Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-05-30
2002-06-25
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S503000, C148S504000, C148S533000, C148S602000, C148S603000, C148S657000, C148S658000, C148S337000, C427S433000, C427S436000, C428S939000
Reexamination Certificate
active
06410163
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a high strength thin steel sheet (substrate for galvanizing) suitable for such uses as an automobile body and a high strength galvannealed steel sheet made from the high strength thin steel sheet, as well as manufacturing methods of the high strength thin steel sheet, the high strength hot-dip galvanized steel sheet and the high strength galvannealed steel sheet.
BACKGROUND ART
From the point of view of achieving a high safety, a smaller weight, a lower fuel/cost ratio, and hence cleaner earth environments, there are increasing applications of high strength steel sheets and high strength hot-dip galvanized steel sheets excellent in corrosion resistance as steel sheets for automobiles.
In order to manufacture high strength hot-dip galvanized steel sheets among others, it is necessary to previously manufacture a material sheet having a good galvanizability, and giving desired strength and workability after passing through a hot-dip galvanizing bath, and after application of a galvannealing treatment.
In order to increase strength of a steel sheet, in general, it is the common practice to add solid solution hardening elements such as P, Mn and Si and precipitation hardening elements such as Ti, Nb and V.
When a steel sheet containing these elements added as described above is treated on a continuous hot-dip galvanizing line (CGL), the steel sheet is subjected to annealing at a temperature of over the Ac1 transformation point, and further, a low cooling rate makes it difficult to obtain a high tensile strength: achievement of a high tensile strength requires addition of alloy elements in large quantities, and this leads to a higher cost.
Addition of alloy elements in large quantities is known to cause serious deterioration of galvanizing property. The quantities of added alloy elements are limited also from the point of view of galvanizability.
Because of the contradictory actions of alloy elements in the substrate steel sheet on strength and galvanizability, it has been very difficult to manufacture a high strength hot-dip galvanized steel sheet excellent in galvanizability on a continuous hot-dip galvanizing line.
In the case of high strength steel sheet, it has further been difficult to manufacture a hot-dip galvanized steel sheet excellent in workability, because of low properties relating to workability such as elongation.
As a high strength steel sheet having a high workability, on the other hand, there has conventionally been proposed a composite (containing residual austenite) mainly comprising martensite with ferrite as the base metal.
This composite structure steel sheet is non-aging at room temperature, has a low yield ratio [:{yield strength (YS)}/{tensile strength (TS)}], and is excellent in workability and hardenability after working.
A known manufacturing method of a composite structure steel sheet is to heat a steel sheet at a temperature within the (&agr;+&ggr;) region, and then rapid cool the steel sheet by water cooling or gas cooling. It is also known that a higher cooling rate leads to the necessity of a smaller number of necessary alloy elements and a smaller amount of addition.
However, when a conventional composite structure steel sheet is subjected to hot-dip galvanizing at a temperature of about 500° C., or further, to a heating-galvannealing treatment, hard martensite, a targeted secondary phase, does not occur, in addition to the primary phase ferrite, but there are generated soft cementite, pearlite and bainite. This results in a decrease in tensile strength and appearance of an upper yield point, leading to an increase in yield point, or further, an yield elongation.
Temper softening tends to be easily caused according as the quantities of added alloy elements become smaller. Large quantities of these alloy elements causes, on the other hand, a decrease in hot-dip galvanizing property.
After all, hard martensite is not generated during the galvanizing step even in the composite structure steel sheet, but soft cementite, pearlite and bainite are produced. It has therefore been difficult to achieve compatibility between workability brought about by the primary phase ferrite and a high strength based on the secondary phase martensite, and a it satisfactory galvanizability in the conventional art.
In a galvanized steel sheet, on the other hand, the galvanized steel sheet is required to be excellent in coating adhesion so as to eliminate the necessity to prevent peeling of the galvanizing layer upon press working and maintain a die.
In order to increase strength of a steel sheet, in general, it is the common practice to add solid solution hardening elements (easily oxidizable elements) such as Mn as described above. These elements however become oxides during reduction-annealing before galvanizing, are concentrated on the steel sheet surface, and reduce wettability by the molten zinc resulting in production of non-galvanized defects on the steel sheet surface in which the galvanizing layer hardly adheres to the steel sheet surface.
The cause is as follows. A recrystallization annealing atmosphere is a reducing atmosphere for Fe, which does not allow production of Fe oxides, but is an oxidizing atmosphere for easily oxidized elements such as Mn. These elements are concentrated on the steel sheet surface, form an oxide film, and thus reduce the contact area between the molten zinc and the steel sheet.
As a manufacturing method of a high strength hot-dip galvanized steel sheet, a method of regulating the cooling rate during annealing upon galvanizing is disclosed in Japanese Unexamined Patent Publication No. 55-50455. The disclosed method contains no description about a method for improving galvanizability. Particularly, when the Mn content in the material steel sheet is over 1%, it is difficult to prevent non-galvanized defects, and the method teaches nothing about a method for improving coating adhesion.
Under the current actual circumstances, therefore, the high strength steel sheet excellent in workability attraction as a high strength material for automobile lacks actual means to be applied as a surface-treated steel sheet excellent also in coating adhesion, though not excellent in workability, in the form of a hot-dip galvanized steel sheet.
Japanese Examined Patent Publication No. 7-9055 discloses a method of applying galvanizing to a steel sheet pickled after annealing as a method for improving the galvannealing rate of a P-added steel. This method has however an object to improve the galvannealing rate, not to prevent non-galvanized defects.
The above-mentioned method teaches nothing about the dew point, the hydrogen concentration and temperature of atmosphere gas upon annealing applied immediately prior to galvanizing. Non-galvanized defects are considered to occur more frequently for certain combinations of the kind of steel and the annealing atmosphere.
Japanese Unexamined Patent Publication No. 7-268584 discloses a method of conducting secondary annealing at a temperature determined in response to the P content in steel. This is however based on a technical idea that the temperature region for preventing brittleness of a steel sheet is dependent upon the P content in steel, not a disclosure of a temperature for improving galvanizability.
The present invention has an object to solve the aforementioned problems involved in the conventional art, and to provide a high strength thin steel sheet serving as a substrate for galvanizing which is excellent in workability and strength even after hot-dip galvanizing or further a galvannealing treatment, and gives an excellent galvanizability as well as an excellent corrosion resistance, a galvannealed steel sheet, made of this high strength thin steel sheet excellent in workability, coating adhesion and corrosion resistance, and manufacturing methods thereof.
More specifically, an object of the present invention is to provide a high strength thin steel sheet excellent in workability which satisfies conditions including a yie
Furukimi Osamu
Kato Chiaki
Osawa Kazunori
Sakata Kei
Shinohara Akio
Kawasaki Steel Corporation
Koehler Robert R.
Young & Thompson
LandOfFree
High strength thin steel sheet, high strength alloyed... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High strength thin steel sheet, high strength alloyed..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High strength thin steel sheet, high strength alloyed... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2936610