
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• 2009

Hot press forming is a new forming process which also names as hot stamping. It can greatly enhance the formability of forming parts. This paper researches the formability of boron steel sheet in hot bending process which is a kind of hot press forming. In the text, the influence of hot press forming processing parameters, such as the heating temperature, blank holding force, punch speed and punch and die radius, on the mechanics properties and microstructure of the hot bending parts was analyzed by tension test and the metallographic observation on the parts with various processing parameters. The relationship between blank holding force and punch load was also presented.

• 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.

• 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.