Cold-rolled steel sheet excellent in shape freezeability and manufacturing method thereof

Abstract

Disclosed are: a thin steel sheet having a low proportional limit; and a process for producing the thin steel sheet. A cold-rolled steel sheet, which has a chemical composition comprising, in mass%, 0.10% or less of C, 0.05% or less of Si, 0.1-1.0% of Mn, 0.05% or less of P, 0.02% or less of S, 0.02-0.10% of Al, less than 0.005% of N and a remainder made up by Fe and unavoidable impurities, is subjected to an annealing treatment which comprises heating at an annealing temperature of 730-850°C for 30 s or longer and cooling to a temperature equal to or lower than 600°C at a cooling rate of 5°C / s or more, thereby producing a cold-rolled annealed sheet having a structure mainly composed of a ferrite phase having an average crystal grain diameter (d) of 5-30 µm. The cold-rolled annealed sheet is subjected to a temper rolling treatment using a roll having a surface roughness (Ra) of 2.0 µm or less and at a temper rolling stretch ratio (R) (%) of (0.05-0.20)d (wherein d (µm) represents the average crystal grain diameter of the thin cold-rolled annealed sheet). In this manner, a cold-rolled thin steel sheet having a proportional limit of 150 MPa or less can be produced. The cold-rolled thin steel sheet may have, formed on the surface thereof, a molten zinc plated layer, an alloyed molten zinc plated layer or an electrolytic zinc plated layer.

Description

technical field [0001] The present invention relates to a cold-rolled steel sheet suitable for structural members such as electrical appliances, business equipment, and automobile members, and in particular to a cold-rolled steel sheet with excellent shape freezing properties suitable for members requiring high dimensional accuracy after stamping. steel plate. It should be noted that the so-called “thin steel plate” here refers to a steel plate with a plate thickness of 0.2 to 2.0 mm, including steel plates and steel strips. Background technique [0002] In recent years, in fields such as the electric motor field, along with rising raw material prices, there has been a strong demand for inexpensive raw materials in order to reduce costs, and thinning of steel sheets used as raw materials has been accelerating. Also, in the automotive field, in addition to cost reduction, there is a strong demand for improvement in fuel efficiency of automobiles from the viewpoint of global ...

AI Technical Summary

Problems solved by technology

However, re-press forming leads to an increase in manufacturing cost, so there is a strong demand for a steel plate with excellent shape freezing properties after press forming.

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