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

[0013] It is an object of the present invention to provide a high carbon hot-rolled steel sheet having 440 MPa or higher tensile strength and giving excellent ductility and stretch-flange formability, generating very few cracks at punched end face, and which steel sheet can be manufactured without applying time-consuming multi-stage annealing.

[0014] The inventors of the present invention conduced intensive studies on the effect of components and microscopic structures of high carbon steel sheet on ductility and stretch-flange formability while securing strength thereof, and found that the ductility and the stretch-flange formability of steel sheet are significantly affected by not only the composition of the steel, the shape and quantity of carbide, but also the dispersed state of carbide. That is, it was found that the ductility and the stretch-flange formability of high carbon hot-rolled steel sheet are improved by controlling each of the carbide shape in terms of average grain size of carbide and volume ratio of carbide having 2.0 μm or larger grain size, and the dispersed state of carbide in terms of volume ratio of carbide-free ferrite grains and average grain size of ferrite.

[0015] The present invention provides a high carbon hot-rolled steel sheet consisting essentially of, 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 balance of Fe and inevitable impurities, and having a structure of ferrite having 6 μm or smaller average grain size and carbide having 0.10 μm or larger and smaller than 1.2 g m of average grain size. The volume ratio of the carbide having 2.0 μm or larger grain size is 10% or less, and the volume ratio of the ferrite containing no carbide is 5% or less. The high carbon steel sheet gives excellent ductility and stretch-flange formability.

[0036] The present invention suppresses the generation of voids at punched end face during punching, and delays the growth of cracks during burring. As a result, the present invention provides a high carbon hot-rolled steel sheet having 440 MPa or higher tensile strength and extremely excellent ductility and stretch-flange formability. By applying the high carbon hot-rolled steel sheet having excellent ductility and stretch-flange formability according to the present invention to highly durable parts such as transmission parts represented by gear, advanced level of forming is attained in the forming step, which provides high product quality and allows manufacturing the parts at low cost with decreased number of manufacturing steps. Also for the parts of driving system, the integrally formed components are requested to have increased strength in the non-heat treating parts for attaining higher durability and lighter weight, thus the steel sheets as the starting material are requested to have 440 MPa class tensile strength (TS). The high carbon hot-rolled steel sheet according to the present invention is useful in this respect.

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.