Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-01-23
2004-02-24
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S603000
Reexamination Certificate
active
06695932
ABSTRACT:
TECHNICAL FIELD
The present invention relates to cold-rolled steel sheets primarily suitable for use in automobile bodies. In particular, the present invention relates to a cold-rolled steel sheet exhibiting a tensile strength (TS) of less than 440 MPa and having superior strain-aging hardenability and to a method for manufacturing the same. While various grades of steel sheets from those suitable for light processing to those suitable for extensive deep drawing are available for the steel sheet for automobile bodies, the cold-rolled steel sheet of the present invention is suitable for use in processes which require relatively low-grade steel sheets with suitable processability. The cold-rolled steel sheets of the present invention are suitable for a wide range of uses from use in relatively light processes such as forming pipes by light bending or roll forming to use in relatively severe drawing. In this invention, the term “steel sheets” includes steel strips.
In the present invention, the statement “superior strain-aging hardenability” means that the deformation stress increment before and after an aging treatment is not less than 80 MPa and that the tensile strength increment before and after the strain aging treatment (predeformation+aging treatment) is not less than 40 MPa, the aging treatment being performed at a temperature of 170° C. held for 20 minutes and being conducted after predeformation at a tensile strain of 5%. The deformation stress increment, which is the difference between the yield strength before the aging treatment and after the aging treatment, is also referred to as a BH increment. The tensile strength increment, which is the difference between the tensile strength before the predeformation and after the aging treatment, is hereinafter represented by &Dgr;TS.
BACKGROUND ART
With current gas-emission regulations concerning global environmental problems, reducing body weight of automobiles has become a matter of vital importance. In order to reduce the body weight of automobiles, increasing the strength of the steel sheets used at a large amount, i.e., applying high-strength steel sheets, and reducing the thickness of the steel sheets are effective.
Unfortunately, steel sheets exhibiting significantly high strength suffer from the following problems during press forming in manufacture of the automobile components:
1. degradation of shape fixability
2. occurrence of cracking and necking during forming due to degraded ductility.
In order to overcome the above problems, steel sheets made from ultra-low carbon steels in which the amount of carbon in the solid-solution state remaining in the final product is controlled within a suitable range is known among cold-rolled steel sheets for use in exterior panels. This type of steel sheet is kept soft during press forming so as to secure shape fixability and ductility. Dent resistance is achieved by increasing the yield stress resulting from a strain-aging hardening phenomenon which occurs during a paint-baking process in which a temperature of 170° C. is held for about 20 minutes. This type of steel sheet having carbon in solid-solution in the steel is soft during press forming. In a paint-baking process subsequent to the press forming process, dislocations caused by press forming are fixed by solute carbon, thereby increasing the yield stress.
In this type of steel sheet, however, the increase in the yield stress caused by strain-aging hardening is kept low in order to prevent stretcher strain during press forming which will cause surface defects. Thus, actual contribution of this steel sheet to weight reduction of the component is small.
In order to reduce the weight of the components, both the increase in the yield stress due to strain aging and the increase in the strength characteristics after progressed deformation are necessary. In other words, an increase in the tensile strength after strain aging is required.
In contrast, for uses where good appearance is not required, a steel sheet in which solute nitrogen is used to improve the bake hardening increment, and a steel sheet in which the bake hardenability is further improved by the composite structure comprising ferrite and martensite have been suggested.
For example, Japanese Unexamined Patent Application Publication No. 60-52528 discloses a hot-rolling process in which a steel containing C: 0.02% to 0.15%, Mn: 0.8% to 3.5%, P: 0.02% to 0.15%, Al: not more than 0.10%, and N: 0.005% to 0.025% is coiled at a temperature of not more than 550° C. and a method for manufacturing a high-strength steel sheet exhibiting good ductility and spot weldability in which controlled-cooling annealing is performed after cold rolling. The steel sheets manufactured by the technology described in Japanese Unexamined Patent Application Publication No. 60-52528 has a mixed structure comprising a phase of low-temperature transformation products mainly including ferrite and martensite and exhibits good ductility. In this steel sheet, strain aging during paint baking caused by deliberately added nitrogen is utilized to obtain high strength.
In the technology described in Japanese Unexamined Patent Application Publication No. 60-52528, the increase in yield strength YS due to strain-aging hardening is large but the increase in the tensile strength TS is small. Moreover, a variation in mechanical characteristics such as a variation in the increase of the yield stress YS is significantly large. Thus, the thickness of the steel sheets cannot be reduced by as much as currently required to reduce the weight of the automobile components.
Japanese Patent Publication No. 5-24979 discloses a bake-hardenable high-tension cold-rolled thin steel sheet having a composition of C: 0.08% to 0.20%, Mn: 1.5% to 3.5% with the balance being Fe and unavoidable impurities. The structure of this steel sheet comprises homogenous bainite containing 5% or less of ferrite, or bainite partially including martensite. The cold-rolled steel sheet disclosed in Japanese Patent Publication No. 5-24979 has a structure primarily including bainite obtained by quenching in the temperature range of 400° C. to 200° C. during cooling step after continuous annealing, the quenching being followed by slow-cooling. The steel sheet attains, through this structure, a high bake-hardening increment which has not been attained before.
In the steel sheet described in above Japanese Patent Publication No. 5-24979, although the yield strength increases after paint-baking and a high degree of bake hardening which has never been achieved before is obtained thereby, the tensile strength is not increased. When applied to components requiring strength, improvements in fatigue resistance and crash resistance after forming cannot be expected. Thus, there is a problem in that the steel sheet cannot be used where high fatigue resistance and crash resistance are required.
Moreover, the conventional steel sheets described above, though excellent in strength when evaluated by simple tensile testing after paint-baking treatment, exhibit a significantly large variation in strength after the steel sheets have been subjected to plastic deformation under actual press conditions. Thus, these steel sheets are not necessarily suitable for the components requiring high reliability.
The present invention aims to overcome the limitations of the above-described related art and to provide a cold-rolled steel sheet exhibiting good formability, stable quality, and superior strain-aging hardenability, the steel sheet having sufficient strength for use in automobile components after the steel sheet is formed into automobile components, thus helping to reduce the weight of automobile bodies. A method for commercially manufacturing the steel sheets at low cost is also provided. The goal of the present invention is to achieve a strain-aging hardenability satisfying an BH increment of 80 MPa or more and &Dgr;TS of 40 MPa or more with the aging conditions being a temperature of 170° C. held for 20 minutes after predeformation at a tensile strain of 5%.
DISCLO
Kami Chikara
Tosaka Akio
JFE Steel Corporation
Piper Rudnick LLP
Yee Deborah
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