• Korean Journal of Materials Research
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• v.13 no.7
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• pp.453-459
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• 2003

In this paper the effect of interstitial heat treatment on the microstructure and mechanical properties was examined both in the 0.15C-6Mn steels and 0.15C-6Mn steels added with Nb or Ti. This result will be applied into the development of a steel which has the properties of high strength and high ductility resulted from the transformation induced plasticity. The strength-elongation combination was increased as the holding time was increased when the temperature is at $625^{\circ}C$. However, the strength-elongation combination was decreased sharply as the holding time was increased when the temperature is at $675^{\circ}C$. The tensile strength and elongation of a reverse transformed steels added with Ti or Nb was 93 kg/$\textrm{mm}^2$ and 40%, respectively. This steel shows higher strength more than 10% of the 0.15C-6Mn steel without loss of ductility. The autenite formed from the reverse transformed treatment has a fine lath type, which has the width size of 0.1-0.3 $\mu\textrm{m}$. The TRIP sequence normally transforms the austenite to martensite, however, some of the sequence will produce retained austenite \longrightarrow deformation twin \longrightarrow martensite

• Korean Journal of Materials Research
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• v.15 no.2
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• pp.115-120
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• 2005

The aim of this research is to develop the TRIP aided high strength low carbon steels using reverse transformation process. The $4\~8\%$ Mn steel sheets were reversely transformed by slow heating to intercritical temperature region and furnace cooling to room temperature. The stability of retained austenite depends on the enrichment of carbon and manganese by diffusion during the reverse transformation. The amount of retained austenite formed after reversely transformed at $625^{\circ}C$ for 6 hrs was about $50\;vol.\%$ in the $8\%Mn$ steel. The change in volume fraction of retained austenite with a holding temperature was consistent with the changes in elongation and the strength-ductility combination. The maximum strength-ductility combination of 40,000 $MPa{\cdot}\%$ was obtained when the $8\%Mn$ steel reversely transformed at $625^{\circ}C$ for 12 hrs. However, it's property was significantly decreased at higher holding temperature of $675^{\circ}C$ resulting from the decrease of ductility.

• Transactions of Materials Processing
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• v.14 no.8 s.80
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• pp.712-717
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• 2005

Microstructure and mechanical properties are investigated for 0.15C-1.5Mn-1.5Al TRIP aided cold rolled steels containing $0.05wt\%P$ and $0.1wt\%P$. Despite of the complete replacement of Si by Al, the TRIP steel shows tensile strength of 700MPa and total elongation of $35\%$ by addition of $0.1wt\%$ P. Tensile strength of P added TRIP steels is not only affected by the solid solution hardening but also the volume fraction of retained austenite. As P content increases from $0.05wt\%$ to $0.1wt\%$, tensile strength and volume fraction of retained austenite are increased, but elongation is decreased. The lower stability of austenite in $0.1wt\%$ P added steel is responsible for the decrease of the elongation.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.08a
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• pp.45-52
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• 2004

Continuing pressure for the weight reduction of vehicles and improvement of collision safety is driving the development of new high strength steel with excellent formability. The formable high strength steels which have excellent drawability have been developed and applied to the complicated inner panels. Although BH steel have mainly occupied the material market for outer panels, it is challenged by DP steel which have low yield strength and good bake hardenability. The advanced high strength steel, TRIP steels and DP steels which have excellent formability are new alternatives to conventional HSLA steel for structural parts such as members and pillars. HSLA steels also have been used for automotive bumper reinforcements due to their high yield ratio. Higher grade complex phase steel(CP) were developed for bumper reinforcements by addition of precipitation hardening to transformation strengthened steel. The usage of the advanced high strength steel ale increasing and will become the main material in structural parts near future. This paper describes the features of newly developed high strength cold rolled steels for automobiles.

• Korean Journal of Metals and Materials
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• v.49 no.12
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• pp.967-976
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• 2011

Conventional fracture tests of resistance spot welds have been performed without consideration of the paint baking process in the automobile manufacturing line. The aim of this paper is to investigate the effect of the paint baking process on load carrying capacity and fracture mode for resistance spot welded 590 dual phase (DP), 780DP, 980DP, 590 transformation in duced plasticity (TRIP), 780TRIP and 1180 complex phase (CP) steels. With paint baking after resistance spot welding, the l-shape tensile test (LTT) and nano-indentation test were conducted on the as-welded and paint baked samples. Paint baking increased the load-carrying capacity of the resistance spot welded samples and improved the fracture appearance from partial interfacial fracture (PIF) to button fracture (BF). Improvement in fracture appearance after LTT is observed on weldments of 780 MPa grade TRIP steels, especially in the low welding current range with paint baking conditions. The higher carbon contents (or carbon equivalent) are attributed to the low weldability of the resistance spot welding of high strength steels. Improvement of the fracture mode and load carrying ability has been achieved with ferrite hardening and carbide formation during the paint baking process. The average nano-indentation hardness profile for each weld zone shows hardening of the base metal and softening of the heat affected zone (HAZ) and the weld metal, which proves that microstructural changes occur during low temperature heat treatment.

• Proceedings of the Korean Society of Laser Processing Conference
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• 2002.05a
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• pp.34-37
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• 2002

• Journal of the Korean Society for Heat Treatment
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• v.16 no.4
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• pp.205-210
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• 2003

Various types of high strength steel sheets were usually used for improving the automobile safety and fuel efficiency by reducing the vehicle weight. The present study aimed to develop the TRIP (transformation induced plasticity) aided high-strength low carbon steel sheets by using a reverse transformation process. The 0.1C-4~8Mn steels were reverse-transformed by slow heating to intercritical temperature region and then furnace cooled to the room temperature. Granular type retained austenite was observed in 4Mn steel and lath type retained austenite was also observed in 6~8Mn steel. The results show that the 6Mn steel under reverse transformed at $625^{\circ}C$ for 6 hrs has maximum elongation up to 39%. The optimum strength-elongation combination was 3,888 ($kg/mm^2{\times}%$) when the 8Mn steel was reverse transformed at $625^{\circ}C$ for 12 h.

• Journal of the Korean Society for Heat Treatment
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• v.10 no.4
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• pp.278-287
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• 1997

The TRIP behavior in tensile deformation of retained austenite formed by reverse transformation treatment in 0.15%C-6%Mn-(Ti, Nb) steels has been investigated. The shape of retained austenite was almost a fine lath type with $0.1{\sim}0.3{\mu}m$ width and the two distinctly different transformation sequences of retained austenite, i) retained austenite${\rightarrow}$martensite and ii) retained austenite${\rightarrow}$deformation twin${\rightarrow}$martensite were revealed. The strength-elongation combination was increased with increasing the holdig time at low temperatures ($625^{\circ}C$) but decreased abruptly with increasing holding time at high temperatures ($675^{\circ}C$), owing to the lowering of ductility. The strength-elongation combination and TRIP effect was lower in tensile deformation in the range of $100{\sim}250^{\circ}C$ than room temperature. The tensile strengh and elongation of a reverse transformed steels with addition of Ti or Nb was 93kg/, 40% respectively, which is higher over 10% of strength without ductility loss than in 0.15%C-6%Mn steels.

• Journal of Welding and Joining
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• v.33 no.2
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• pp.69-77
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• 2015

High strength steels have been continually being developed to improve in fuel economy in automotive and ensure safety of passengers. New bonding and welding methods have been required for improving weldability on high strength steels. In this study, resistance spot welding and Weld-bond with nugget diameters of 4.0mm, 5.0mm, 6.0mm and 7.0mm were produced and tested, respectively. In order to confirm the effect of nugget diameters on tensile shear characteristic of the Weld-bond, tensile shear characteristics of Weld-bond were compared with those of resistance spot welding and adhesive bonding. Peak load of Weld-bond were increased as the nugget diameter increases. After appearing maximum peak load continuous fracture followed with second peak owing to load being carried by resistance spot weldment. Fracture modes of the adhesive layer in Weld-bond fractures were represented by mixed fracture mode, which are cohesive failure on adhesive part and button failure at resistance spot welds. The results showed that the tensile shear properties can be improved by applying Weld-bond on TRIP steel, and more apparent with nugget diameter higher than 5${\surd}$t.

• Korean Journal of Materials Research
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• v.14 no.2
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• pp.126-132
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• 2004

The high strength steel sheets has been widely used as the automobile parts to reduce the weight of a vehicle. The aim of this research is to develop the TRIP aided high strength low carbon steels using reverse transformation process. The 0.15C-4Mn and 0.15C-6.5Mn steel sheets were reversely transformed by slow heating to intercritical temperature region and air cooling to room temperature. The stability of retained austenite depends on the enrichment of carbon and manganese by diffusion during the reverse transformation. The amount of retained austenite formed after reversely transformed at $645^{\circ}C$ for 12 hrs. was about 46vol.% in hot rolled 0.lC-6.5Mn steel. The change in volume fraction of retained austenite with a holding temperature was consistent with the changes in elongation and the strength-ductility combination. The tendency of tensile strength to increase with increasing the holding temperature was due to the decrease of retained austenite after cooling from the higher temperature of $670 ^{\circ}C$. The maximum strength-ductility combination was about 4,250 kg/$\textrm{mm}^2$ㆍ% when the hot rolled 0.lC-6.5Mn steel was reversely transformed at $645^{\circ}C$ for 12 hrs.