Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-10-11
2001-10-23
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S533000, C427S433000, C427S436000, C428S939000
Reexamination Certificate
active
06306527
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hot-dip galvanized steel sheet (including a hot-dip galvannealed steel sheet) superior in strength-ductility balance, with low strength and high ductility. The present invention relates also to a process for producing said hot-dip galvanized steel sheet.
2. Description of the Related Art
Automotive steel sheets often require both good press-workability and good corrosion resistance. Steel sheets meeting this requirement include hot-dip galvanized steel sheets and hot-dip galvannealed steel sheets. The latter are produced from cold-rolled steel sheets by hot-dip galvanization and ensuing alloying (heating at about 550° C.) to improve adhesion between the zinc plating layer and the base steel sheet. In this specification, the term “hot-dip galvanized steel sheets” covers hot-dip galvannealed steel sheets.
OBJECT AND SUMMARY OF THE INVENTION
Hot-dip galvanized steel sheets designed for high strength are sometimes produced from base steel sheets with a composite structure containing martensite and bainite in addition to ferrite. For example, Japanese Patent Laid-open No. 39770/1983 discloses a steel sheet with a three-phase structure containing ferrite, martensite, and bainite. Japanese Patent Laid-open No. 122821/1980 discloses a hot-dip galvanized steel sheet produced from a steel sheet with a two-phase structure containing ferrite and martensite. These steel sheets with a composite structure have a low yield ratio despite high strength and hence they are superior in shape freezing property.
Unfortunately, the steel sheet with a composite structure gives hot-dip galvanized steel sheets containing a large amount of martensite and bainite, with strength exceeding 500 MPa, mostly exceeding 600 MPa. The steel sheet with such properties poses a problem when formed by a press for mild steel sheets; it needs a special press for high-strength steel sheets.
The present invention was completed to address the problem mentioned above. It is an object of the present invention to provide a hot-dip galvanized steel sheet (including a hot-dip galvannealed steel sheet) superior in ductility, having a good strength-ductility balance (TS*El), with strength lower than 500 MPa, despite its composite structure containing martensite.
The first aspect of the present invention resides in a hot-dip galvanized steel sheet which is produced from a cold-rolled steel sheet, as a base steel sheet, consisting essentially of:
C: 0.010-0.06 wt %,
Si: no more than 0.5 wt %,
Mn: no less than 0.5 wt % and less than 2.0 wt %,
P: no more than 0.20 wt %,
S: no more than 0.01 wt %,
Al: 0.005-0.10 wt %,
N: no more than 0.005 wt %,
Cr: no more than 1.0 wt %,
Mn +1.3Cr: 1.9-2.3 wt %,
Fe: remainder
and having a structure composed of ferrite and a second phase containing martensite, said second phase in the structure accounting for no more than 20% in terms of area and martensite in the second phase accounting for no less than 50% (in terms of area), and which has a zinc-plated layer formed on the surface thereof by hot-dip galvanizing or hot-dip galvannealing.
The cold-rolled steel sheet (as the base steel sheet) should preferably have a structure in which the second phase accounts for no more than 10% (in terms of area) and martensite in the second phase accounts for no less than 90% (in terms of area).
The second aspect of the present invention resides in a process for producing a hot-dip galvanized steel sheet, said process comprising the steps of heating, for recrystallization annealing, a cold-rolled steel sheet having the chemical composition shown in the first aspect to a temperature at which two phases of ferrite and austenite coexists, cooling it from the annealing temperature to the plating temperature at a first cooling rate of 1-10° C./s, performing hot-dip galvanizing, and finally cooling, said steps being accomplished by using a continuous annealing-plate line. This process may be modified such that the first cooling is carried out at a first cooling rate of 1-3° C./s and the second cooling is carried out at a second cooling rate no smaller than 10° C./s. The result of this modification is that the ratio of the second phase in the structure decreases to 10% or less (in terms of area) and the ratio of martensite in the second phase increases to 90% or more (in terms of area). The process may be modified further such that the hot-dip galvanizing step is followed by an alloying step and subsequently cooling is carried out at a second cooling rate greater than 10° C./s.
REFERENCES:
patent: 55-122821 (1980-09-01), None
patent: 58-39770 (1983-03-01), None
patent: 4-26744 (1992-01-01), None
patent: 4-128321 (1992-04-01), None
patent: 4-128320 (1992-04-01), None
patent: 4-173945 (1992-06-01), None
patent: 5-331537 (1993-12-01), None
patent: 8-134591-A (1996-05-01), None
patent: 9-25537 (1997-01-01), None
patent: 9-263883 (1997-10-01), None
Akamizu Hiroshi
Hashimoto Shunichi
Ikeda Shushi
Kashima Takahiro
Makii Koichi
Kabushiki Kaisha Kobe Seiko Sho
Koehler Robert R.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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