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High strength cold-rolled bake hardenable steel sheets for automotive use have been developed. In the last decade, the major strengthening methods of extra low carbon (ELC) steel and interstitial free (IF) steel were the solid-solution strengthening with silicon, manganese and phosphorous and the precipitation strengthening with microalloying elements, such as niobium and titanium. When the steels are strengthened with the high amount of solid-solution elements such as silicon and manganese, it shows the low surface quality due to surface oxidation. In addition, phosphorous added ELC and IF steels become susceptible to the secondary work embrittlement because of the lack of grain boundary strength, which is the essential drawback of the steels.In this paper, we propose the extra low carbon bake hardenable steel which is precipitation strengthened by means of the fine distribution of Copper sulfide (Cu 2 S) precipitates instead of solid-solution strengthening and precipitation strengthening using silicon, manganese, niobium and titanium. These Cu sulfides are oriented to the body-centered cubic a-Fe matrix in (001) KEY WORDS: extra low carbon steel; Cu precipitate; sulfide; precipitation hardening; bake hardenability; cold-rolled sheet. 109© 2009 ISIJ Cu and to treat temper annealing.9-11) Thus, in case of small addition of Cu, it is difficult to form the Cu precipitate (eCu).On the other hand, some researchers studied about the solid-solution strengthening and Cu-S precipitation in Ti added ELC steel. Yamada et al. reported that the addition of 0.22 % Cu increases tensile strength about 10 MPa in Tistabilized IF steel. 12) They described that the increment of tensile strength is mainly resulted from grain refinement and solid-solution strengthening. In addition, Kejian et al. and Ishiguro et al. suggested that a residual level of Cu can make large sulfides and inclusion in Ti added steels. 13,14) However, they did not consider the possibility of precipitation hardening of Cu sulfides.Guilet et al. reported that nano-sized Cu-S precipitates can be formed in batch annealing process for Ti free low carbon steels.15) Recently, Liu et al. studied the crystallography and precipitation kinetics of copper sulfide for low carbon steel in strip casting process. 16,17) In this work, we examine the evidence of Cu-S precipitation hardening due to fine Cu 2 S for extra low carbon steels in continuous annealing process. Also, the performance of the Cu added ELC steel is compared with conventional Nb-Ti BH steel. Experiments MaterialThe materials used in this study are extra low carbon steels with a bake hardenability are listed in Table 1. In order to investigate the effect of Cu on the strengthening for the ELC steel, three ELC steels (0Cu ELC, 01Cu ELC and 02Cu ELC) that contain different content of Cu were selected. In addition, the mechanical properties of the 02Cu ELC steel are compared with conventional Nb-Ti added BH steel (Nb-Ti BH). The Nb-Ti added BH steel means that solute elements of C, N, and S are partially sc...

This paper presents a method to determine a constitutive equation of AISI4140 steel when it deforms at a wide range of strain rates (0.05~1900s−1) under high temperatures. The fundamental idea considered here is that the dependency of flow stress upon strain rate and temperature is described by the power law model of the Arrhenius equation. Strengthening coefficient and work hardening exponent are introduced to take a simplified treatment to the flow stress behaviour characteristics without losing accuracy of the model. The validity of the proposed constitutive equation has been examined using results obtained from the hot compression test (GLEEBLE® 3500C) and Split Hopkinson‐Pressure Bar test at high temperature. Results revealed the proposed constitutive equation could predict flow stress‐strain curve of the steel with an acceptable amount of error at the ranges of strain rates.

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