Bake-hardening hot-rolled steel sheet with excellent workability and method for manufacturing the same

Abstract

According to an exemplary embodiment of the present invention, a bake-hardening hot-rolled steel sheet with excellent workability can be provided. The exemplary steel sheet can contain, in terms of mass %, C of about 0.01 to 0.2%, Si of about 0.01 to 2%, Mn of about 0.1 to 2%, P of about 0.1% or less, S of about 0.03% or less, Al of about 0.001 to 0.1%, N of about 0.01% or less, Nb of about 0.005 to 0.05%, and as the remainder, Fe and unavoidable impurities. A microstructure of the steel sheet can be a polygonal ferrite and / or a continuous-cooled microstructure having an average particle diameter of about 2 μm to 8 μm, and the grain boundary abundance ratio of solute C and / or solute N may be about 0.28 or lower. An exemplary embodiment of the present invention can also be directed to a method for manufacturing a hot-rolled steel sheet. In this exemplary method, a slab containing aforementioned components can be heated to at least a temperature satisfying the equation of SRT (° C.)=6670 / {2.26−log(% Nb)(% C)}−273. The heated slab can be subjected to a rough rolling, a finish rolling under a condition where the final temperature can be in a range of the Ar3 transformation point temperature or higher and (Ar3 transformation point temperature+100° C.) or lower to obtain a rolled steel. The rolled steel can be cooled at a cooling rate of about 80° C. / sec or higher from cooling initiation down to a temperature range of about 500° C. or lower to obtain a hot rolled steel sheet; and such hot rolled steel sheet can be coiled.

Description

TECHNICAL FIELD[0001]The present invention relates to a bake-hardening hot-rolled steel sheet with excellent workability and a method for manufacturing the same. In detail, it relates to a steel sheet including a microstructure of polygonal ferrite and / or a continuous-cooled microstructure having an average particle diameter of 2 to 8 μm, by which not only parts requiring severe working can be readily formed, but also, even in a steel sheet having a tensile strength of 370 to 640 MPa class, a strength of press material corresponding to the design strength when a steel sheet of 540 to 780 MPa class is applied, can be obtained by strain introduction by means of pressing, and a baking finish treatment.[0002]This application claims priority from Japanese Patent Application No. 2005-010210, filed on Jan. 18, 2005, the content of which is incorporated herein by reference.BACKGROUND ART[0003]Recently, in order to improve the fuel efficiency of automobiles and the like, with an object of re...

AI Technical Summary

Benefits of technology

[0029]By using the bake-hardening hot-rolled steel sheet with excellent workability of the present invention, not only parts requiring severe working can be readily formed, but also a bake-hardening amount of 50 MPa or more can be obtained stably in a strength range of 370 to 640 MPa class. As a result, even when the tensile strength of the steel sheet is 370 to 640 MPa, a pressed product having strength equivalent to the design strength of pressed product manufactured by applying a 540 to 780 MPa class steel sheet can be manufactured as a result of the introduction of pressing stress and baking finish treatment. From the above, the present invention can be said to be a highly industrially valuable invention.

Problems solved by technology

However, regardless of the advantage of high specific strength, light metals such as Al alloy are remarkably expensive compared to steels, and therefore the application thereof is limited to specific usages.

If the elements such as C, Si, and Mn are reduced to realize the strength within the range of 370 to 540 MPa class, there is a problem of being unable to maintain amount of retained austenite required for obtaining the TRIP phenomenon in the microstructure at room temperature.

Moreover, the above techniques do not take into consideration of the improvement of burring workability.

Consequently, in the current state, it is difficult to apply high strength steel sheets having strength of 590 MPa or more class to members in which steel sheet having strength on the order of about 270 to 340 MPa class is currently used, without first improving operations and equipment used during pressing.

It is effective to increase solute C and solute N so as to improve bake hardenability; however, increases in these solute elements present in the solid solution worsen aging deterioration at normal temperatures.

However, refining the crystal grains gives concern of deterioration of press formability.

Moreover, if suspension members and inner plate members are the subject, regardless of the needs for excellent burring workability, the burring workability is considered to be inappropriate since the microstructure is ferrite-pearlite.

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