Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-06-02
2002-04-02
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S330000, C148S333000, C148S334000, C148S335000, C148S336000, C148S602000, C148S654000
Reexamination Certificate
active
06364968
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hot-rolled steel sheet for use as high-strength parts such as bumper parts and impact beams of motor vehicles and, more particularly, to a high-strength hot-rolled steel sheet having excellent stretch flangeability with a tensile strength TS of not less than about 780 MPa. The invention also relates to a method of producing the hot-rolled steel sheet.
2. Description of the Related Art
In a recent trend toward lighter weight vehicle bodies, attention has been focused on application of high-strength steel sheets to a wider range of vehicle parts. In particular, high-strength steel sheets exceeding 1000 MPa have been employed as bumper parts, impact beams, etc. which are used to suppress deformation of cabins or passenger compartments upon collision of vehicles. Those high-strength steel sheets are cold-rolled steel sheets produced through a cold rolling process except for steel plate having thicknesses in excess of 3.2 mm. The main reason is that, in the case of employing cold-rolled steel sheets, disorder in shape of the steel sheet can be relatively easily suppressed by in-furnace rolls during continuous annealing and a good product shape can be obtained.
On the other hand, it has hitherto been difficult to employ hot-rolled steel sheets as thin high-strength steel sheets having thickness of not more than 3.2 mm, especially not more than 3.0 mm. One major reason is that, in a cooling step after hot rolling, effective tensile forces cannot be imparted to the steel sheet and disorder in shape of the steel sheet cannot be suppressed as with cold-rolled steel sheets.
In addition to the above-mentioned disorder in shape of the steel sheet, another reason why hot-rolled steel sheets have not been practically used as thin high-strength steel sheets having thickness not more than the above value is that the hot-rolled steel sheet is disadvantageous in ensuring satisfactory mechanical properties. More specifically, the structure just subjected to hot rolling without undergoing cold rolling and annealing is generally difficult to make uniform and achieve a fine structure comparable to that obtainable in the case of structures undergoing cold rolling and annealing. With the poor structure, it is difficult to obtain superior workability represented by stretch flangeability (bending workability and barring (Hole Expanding) workability).
To improve stretch flangeability of high-tensile hot-rolled steel sheets, several proposals have been made in the past. For example, Japanese Unexamined Patent Publication Nos. 61-19733 and 62-196336 disclose that the bainite phase is superior as a microstructure in consideration of stretch flangeability. In other words, according to those Publications, stretch flangeability is improved when a component system comprising a simple C—Si—Mn system is subjected to accelerated cooling after hot rolling to thereby develop a structure mainly comprising bainite.
The steel sheets produced by the methods disclosed in the above-cited Japanese Unexamined Patent Publication Nos. 61-19733 and 62-196336 have excellent stretch flangeability relative to that of a steel sheet having the ferrite-martensite structure, etc., but the stretch flangeability is not sufficient to reach a level (TS×El≧15500 MPa·% and hole expanding ratio ≧150%) demanded today. Further, the disclosed related art is disadvantageous in that the structure is likely to change with a comparatively high sensitivity depending on variations in the cooling start time after hot rolling and the hot rolling conditions such as the cooling rate and, therefore, the mechanical properties tend to vary to a larger extent. Such a tendency is not compatible with continuous and automatic pressing to be implemented by automobile makers and so on.
Further, Japanese Unexamined Patent Publication No. 5-320773 discloses that the effect of improving the stretch flangeability is improved by specifying the contents of S, N and O which are apt to easily produce inclusions in steel, and by adding Ti, Nb to obtain a finer structure. According to this Publication, the tensile strength of not less than 100 kgf/mm
2
is satisfied by setting the coiling temperature after hot rolling to be not higher than 400° C., and the stretch flangeability is improved by controlling the total content of (S+N+O) to be not more than 100 ppm.
With the producing method disclosed in the above-cited Japanese Unexamined Patent Publication No. 5-320773, however, the coiling temperature of not higher than 400° C. is required to obtain the tensile strength of not less than 100 kgf/mm
2
and, at such a temperature level, the mechanical properties are easily susceptible to significant variations while being in the form of a coil. Although the above-cited Japanese Unexamined Patent Publication No. 5-320773 does not clearly describe the microstructure of a hot-rolled sheet obtained by the disclosed producing method, the microstructure is presumably bainite or martensite. Then, the above disadvantage is attributable to the fact that the tensile strength can be improved, but the microstructure varies significantly and so does the tensile strength correspondingly due to the effect of variations in the steel components, the cooling conditions after hot rolling, and the temperature distribution in a coil obtained after winding the hot-rolled sheet. Such variations in the material characteristic are not compatible with continuous and automatic pressing to be implemented by automobile makers and so on.
In addition, the above-cited Japanese Unexamined Patent Publication No. 5-320773 describes the necessity of controlling the steel components to improve stretch flangeability, but the concrete relationship between the microstructure, crystal grain size, etc. and the stretch flangeability is not disclosed at all. Also, nothing is disclosed with regard to finish rolling start temperature, and coiling temperature after hot rolling is only specified to obtain the required strength.
Meanwhile, as a means for achieving the high tensile strength without performing accelerated cooling after hot rolling, there is a method of adding elements capable of improving quench hardening, such as Cu, Ni, Cr and Mo, which have been conventionally employed in the field of steel plate.
However, the method of adding the quench-hardening improving elements, such as Cu, Ni, Cr and Mo, has the problems that the necessity of using a large amount of expensive alloy elements is disadvantageous from the cost-effective point of view and renders the scrap management complicated from the viewpoint of recycling the used materials.
Further, the above known method requires the alloy elements to be added in such an amount that the added elements become perfectly a martensite single-phase. If the amount of the added alloy elements is insufficient, the resulting structure would be a mixed structure of ferrite and martensite, or a structure partly containing perlite and bainite in small amounts. Therefore, satisfactory stretch flangeability is not easy to attain as intended.
As described above, it has been very difficult to produce a high-strength hot-rolled steel sheet which has the tensile strength of not less than 780 MPa, particularly in the range of 780-1300 MPa, has good stretch flangeability, high uniformity in shape and mechanical properties of the steel sheet, and has quality enough to stand in practical use over a wide range of thickness from thickness not more than 3.0 mm corresponding to a thin steel sheet to a thickness of more than 3.0 mm corresponding to a thick steel sheet that is produced as an ordinary hot-rolled steel sheet. Accordingly, there has been a strong demand for development of the technique for producing a hot-rolled steel sheet, which can succeed in overcoming the problems set forth above. From the viewpoint of reducing the cost of steel sheets, in particular, there has been demanded a technique of producing a hot-rolled steel sheet with a composition of low-alloy system containin
Furukimi Osamu
Tosaka Akio
Uchiyama Takao
Yamada Nobuo
Yasuhara Eiko
Kawasaki Steel Corporation
Schnader Harrison Segal & Lewis LLP
Yee Deborah
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