Hot-rolled high-carbon steel sheets and process for production of the same

Abstract

The invention provides a hot-rolled high-carbon steel sheet excellent in the homogeneity in the width direction. The steel sheet contains C: 0.2 to 0.7%, Si: 0.01 to 1.0%, Mn: 0.1 to 1.0%, P: 0.03% or below, S: 0.035% or below, Al: 0.08% or below and N: 0.01% or below with the balance consisting of Fe and unavoidable impurities. The texture of the sheet is characterized by a mean ferrite grain size in the sheet edge of less than 35[mu]m, a mean ferrite grain size in the central area of less than 20[mu]m, and a mean carbide grain size of 0.10mum to less than 2.0mum. The steel sheet of the invention is manufactured by rough rolling, finish rolling at a finishing temperature of (Ar3+40 DEG C) or above, cooling initiated within two seconds after the finish rolling at a cooling rate exceeding 120 DEG C / s until a cooling stop temperature exceeding 550 DEG C and below 650 DEG C, coiling at a temperature of 550 DEG C or below, pickling, and then spheroidizing at a temperature of 670 DEG C to Ac1 transformation point.

Description

technical field [0001] The present invention relates to a high-carbon hot-rolled steel sheet and a manufacturing method thereof, and particularly to a high-carbon hot-rolled steel sheet excellent in width direction homogeneity and a manufacturing method thereof. Background technique [0002] High-carbon steel sheets used in tools and automotive parts (gears, transmissions), etc. are stamped and then subjected to heat treatment such as quenching and tempering. In recent years, tool and component manufacturers, that is, users of high-carbon steel plates, have been studying the transformation from the conventional cutting and hot forging of cast materials to stamping (including cold forging) of steel plates in order to reduce costs. Simplification of processing procedures. At the same time, hardenability and stability in processing into complex shapes are strongly desired for high-carbon steel sheets as raw materials. Furthermore, from the standpoint of maintenance and manage...

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Problems solved by technology

[0011] In the technology described in Patent Document 1, coarse spheroidized cementite is obtained by annealing the high-carbon steel strip in the ferrite-austenite two-phase region above the Ac1 point, but such coarse cementite It is a structure that is difficult to stabilize hardenability and workability

[0012] In the technology described in Patent Document 2, since the annealing process is complicated, when it is assumed that the actual operation is performed, the productivity deteriorates and the cost increases

[0013] In addition, in the technique described in Patent Document 3, ferrite grain size is coarsened by spheroidizing and annealing a hot-rolled steel sheet containing bainite with a volume ratio exceeding 70%, thereby making it extremely soft. After hot rolling at a finish rolling temperature above (Ar3 transformation point - 20°C), due to rapid cooling at a cooling rate exceeding 120°C / s, a phase change exotherm occurs after cooling, and the temperature rises, resulting in hot rolling The problem of poor stability of steel plate structure

Moreover, for the hardness after spheroidizing annealing, the plate surface of the sample can only be evaluated by Rockwell B scale hardness (HRB), and because the coarse ferrite grains are not uniformly formed in the direction of plate thickness after spheroidizing annealing , prone to material unevenness, so stable softening cannot be obtained

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