Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-05-13
2004-09-28
La Villa, Michael (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S653000, C428S681000, C427S431000, C427S433000, C427S435000, C427S436000, C148S516000, C148S579000, C148S306000, C148S307000
Reexamination Certificate
active
06797410
ABSTRACT:
TECHNICAL FIELD
This invention relates to a high tensile strength hot-dipped steel sheet usable for a vehicle body of an automobile or the like formed by subjecting a surface of a high tensile strength steel sheet to a hot dipping of zinc (including an alloy thereof, the same is applied hereinafter), aluminum, zinc-aluminum alloy, zinc-aluminum-magnesium alloy or the like, and a method of producing the same.
BACKGROUND ART
Recently, the application of high tensile strength hot-dipped steel sheets formed by subjecting a surface of a steel sheet to galvanizing or the like is increasing as a steel sheet for an automobile from viewpoints of safety, weight reduction and low fuel consumption of the automobile and hence global environment protection.
In order to obtain such a high tensile strength hot-dipped steel sheet, it is important to use a steel sheet having an excellent plating property and providing desired strength and workability (press formability or the like) after being passed through a hot dipping bath or further subjected to an alloying treatment as an original sheet.
In general, Si, Mn and so on are added to the steel sheet for increasing the strength of the steel sheet. However, it is known that when the steel sheet added with such elements is subjected to a plating in, for example, a continuous galvanizing line (CGL), the plating property is degraded because oxides of Si, Mn and so on are formed on the surface of the steel sheet at an annealing step before the plating.
This phenomenon is caused due to the fact that when the annealing is carried out in a reducing atmosphere before the plating, since such an atmosphere is reducing for Fe but is oxidative for Si, Mn and the like in steel, Si, Mn and the like are selectively oxidized on the surface of the steel sheet to form oxides.
Since such surface oxides considerably lower a wettability of fused zinc to the steel sheet, the plating property is degraded in the galvanized steel sheet using a high tensile strength steel sheet as an original plating sheet, and particularly, when the contents of Si, Mn and the like are high, there is a problem that the plating is not locally conducted or so-called non-plated portion is formed.
As means for improving the degradation of the plating property in such a high tensile strength steel sheet, for example, JP-A-55-122865 and JP-A-9-13147 propose a method of forcedly oxidizing the steel sheet under a high oxygen partial pressure and then reducing it prior to the heating during the plating. And also, a method of conducting a preliminary plating before the hot dipping is proposed in JP-A-58-104163.
However, the former method has problems that the control of the surface oxide through forcible oxidation is not sufficiently carried out and the stable plating property is not necessarily guaranteed in accordance with components in steel and plating conditions. On the other hand, the latter method has a problem that the production cost rises because an extra process should be added.
Besides, JP-A-6-287684 discloses a high strength steel sheet having an improved plating property by optimizing addition amounts of P, Si and Mn. And also, JP-A-7-70723 and JP-A-8-85858 propose a method wherein a recrystallization annealing is previously carried out before the plating to form a surface oxide and then a galvanizing is carried out after such an oxide is removed by pickling.
By these method could be prevented the occurrence of the non-plated portion in a substantial quantity of high-strength steels.
Even in these methods, however, there is still a problem that the occurrence of the non-plated portion can not be completely prevented as to a type of steel having a higher Si content.
DISCLOSURE OF THE INVENTION
It is an object of the invention to advantageously solve the aforementioned problems and to propose a high tensile strength hot-dipped steel sheet capable of effectively preventing the occurrence of non-plated portions even if a high tensile strength steel sheet having higher contents of Si and Mn is used as an original plating sheet as well as a production method usable therefor.
The inventors have made various studies in order to solve the above problems and obtained a knowledge that
a) as components, Nb and Cu or Ni, Mn are compositively added while regulating Si content to a given range,
b) an internal oxide layer is formed just beneath a surface of a steel sheet through an annealing in a continuous annealing line (CAL)(hereinafter referred to as a recrystallization annealing) and a simultaneously formed surface oxide is removed by pickling after the annealing,
c) in a subsequent heating before a plating in a continuous galvanizing line (CGL)(hereinafter referred to as a heating before plating), the formation of oxides of Si, Mn and the like is considerably decreased on the surface of the steel sheet as the above internal oxide layer acts as a diffusion barrier, and hence a big improvement of the plating property can be attained.
The invention is accomplished based on the above knowledge.
That is, the gist and construction of the invention are as follows.
1. A high tensile strength hot-dipped steel sheet provided on a surface of a steel sheet with a hot-dipped layer, characterized in that the hot-dipped steel sheet is obtained by subjecting a steel sheet of a composition comprising
C: not more than 0.010 mass % or not less than 0.03 mass % but not more than 0.20 mass %,
Nb: not less than 0.005 mass % but not more than 0.2 mass %,
not less than 0.03 mass % but not more than 1.5 mass % in total of one or more selected from Cu: less than 0.5 mass %, Ni: less than 1.0 mass % and Mo: less than 1.0 mass %,
Al: not more than 0.10 mass %,
P: not more than 0.100 mass %,
S: not more than 0.010 mass %,
N: not more than 0.010 mass %,
and further containing, in case of C: not more than 0.010 mass %,
Si: not less than 0.25 mass % but not more than 1.2 mass %,
Mn: not less than 0.50 mass % but not more than 3.0 mass %,
Ti: not more than 0.030 mass %,
B: not more than 0.005 mass %, or in case of C: not less than 0.03 mass % but not more than 0.20 mass %,
Si: not less than 0.5 mass % but not more than 1.5 mass %,
Mn: not less than 1.2 mass % but not more than 3.5 mass % in a range satisfying 1.5×Si(mass %)<Mn(mass %), and the remainder being Fe and inevitable impurities to a recrystallization annealing in a reducing atmosphere having a dew point of not higher than 0° C. but not lower than −45° C. at an annealing temperature of not lower than 750° C., removing oxides formed on a surface of the steel sheet by pickling after the cooling, again heating to a temperature of not lower than 650° C. but not higher than 850° C. in a reducing atmosphere having a dew point of not higher than −20° C., and subjecting to a hot dipping treatment on the way of dropping temperature from the reheating temperature.
2. A high tensile strength hot-dipped steel sheet according to the above item 1, wherein when the C amount is not less than 0.03 mass % but not more than 0.20 mass %, one or two of Ti and V are further included in the steel sheet in a range satisfying one of or two in total of Ti and V: not more than 0.5 mass % and Ti(mass %)<5×C(mass %).
3. A high tensile strength hot-dipped steel sheet according to the above item 1 or 2, wherein when the C amount is not less than 0.03 mass % but not more than 0.20 mass %, Cr is further included in the steel sheet in a range satisfying Cr: not more than 0.25 mass % and Si(mass %)>3×Cr(mass %).
4. A method of producing a high tensile strength hot-dipped steel sheet, characterized in that a steel sheet of a composition comprising
C: not more than 0.010 mass % or not less than 0.03 mass % but not more than 0.20 mass %,
Nb: not less than 0.005 mass % but not more than 0.2 mass %,
not less than 0.03 mass % but not more than 1.5 mass % in total of one or more selected from Cu: less than 0.5 mass %, Ni: less than 1.0 mass % and Mo: less than 1.0 mass %,
Al: not more than 0.10 mass %,
P: not more than 0.100 mass %,
S: not more than 0.010 mass %,
N: no
Ishii Kazuhide
Kato Chiaki
Kyono Kazuaki
Mochizuki Kazuo
JFE Steel Corporation
La Villa Michael
Young & Thompson
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