Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
2000-10-19
2002-06-04
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S651000, C148S653000
Reexamination Certificate
active
06398884
ABSTRACT:
TECHNICAL FIELD
This invention relates to steel sheets, hot-dipped steel sheets and alloyed hot-dipped steel sheets suitable for use in automobile parts and the like as well as a method of producing the same, and particularly is to advantageously improve the hot dipping property and conversion treating property.
BACKGROUND ART
In automobile members, it is recently intended to increase the strength from a viewpoint that a weight of a vehicle body is reduced and a reliability and a safeness are improved. At the same time, the improvement of formability is demanded.
This tendency is also true in hot-dipped steel sheets and alloyed hot-dipped steel sheets such as galvanized steel sheets, alloyed galvanized steel sheets, frequently used as a steel sheet for automobiles and then, many methods haven been proposed for increasing strength of these steels.
For example, JP-A-59-193221 proposes a method for increasing the strength of the steel sheet by adding a relatively large amount of a solid-solution strengthening element such as Si, Mn or the like.
In this method, however, there are caused another problems resulted from the addition of the greater amount of Si or Mn, i.e. degradation of hot-dipping property due to the surface enrichment of Si or Mn (formation of portion not hot-dipped or occurrence of bare spot) and degradation of conversion treating property (no formation of chemical conversion coatings such as zinc phosphate or the like applied onto a cold rolled steel sheet as an undercoating treatment), so that the resulting steel sheets can not be put into practical use.
And also, high-strength cold rolled steel sheets and high-strength galvanized steel sheets having a deep drawability improved by subjecting to &agr;-region lubrication rolling at a hot finish temperature of not lower than 500° C. but not higher than Ar3 transformation point are proposed in JP-A-5-339643.
In this way, the excellent deep drawability is surely obtained, but the degradation of the hot-dipping property is not avoided in the galvanization.
As a countermeasure for solving the above problems, there are proposed a method wherein a steel sheet is forcedly oxidized under a high oxygen partial pressure and subjected to reduction and hot dipping (JP-A-55-122865), a method wherein a pre-plating is carried out before the hot dipping (JP-A-58-104163) and the like. In these methods, however, the control of surface oxide in the heat treatment is not sufficient, so that stable hot-dipping property and conversion treating property are not always obtained in accordance with the chemical composition of steel and the plating conditions, and also an extra process is added to increase the production cost.
Furthermore, JP-A-9-310163 proposes a method wherein a high-temperature coiling is carried out after the hot rolling to form an oxide in a crystal grain boundary or an inside of a crystal grain at a surface layer portion of a matrix in the steel sheet or form an internal oxide layer for improving the aforementioned degradation of the hot-dipping property.
Such a method of forming the internal oxide layer is very useful as a method for preventing the occurrence of bare spot.
In the above method, however, the sufficient internal oxide layer can not be ensured in accordance with the kind of steel or the production history, so that there is remained a problem that excellent hot-dipping property and conversion treating property are not necessarily obtained to a satisfactory level.
Particularly, this tendency is large when recrystallization annealing before the hot dipping is carried out in a radiation type heating system such as a radiant tube or the like.
Moreover, when the heating system is a direct heating system, the internal oxide layer is somewhat strengthened during the annealing, so that the properties are improved as compared with the radiation type heating system, but it is difficult to stably form the desired internal oxide layer.
Lately, hot rolled steel sheets are used instead of the conventional cold rolled steel sheet as a part of the automobile members.
In the hot rolled steel sheet, the recrystallization annealing as in the cold rolled steel sheet is not required, so that it is considered that the surface enrichment of Si or Mn mainly produced in the recrystallization annealing and the occurrence of troubles resulted from such a surface enrichment are less.
However, when the hot-dipping property and conversion treating property are examined with respect to the actual hot rolled steel sheets, the sufficiently satisfactory results are not obtained.
The invention is to advantageously solve the aforementioned problems.
That is, a first object of the invention is to propose steel sheets, hot-dipped steel sheets and alloyed hot-dipped steel sheets capable of stably developing the excellent hot-dipping property and conversion treating property when being used as a hot rolled steel sheet as well as a method of advantageously producing the same.
And also, a second object of the invention is to propose steel sheets, hot-dipped steel sheets and alloyed hot-dipped steel sheets capable of stably developing the excellent hot-dipping property and conversion treating property irrespectively of a chemical steel composition and production history when being used as a cold rolled steel sheet and even when a radiation type heating such as a radiant tube or the like is used in the recrystallization annealing before a hot dipping treatment as well as a method of advantageously producing the same.
Furthermore, a third object of the invention is to propose steel sheets, hot-dipped steel sheets and alloyed hot-dipped steel sheets having the excellent hot-dipping property and conversion treating property and an excellent workability with respect to a cold rolled steel sheet particularly improving the workability among these cold rolled steel sheets as well as a method of advantageously producing the same.
Moreover, the “conversion treating property” used in the invention means an ability forming chemical conversion coatings such as zinc phosphate or the like when the steel sheet is used as an automobile member as it is.
DISCLOSURE OF INVENTION
As mentioned above, the cause degrading the hot-dipping property and conversion treating property when a greater amount of Si or Mn is added is the surface enrichment of Si or Mn in the annealing (Si or Mn is selectively oxidized during the annealing to largely appear on the surface).
And also, it has been elucidated in the hot rolled steel sheets that an essential cause lies in the surviving of oxides of Si, Mn, P and the like on the surface of the hot rolled steel sheet after pickling in addition to the surface enrichment of Si or Mn in the heating before the hot dipping as previously mentioned. This cause is considered due to the fact that the oxides of Si, P and the like and composite oxide thereof with iron are hardly dissolved in the pickling.
As a solution for the above problem, therefore, it is considered that converting an outermost surface layer of iron matrix into an iron layer containing less of a solid solution element such as Si, Mn or the like is effective.
Now, the inventors have made various studies in order to achieve the above object, and found out that it is advantageous that an internal oxide layer is formed in the vicinity of a surface of an iron matrix, namely in a surface layer portion of the iron matrix, to enclose Si, Mn, P or the like on the surface of the iron matrix as an element forming the internal oxide layer in the inside thereof, and that it is very effective to conduct a heat treatment in an atmosphere substantially not causing reduction while being adhered with a black skin scale after the hot rolling for sufficiently and stably forming the above internal oxide layer.
The invention is based on the above knowledge.
That is, the gist and construction of the invention are as follows.
1. A hot rolled steel sheet characterized by subjecting a base steel after a hot rolling to a heat treatment at a temperature range of 650-950° C. in an atmosphere substantially n
Hayashi Hiromasa
Kyono Kazuaki
Nishimura Keiji
Tosaka Akio
Umino Shigeru
Kawasaki Steel Corporation
Oliff & Berridg,e PLC
Yee Deborah
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