High-strength cold rolled steel sheet having excellent shear processability, and manufacturing method therefor

Abstract

Provided is a high-strength cold rolled steel sheet having high shear processability, including, by wt %, C: 0.05% to 0.10%, Si: 0.01% to 0.5%, Mn: 1.2% to 2.0%, Al: 0.01% to 0.1%, Cr: 0.005% to 0.3%, B: 0.0003% to 0.0010%, Mo: 0.005% to 0.2%, P: 0.001% to 0.05%, S: 0.001% to 0.01%, N: 0.001% to 0.01%, Nb: 0.005% to 0.08%, Ti: 0.005% to 0.13%, V: 0.005% to 0.2%, and a balance of Fe and inevitable impurities. The high-strength cold rolled steel sheet satisfies Formulas 1 and 2 below, and the high-strength cold rolled steel sheet includes at least one of carbides, nitrides, and carbonitrides. Formula 1: 2.0≤[Mn]+2.5[Mo]+1.5[Cr]+300[B]≤2.5, Formula 2: 0.2≤([Nb] / 93+[Ti] / 48+[V] / 51) / ([C] / 12+[N] / 14)≤0.5 (in Formulas 1 and 2, each element symbol refers to a weight percent (wt %) of a corresponding element).

Description

TECHNICAL FIELD[0001]The present disclosure relates to a high-strength cold rolled steel sheet having high shear processability, and a method for manufacturing the high-strength cold rolled steel sheet.BACKGROUND ART[0002]Parts, such as the friction plates of automatic transmissions of automobiles, are required to have an ability of suppressing the propagation of cracks in under frictional heat conditions and to have high strength and hardness in addition to having crack resistance in a shearing process.[0003]In the related art, as disclosed in Patent Document 1, a technique, in which low carbon steel or steel having various alloying elements is annealed by a recovery annealing method after a cold rolling process, has been applied to high-strength cold rolled steel sheets used for friction plates or guaranteeing hardness. In addition, a method of performing a spheroidizing heat treatment on high carbon steel has generally been used, and Patent Document 2 has proposed a technique in ...

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Benefits of technology

[0015]The present disclosure provides a high-strength cold rolled steel sheet having high shear processability and capable of suppressing the formation of cracks in a shearing process and under frictional heat conditions in addition to having high strength and high hardness, and a method of manufacturing the high-strength cold rolled steel sheet.

Problems solved by technology

However, it is difficult to manufacture high-strength steel sheets using the recovery annealing method, and techniques using the spheroidizing heat treatment method and performing cold rolling twice incur high manufacturing costs.

In addition, although alloying elements such as carbon (C), silicon (Si), manganese (Mn), molybdenum (Mo), or chromium (Cr), mainly used to manufacture high-strength steel sheets through cold rolling, are effective in improving the strength of steel sheets through solid-solution strengthening, the excessive addition of such elements causes segregation of the elements and formation of non-uniform microstructures.

In particular, ferrite transformation is markedly delayed because of an increase in the hardenability of steel during cooling, low-temperature phases (martensite and austenite) are formed, and grain boundaries become non-uniform.

Thus, cracks increase during a shearing process, and if frictional heat is generated during operations, cracks may easily propagate and form defects.

In addition, if alloying elements such as titanium (Ti), niobium (Nb), or vanadium (V) used for additional strength improvements are improperly added, coarse carbides, nitrides, and precipitates are formed along grain boundaries, thereby increasing formation of cracks and facilitating propagation of cracks during a shearing process.

Furthermore, if frictional heat is generated on a sheared portion during an operation, the propagation of cracks may more easily occur.

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