U.S. patent application number 16/093369 was filed with the patent office on 2019-07-11 for surface treated steel strip and manufacturing method of surface treated steel strip.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Takeshi FUJIWAKI, Yoshihiro KAWANISHI, Norihisa NISHITANI, Kazuhiro SATO, Keisuke SHIMAYA, Hiroki TAGUCHI, Tomoya TESHIGAWARA, Atsushi YASUI.
Application Number | 20190211456 16/093369 |
Document ID | / |
Family ID | 60042513 |
Filed Date | 2019-07-11 |
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United States Patent
Application |
20190211456 |
Kind Code |
A1 |
YASUI; Atsushi ; et
al. |
July 11, 2019 |
SURFACE TREATED STEEL STRIP AND MANUFACTURING METHOD OF SURFACE
TREATED STEEL STRIP
Abstract
A surface treated steel strip according to the present invention
includes: a base steel sheet; a zinc phosphate coating film layer
made of needle zinc phosphate crystals which are formed in an
island shape on a surface of the base steel sheet; and a
lubricating coating film layer covering the surface of the base
steel sheet and a part of the zinc phosphate coating film layer,
and containing at least a lubricating component, in which an area
ratio of the zinc phosphate crystals exposed from a surface of the
lubricating coating film layer is 25% to 90%.
Inventors: |
YASUI; Atsushi; (Tokyo,
JP) ; SATO; Kazuhiro; (Tokyo, JP) ; NISHITANI;
Norihisa; (Tokyo, JP) ; KAWANISHI; Yoshihiro;
(Tokyo, JP) ; SHIMAYA; Keisuke; (Tokyo, JP)
; TESHIGAWARA; Tomoya; (Tokyo, JP) ; FUJIWAKI;
Takeshi; (Tokyo, JP) ; TAGUCHI; Hiroki;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
60042513 |
Appl. No.: |
16/093369 |
Filed: |
April 13, 2017 |
PCT Filed: |
April 13, 2017 |
PCT NO: |
PCT/JP2017/015160 |
371 Date: |
October 12, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C10M 2205/022 20130101;
C10M 2223/04 20130101; C23C 28/00 20130101; C10M 103/06 20130101;
C10M 125/26 20130101; C10N 2020/06 20130101; C10N 2020/04 20130101;
C10N 2040/246 20200501; C23C 28/04 20130101; C23C 22/80 20130101;
C10N 2080/00 20130101; C10M 143/04 20130101; C10M 169/044 20130101;
C10M 143/02 20130101; C10M 2201/102 20130101; C10M 2209/084
20130101; C10N 2050/02 20130101; C10N 2040/24 20130101; C10M
2201/066 20130101; C10M 2207/126 20130101; C10N 2030/06 20130101;
C23C 22/12 20130101; C23C 22/73 20130101; C10M 2205/14 20130101;
C10M 2205/024 20130101; C10M 2201/102 20130101; C10N 2010/02
20130101; C10M 2207/126 20130101; C10N 2010/02 20130101 |
International
Class: |
C23C 22/12 20060101
C23C022/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2016 |
JP |
2016-080549 |
Claims
1-8. (canceled)
9. A surface treated steel strip, comprising: a base steel sheet; a
zinc phosphate coating film layer made of needle zinc phosphate
crystals which are formed in an island shape on a surface of the
base steel sheet; and a lubricating coating film layer covering the
surface of the base steel sheet and a part of the zinc phosphate
coating film layer, and containing at least a lubricating
component, wherein an area ratio of the zinc phosphate crystals
exposed from a surface of the lubricating coating film layer is 25%
to 90%.
10. The surface treated steel strip according to claim 9, wherein
the zinc phosphate crystals have an average grain diameter in a
major axis direction of 25 .mu.m to 70 .mu.m, and have an average
grain diameter in a minor axis direction of 3 .mu.m to 10
.mu.m.
11. The surface treated steel strip according to claim 9, wherein a
coating weight of the zinc phosphate coating film layer is 1.5
g/m.sup.2 to 15.0 g/m.sup.2 per one side.
12. The surface treated steel strip according to claim 10, wherein
a coating weight of the zinc phosphate coating film layer is 1.5
g/m.sup.2 to 15.0 g/m.sup.2 per one side.
13. The surface treated steel strip according to claim 9, wherein a
coating weight of the lubricating coating film layer is 1.0
g/m.sup.2 to 12.0 g/m.sup.2 per one side.
14. The surface treated steel strip according to claim 10, wherein
a coating weight of the lubricating coating film layer is 1.0
g/m.sup.2 to 12.0 g/m.sup.2 per one side.
15. The surface treated steel strip according to claim 11, wherein
a coating weight of the lubricating coating film layer is 1.0
g/m.sup.2 to 12.0 g/m.sup.2 per one side.
16. The surface treated steel strip according to claim 12, wherein
a coating weight of the lubricating coating film layer is 1.0
g/m.sup.2 to 12.0 g/m.sup.2 per one side.
17. The surface treated steel strip according to claim 9, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
18. The surface treated steel strip according to claim 10, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
19. The surface treated steel strip according to claim 11, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
20. The surface treated steel strip according to claim 12, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
21. The surface treated steel strip according to claim 13, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
22. The surface treated steel strip according to claim 14, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
23. The surface treated steel strip according to claim 15, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
24. The surface treated steel strip according to claim 16, wherein
the lubricating coating film layer contains an alkali metal
silicate whose molar ratio represented by SiO.sub.2/M.sub.2O (M is
alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m; and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
25. A manufacturing method of a surface treated steel strip,
comprising a thermal refining step of performing thermal refining
on a surface of a base steel sheet by utilizing a surface
conditioner containing titanium colloid; a zinc phosphate coating
film layer forming step of making needle crystals of zinc phosphate
grow in an island shape on the surface of the base steel sheet
after being subjected to the thermal refining of surface to form a
zinc phosphate coating film layer on the surface of the base steel
sheet; and a lubricating coating film layer forming step of forming
a lubricating coating film layer by coating a lubricating treatment
agent containing at least a lubricating component to the surface of
the base steel sheet and the zinc phosphate coating film layer so
that a coating weight of the lubricating treatment agent becomes
1.0 g/m.sup.2 to 12.0 g/m.sup.2 per one side, and making an area
ratio of the zinc phosphate crystals exposed from a surface of the
lubricating coating film layer to be 25% to 90%.
26. The manufacturing method of the surface treated steel strip
according to claim 25, wherein the base steel sheet is heated in
the zinc phosphate coating film layer forming step.
27. The manufacturing method of the surface treated steel strip
according to claim 26, wherein the base steel sheet is heated by
applying steam to the base steel sheet.
Description
TECHNICAL FIELD
[0001] The present invention relates to a surface treated steel
strip and a manufacturing method of a surface treated steel
strip.
BACKGROUND ART
[0002] Among plastic workings of a steel sheet, in the working of
an automobile transmission component or the like which has to be
subjected to multistage press forming under a high contact
pressure, there has been performed a phosphate soap treatment in
which phosphate crystals mainly containing zinc phosphate are
precipitated on a surface of a steel sheet to form a phosphate
coating film, and then a reactive soap coating film containing
sodium stearate (alkali soap) as a main component covers, as an
upper layer, the phosphate coating film, in order to prevent
seizing or die galling of the steel sheet with respect to a die.
However, the phosphate soap treatment requires a long chemical
reaction time for forming the phosphate coating film and the
reactive soap coating film, so that a production cost is increased.
Further, an unreacted soap component adheres to a die as a press
scum when performing press forming, so that it is important to
frequently clean the die.
[0003] Accordingly, in order to prevent seizing or die galling in a
multistage plastic working such as multistage press forming under a
high contact pressure, without performing the reactive soap
treatment which requires a long time, there has been disclosed a
technique of forming a lubricating coating film containing a
lubricating component as an upper layer of a phosphate coating film
(refer to the following Patent Document 1, for example).
PRIOR ART DOCUMENT
Patent Document
[0004] [Patent Document 1] Japanese Laid-open Patent Publication
No. 2013-104125
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0005] Here, the treatment such as one disclosed in the
aforementioned Patent Document 1 is often carried out in a state of
a steel strip in a practical sense. When an object to be treated is
a steel strip, in a slit process to be performed in a previous
stage of a multistage plastic working, it is necessary to feed a
steel sheet from the steel strip with the use of pinch rolls.
Further, in order to secure die galling resistance when performing
multistage press forming, it becomes important to lower a
coefficient of static friction of the steel sheet fed from the
steel strip, but, an excessive reduction in the coefficient of
static friction increases a probability that the steel sheet slips
at the pinch rolls to cause blanking. As described above, roll
feedability (roll slip resistance) and the die galling resistance
when performing the multistage press forming under the high contact
pressure are properties which are in a mutual trade-off relation.
In order to continuously perform the multistage plastic working
while feeding the steel sheet from the steel strip, it is demanded
to satisfy both of the roll slip resistance and the die galling
resistance as described above.
[0006] Accordingly, the present invention has been made in view of
the above-described problems, and an object of the present
invention is to provide a surface treated steel strip and a
manufacturing method of a surface treated steel strip capable of
realizing both of roll slip resistance and die galling resistance
being properties which are contrary to each other.
Means for Solving the Problems
[0007] As a result of earnest studies for solving the
above-described problems, the present inventor obtained the
following findings.
[0008] (A) Before forming a lubricating coating film layer on a
surface of a base steel sheet being a base material, zinc phosphate
crystals are precipitated in an island shape on a surface of the
base steel sheet. At this time, shapes of the zinc phosphate
crystals are controlled to make a part of the zinc phosphate
crystals to be exposed (protruded) from a surface of the
lubricating coating film layer. Consequently, a surface treated
steel strip in a form in which concave and convex made by the zinc
phosphate crystals are exposed (protruded) from the surface of the
lubricating coating film layer, is finally obtained.
[0009] (B) The zinc phosphate crystals do not have a lubricating
property, so that a coefficient of static friction of the surface
treated steel strip in which the concave and the convex made by the
zinc phosphate crystals are formed on the surface of the
lubricating coating film layer, becomes larger than a coefficient
of static friction of the lubricating coating film layer itself.
The coefficient of static friction of the surface treated steel
strip is correlative to an area ratio of the zinc phosphate
crystals exposed from the surface of the lubricating coating film
layer (sometimes referred to as an exposed area ratio,
hereinafter). Specifically, by controlling the exposed area ratio
of the zinc phosphate crystals, it becomes possible to control the
coefficient of static friction of the surface treated steel
strip.
[0010] (C) When the surface treated steel strip is conveyed in a
state of being sandwiched between a pair of pinch rolls, a pressure
(contact pressure) is applied to the surface treated steel strip by
the pinch rolls, but, the pressure is not high enough to crush the
zinc phosphate crystals exposed from the lubricating coating film
layer. In other words, the concave and the convex at the surface of
the lubricating coating film layer are not flattened by the pinch
rolls. Accordingly, a value of the coefficient of static friction
of the surface treated steel strip under the pinch rolls is
maintained to be a large value. As a result of this, a slip of the
surface treated steel strip under the pinch rolls is suppressed,
which improves the roll slip resistance under the pinch rolls
(under a low contact pressure).
[0011] (D) On the other hand, when multistage press forming is
performed on the surface treated steel strip by a pressing device
disposed on a downstream side of the pinch rolls, a very large
pressure (contact pressure) is applied to the surface treated steel
strip, so that the zinc phosphate crystals exposed from the
lubricating coating film layer are crushed to flatten the concave
and the convex at the surface of the lubricating coating film
layer. Accordingly, the coefficient of static friction of the
surface treated steel strip under the pressing device becomes a
coefficient of static friction which the lubricating coating film
layer originally has. As a result of this, since the original
lubricating property (slidability) of the surface treated steel
strip is exhibited under the pressing device, the die galling
resistance under the pressing device (under a high contact
pressure) improves.
[0012] The present invention has been completed based on the
above-described findings, and the gist thereof is as follows.
[0013] [1]
[0014] A surface treated steel strip includes: [0015] a base steel
sheet; [0016] a zinc phosphate coating film layer made of needle
zinc phosphate crystals which are formed in an island shape on a
surface of the base steel sheet; and [0017] a lubricating coating
film layer covering the surface of the base steel sheet and a part
of the zinc phosphate coating film layer, and containing at least a
lubricating component, in which [0018] an area ratio of the zinc
phosphate crystals exposed from a surface of the lubricating
coating film layer is 25% to 90%.
[0019] [2]
[0020] In the surface treated steel strip described in [1], the
zinc phosphate crystals have an average grain diameter in a major
axis direction of 25 .mu.m to 70 .mu.m, and have an average grain
diameter in a minor axis direction of 3 .mu.m to 10 .mu.m.
[0021] [3]
[0022] In the surface treated steel strip described in [1] or [2],
a coating weight of the zinc phosphate coating film layer is 1.5
g/m.sup.2 to 15.0 g/m.sup.2 per one side.
[0023] [4]
[0024] In the surface treated steel strip described in any one of
claims [1] to [3], a coating weight of the lubricating coating film
layer is 1.0 g/m.sup.2 to 12.0 g/m.sup.2 per one side.
[0025] [5]
[0026] In the surface treated steel strip described in any one of
[1] to [4], the lubricating coating film layer contains an alkali
metal silicate whose molar ratio represented by SiO.sub.2/M.sub.2O
(M is alkali metal) is 2 to 5, and a polymeric wax made of at least
either a polyethylene wax or a polypropylene wax having an average
grain diameter of 0.1 .mu.m to 3.0 .mu.m, and a solid content of
the alkali metal silicate and a solid content of the polymeric wax
with respect to a total solid content mass of the lubricating
coating film layer are 60 mass % to 90 mass % and 5 mass % to 40
mass %, respectively.
[0027] [6]
[0028] A manufacturing method of a surface treated steel strip
includes: [0029] a thermal refining step of performing thermal
refining on a surface of a base steel sheet by utilizing a surface
conditioner containing titanium colloid; [0030] a zinc phosphate
coating film layer forming step of making needle crystals of zinc
phosphate grow in an island shape on the surface of the base steel
sheet after being subjected to the thermal refining of surface to
form a zinc phosphate coating film layer on the surface of the base
steel sheet; and a lubricating coating film layer forming step of
forming a lubricating coating film layer by coating a lubricating
treatment agent containing at least a lubricating component to the
surface of the base steel sheet and the zinc phosphate coating film
layer so that a coating weight of the lubricating treatment agent
becomes 1.0 g/m.sup.2 to 12.0 g/m.sup.2 per one side, and making an
area ratio of the zinc phosphate crystals exposed from a surface of
the lubricating coating film layer to be 25% to 90%.
[0031] [7]
[0032] In the manufacturing method of the surface treated steel
strip described in [6], the base steel sheet is heated in the zinc
phosphate coating film layer forming step.
[0033] [8]
[0034] In the manufacturing method of the surface treated steel
strip described in [7], the base steel sheet is heated by applying
steam to the base steel sheet.
Effect of the Invention
[0035] According to the present invention, it is possible to
realize both of roll slip resistance under a low contact pressure
and die galling resistance under a high contact pressure being
properties which are contrary to each other.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is an explanatory diagram of a surface treated steel
strip according to one embodiment of the present invention.
[0037] FIG. 2(A) is an explanatory diagram schematically
illustrating a configuration of the surface treated steel strip
according to the same embodiment. FIG. 2(B) is an explanatory
diagram schematically illustrating a configuration of the surface
treated steel strip according to the same embodiment.
[0038] FIG. 3(A) is a front view illustrating one example of a zinc
phosphate treatment bath with which a zinc phosphate coating film
layer is formed on a surface of a steel strip. FIG. 3(B) is a plan
view illustrating one example of the zinc phosphate treatment bath
with which the zinc phosphate coating film layer is formed on the
surface of the steel strip.
[0039] FIG. 4 is a flow chart illustrating one example of a flow of
a manufacturing method of the surface treated steel strip according
to the same embodiment.
[0040] FIG. 5 is an explanatory diagram schematically illustrating
a test method regarding die galling resistance.
[0041] FIG. 6 is an explanatory diagram schematically illustrating
a test method regarding roll slip resistance.
[0042] FIG. 7 is an enlarged photograph of a surface treated steel
strip of a comparative example.
[0043] FIG. 8 is an enlarged photograph of a surface treated steel
strip of a comparative example.
[0044] FIG. 9 is an enlarged photograph of a surface treated steel
strip of an example of the present invention.
[0045] FIG. 10 is an enlarged photograph of a surface treated steel
strip of an example of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0046] Hereinafter, an example of embodiments of the present
invention will be described while referring to the attached
drawings. Note that in the present specification and the drawings,
constituent elements having substantially the same functional
configurations are denoted by the same reference numerals, and
overlapped explanation thereof will be omitted.
[0047] (Regarding Surface Treated Steel Strip)
[0048] First, a surface treated steel strip according to one
embodiment of the present invention will be described in detail
while referring to FIG. 1 to FIG. 2B. FIG. 1 is an explanatory
diagram for explaining a surface treated steel strip according to
the present embodiment, and FIG. 2A and FIG. 2B are explanatory
diagrams each schematically illustrating a configuration of the
surface treated steel strip according to the present
embodiment.
[0049] A surface treated steel strip 10 according to the present
embodiment is obtained by performing a high-level lubricating
treatment on a base steel sheet 101, as will be described below in
detail. As schematically illustrated in FIG. 1, in a slit process
to be performed in a previous stage of a multistage plastic
working, such a surface treated steel strip 10 in a state of being
wound in a coil shape is unwound by pinch rolls 1 and continuously
passed in a predetermined sheet passage direction X. The passed
surface treated steel strip 10 is subjected to a multistage
presswork utilizing a die 2 in accordance with an intended product,
to be worked into the intended product.
[0050] As already described above, in order to prevent seizing or
die galling of the passed surface treated steel strip 10 (base
steel sheet 101) with respect to the die 2, it is important to
lower a coefficient of static friction of the surface treated steel
strip 10, but, on the other hand, in order to stably unwind, by
using the pinch rolls 1, the surface treated steel strip 10 in the
state of being wound in the coil shape, it is important that the
surface treated steel strip 10 has a certain level of the
coefficient of static friction. Accordingly, in the surface treated
steel strip 10 according to the present embodiment, a surface
treatment such as one to be described below in detail is performed
on a surface of the base steel sheet 101 to be a base material to
form a surface treated layer made of two layers, as illustrated in
FIG. 2A and FIG. 2B.
[0051] As illustrated in FIG. 2A and FIG. 2B, the surface treated
steel strip 10 according to the present embodiment has the base
steel sheet 101 to be the base material, a zinc phosphate coating
film layer 103 formed on the base steel sheet 101, and a
lubricating coating film layer 105 formed on the zinc phosphate
coating film layer 103. Note that the zinc phosphate coating film
layer 103 and the lubricating coating film layer 105 may be formed
only on one surface of the base steel sheet 101 as illustrated in
FIG. 2A, and they may also be formed on two surfaces, which face
each other, of the base steel sheet 101 as illustrated in FIG.
2B.
[0052] [Regarding Base Steel Sheet 101]
[0053] The base steel sheet 101 is used as a base material of the
surface treated steel strip 10. The base steel sheet 101 according
to the present embodiment is not particularly limited, and it is
possible to use a publicly-known steel sheet capable of realizing
characteristics demanded with respect to an object manufactured
through multistage plastic working processing at a subsequent
stage. A manufacturing method and a material of such a
publicly-known steel sheet are also not particularly limited, and
it is possible to use a steel sheet manufactured by being
appropriately subjected to publicly-known various processes of hot
rolling, pickling, cold rolling, annealing, temper rolling, and so
on after being subjected to a normal cast slab manufacturing
process. Further, it is needless to say that such a base steel
sheet 101 may be not only a carbon steel sheet but also a special
steel sheet such as a stainless steel sheet or a high-alloy steel
sheet.
[0054] As one example of such a base steel sheet 101, there can be
cited a hot-rolled steel sheet containing chemical components as
follows, for example.
[0055] A hot-rolled steel sheet being one example of the base steel
sheet 101 contains, in mass %, C: 0.070% to 0.080%, Si: 0.030% to
0.080%, Mn: 1.15% to 1.30%, P: 0.015% to 0.028%, S: 0.000% to
0.040%, and the balance being composed of Fe and impurities.
[0056] By using such a hot-rolled steel sheet as the base steel
sheet 101, it becomes possible to improve strength of an object to
be manufactured.
[0057] [Regarding Zinc Phosphate Coating Film Layer 103]
[0058] The zinc phosphate coating film layer 103 plays a role for
increasing adhesiveness between the base steel sheet 101 and the
lubricating coating film layer 105. This zinc phosphate coating
film layer 103 is an aggregate of needle zinc phosphate crystals
constituted of needle crystals of zinc phosphate precipitated
through a chemical reaction on a surface of the base steel sheet
101. As schematically illustrated in FIG. 2A and FIG. 2B, the
precipitated needle crystals of zinc phosphate do not completely
cover the surface of the base steel sheet 101, and a part of the
surface of the base steel sheet 101 remains in a state where it is
not covered by the crystals of zinc phosphate. As a result of this,
the needle crystals of zinc phosphate are distributed in an island
shape on the surface of the base steel sheet 101, and the surface
of the base steel sheet 101 where there is no needle crystal of
zinc phosphate, exists as a flat portion. Note that the zinc
phosphate coating film layer 103 appears in a state of being
divided at a plurality of places on the surface of the base steel
sheet 101 in a mutually independent manner. In the present
invention, a state of the zinc phosphate coating film layer 103
which appears in a state of being divided at a plurality of places
on the surface of the base steel sheet 101 in a mutually
independent manner as described above, is referred to as an "island
shape".
[0059] The zinc phosphate coating film layer 103 according to the
present embodiment is formed on the base steel sheet 101 whose
surface is subjected to thermal refining by using a specific
surface conditioner as will be described later, and further, in a
zinc phosphate coating film layer forming step of forming the zinc
phosphate coating film layer 103, the base steel sheet 101 is
heated, so that the zinc phosphate coating film layer 103 is
constituted of needle crystals of zinc phosphate with larger ratio
of grain diameters in a major axis direction and a minor axis
direction. A coating weight of such a zinc phosphate coating film
layer 103 is preferably set to 1.5 g/m.sup.2 to 15.0 g/m.sup.2 per
one side. The coating weight of the zinc phosphate coating film
layer 103 is more preferably 3.0 g/m.sup.2 to 15.0 g/m.sup.2 per
one side. By making the coating weight of the zinc phosphate
coating film layer 103 satisfy the range as described above, it
becomes possible to make the lubricating coating film layer 105 to
be brought into close contact with the base steel sheet 101 more
securely, and even in a forming work under a high contact pressure,
the zinc phosphate coating film layer 103 does not disappear, and
the lubricating coating film layer 105 can be maintained to a final
process of the forming work more securely.
[0060] Note that it is preferable that the needle crystals of zinc
phosphate that constitute the zinc phosphate coating film layer 103
according to the present embodiment have an average grain diameter
in a major axis direction of 25 .mu.m to 70 .mu.m, and have an
average grain diameter in a minor axis direction of 3 .mu.m to 10
.mu.m. It is more preferable that the average grain diameter of the
needle crystals of zinc phosphate is 25 .mu.m to 50 .mu.m in the
major axis direction, and is 3 .mu.m to 5 .mu.m in the minor axis
direction. When the zinc phosphate crystals that constitute the
zinc phosphate coating film layer 103 have such average grain
diameters, it becomes possible to realize the adhesiveness as
described above more securely.
[0061] Further, the needle crystals of zinc phosphate constituting
the zinc phosphate coating film layer 103 according to the present
embodiment are preferably needle crystals having a ratio of average
grain diameters in the major axis direction and the minor axis
direction of 2.5 or more. When the needle crystals of zinc
phosphate having the average grain diameters as above are
precipitated, it becomes possible to realize the adhesiveness
between the base steel sheet 101 and the lubricating coating film
layer 105 more securely.
[0062] This zinc phosphate coating film layer 103 can be formed by
using a publicly-known treatment solution containing zinc phosphate
and capable of precipitating needle crystals of zinc phosphate.
Such a treatment solution is not particularly limited, and there
can be cited, for example, a reactive zinc phosphate treatment
solution for plastic working (more specifically, a reactive zinc
phosphate treatment solution for plastic working in which a
treatment time for complete covering is 20 seconds or more). When
forming the zinc phosphate coating film layer 103, it is possible
to make the zinc phosphate treatment solution as described above to
be brought into contact with the base steel sheet 101 through a
spray method or an immersion method, or it is possible to perform
an electrolysis treatment by utilizing a reaction tank
accommodating the zinc phosphate treatment solution as described
above.
[0063] In order to make the zinc phosphate crystals precipitate in
an island shape on the base steel sheet 101, it is only required to
terminate the zinc phosphate treatment before the zinc phosphate
crystals completely cover the entire surface of the base steel
sheet 101. In order to achieve this, it is only required to
regulate a contact time between the base steel sheet 101 and the
zinc phosphate treatment solution or an electrolysis time to a
short period of time. Specifically, it is only required that a
commercially available zinc phosphate treatment solution for
plastic working is used, and the treatment is terminated at a time
shorter than an indicated treatment time. Regarding concrete
treatment time and electrolysis condition, it is only required to
specify the treatment time (electrolysis condition) capable of
realizing the preferable coating weight as described above by
previously examining a correspondence between the treatment time
(or the electrolysis condition) and the coating weight. Further, it
is also effective to heat the base steel sheet 101 in the zinc
phosphate coating film layer forming step of forming the zinc
phosphate coating film layer 103, in order to make the zinc
phosphate crystals precipitate in an island shape on the base steel
sheet 101.
[0064] It is possible to judge whether or not the zinc phosphate
crystals precipitated through the treatment as described above are
in an island shape, by performing a microscopic observation on the
surface of the base steel sheet 101 after being subjected to the
zinc phosphate treatment. Concretely, the surface of the base steel
sheet 101 after being subjected to the zinc phosphate treatment is
observed by a scanning electron microscope (SEM), and a case where
an area of a flat portion determined through image processing is
30% or more may be judged that the zinc phosphate crystals are
formed in an island shape. Such an observation of the zinc
phosphate crystals may be performed before the formation of the
lubricating coating film layer 105 to be described later, or it may
be performed after the formation. However, when the observation is
performed after the formation of the lubricating coating film layer
105, the zinc phosphate crystals are observed through the
lubricating coating film layer 105, so that the SEM observation at
a high acceleration voltage is performed. Concretely, by setting an
acceleration voltage to 20 kV or more, it is possible to observe
the crystals of zinc phosphate through the lubricating coating film
layer 105, to thereby determine the area ratio of the flat portion.
Note that in the observation before the formation of the
lubricating coating film layer 105, it is possible to observe the
zinc phosphate crystals even at a lower acceleration voltage.
[0065] Here, when forming the zinc phosphate coating film layer 103
on the base steel sheet 101, a thermal refining treatment of the
surface of the base steel sheet 101 is performed by using a surface
conditioner containing titanium colloid, previous to the zinc
phosphate treatment as described above. The zinc phosphate crystals
are precipitated from the surface of the base steel sheet 101 where
a component of the surface conditioner is not adhered, and since
the titanium colloid is a coarse colloidal particle, an area of the
surface of the exposed base steel sheet 101 is suppressed. As a
result of this, by performing the thermal refining treatment of
surface by using the surface conditioner containing the titanium
colloid, it becomes possible to make the needle crystals of zinc
phosphate having the preferable average grain diameter ratio as
described above to be precipitated more securely.
[0066] [Regarding Lubricating Coating Film Layer 105]
[0067] As schematically illustrated in FIG. 2A and FIG. 2B, the
lubricating coating film layer 105 is a layer positioned on the
surface of the base steel sheet 101, covering the surface of the
base steel sheet 101 and at least a part of the zinc phosphate
coating film layer 103, and having a surface from which at least a
part of the zinc phosphate crystals is exposed. This lubricating
coating film layer 105 is a layer containing at least a lubricating
component, and preferably made of a binder component and a
lubricating component.
[0068] In the surface treated steel strip 10 according to the
present embodiment, since the zinc phosphate crystals in an island
shape as described before are formed, the lubricating component and
the binder component contained in the lubricating coating film
layer 105 are held among the zinc phosphate crystals in an island
shape, as schematically illustrated in FIG. 2A and FIG. 2B. If a
high contact pressure is applied to the surface of the surface
treated steel strip 10 at a time of performing the multistage
plastic working (at a time of performing the multistage presswork,
for example), the lubricating component held among the zinc
phosphate crystals in an island shape flows out between the base
steel sheet 101 and a die. As a result of this, a coefficient of
static friction of the lubricating coating film layer 105 is
lowered to exhibit the lubricating property, and the die galling
resistance is realized.
[0069] A coating weight of such a lubricating coating film layer
105 is 1.0 g/m.sup.2 to 12.0 g/m.sup.2 per one side, and an area
ratio (an area ratio per unit area) of the zinc phosphate crystals
exposed from the surface of the lubricating coating film layer 105
is 25% to 90%. By making the coating weight of the lubricating
coating film layer 105 and the area ratio of the zinc phosphate
crystals exposed from the surface of the lubricating coating film
layer 105 fall within such ranges, it becomes possible to realize
both of the die galling resistance and the roll slip resistance as
described above.
[0070] A case where the coating weight of the lubricating coating
film layer 105 becomes less than 1.0 g/m.sup.2 is not preferable
since an amount of the lubricating component held as the
lubricating coating film layer 105 becomes insufficient, and
sufficient die galling resistance cannot be realized. Further, a
case where the coating weight of the lubricating coating film layer
105 exceeds 12.0 g/m.sup.2 is not preferable since an amount of the
lubricating component held as the lubricating coating film layer
105 becomes excessive, and the roll slip resistance cannot be
realized. The coating weight per one side of the lubricating
coating film layer 105 is more preferably 2.0 g/m.sup.2 to 9.0
g/m.sup.2.
[0071] Note that the coating weight of the lubricating coating film
layer 105 is also influenced by the coating weight of the zinc
phosphate coating film layer 103. Specifically, when the coating
weight of the zinc phosphate coating film layer 103 is small, the
amount of the lubricating coating film layer 105 capable of being
held also becomes small and, on the contrary, when the coating
weight of the zinc phosphate coating film layer 103 is large, the
amount of the lubricating coating film layer 105 capable of being
held also becomes large. For example, when the coating weight of
the zinc phosphate coating film layer 103 is 1.5 g/m.sup.2 to 8.0
g/m.sup.2, the coating weight of the lubricating coating film layer
105 is preferably about 1.0 g/m.sup.2 to 6.0 g/m.sup.2, and when
the coating weight of the zinc phosphate coating film layer 103 is
greater than 8.0 g/m.sup.2 to 15.0 g/m.sup.2, the coating weight of
the lubricating coating film layer 105 is preferably about greater
than 6.0 g/m.sup.2 to 12.0 g/m.sup.2.
[0072] When the coating weight of the zinc phosphate coating film
layer 103 is 1.5 g/m.sup.2 to 8.0 g/m.sup.2 per one side, it
becomes possible to make the lubricating coating film layer 105
with the coating weight of about 1.0 g/m.sup.2 to 6.0 g/m.sup.2 to
be favorably brought into close contact with the base steel sheet
101, resulting in that even in the forming work under a high
contact pressure, the zinc phosphate coating film layer 103 does
not disappear, and it becomes possible to maintain the lubricating
coating film layer 105 to a final process of the forming work.
[0073] Meanwhile, among automotive parts such as a direct clutch,
there exist parts each of which is subjected to multistage forming
in a cylindrical shape, and then a periphery thereof is formed in a
tooth shape. These parts are repeatedly subjected to sliding
forming under a higher contact pressure, so that when the coating
weight of the zinc phosphate coating film layer 103 is equal to or
less than 8.0 g/m.sup.2 per one side, there is a possibility that
the zinc phosphate coating film layer 103 disappears due to sliding
in a process in the middle of the forming, which causes reduction
in dimensional accuracy of a finished product and a crack in the
process in the middle of the forming. In such a case, it is
preferable that the coating weight of the zinc phosphate coating
film layer 103 is greater than 8.0 g/m.sup.2 to 15.0 g/m.sup.2, and
the coating weight of the lubricating coating film layer 105 is
about greater than 6.0 g/m.sup.2 to 12.0 g/m.sup.2.
[0074] Here, it is preferable that the lubricating coating film
layer 105 according to the present embodiment contains an alkali
metal silicate whose molar ratio represented by SiO.sub.2/M.sub.2O
(M is alkali metal selected from Li, Na, K, and the like) is 2 to 5
as the aforementioned binder component, and a polymeric wax made of
at least either a polyethylene wax or a polypropylene wax having an
average grain diameter of 0.1 .mu.m to 3.0 .mu.m as the
aforementioned lubricating component.
[0075] By using the alkali metal silicate as described above as the
binder component, it becomes possible to properly hold the
lubricating component in the coating film, and in addition to that,
it becomes possible to form a strong continuous coating film
excellent in heat resistance on the surface of the steel strip. As
a result of this, it becomes possible to exhibit a seizing
resistance function for preventing a direct contact of metal
between the surface treated steel strip according to the present
embodiment and the die, a rust resistance function ascribable to a
barrier property of a dense alkaline coating film, and the like.
Here, a case where the aforementioned molar ratio is less than 2 is
not preferable since a coating film strength cannot be sufficiently
obtained and a plastic working performance deteriorates. Further, a
case where the aforementioned molar ratio exceeds 5 is not
preferable since the plastic working performance deteriorates, and
in addition to that, stability of an alkali metal silicate aqueous
solution used when forming the coating film deteriorates and
practicality becomes poor. The molar ratio represented by
SiO.sub.2/M.sub.2O in the alkali metal silicate is more preferably
3 to 4.
[0076] By using the polymeric wax as described above as the
lubricating component, it is possible to suppress expansion of the
lubricating coating film layer 105, and it is possible to further
improve the plastic working performance of the lubricating coating
film layer 105. Here, a case where the average grain diameter of
the polymeric wax is less than 0.1 .mu.m is not preferable since
oil is significantly dispersed from an interface of the polymeric
wax into the coating film to deteriorate oil resistance of the
coating film, and a case where the average grain diameter of the
polymeric wax is greater than 3.0 .mu.m is not preferable since the
polymeric wax in a chemical solution is not dispersed well, and it
becomes difficult to form a uniform coating film. The average grain
diameter of the polymeric wax is more preferably 0.5 .mu.m to 1.5
.mu.m. By using the polymeric wax having the average grain diameter
as described above as the lubricating component, the lubricating
component is easily filled in a concave portion of convex and
concave of the zinc phosphate crystals, and as a result of this,
the zinc phosphate crystals are easily exposed from the surface of
the lubricating coating film layer 105.
[0077] A solid content of the aforementioned alkali metal silicate
is preferably 60 mass % to 90 mass % with respect to a total solid
content mass of the lubricating coating film layer 105, and a solid
content of the aforementioned polymeric wax is preferably 5 mass %
to 40 mass % with respect to the total solid content mass of the
lubricating coating film layer 105.
[0078] A case where the solid content of the alkali metal silicate
is less than 60 mass % is not preferable since continuity of a
vitreous coating film formed of the alkali metal silicate becomes
poor, and a possibility at which the coating film strength capable
of enduring the plastic working can be obtained is lowered.
Further, a case where the solid content of the alkali metal
silicate exceeds 90 mass % is not preferable since the obtained
coating film strength is saturated, which is disadvantageous in
terms of cost. The solid content of the alkali metal silicate
described above is more preferably 70 mass % to 80 mass % with
respect to the total solid content mass of the lubricating coating
film layer 105.
[0079] A case where the solid content of the polymeric wax is less
than 5 mass % is not preferable since an amount of the lubricating
component held by the lubricating coating film layer 105 becomes
insufficient, which reduces a possibility that a sufficient
lubricating property is exhibited. Further, a case where the solid
content of the polymeric wax is greater than 40 mass % is not
preferable since the amount of the lubricating component held by
the lubricating coating film layer 105 becomes excessive, which
reduces a possibility that sufficient roll slip resistance is
exhibited. The solid content of the polymeric wax described above
is more preferably 3 mass % to 10 mass % with respect to the total
solid content mass of the lubricating coating film layer 105.
[0080] The lubricating coating film layer 105 according to the
present embodiment can be formed by using a lubricating treatment
agent obtained by mixing the lubricating component as described
above in a solution or a dispersion liquid of the binder component
as described above, and coating the lubricating treatment agent.
Here, as a solvent to be used, any of water, an organic solvent,
and a mixture of these may be employed, but, on a working
environment, it is preferable to use a water-based solvent (water,
or a mixed solvent between water and a water-miscible organic
solvent such as alcohol). By adding, to such a solvent, the binder
component (the alkali metal silicate, for example) of 60 mass % to
90 mass % with respect to the total solid content mass of the
lubricating treatment agent and the lubricating component (the
polymeric wax, for example) of 5 mass % to 40 mass % with respect
to the total solid content mass of the lubricating treatment agent,
and properly performing coating and drying, it is possible to form
the lubricating coating film layer 105 having the solid content as
described above.
[0081] Note that, in order to increase the dispersibility of the
lubricating component, a publicly-known surface active agent may be
added to the aforementioned lubricating treatment agent. Further,
in order to modify viscosity of the aforementioned lubricating
treatment agent, a viscosity modifier may be added in a range of
exerting no influence on the coating film strength of the
lubricating coating film layer 105. Such a viscosity modifier can
employ a generally used one, and there can be cited, for example,
an organic polymer thickener such as hydroxyethyl cellulose,
carboxymethyl cellulose, polyacrylic amide, sodium polyacrylate,
polyvinyl pyrrolidone, or polyvinyl alcohol. When such a viscosity
modifier is used, a content thereof is preferably less than 10 mass
% with respect to the total solid content mass of the lubricating
coating film layer 105.
[0082] When coating the aforementioned lubricating treatment agent
onto a base steel sheet 101 and the zinc phosphate coating film
layer 103, it is possible to use a publicly-known method of an
immersion treatment, a shower ringer treatment, a roll coating
treatment, and the like. Further, the coating is only required to
be performed to make the surface of the base steel sheet 101 and
the zinc phosphate coating film layer 103 to be sufficiently
covered by the aforementioned lubricating treatment agent, and a
temperature and a coating time of the lubricating treatment agent
are not particularly limited. Further, a drying temperature of the
lubricating treatment agent is also not particularly limited, and
it may be appropriately set in accordance with the component
contained in the lubricating treatment agent.
[0083] [Regarding Area Ratio (25% to 90%) of Zinc Phosphate
Crystals Exposed from Surface of Lubricating Coating Film Layer
105]
[0084] In the lubricating coating film layer 105 according to the
present embodiment, a part of the zinc phosphate crystals (zinc
phosphate coating film layer 103) is exposed from a part of the
surface of the lubricating coating film layer 105, as illustrated
in FIG. 2A and FIG. 2B, so that the reduction in the coefficient of
static friction of the lubricating coating film layer 105 is
properly suppressed. As a result of this, it becomes possible to
realize even the roll slip resistance when the steel sheet is
unwound from the steel strip.
[0085] A case where the area ratio of the zinc phosphate crystals
exposed from the surface of the lubricating coating film layer 105
is less than 25% is not preferable since the degree of suppression
of the reduction in the coefficient of static friction of the
lubricating coating film layer 105 becomes insufficient, and it is
not possible to realize sufficient roll slip resistance. Further, a
case where the area ratio of the zinc phosphate crystals exposed
from the surface of the lubricating coating film layer 105 is
greater than 90% is not preferable since the degree of suppression
of the reduction in the coefficient of static friction of the
lubricating coating film layer 105 becomes excessive, and it is not
possible to realize sufficient die galling resistance. The area
ratio of the zinc phosphate crystals exposed from the surface of
the lubricating coating film layer 105 is more preferably 30% to
60%.
[0086] When the coating weight of the lubricating coating film
layer 105 and the area ratio of the zinc phosphate crystals exposed
from the surface of the lubricating coating film layer 105 fall
within the ranges as described above, the coefficient of static
friction of the lubricating coating film layer 105 according to the
present embodiment becomes 0.10 to 0.20. The coefficient of static
friction of the lubricating coating film layer 105 is more
preferably 0.12 to 0.15.
[0087] Note that by making the coating weight of the zinc phosphate
coating film layer 103 fall within the preferable range as
described above, it becomes possible to make the area ratio of the
zinc phosphate crystals exposed from the lubricating coating film
layer 105 fall within the aforementioned range more securely.
[0088] Further, for making the area ratio of the zinc phosphate
crystals (zinc phosphate coating film layer 103) exposed from the
surface of the lubricating coating film layer 105 to be 25% to 90%,
it is necessary to make the zinc phosphate coating film layer 103
of 1.5 g/m.sup.2 to 15.0 g/m.sup.2 per one side to be formed in an
island shape, and in order to achieve this, there can be considered
a method such that a temperature in the zinc phosphate treatment is
increased or the treatment time is extended. However, regarding the
temperature of the zinc phosphate treatment bath, it is practically
difficult to increase the treatment temperature to 100.degree. C.
or more since water is used as the solvent, and meanwhile, if the
treatment time is extended, the crystals of zinc phosphate coating
film to be precipitated are densely generated on the surface of the
steel strip, resulting in that the zinc phosphate coating film
layer 103 loses the island shape. Further, it is required to
increase a temperature of the entire treatment bath for increasing
the temperature of the zinc phosphate treatment bath, which causes
a problem in terms of increase in energy cost. Furthermore, the
extension of the treatment time also causes a problem in terms of
reduction in productivity when the zinc phosphate treatment is
performed in the zinc phosphate treatment bath with a certain
length.
[0089] Accordingly, as one example of a method of forming the zinc
phosphate coating film layer 103 of 1.5 g/m.sup.2 to 15.0 g/m.sup.2
per one side in an island shape, there is proposed a method in
which the temperature of the zinc phosphate treatment bath is
partially increased by steam. With the use of such a method, a
coating weight of the zinc phosphate crystals is increased, and in
addition to that, by making the zinc phosphate crystals have more
acicular shapes (sharper shapes), bulkiness of the zinc phosphate
crystals is increased, which makes a lot of heads of the zinc
phosphate crystals protrude from the lubricating coating film
layer. The zinc phosphate crystal itself does not have slidability
under a low contact pressure, so that the coefficient of static
friction becomes large. On the other hand, under a high contact
pressure, the zinc phosphate crystals are crushed to contribute to
the slidability together with the lubricating coating film layer,
so that the formability under the high contact pressure=the die
galling resistance (simulated in L-shape press forming) is
maintained to one equal to a conventional one.
[0090] Here, an example of an apparatus for forming the zinc
phosphate coating film layer 103 of 1.5 g/m.sup.2 to 15.0 g/m.sup.2
per one side in an island shape, is illustrated in FIG. 3A and FIG.
3B. The apparatus illustrated in FIG. 3A and FIG. 3B is configured
such that steam generated by heaters 21 is applied to a surface of
the base steel sheet 101 which is passed in a zinc phosphate
treatment bath 20. In the zinc phosphate treatment bath 20, the
steam generated through heating by the heaters 21 is agitated by
agitators 22 to be applied to the surface of the base steel sheet
101. If partial heating is performed in the zinc phosphate
treatment bath 20 and the steam is applied to the surface of the
base steel sheet 101 to increase the treatment temperature at the
time of forming the zinc phosphate coating film layer 103, it
becomes possible to easily form the zinc phosphate coating film
layer 103 in an island shape at a short treatment time with which
the productivity is not impaired, without creating a state where
the temperature of the entire zinc phosphate coating film treatment
bath 20 is significantly increased. As a steam temperature
increases, the formation of the zinc phosphate coating film layer
103 is accelerated more. For this reason, the steam temperature is
preferably 100.degree. C. or more, and more preferably 120.degree.
C. or more. When the temperature of steam becomes high, an energy
cost increases and an effect of the steam temperature is saturated,
so that the steam temperature is preferably set to 200.degree. C.
or less.
[0091] The reason why the zinc phosphate coating film of 1.5
g/m.sup.2 to 15.0 g/m.sup.2 per one side is formed in an island
shape on the steel strip when the steam is applied to the surface
of the base steel sheet 101 in the zinc phosphate treatment bath 20
as described above, is not clear. However, it is estimated that, on
the surface of the base steel sheet 101, growth of the zinc
phosphate crystals from a part of growing points of the zinc
phosphate crystals is inhibited and the zinc phosphate coating film
is formed in an island shape, and meanwhile, the zinc phosphate
crystals formed in an island shape are heated to 100.degree. C. or
more by the steam to be activated, and are brought into contact
with the zinc phosphate treatment solution in the zinc phosphate
treatment bath 20, which further accelerates the growth of the zinc
phosphate crystals, resulting in that the zinc phosphate coating
film of 1.5 g/m.sup.2 to 15.0 g/m.sup.2 per one side can be formed
in an island shape.
[0092] Note that the apparatus for forming the zinc phosphate
coating film layer 103 of 1.5 g/m.sup.2 to 15.0 g/m.sup.2 per one
side in an island shape, is not particularly limited. An apparatus
of a type in which the steam is directly applied to the base steel
sheet 101 may be employed, or an apparatus of a type in which after
mixture with the zinc phosphate treatment solution, the steam is
directly applied to the base steel sheet 101 may be employed.
Further, it is also possible to employ an apparatus of a type in
which steam generated by the heaters 21 installed on wall surfaces
of the zinc phosphate treatment bath 20 is agitated by the
agitators 22, and the zinc phosphate treatment solution and the
steam are simultaneously applied to the base steel sheet 101.
[0093] Note that by using the apparatus illustrated in FIG. 3A and
FIG. 3B, it becomes possible to form the zinc phosphate coating
film layer 103 in an island shape, and a maximum value of a coating
weight of the zinc phosphate coating film layer 103 obtained by the
blowing of steam is about 15.0 g/m.sup.2. By setting the coating
weight of the zinc phosphate coating film layer 103 to 1.5
g/m.sup.2 to 15.0 g/m.sup.2 per one side, it becomes possible to
make the lubricating coating film layer 105 to be more securely
brought into close contact with the base steel sheet 101, and even
in the forming work under a high contact pressure, the zinc
phosphate coating film layer 103 does not disappear, and it becomes
possible to maintain the lubricating coating film layer 105 to a
final process of the forming work more securely.
[0094] The surface treated steel strip 10 according to the present
embodiment has been described above in detail while referring to
FIG. 1 to FIG. 3.
[0095] <Regarding Measuring Methods of Various Physical Property
Values>
[0096] Subsequently, measuring methods of various physical property
values realized by the surface treated steel strip 10 according to
the present embodiment will be briefly described.
[0097] First, the average grain diameter of the zinc phosphate
crystals which form the zinc phosphate coating film layer 103, and
the average grain diameter of the polymeric wax contained in the
lubricating coating film layer 105 can be measured through a
publicly-known measuring method such that a field emission scanning
electron microscope (FE-SEM) is used to observe the steel sheet
surface at a low acceleration voltage.
[0098] Further, the coating weight per one side of each of the zinc
phosphate coating film layer 103 and the lubricating coating film
layer 105 can be measured through a publicly-known measuring method
such as a gravimetric method.
[0099] Note that the thicknesses of the zinc phosphate coating film
layer 103 and the lubricating coating film layer 105 can be
measured by observing a cross section of the surface treated steel
strip with the use of an electron microscope such as a SEM, but,
the thickness of the lubricating coating film layer 105 can also be
measured through a method to be described below. First, by using a
glow discharge spectroscopy (GDS), emission spectrum intensities of
a component (Si, for example) of the lubricating coating film layer
105 and the zinc phosphate crystal component (Zn, for example) are
measured on a cross section of the surface treated steel strip 10
from a surface along a depth direction. By such measurement, it is
possible to obtain two peaks corresponding to the respective
components in the emission spectrum. Here, a thickness
corresponding to a value between 50% values of respective peak
intensities of the two components (a 50% value on the base steel
sheet side regarding Si, and a 50% value on a surface layer side of
the steel strip regarding Zn), can be set to the thickness of the
lubricating coating film layer 105.
[0100] Further, the area ratio of the zinc phosphate crystals
exposed from the surface of the lubricating coating film layer 105
can be obtained in a manner that the surface of the surface treated
steel strip 10 is observed by an electron microscope such as a SEM,
and an area of the zinc phosphate crystals detected within a field
of view is specified by a publicly-known method. Here, regarding
the specification of the area ratio, it is preferable to observe a
plurality of places on the surface of the surface treated steel
strip 10 and calculate an average of the specified area ratios.
[0101] Further, the coefficient of static friction of the
lubricating coating film layer 105 can be measured through various
test methods such as a round bead drawing test to be described
below in detail.
[0102] The measuring methods of the various physical property
values realized by the surface treated steel strip 10 according to
the present embodiment have been briefly described above. Note that
the above-described measuring methods are only examples, and it is
also possible to specify the respective physical property values by
using the other publicly-known measuring methods.
[0103] (Regarding Manufacturing Method of Surface Treated Steel
Strip)
[0104] Subsequently, a manufacturing method of the surface treated
steel strip 10 according to the present embodiment will be
described while referring to FIG. 4. FIG. 4 is a flow chart
illustrating one example of a flow of the manufacturing method of
the surface treated steel strip 10 according to the present
embodiment.
[0105] In the manufacturing method of the surface treated steel
strip 10 according to the present embodiment, a pretreatment such
as a degreasing treatment or a cleaning treatment is first
performed, according to need, on a steel strip formed by winding a
predetermined base steel sheet 101 (pretreatment step S101).
[0106] After that, a surface of the base steel sheet 101 is
subjected to thermal refining by using a surface conditioner
containing titanium colloid (thermal refining step S103).
Accordingly, the titanium colloid having a coarse grain diameter is
adhered to the surface of the base steel sheet 101.
[0107] Subsequently, by the method as described before, needle
crystals of zinc phosphate are precipitated on the surface of the
base steel sheet 101 after being subjected to the thermal refining
of surface (zinc phosphate coating film layer forming step S105).
Accordingly, the needle crystals of zinc phosphate are precipitated
in an island shape on the surface of the base steel sheet 101,
resulting in that the zinc phosphate coating film layer 103 is
formed. Note that as described above, by performing the
precipitation of zinc phosphate in the apparatus illustrated in
FIG. 3A and FIG. 3B, it becomes possible to favorably form the zinc
phosphate coating film layer 103 in an island shape.
[0108] Next, a lubricating treatment agent is coated onto the base
steel sheet 101 and the zinc phosphate coating film layer 103, and
dried under a proper drying condition, to thereby form the
lubricating coating film layer 105 (lubricating coating film layer
forming step S107). Consequently, the surface treated steel strip
10 as illustrated in FIG. 2A and FIG. 2B is manufactured.
[0109] After that, it is also possible to perform a publicly-known
post-treatment on the manufactured surface treated steel strip 10
according to need (post-treatment step S109).
[0110] The one example of the flow of the manufacturing method of
the surface treated steel strip 10 according to the present
embodiment has been briefly described above while referring to FIG.
4.
Examples
[0111] Hereinafter, the surface treated steel strip and the
manufacturing method of the surface treated steel strip according
to the present invention will be concretely described while showing
Examples and Comparative Examples. Note that the Examples to be
described below are only examples of the surface treated steel
strip and the manufacturing method of the surface treated steel
strip according to the present invention, and the surface treated
steel strip and the manufacturing method of the surface treated
steel strip according to the present invention are not limited to
the examples to be described below.
[0112] Note that in the following Examples, the indication of "%"
means "mass %" unless otherwise specified. Further, in the
following Examples, each of coating weights means a coating weight
per one side.
[0113] (1) Zinc Phosphate Treatment of Steel Sheet
[0114] On both surfaces of a cut sheet (300 mm.times.300 mm) of
SPH590 steel sheet (hot-rolled steel sheet having tensile strength
of 590 MPa or more) with a sheet thickness of 3.2 mm, a zinc
phosphate treatment was performed through an immersion treatment.
At that time, in the Examples, by agitating steam generated by
heaters installed on wall surfaces of an apparatus with agitators,
the steam of 100.degree. C. to 120.degree. C. was applied for 5
seconds to the both surfaces of the steel sheet during the
immersion treatment. An immersion time was changed from 5 seconds
to 120 seconds to adjust a coating weight of zinc phosphate
crystals, to thereby obtain zinc phosphate treated steel sheets
shown in Table 1. The coating weight of the zinc phosphate crystals
was determined from a mass change of the steel sheet before and
after the zinc phosphate treatment.
[0115] (2) Formation of Lubricating Coating Film
[0116] The following binder components, lubricating components, and
extreme-pressure additive were added to pure water (deionized
water), sufficiently agitated to be dispersed, and diluted by pure
water to realize a solid content of 20%, to thereby adjust a
chemical solution used for coating. This chemical solution was
coated to surfaces of the above-described zinc phosphate treated
steel sheet one side by one side with the use of a bar coater,
dried at 60.degree. C. to form a lubricating coating film on each
of the surfaces, to thereby form a lubricated steel sheet. A
coating weight of the lubricating coating film was calculated from
a mass change of the steel sheet before and after the formation of
the lubricating coating film.
[0117] Chemical Solution (2-1)
A) Binder Components
[0118] Sodium metasilicate (molar ratio of SiO.sub.2/Na.sub.2O: 4)
[0119] Acryl-based resin (monomer composition: 8% of acrylic acid,
52% of methacrylic acid, 40% of butyl acrylate)
B) Lubricating Components
[0119] [0120] Organic polymeric compound: polyethylene wax
(molecular weight of 20000, average grain diameter of 3 .mu.m)
[0121] Solid lubricant: molybdenum disulfide (average grain
diameter of 2 .mu.m) [0122] Soap: sodium stearate
C) Extreme-Pressure Additive
[0122] [0123] Phosphorous acid ester
D) Solid Content
[0123] [0124] Solid content of sodium metasilicate with respect to
total solid content of lubricating coating film layer: 88% [0125]
Solid content of polyethylene wax with respect to total solid
content of lubricating coating film layer: 5%
[0126] Chemical Solution (2-2)
A) Binder Components
[0127] Sodium metasilicate (molar ratio of SiO.sub.2/Na.sub.2O: 5)
[0128] Acryl-based resin (monomer composition: 8% of acrylic acid,
52% of methacrylic acid, 40% of butyl acrylate)
B) Lubricating Components
[0128] [0129] Organic polymeric compound: polyethylene wax
(molecular weight of 2000, average grain diameter of 0.5 .mu.m)
[0130] Solid lubricant: molybdenum disulfide (average grain
diameter of 2 .mu.m) [0131] Soap: sodium stearate
C) Extreme-Pressure Additive
[0131] [0132] Phosphorous acid ester
D) Solid Content
[0132] [0133] Solid content of sodium metasilicate with respect to
total solid content of lubricating coating film layer: 80% [0134]
Solid content of polyethylene wax with respect to total solid
content of lubricating coating film layer: 5%
[0135] Chemical Solution (2-3)
A) Binder Components
[0136] Sodium metasilicate (molar ratio of SiO.sub.2/Na.sub.2O: 2)
[0137] Acryl-based resin (monomer composition: 8% of acrylic acid,
52% of methacrylic acid, 40% of butyl acrylate)
B) Lubricating Components
[0137] [0138] Organic polymeric compound: polyethylene wax
(molecular weight of 10000, average grain diameter of 1.0 .mu.m)
[0139] Solid lubricant: molybdenum disulfide (average grain
diameter of 2 .mu.m) [0140] Soap: sodium stearate
C) Extreme-Pressure Additive
[0140] [0141] Phosphorous acid ester
D) Solid Content
[0141] [0142] Solid content of sodium metasilicate with respect to
total solid content of lubricating coating film layer: 60% [0143]
Solid content of polyethylene wax with respect to total solid
content of lubricating coating film layer: 30%
[0144] (3) Size of Crystal
[0145] The steel sheet surface after being subjected to the zinc
phosphate treatment was observed by a SEM of 500 magnifications at
an acceleration voltage of 5 kV. An area ratio of zinc phosphate
crystals exposed from the surface was calculated based on an area
of crystal portions by performing binarization of an image.
Further, four or more crystal exposed from the surface were
selected in an arbitrary manner, lengths of a major axis and a
minor axis were measured to calculate an average crystal size, and
the size was set to a crystal size of zinc phosphate.
[0146] (4) Evaluation Method
a-1) Die Galling Resistance
[0147] In order to simulate a multistage plastic working under a
high contact pressure, an L-shape press forming equipment was used
to perform multistage ironing on a side surface of an L-bent sample
of the lubricated steel sheet through the following procedures, as
in FIG. 5 which illustrates an outline, and a lubricating property
of the steel sheet was evaluated. At this time, if the galling
occurs on the steel sheet surface when the number of times of
forming is two or less, the multistage forming is not realized, so
that a steel sheet on which the galling does not occur when the
number of times of forming is three or more was evaluated as
acceptable. Note that a steel sheet on which the galling occurred
when the number of times of forming was two or less was represented
as x, and a steel sheet on which the galling did not occur when the
number of times of forming was three or more was represented as
a.
[0148] a-2) Procedures
[0149] Procedure (1) A general rust preventive oil is coated to
both surfaces of a sample so that a coating weight becomes 1.5
g/m.sup.2.
[0150] Procedure (2) A clearance between a punch and a die is set
to satisfy a sheet thickness -0.25 (sheet thickness reduction
width=0.15 mm), and first ironing (L-shape forming) is performed on
the sample.
[0151] Procedure (3) On the L-shaped sample after being subjected
to the ironing in the procedure (2), the ironing is repeatedly
performed while further reducing the clearance at a pitch of -0.25
until the die galling occurs on the steel sheet surface, and the
number of times of ironing at which the die galling occurs is
determined.
[0152] b-1) Roll Slip Resistance
[0153] In order to simulate roll feed resistance under a high
contact pressure, front and rear surfaces of a steel sheet were
sandwiched by round beads, and a coefficient of static friction was
calculated from a load at a time of drawing the steel sheet, as in
FIG. 6 which illustrates an outline. It is confirmed that when the
coefficient of static friction is 0.09 or less, the force for
feeding the steel sheet cannot be applied and it is difficult to
pass the sheet in an actual line, and when the coefficient of
static friction is greater than 0.20, the force required for
feeding the steel sheet becomes too large and it is difficult to
pass the sheet in an actual line. Therefore, a steel sheet having
the coefficient of static friction of not less than 0.10 nor more
than 0.20 was set as acceptable. Note that a steel sheet whose
coefficient of static friction is out of the range was represented
as x, and a steel sheet whose coefficient of static friction is
within the range was represented as a.
[0154] b-2) Procedures
[0155] Procedure (1) A general rust preventive oil is coated to
both surfaces of a sample so that a coating weight becomes 1.5
g/m.sup.2.
[0156] Procedure (2) Round beads (R5) are pressed against the both
surfaces of the steel sheet to sandwich the steel sheet at a load
of 1 [kN], the steel sheet is drawn, and a coefficient of static
friction when drawing the steel sheet is calculated.
TABLE-US-00001 TABLE 1 ZINC PHOSPHATE LUBRICATING ZINC COATING FILM
COATING PHOSPHATE LAYER FILM LAYER EXPOSED COATING MAJOR MINOR
COATING AREA WEIGHT AXIS AXIS CHEMICAL WEIGHT RATIO TYPE No.
[g/m.sup.2] [.mu.m] [.mu.m] SOLUTION [g/m.sup.2] [%] COMPARATIVE 1
1 33 2 2-1 3 5 EXAMPLE EXAMPLE 2 1.5 32 4 2-1 1.5 50 EXAMPLE 3 1.5
30 4 2-1 3.7 40 EXAMPLE 4 1.5 29 3 2-1 5.9 25 COMPARATIVE 5 3.3 29
5 2-1 1 95 EXAMPLE EXAMPLE 6 3.3 38 5 2-1 1.5 90 EXAMPLE 7 3.3 31 4
2-1 3.7 60 EXAMPLE 8 3.3 30 4 2-1 4 60 EXAMPLE 9 3.3 30 4 2-1 5 40
EXAMPLE 10 3.3 27 3 2-1 5.9 30 COMPARATIVE 11 3.3 24 3 2-1 8 10
EXAMPLE EXAMPLE 12 5 58 6 2-1 3.7 70 EXAMPLE 13 8 66 7 2-1 1.5 90
EXAMPLE 14 8 64 7 2-1 3.7 80 EXAMPLE 15 8 62 7 2-1 5.9 40
COMPARATIVE 16 8 61 6 2-1 7 23 EXAMPLE EXAMPLE 17 6.5 62 7 2-2 3 70
EXAMPLE 18 5.5 60 6 2-2 5 50 COMPARATIVE 19 0 -- -- 2-2 5 0 EXAMPLE
EXAMPLE 20 2 35 4 2-3 2 44 EXAMPLE 21 12 60 8 2-1 6 70 EXAMPLE 22
12 60 8 2-1 8 60 EXAMPLE 23 12 60 8 2-1 12 45 COMPARATIVE 24 12 50
25 2-1 8 10 EXAMPLE EXAMPLE 25 15 70 10 2-1 2 90 EXAMPLE 26 15 70
10 2-1 6 80 EXAMPLE 27 15 70 10 2-1 8 75 EXAMPLE 28 15 70 10 2-1 12
55 COMPARATIVE 29 15 85 40 2-1 12 20 EXAMPLE COMPARATIVE 30 1.9 100
20 2-1 3.3 10 EXAMPLE COMPARATIVE 31 4.5 200 50 2-1 3.3 24 EXAMPLE
COEFFICIENT OF PROPERTY STATIC DIE PRESENCE/ FRICTION GALLING ROLL
SLIP ABSENCE TYPE No. [--] RESISTANCE RESISTANCE OF STEAM
COMPARATIVE 1 0.07 X X ABSENCE EXAMPLE EXAMPLE 2 0.12 .largecircle.
.largecircle. PRESENCE EXAMPLE 3 0.11 .largecircle. .largecircle.
PRESENCE EXAMPLE 4 0.10 .largecircle. .largecircle. PRESENCE
COMPARATIVE 5 0.14 X .largecircle. ABSENCE EXAMPLE EXAMPLE 6 0.14
.largecircle. .largecircle. PRESENCE EXAMPLE 7 0.12 .largecircle.
.largecircle. PRESENCE EXAMPLE 8 0.12 .largecircle. .largecircle.
PRESENCE EXAMPLE 9 0.11 .largecircle. .largecircle. PRESENCE
EXAMPLE 10 0.11 .largecircle. .largecircle. PRESENCE COMPARATIVE 11
0.09 .largecircle. X ABSENCE EXAMPLE EXAMPLE 12 0.13 .largecircle.
.largecircle. PRESENCE EXAMPLE 13 0.14 .largecircle. .largecircle.
PRESENCE EXAMPLE 14 0.13 .largecircle. .largecircle. PRESENCE
EXAMPLE 15 0.11 .largecircle. .largecircle. PRESENCE COMPARATIVE 16
0.09 .largecircle. X ABSENCE EXAMPLE EXAMPLE 17 0.11 .largecircle.
.largecircle. PRESENCE EXAMPLE 18 0.13 .largecircle. .largecircle.
PRESENCE COMPARATIVE 19 0.06 X X ABSENCE EXAMPLE EXAMPLE 20 0.13
.largecircle. .largecircle. PRESENCE EXAMPLE 21 0.13 .largecircle.
.largecircle. PRESENCE EXAMPLE 22 0.12 .largecircle. .largecircle.
PRESENCE EXAMPLE 23 0.11 .largecircle. .largecircle. PRESENCE
COMPARATIVE 24 0.07 .largecircle. X ABSENCE EXAMPLE EXAMPLE 25 0.20
.largecircle. .largecircle. PRESENCE EXAMPLE 26 0.14 .largecircle.
.largecircle. PRESENCE EXAMPLE 27 0.13 .largecircle. .largecircle.
PRESENCE EXAMPLE 28 0.12 .largecircle. .largecircle. PRESENCE
COMPARATIVE 29 0.08 .largecircle. X ABSENCE EXAMPLE COMPARATIVE 30
0.08 .largecircle. X ABSENCE EXAMPLE COMPARATIVE 31 0.07
.largecircle. X ABSENCE EXAMPLE
[0157] (5) Evaluation Results
[0158] The above Table 1 shows evaluation results when forming
lubricating coating films on various zinc phosphate treated steel
sheets with different coating weights of zinc phosphate
crystals.
[0159] As is apparent from Table 1, in Nos. 2 to 4, 6 to 10, 12 to
15, 17, 18, 20 having preferable zinc phosphate coating film layers
and lubricating coating film layers and satisfying the exposed area
ratios of zinc phosphate crystals, sufficient properties realizing
both of the die galling resistance and the roll slip resistance
were exhibited.
[0160] On the other hand, in Nos. 1, 19 in which the coating weight
of the zinc phosphate coating film layer was 0 or insufficient, the
coefficient of static friction became low, resulting in that the
die galling resistance and the roll slip resistance deteriorated.
In each of Nos. 11, 24, 29, 30, 31 in which the average grain
diameter in the major axis direction or the average grain diameter
in the minor axis direction of the zinc phosphate crystals was out
of the range of the present invention although the coating weight
of the zinc phosphate coating film layer and the lubricating
coating film of the upper layer satisfied the preferable
conditions, the coefficient of static friction became low,
resulting in that the roll slip resistance deteriorated. In No. 5
in which a lot of zinc phosphate crystals were exposed, an amount
of the lubricating coating film itself was small, resulting in that
the die galling resistance deteriorated. In No. 16, the exposed
area ratio of the zinc phosphate crystals was small, and the
coefficient of static friction did not become 0.10 or more.
[0161] FIG. 7 to FIG. 10 illustrate enlarged photographs of surface
treated steel strips of comparative examples and examples of the
present invention. A comparative example (No. 30) illustrated in
FIG. 7 has a coating weight of an island-shaped zinc phosphate
coating film layer of 1.9 g/m.sup.2, a major axis of 100 .mu.m, a
minor axis of 20 .mu.m, a coating weight of the lubricating coating
film layer (chemical solution 2-1) of 3.3 g/m.sup.2, and a zinc
phosphate exposed area ratio of 10%, and a comparative example (No.
31) illustrated in FIG. 8 has a coating weight of an island-shaped
zinc phosphate coating film layer of 4.5 g/m.sup.2, a major axis of
200 .mu.m, a minor axis of 50 .mu.m, a coating weight of the
lubricating coating film layer (chemical solution 2-1) of 3.3
g/m.sup.2, and a zinc phosphate exposed area ratio of 24%. An
example of the present invention (No. 14) illustrated in FIG. 9 has
a coating weight of an island-shaped zinc phosphate coating film
layer of 8.0 g/m.sup.2, a major axis of 64 .mu.m, a minor axis of 7
.mu.m, a coating weight of the lubricating coating film layer
(chemical solution 2-1) of 3.7 g/m.sup.2, and a zinc phosphate
exposed area ratio of 80%, and an example of the present invention
illustrated in FIG. 10 has a coating weight of an island-shaped
zinc phosphate coating film layer of 8.0 g/m.sup.2, a major axis of
31 .mu.m, a minor axis of 4 .mu.m, a coating weight of the
lubricating coating film layer (chemical solution 2-1) of 3.7
g/m.sup.2, and a zinc phosphate exposed area ratio of 60%.
[0162] In the examples of the present invention, the temperature of
the zinc phosphate treatment bath was partially increased by the
steam to increase the coating weight of the zinc phosphate
crystals, and in addition to that, by making the zinc phosphate
crystals have more acicular shapes (sharper shapes), bulkiness of
the zinc phosphate crystals was increased, which made a lot of
heads of the zinc phosphate crystals protrude from the lubricating
coating film layer. The zinc phosphate crystal itself does not have
slidability under a low contact pressure, so that the coefficient
of static friction became large. On the other hand, under a high
contact pressure, the zinc phosphate crystals were crushed to
contribute to the slidability together with the lubricating coating
film layer, so that the formability under the high contact
pressure=the die galling resistance (simulated in L-shape press
forming) was maintained to one equal to a conventional one. As
described above, it became possible to realize both of the roll
slip resistance and the die galling resistance being properties
which are contrary to each other.
[0163] In the foregoing, the preferred embodiments of the present
invention have been described in detail while referring to the
attached drawings, but, the present invention is not limited to
such examples. It is apparent that a person ordinary skilled in the
art to which the present invention pertains is able to devise
various variation or modification examples within the scope of the
technical spirit described in the claims, and it should be
understood that such examples belong to the technical scope of the
present invention as a matter of course.
EXPLANATION OF CODES
[0164] 1 pinch rolls [0165] 2 die [0166] 10 surface treated steel
strip [0167] 20 zinc phosphate treatment bath [0168] 21 heater
[0169] 22 agitator [0170] 101 base steel sheet [0171] 103 zinc
phosphate coating film layer [0172] 105 lubricating coating film
layer
* * * * *