High carbon hot-rolled steel sheet and method for manufacturing the same

Abstract

The high carbon hot-rolled steel sheet contains, in terms of percentages of mass, 0.10 to 0.7% C, 2.0% or less Si, 0.20 to 2.0% Mn, 0.03% or less P, 0.03% or less S, 0.1% or less Sol.Al, 0.01% or less N, and the balance being Fe and inevitable impurities, and has a structure of ferrite having 6 μm or less average grain size and carbide having 0.10 μm or more and less than 1.2 μm of average grain size. The volume ratio of the carbide having 2.0 μm or more of grain size is 10% or less. The volume ratio of the ferrite containing no carbide is 5% or less. The manufacturing method thereof has the steps of hot-rolling, primary cooling, holding, coiling, acid washing, and annealing. The primary cooling step is to cool the hot-rolled steel sheet down to cooling termination temperatures ranging from 450° C. to 600° C. at cooling rates of higher than 120° C. / sec. The holding step is to apply secondary cooling to hold the primarily cooled hot-rolled steel sheet at a temperature range from 450° C. to 650° C. until coiling.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a high carbon hot-rolled steel sheet having excellent ductility and stretch-flange formability, and a manufacturing method thereof.DESCRIPTION OF THE RELATED ARTS [0002] High carbon steel sheets employed for tools, automobile parts (gear, transmission), and the like are subjected to heat treatment such as quenching and tempering after punching and forming thereof. The requests of users who conduct the working on these components include improvement in bore expansion (burring) property in the forming process after punching, as well as the elongation characteristic which is an index of ductility for forming the steel sheet into complex shapes. The burring property is evaluated by the stretch-flange formability as one of press-forming properties. Consequently, there are wanted the materials having excellent stretch-flange formability as well as ductility. [0003] Regarding the improvement in the stretch-flange formability of...

AI Technical Summary

Benefits of technology

The present invention provides a high carbon hot-rolled steel sheet with excellent ductility and stretch-flange formability. The steel sheet has a structure of ferrite with small average grain size and carbide having a specific shape and quantity, which significantly affects its ductility and stretch-flange formability. The steel sheet contains at least one element selected from the group consisting of Cr, Mo, B, Cu, Ni, and W, and further contains at least one element selected from the group consisting of Ti, Nb, V, and Zr. The steel sheet can be manufactured by hot-rolling, primary cooling, holding, coiling, acid washing, and annealing. The primary cooling step is primary cooling of the hot-rolled steel sheet down to the cooling termination temperatures ranging from 450 to 650°C.

Problems solved by technology

As a result, the stretch-flange formability is not necessarily favorable, though the material is soft and shows excellent ductility.

As a result, stress concentrates to the peripheral zones of grain boundary where the deformation significantly differs therebetween, thereby generating voids at interface between the spheroid structure and the ferrite.

Since the voids grow to cracks, the stretch-flange formability is ultimately deteriorated.

In this measure, however, the spheroidized carbide becomes coarse to become the origin of void during the forming step, and the carbide becomes less soluble in the heat treatment step after the forming to cause the decrease in quenched strength.

Consequently, burring in high carbon steel sheet also likely induces crack generation at punched end face caused by the advanced level of forming.

According to the technology disclosed in JP-A-2003-13145, however, achieving TS=440 MPa (73 point or more as HRB hardness) not necessarily attains satisfactory stretch-flange formability.

That is, the technology cannot satisfy stably the requirements of both that level of TS and the stretch-flange formability.

Adding to the above technology, the technology disclosed in JP-A-2003-13145 generates transformation heat after cooling, which increases the temperature to enhance the precipitation of proeutectoid ferrite and the pearlite transformation, thereby inducing growth of coarse carbide and uneven carbide distribution to likely deteriorate the characteristics.