Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-12-17
2003-11-11
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C420S121000, C420S123000, C420S104000, C420S105000, C420S106000
Reexamination Certificate
active
06645320
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a steel sheet to be formed for use in manufacturing a structural component in an industrial field of automobile, electric machinery, machine, or the like. More particularly, it relates to a steel sheet which, as characteristics, has excellent ductility, and ensures high strength irrespective of variations in the heat treatment conditions (below, such a characteristic may be referred to as “strength stability after a heat treatment” or “strength stability after quenching”), and is further also excellent in corrosion resistance, plating properties, and spot weldability. It is noted that the steel sheet of the present invention is used in the aforesaid various fields. Below, a description will be proceeded centering on the case where the steel sheet is used as a steel sheet for an automobile as a typical use example.
2. Description of the Related Art
As the characteristic required of a structural component for an automobile obtained by forming a steel sheet, mention may be made of a characteristic that the structural component is deformed to absorb a shock without being completely destructed upon automobile crash from the viewpoint of safety. In order to ensure such a characteristic, an increase in strength has been accomplished by increasing the sheet thickness of a part of the structural component, superimposing a reinforcing member thereon, and the like. Incidentally, in recent years, weight reduction has been pursued from the viewpoint of fuel economy enhancement of an automobile. Accordingly, a more increase in strength of a steel sheet has been pursued so as to ensure the safety even without achieving the reinforcement, and the like. However, since a high strength steel sheet is generally poor in workability, it is also required to simultaneously ensure the workability at the time of forming the structural component. As a means for attaining such an object, JP-A-152541/1999 proposes a high strength steel sheet member of which strength has been partially increased by quenching the required portions after forming a steel sheet having a relatively high ductility. Further, JP-A-144319/2000 discloses a technology in which the strength and the workability are ensured by addition of Mn.
In such a steel product, C, Mn, or the like is added in a relatively large amount for the purpose of increasing the strength after quenching. An increase in amount of C added increases the strength after quenching. However, the weldability, and the like become more likely to be deteriorated inversely with the strength improvement. Therefore, the content of Mn in place of C is increased. However, if the Mn amount is increased, the two phase region temperature of the steel is decreased. Accordingly, a hard phase such as martensite or bainite tends to be formed upon recrystallization annealing after cold rolling. In consequence, the ductility of the material is reduced. For this reason, when the steel sheet is used as a steel sheet for an automobile, or the like, which is subjected to a complex processing, it is important that the Mn content is controlled to ensure more excellent ductility.
Incidentally, a quenching treatment is performed for increasing the strength of the structural component as described above. However, when quenching is accomplished by any of the methods of high frequency induction quenching, press quenching, and the like, the heating temperature or the cooling start temperature tends to vary by about 50° C. Accordingly, the strength after quenching also becomes likely to vary with such a variation in quenching temperature. For this reason, there is a problem that a given high strength cannot be ensured as the structural component.
FIG. 1
is a graph showing the relationship between the quenching temperature and the tensile strength after quenching by Mn concentration. The experiment conditions are as follows. Namely, a high Mn steel containing C: 0.13% and Mn: 1.5%, and a low Mn steel containing C: 0.16% and Mn: 0.38% are respectively hot rolled under the conditions of a finishing delivery temperature (FDT) of 890° C., and a coiling temperature (CT) of 650° C. to a sheet thickness of 2 mm. Then, the respective sheets are cold rolled to a sheet thickness of 1 mm, followed by annealing at 720° C. for 60 seconds. Finally, the respective sheets are skin passed for 1% rolling. Flat sheets each with dimensions of 1.0 mm×30 mm×300 mm are cut from the respective rolled steel sheets. The cut sheets are respectively quenched at an each temperature of 700° C., 800° C., 850° C., 900° C., 950° C., or 1050° C. Then, JIS No. 5 test specimens are collected therefrom. Each specimen is subjected to a tensile test to determine the tensile strength.
As shown in
FIG. 1
, it is possible to suppress the variations in strength after quenching with changes in quenching temperature by raising the quenching temperature, adding a large amount of Mn, or achieving improvement in terms of facilities. However, if the quenching temperature is raised, the plating adhesion of the quenched site in a plated steel sheet is deteriorated, the plated layer disappears, or coatability of the hot rolled steel sheet or the cold rolled steel sheet is deteriorated. As a result, the corrosion resistance is undesirably deteriorated.
FIG. 2
is a graph showing the relationship between the quenching temperature and the iron content of the plating layer.
FIG. 3
is a graph showing the relationship between the iron content of the plating layer and the maximum hole depth in a corrosion resistance test.
FIG. 2
is based on the experiment conditions as follows. Namely, each continuously cast slab is hot rolled to a thickness of 4.0 mm, followed by acid cleaning. Then, the rolled slab is rolled to a thickness of 2.0 mm by cold rolling, and then subjected to a plating treatment (coating weight of plating: 45 g/m
2
per side for both sides) in a hot dip galvanizing line, annealing, and alloying, and quenching is performed in the same manner as with FIG.
1
. Further,
FIG. 3
is based on the following experiment conditions. Namely, by using each of the steel sheets subjected to quenching as described above, a corrosion resistance test is performed under the conditions in accordance with JASO (automotive material corrosion testing method). In the test, by using each test specimen with dimensions of 2.0 mm×70 mm×150 mm, the maximum hole depth has been determined after 170 cycles, wherein one cycle covers 8-hour salt spray (35° C., 5% salt water), 4-hour drying (60° C., relative humidity 30%), and 2-hour wetting (50° C., relative humidity 90%).
FIGS. 2 and 3
indicate as follows. Namely, if the quenching temperature of the plated steel sheet is too high, plating alloying proceeds to excess, so that the Fe content of the plating layer tends to increase. If the Fe content of the plating layer increases in such a manner, rust tends to occur. Accordingly, the maximum hole depth in the corrosion resistance test is increased. In other words, the corrosion resistance is deteriorated.
In such a case where the material is a plated steel sheet the corrosion resistance of the quenched site depends upon the alloying degree due to quenching or the residual degree of the plating layer. If the quenching temperature is raised, the plating alloying proceeds to excess, or the plating layer disappears. As a result, the anti-corrosive effect due to the plating layer is lost.
FIG. 4
is a graph showing the relationship between the quenching temperature of the cold rolled steel sheet and the coating residual rate, and based on the following experiment conditions. Namely, each steel sheet is manufactured under the same conditions as those for
FIGS. 2 and 3
, except that a plating treatment is performed. The coating residual rate is determined by subjecting the quenched steel sheet to a phosphate treatment and electrodeposition coating, and then performing a cross-cut adhesion test.
FIG. 4
indicates as follows. If the quenching temperature of the col
Asai Tatsuya
Soshiroda Tetsuo
(Kobe Steel, Ltd.)
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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