• Journal of the Korean Society for Heat Treatment
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• v.12 no.3
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• pp.231-239
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• 1999

Volume fraction and morphology of retained austenite, tensile properties of TRIP type high strength steel sheet with Fe-C-Si-Mn-Cu chemical composition have been investigated. The retained austenite of granular, bar and film type existing in specimen was obtained after intercritical annealing and austempering. The granular type retained austenite increased with increase of intercritical annealing and austempering temperature. With increase of intercritical annealing temperature, retained austenite and carbon contents increased. Maximum contents of retained austenite was obtained by austempering at $400^{\circ}C$. The maximum tensile strength was obtained by austempering at $450^{\circ}C$ and maximum elongation was obtained at $400^{\circ}C$. T.S${\times}$E.L value increased with increase of retained austenite contents due to the elongation strongly controlled by contents of retained austenite, but tensile strength was affected with various factors such as bainitic structure etc.

• Transactions of Materials Processing
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• v.12 no.8
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• pp.718-723
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• 2003

Tn order to develop springback control technology for high-strength steel sheets, several studies have been conducted: dome stretching test, stepped s-rail forming and springback measurement, and optimally shaped initial blank design. First, to find out the formability of TRIP60, dome stretching test was performed. Next, the stepped s-rail die, which was designed to form a channel type panel with large twist and wall curl, was manufactured and used to evaluate the effect of controlling forming variables, such as blank holding force and flange amount on the springback. Furthermore, new measurement method of the springback was introduced to define wall curl and twist in geometrically complex panels. Finally, the optimally shaped initial blank was employed to verify one of the best ways to control the springback in channel type. high-strength sheet panels.

• Journal of the Korean Society for Heat Treatment
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• v.11 no.4
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• pp.295-304
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• 1998

The effects of retained austenite and carbon content in the retained austenite on the tensile strength-elongation balance and spot weldability of TRIP high strength steel sheet have been investigated. The retained austenite of granular type increased with increasing intercritical annealing and austempering temperature, and film type was increased with the increase of austempering time. The volume fraction of retained austenite increased with decreasing intereritical annealing temperature, and the maximum value was obtained at austempering temperature of $400^{\circ}C$. The values of tensile strength-elongation balance increased with decreasing intercritical annealing temperature and maximum value was obtained at austempering temperature of $400^{\circ}C$. The maximum value of tensile strength-elongation balance was obtained at a retained austenite content of about 12%. Tensile shear strength of the specimens with retained austenite was higher than that of the normalizing specimens. With increasing welding current and time, the tensile shear strengh and nugget diameter increased, while nugget thickness showed the peak value and then decreased. The optimum range of welding condition at the given welding pressure of 350kgf was 7~11kA and 10~15 cycles.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.08a
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• pp.35-40
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• 2003

In order to develope springback control technology for high-strength steel sheets, some studies have been conducted: dome stretching test, stepped s-rail forming and springback measurement, and optimally shaped initial blank design. First, to find out the formability of TRIP60, dome stretching test was performed. Next the stepped s-rail die, which was designed to form a channel type panel with large twist and wall curl, was manufactured and used to know the effect of controlling forming variables, such as blank holding force and flange amount on the springback. Furthermore, new measurement method of the springback was introduced to define wall curl and twist in geometrically complex panels. Finally, the optimally shaped initial blank was employed to verify one of the best ways to control the springback in channel type, high strength sheet panels.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.05a
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• pp.106-109
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• 2005

Springback is a common phenomenon in sheet metal forming, caused by the elastic recovery of the internal stresses after removal of the tooling. Recently, advanced high strength steels (AHSS) such as TRIP and DP are finding acceptance in the automotive industry because their superior strength to weight ratio can lead to improved fuel efficiency and assessed crashworthiness of vehicles. The major troubles of the automotive structural members stamped with high strength steel sheets are the tendency of the large amount of springback due to the high yield strength and the tensile strength. The amount of springback is mainly influenced by the type of the yield function and anisotropic model induced by rolling. The discrepancy of the deep drawn product comparing the data of from the product design induced by springback must be compensated at the tool design stage in order to guarantee its function and assembly with other parts. The methodology of compensation of the low shape accuracy induced by large amount of springback is developed by the expert engineer in the industry. Recently, the numerical analysis is introduced in order to predict the amount of springback and to improve the shape accuracy prior to tryout stage of press working. In this paper, the tendency of springback is evaluated with respect to the blank material. The stamping process is analyzed fur the front side member formed with AHSS sheets such as TRIP60 and DP60. The analysis procedure fully covers the binderwrap, stamping, trimming and springback process with the commercial elasto-plastic finite element code LS-DYNA3D.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.1115-1120
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• 2003

The recent tendency in the automobile industries is toward light weighting vehicle body to improve the problems by environmental pollution as well as improving fuel cost. The effective way to reduce the weight of vehicle body seems to be application of new materials for body structure and such trend is remarkable. Among the various materials for vehicle body, stainless steel sheet (for example, 301L and 304L), TRIP steel and cold rolled steel sheets are under the interests. However, in order to guarantee reliability of new material and to establish the long life design criteria of body structure, it is important and require condition to assess spot weldability of them and fatigue strength of spot welded lap joints which were fabricated under optimized spot welding condition. And, recently, a new issue in the design of the spot welded structure is to predict economically fatigue design criterion without additional fatigue tests. In general, for fatigue design of the spot-welded thin sheet structure, additional fatigue tests according to the welding condition, material, joint type, and fatigue loading condition are generally required. This indicates that much cost and time for it should be consumed. Therefore, in this paper, the maximum stresses at nugget edge of spot weld were calculated through nonlinear finite element analysis first. And next, obtained the ${\Delta}P-N_{f}$ relation through the actual fatigue tests on spot welded lap joints of similar and dissimilar high strength steel sheets. And then, the ${\Delta}P-N_{f}$ relation was rearranged in the ${\Delta}{\sigma}-N_{f}$ relation. From this ${\Delta}{\sigma}-N_{f}$ relation, developed the fatigue design technology for spot welded lap joints of them welded using the optimized welding conditions.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.1
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• pp.10-19
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• 1997

This study has been conducted to investigate the effects of initial structure on the microstructure and tensile properties of high strength trip steel sheet. The initial structure before austempering remarkably influenced the second phase. The specimen with normalized initial structure showed mainly bainitic ferrite and retained austenite, while the as rolled specimen and spherodized specimen showed martensite plus retained austenite and martensite plus bainitic ferrite with small retained austenite, respectively. Two type of retained austenite, film type and granual type were observed in all specimens. The as rolled specimen appeared the highest contents of retained austenite owing to the compressive stress by cold rolling. The contents of retained austenite increased with increasing intercritical annealing temperature and austempering time. Tensile strength showed the highest in the as rolled specimen, while the highest elongation were obtained in the normalized specimen. The maximum T.S.${\times}$El. Value showed in normalized initial structure and increased with increasing intercritical annealing and austempering time. The highest Value of T.S.${\times}$El. obtained at austempering temperature of $400^{\circ}C$ and retained austenite of 12%.