• Korean Journal of Metals and Materials
• /
• v.47 no.6
• /
• pp.331-337
• /
• 2009

To lower the annealing temperature and the deviation of the mechanical properties of bake hardening steels, high purity steels were investigated. The steels were characterized by treating at low recrystallization temperature. It was confirmed that the strengthening originated from the solid solution of carbon and the ferrite grain refinement by fine MnS precipitates as carbon and sulfur contents increased in high purity steels. However, it was observed that there was no more increase of strength in steels containing over 40 ppm of carbon. It was considered that the excess carbon formed either the carbon cluster or the low temperature unstable carbides which had the negligible effect on the strengthening because they were reported to be highly coherent with the matrix. The carbon cluster and unstable carbides could be transformed to the stable cementite during bake hardening treatment. MnS was not observed in the high purity steel containing 5 ppm S, resulting in very coarse recrystallized grains and good ductility. As sulfur content increased, the recrystallized grain size decreased due to the formation of the fine MnS precipitates.

• Korean Journal of Metals and Materials
• /
• v.46 no.10
• /
• pp.609-616
• /
• 2008

Conventional bake-hardenable(BH) steels should be annealed at higher temperatures because of the addition of Ti or/and Nb which forms carbides and raises recrystallization start temperature. In this study, the development of new BH steels without Ti or Nb addition has been reviewed. The new BH steels have nearly same mechanical properties as the conventional BH steels even though it is annealed at lower temperature. The steels also show smaller deviation of the mechanical properties than that of the conventional BH steels because of the conarol of solute carbon content during steel making processes. The deviation of mechanical properties in conventional BH steels is directly dependent on the deviation of solute carbon which is greatly influenced by the amount of the carbide formers in conventional BH steels. Less alloy addition in the newly developed BH steels gives economical benefits. By taking the advantage of sulfur and/or nitrogen which scarenge in Interstitial-Free or conventional BH steels, fine manganese sulfides or nano size copper sulfides were designed to precipitate, and result in refined ferrite grains. Aluminum nitrides used as a precipitation hardening element in the developed steels were also and resull in fine and well dispersed. As a result, the developed steels with less production cost and reduced deviation of mechanical properties are under commercial production. Note that the developed BH steels are registered as a brand name of MAFE(R) and/or MAF-E(R).

• Transactions of Materials Processing
• /
• v.15 no.8 s.89
• /
• pp.581-585
• /
• 2006

Effects of V on both the formation of Ti-Nb-V carbonitrides and mechanical properties of Ti-Nb bearing low carbon HSLA steels were investigated. Hot rolling process was simulated by using Gleeble 3500 system with the steels containing three different levels of V ($0{\sim}0.1wt.%$). Vanadium precipitated as Ti-Nb-V carbonitrides at austenite region but it did not precipitate as VC during austenite to acicular ferrite or bainitic ferrite phase transformation. As V content increased, the amount of Nb precipitates was decreased but the average size of Ti-Nb-V carbonitrides was increased due to larger diffusivity of V than that of Nb. Coarsened Ti-Nb-V carbonitrides could act as heterogeneous nucleation site during ${\gamma}{\rightarrow}{\alpha}$ phase transformation, thus, acicular ferrite transformation was promoted as V content increased, resulting in increase of upper shelf energy.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2008.05a
• /
• pp.431-434
• /
• 2008

Effects of microstructure on the toughness of low carbon HSLA steels were investigated. Nickel decreased the ferrite-austenite transformation temperature, resulted in increase of the fraction of bainitic ferrite. However, it was decreased with increasing deformation amount at austenite region. Since fine austenite grains formed by dynamic recrystallization under large strain transformed to acicular ferrite or granular bainite rather than bainitic ferrite. The effective grain size, thus, was decreased by deformation and it resulted in lower ductile-brittle transition temperature (DBTT). The bainitic ferrite was thought to inhibit the fracture crack initiation and to delay the crack propagation by its high dislocation density and hard interlath $2^{nd}$ phase constituents, respectively. Thus, DBTT was also decreased by Ni addition in low carbon HSLA steels.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 1996.04a
• /
• pp.97-101
• /
• 1996

In this paper, We have studied internal quality incluiding chemical compositions, microscopic structrue and nonmetalic inclusion of test materials. We have analyzed dynamic characteristics of cutting resistence including tensile strength value, hardeness value, impact value etcs. We have compared chip treatments of test materials. In analyzing internal quality, all of test materials have typical ferrite+pearlite structure. But, nonmetallic inclusion have oxide and sulfide inclusion in medium carbon steels, mainly sulfide inclusion is existed in S-free cutting steels. In Ca+S-free cutting steels, calcium aluminate and sulfide complex inclusion, had low-melting points, as deformation of sulfide and oxide inclusion is existed. machining characteristics, cutting resistence is maximum in Ca+S-free cutting steels, minimum in medium carbon steels. Chip treatements are excellent in S-free cutting steels, similar to the Ca+S free cutting steels and medium carbon steels.

• Journal of the Korean Society of Manufacturing Technology Engineers
• /
• v.9 no.3
• /
• pp.48-54
• /
• 2000

In this study the characteristics of chip formation of the cold drawn Bi-S free machining steels were assessed. And for comparison those of the cold drawn Pb-S free machining steel the hot rolled low carbon steel which has MnS as free machining inclusions and the conventional steels were also investigated. During chip formation the cold drawn free machining steels show relatively little change in thickness and width of chip compare to those of the conventional carbon steels. And a single parameter which indicates the degree of deformation during chip formation chip cross-section area ratio is introduced. The chip cross-section area ratio is defined as chip cross-section area is divided by undeformed chip cross-section area. The variational patters of the chip cross-section area ratio of the materials cut are similar to those of the shear strain values. The shear stress however seems to be dependent on the carbon content of the materials. The cold drawn Bi-S and Pb-S steels show nearly the same chip forming behaviors and the energy consumed during chip formation is almost same. A low carbon steel without free machining aids shows poor chip breakability due to its high ductility. By introducing a small amount of free machining inclusions such as MnS Bi, Pb or merely increasing carbon content the chip breakability improves significantly.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
• /
• 1999.10a
• /
• pp.351-356
• /
• 1999

In this study, the characteristics of chip formation of the cold drawn Bi-S free machining steels were assessed. And for comparison, those of the cold drawn Pb-S free machining steel, the hot rolled low carbon steel which has MnS as free machining inclusions and the conventional steels were also investigated. During chip formation, the cold drawn free machining steels show relatively little change in thickness and width of chip compare to those of the conventional carbon steels. And a single parameter which indicates the degree of deformation during chip formation, 'chip cross-section area ratio' is introduced. The chip cross-section area. The variational patterns of cross-section area is divided by undeformed chip cross-section area. The variational patterns of the chip cross-section area ratio of the materials cut are similar to those of the shear strain values. The shear stress, however, seems to be dependent on the carbon content of the materials. The cold drawn BiS and Pb-S steels show nearly the same chip forming behaviors and the energy consumed during chip formation is almost same. A low carbon steel without free machining aids shows poor chip breakability due to its high ductility. By introducing a small amount of non-metallic inclusions such as MnS, Bi, Pb or merely increasing carbon content the chip breakability improves significantly.

• Proceedings of the Korean Society for Technology of Plasticity Conference
• /
• 2003.05a
• /
• pp.307-310
• /
• 2003

Non-heat treated steels are attractive in the steel-wire industry since the spheroidization and quenching-tempering treatment are not involved during the processing. In this study, three different steels such as dual phase steel, low-Si steel, and ultra low carbon bainitic steel were used to investigate their deformation resistance and forming limit. Deformation resistance was estimated by calculating the deformation energy and the forming limit was evaluated by measuring the critical strain revealing crack initiation at the notch tip of the specimens. The results showed that deformation resistance was the lowest in the low-Si steel, and the forming limit strain was the highest in the ultra low carbon bainitic steel.

• Journal of the Korean Society for Heat Treatment
• /
• v.27 no.2
• /
• pp.79-89
• /
• 2014

The effect of carbon on the microstructure and texture of low carbon steels was investigated in a series of 1.6 Mn-0.3Cr-0.2Mo-0.001B steels with carbon ranging from 0.021 to 0.048%. Intensity of {111} orientation increased with decreasing the carbon content, resulting in the increase in $r_m$ value. The highest $r_m$ value of 1.30 was obtained in 0.021%C steel annealed at $820{\sim}850^{\circ}C$ according to the typical galvannealing heat cycle. Martensite volume fraction was not substantially affected by the annealing temperature. It was found that the fine and uniformly distributed martensite particles which were present in amounts of about 5% volume fraction were desirable for the highest $r_m$ value. The other factor affecting the high $r_m$ value was the preferred epitaxial growth of retained ferrite with {111} orientation into austenite during cooling.

• Corrosion Science and Technology
• /
• v.5 no.1
• /
• pp.23-26
• /
• 2006

Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks. In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel. The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties. By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability.