U.S. patent application number 14/888434 was filed with the patent office on 2016-03-10 for surface layer grain refining hot-shearing method and workpiece obtained by surface layer grain refining hot-shearing.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Kaoru KAWASAKI, Takashi MATSUNO, Yoshihito SEKITO, Atsushi SETO, Tamaki SUZUKI.
Application Number | 20160067760 14/888434 |
Document ID | / |
Family ID | 51867354 |
Filed Date | 2016-03-10 |
United States Patent
Application |
20160067760 |
Kind Code |
A1 |
MATSUNO; Takashi ; et
al. |
March 10, 2016 |
SURFACE LAYER GRAIN REFINING HOT-SHEARING METHOD AND WORKPIECE
OBTAINED BY SURFACE LAYER GRAIN REFINING HOT-SHEARING
Abstract
Provided is a surface layer grain refining hot-shearing method
including: heating and keeping a steel sheet in a temperature range
of from Ac3 to 1400.degree. C. to austenitize the steel sheet;
subsequently shearing the steel sheet in a state in which the steel
sheet is placed on a die; and quenching by rapidly cooling the
sheared steel sheet, wherein a start temperature of the shearing is
set to be a temperature (.degree. C.) obtained by adding a
temperature of from 30.degree. C. to 140.degree. C. to a previously
measured Ar3 of the steel sheet.
Inventors: |
MATSUNO; Takashi; (Tokyo,
JP) ; SEKITO; Yoshihito; (Tokyo, JP) ; SUZUKI;
Tamaki; (Tokyo, JP) ; KAWASAKI; Kaoru; (Tokyo,
JP) ; SETO; Atsushi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
51867354 |
Appl. No.: |
14/888434 |
Filed: |
May 9, 2013 |
PCT Filed: |
May 9, 2013 |
PCT NO: |
PCT/JP2014/062534 |
371 Date: |
October 31, 2015 |
Current U.S.
Class: |
148/500 ;
148/320; 148/661 |
Current CPC
Class: |
B21D 37/16 20130101;
C22C 38/00 20130101; C21D 9/46 20130101; C21D 9/00 20130101; B21D
28/00 20130101; C22C 38/18 20130101; C21D 1/18 20130101; C21D 1/673
20130101; C21D 2211/008 20130101; B21D 28/24 20130101; C21D
2211/005 20130101; C21D 2221/10 20130101 |
International
Class: |
B21D 37/16 20060101
B21D037/16; B21D 28/00 20060101 B21D028/00; C22C 38/00 20060101
C22C038/00; C21D 1/18 20060101 C21D001/18; C21D 9/46 20060101
C21D009/46 |
Foreign Application Data
Date |
Code |
Application Number |
May 9, 2013 |
JP |
2013-099243 |
Claims
1. A surface layer grain refining hot-shearing method comprising:
heating and keeping a steel sheet having a carbon content of 0.15%
or more by mass in a temperature range of from Ac3 to 1400.degree.
C. to austenitize the steel sheet; subsequently shearing the steel
sheet in a state in which the steel sheet is placed on a die; and
quenching by rapidly cooling the sheared steel sheet, wherein a
start temperature of the shearing is set to be a temperature
(.degree. C.) obtained by adding a temperature of from 30.degree.
C. to 140.degree. C. to a previously measured Ar3 of the steel
sheet.
2. A surface layer grain refining hot-shearing method comprising:
heating and keeping a steel sheet having a carbon content of 0.15%
or more by mass in a temperature range of from Ac3 to 1400.degree.
C. to austenitize the steel sheet; subsequently shearing the steel
sheet in a state in which the steel sheet is placed on a die; and
quenching by rapidly cooling the sheared steel sheet, wherein a
start temperature of the shearing is set to be a temperature
(.degree. C.) obtained by adding a value, which is calculated by
multiplying an amount of equivalent plastic strain of a surface
layer in a sheared portion by a coefficient from 40 to 60, to a
previously measured Ar3 of the steel sheet.
3. The surface layer grain refining hot-shearing method according
to claim 2, wherein the amount of equivalent plastic strain of the
surface layer in the sheared portion is calculated as an average
value of an amount of equivalent plastic strain of a region in a
range of from 5% to 20% of a thickness of the steel sheet from a
shear plane of the sheared portion to an inside of the steel sheet
in a normal direction of the shear plane and in a range of from 20%
to 50% of the thickness of the steel sheet in a thickness direction
of the steel sheet from a bottom on a burr side of the sheared
portion.
4. The surface layer grain refining hot-shearing method according
to claim 2, wherein the amount of equivalent plastic strain of the
surface layer in the sheared portion is calculated by a numerical
simulation that is performed based on a stress-strain diagram at a
steel sheet temperature of from 500.degree. C. to 800.degree.
C.
5. The surface layer grain refilling hot-shearing method according
to claim 2, wherein the amount of equivalent plastic strain of the
surface layer in the sheared portion is calculated based on a Mises
yield function represented by the following Formula (1) d _ P = 2 3
( d xx 2 + d yy 2 + d zz 2 + 2 d xy 2 + 2 d yz 2 + 2 d zx 2 ) . ( 1
) ##EQU00003##
6. The surface layer grain refining hot-shearing method according
to claim 1, wherein the shearing of the steel sheet starts within
three seconds after the steel sheet comes in contact with the
die.
7. The surface layer grain refining hot-shearing method according
to claim 1, wherein the rapid cooling is performed when the steel
sheet comes in contact with the die.
8. The surface layer grain refining hot-shearing method according
to claim 1, wherein the rapid cooling is performed when water
jetting from a puncture formed in a contacting portion of the steel
sheet with the die passes through a groove provided in the
contacting portion of the steel sheet.
9. The surface layer grain refining hot-shearing method according
to claim 1, wherein press forming not accompanying fracture of the
steel sheet is performed between the heating and the shearing of
the steel sheet.
10. A workpiece obtained by surface layer grain refining
hot-shearing, comprising: a steel sheet having a carbon content of
0.15% or more by mass, a surface layer of a sheared portion of a
steel sheet having a carbon content of 0.15% or more by mass
including a ferrite phase and a remainder, the surface layer being
defined as a range up to 100 .mu.m inside of the steel sheet in a
normal direction of a shear plane from a fracture plane of the
sheared portion, wherein the remainder includes at least one phase
of a bainite phase, a martensite phase, or a residual austenite
phase which has a crystal grain diameter of 3 .mu.m or less, and
includes cementite and inevitably generated inclusions, wherein the
ferrite phase has an average grain size of 3 .mu.m or less, wherein
the surface layer contains 5% or more grains by number having an
aspect ratio of 3 or more, and wherein a region out of the range of
100 .mu.m includes: martensite and inevitably generated inclusions;
or bainite, martensite, and inevitably generated inclusions.
11. The workpiece obtained by surface layer grain refining
hot-shearing according to claim 10, wherein, in the surface layer,
the cementite has a number density of 0.8 pieces/.mu.m.sup.3 or
less and the cementite has a maximum length of 3 .mu.m or less.
12. The workpiece obtained by surface layer grain refining
hot-shearing according to claim 10, wherein a total area ratio of
the bainite phase, the martensite phase, and the residual austenite
phase, which are measured by an electron-beam backscattering
diffraction (EBSD) method, is from 10% to 50% in the surface
layer.
13. A workpiece obtained by surface layer grain refining
hot-shearing, the workpiece produced by: heating and keeping a
steel sheet having a carbon content of 0.15% or more by mass in a
temperature range of from Ac3 to 1400.degree. C. to austenitize the
steel sheet; subsequently shearing the steel sheet in a state in
which the steel sheet is placed on a die; and quenching by rapidly
cooling the sheared steel sheet, wherein a start temperature of the
shearing is set to be a temperature (.degree. C.) obtained by
adding a temperature of from 30.degree. C. to 140.degree. C. to a
previously measured Ar3 of the steel sheet.
14. A workpiece obtained by surface layer grain refining
hot-shearing, the workpiece produced by heating and keeping a steel
sheet having a carbon content of 0.15% or more by mass in a
temperature range of from Ac3 to 1400.degree. C. to austenitize the
steel sheet; subsequently shearing the steel sheet in a state in
which the steel sheet is placed on a die; and quenching by rapidly
cooling the sheared steel sheet, and a start temperature of the
shearing is set to be a temperature (.degree. C.) obtained by
adding a value, which is calculated by multiplying an amount of
equivalent plastic strain of a surface layer in a sheared portion
by a coefficient from 40 to 60, to a previously measured Ar3 of the
steel sheet.
15. The surface layer grain refining hot-shearing method according
to claim 2, wherein the shearing of the steel sheet starts within
three seconds after the steel sheet comes in contact with the
die.
16. The surface layer grain refining hot-shearing method according
to claim 2, wherein the rapid cooling is performed when the steel
sheet comes in contact with the die.
17. The surface layer grain refining hot-shearing method according
to claim 2, wherein the rapid cooling is performed when water
jetting from a puncture formed in a contacting portion of the steel
sheet with the die passes through a groove provided in the
contacting portion of the steel sheet.
18. The surface layer grain refining hot-shearing method according
to claim 2, wherein press forming not accompanying fracture of the
steel sheet is performed between the heating and the shearing of
the steel sheet.
Description
TECHNICAL FIELD
[0001] The present invention relates to a surface layer grain
refining hot-shearing method of a steel sheet, which has a carbon
content of 0.15% or more by mass and is used in automobiles, ships,
bridges, construction equipment, various plants, or the like, and a
workpiece obtained by surface layer grain refining
hot-shearing.
BACKGROUND ART
[0002] From the past, a metal material (steel sheet) to be used in
automobiles, ships, bridges, construction equipment, various
plants, or the like has been often subjected to shearing by a punch
and a die. Recently, from the viewpoint of safety and weight
lightening, various members become high strengthening, and as
disclosed in "Press Technology", Vol. 46, No. 7, p. 36-41
(hereinafter, referred to as "Non-Patent Literature 1"), a
quenching press is performed in which press forming and heat
treatment are almost simultaneously performed to form a
high-strength member.
[0003] A general cold-pressed workpiece is subjected to shearing
such as punching and trimming after being subjected to press
forming. However, when the quenching-pressed workpiece is subjected
to shearing after being subjected to forming, a service life of a
shearing tool becomes significantly shorter due to high hardness of
the member. In addition, there is a concern that delayed fracture
occurs due to residual stress in a sheared portion. Thus, the
quenching-pressed workpiece is often subjected to laser cutting
rather than the shearing.
[0004] However, since the laser cutting requires costs, for
example, the following methods have been proposed so far: a method
of performing a heat treatment after shearing (for example, see
Japanese Patent Application Laid-Open (JP-A) No. 2009-197253
(hereinafter, referred to as "Patent Literature 1")); methods of
reducing residual stress in a sheared portion by simultaneously
performing shearing and hot pressing before quenching (for example,
see JP-A No. 2005-138111 (hereinafter, referred to as "Patent
Literature 2"), JP-A No. 2006-104526 (hereinafter, referred to as
"Patent Literature 3"), and JP-A No. 2006-83419 (hereinafter,
referred to as "Patent Literature 4")); a method of reducing
quenching hardness by gradually lowering a cooling rate of a
sheared portion (for example, see JP-A No. 2003-328031
(hereinafter, referred to as "Patent Literature 5")); a method of
working to soften only a shearing scheduled portion by performing
local electric-heating (for example, see "CIRP Annals-Manufacturing
Technology" 57 (2008), p. 321-324 (hereinafter, referred to as
"Non-Patent Literature 2")); and a shearing-related technology for
controlling structures in a surface layer of a shear plane in a
high-strength steel sheet to improve delayed fracture resistance
(see JP-A No. 2012-237041 (hereinafter, referred to as "Patent
Literature 6")).
SUMMARY OF INVENTION
Technical Problem
[0005] There are several problems in the methods disclosed in
Patent Literatures 1 to 6 and the method disclosed in Non-Patent
Literature 2. According to the method disclosed in Patent
Literature 1, since the method can be used for only a specific
material and is used to perform shearing on a quenched material,
the problem such as deterioration in service life of the tool is
not solved.
[0006] According to the methods disclosed in Patent Literatures 2
to 4, the residual stress in the sheared portion caused by
deformation resistance of the steel sheet can be reduced, but it is
not possible to reduce thermal stress caused by seizure of the tool
and non-uniformity of a contact with a die during quenching and to
reduce residual stress caused by transformation of the steel sheet.
Therefore, when ductility of the hot-sheared portion is low, the
problem such as occurrence of the delayed fracture is not solved. A
method of improving the ductility of the hot-sheared portion is not
disclosed in Patent Literatures 2 to 4.
[0007] According to the method disclosed in Patent Literature 5, it
is considered that ductility can be improved because the sheared
portion of the steel sheet is not hardened, but a shearing time
becomes longer and thus costs increase as the cooling rate becomes
slower. According to the method disclosed in Non-Patent Literature
2, it is necessary to prepare a new die formed with an electric
heating apparatus for shearing and thus costs increase.
[0008] According to the method disclosed in Patent Literature 6, it
has an excellent effect of improving the delayed fracture
resistance, but a shearing start temperature of from 400.degree. C.
to 900.degree. C. is defined regardless of a material of a member
to be sheared or a cooling rate. For this reason, the shearing may
occur at a temperature range (low-temperature side), at which the
delayed fracture occurs, depending on the materials of the member
to be sheared or shearing conditions. Conversely, when the shearing
is performed at a high temperature more than necessary such that
the delayed fracture does not occur, the amount of thermal
expansion becomes larger and a dimensional change becomes larger at
the time of returning to an ambient temperature. As a result, the
dimensional error of the workpiece becomes greater. Therefore, in a
case in which the shearing temperature is precisely controlled at
the lower temperature according to actual shearing conditions,
there still remains a possibility of suppressing the delayed
fracture while further improving shearing accuracy of the
workpiece.
[0009] Patent Literature 6 discloses that the delayed fracture does
not occur when fine ferrite is present in the surface of a shearing
portion. However, for example, in experimental numbers 36 to 40 in
which a steel sheet A8 indicated in Table 5 obtained by steel sheet
component A8 or A9 indicated in Table 1 of Example is used, even
when the shearing is performed at the same shearing temperature and
cooling rate under the same heating conditions and keeping
conditions, structures vary and thus the delayed fracture may occur
in some cases. Even when a steel sheet A9 indicated in Table 5 is
used, the same results were obtained.
[0010] In order to solve the above problems, the invention has
tasks to prevent delayed fracture occurring in a hot-sheared
portion and to improve shearing accuracy of a workpiece without
increasing the shearing time and new steps, and an object thereof
is to provide a surface layer grain refining hot-shearing method
and a workpiece obtained by surface layer grain refining
hot-shearing, which meets these requirements, for the purpose of
achieving of these tasks.
Solution to Problem
[0011] The present inventors have intensively studied on a
technique for solving the above problems. As a result, the
inventors found that in a case in which a temperature for staring
shearing (shearing start temperature) is set to an appropriate
range based on the amount of equivalent plastic strain of a surface
layer of a sheared portion, delayed fracture does not occur even
when high residual stress remains in the sheared portion.
[0012] That is, the amount of equivalent plastic strain of the
sheared portion is affected by a temperature during the shearing
and a structure before the shearing (ferrite or austenite), but a
structure after the shearing is differently changed depending on
the amount of equivalent plastic strain of the sheared portion and
the shearing temperature. As to how the structure differs,
compositions of the steel sheet, pressing conditions and
temperature histories associated with these pressing conditions
when pressing is performed before the shearing contribute thereto.
The inventors found conditions in which even when high residual
stress remains in the sheared portion, the dimension accuracy is
improved without an occurrence of the delayed fracture by
optimizing the shearing temperature in view of all these
factors.
[0013] In particular, the inventors confirmed, in a carbon steel
for machine structural use defined in JIS G 4051 having a carbon
content of 0.15% or more by mass or having preferably a carbon
content of 0.48% or less by mass in view of cold workability after
shear cooling, that the invention was applicable to cold-rolled
steel sheets of S17C, S25C, S35C, and S45C defined in JIS G 4051
when an actually measured Ar3 point is approximately 500.degree. C.
or lower at the time of cooling by leaving.
[0014] The invention has been made based on the above findings and
the gist thereof is as follows.
[0015] A first aspect of the invention is to provide a surface
layer grain refining hot-shearing method including: heating and
keeping a steel sheet having a carbon content of 0.15% or more by
mass in a temperature range of from Ac3 to 1400.degree. C. to
austenitize the steel sheet; subsequently shearing the steel sheet
in a state in which the steel sheet is placed on a die; and
quenching by rapidly cooling the sheared steel sheet, wherein a
start temperature of the shearing is set to be a temperature
(.degree. C.) obtained by adding a temperature of from 30.degree.
C. to 140.degree. C. to a previously measured Ar3 of the steel
sheet.
[0016] A second aspect of the invention is to provide a surface
layer grain refining hot-shearing method including: heating and
keeping a steel sheet having a carbon content of 0.15% or more by
mass in a temperature range of from Ac3 to 1400.degree. C. to
austenitize the steel sheet; subsequently shearing the steel sheet
in a state in which the steel sheet is placed on a die; and
quenching by rapidly cooling the sheared steel sheet, wherein a
start temperature of the shearing is set to be a temperature
(.degree. C.) obtained by adding a value, which is calculated by
multiplying an amount of equivalent plastic strain of a surface
layer in a sheared portion by a coefficient from 40 to 60, to a
previously measured Ar3 of the steel sheet.
[0017] A third aspect of the invention is to provide the surface
layer grain refining hot-shearing method according to second aspect
of the invention, wherein the amount of equivalent plastic strain
of the surface layer in the sheared portion is calculated as an
average value of an amount of equivalent plastic strain of a region
in a range of from 5% to 20% of a thickness of the steel sheet from
a shear plane of the sheared portion to an inside of the steel
sheet in a normal direction of the shear plane and in a range of
from 20% to 50% of the thickness of the steel sheet in a thickness
direction of the steel sheet from a bottom on a burr side of the
sheared portion.
[0018] A fourth aspect of the invention is to provide the surface
layer grain refining hot-shearing method according to the second or
third aspect of the invention, wherein the amount of equivalent
plastic strain of the surface layer in the sheared portion is
calculated by a numerical simulation that is performed based on a
stress-strain diagram at a steel sheet temperature of from
500.degree. C. to 800.degree. C.
[0019] A fifth aspect of the invention is to provide the surface
layer grain refining hot-shearing method according to any one of
the second aspect to the fourth aspect of the invention, wherein
the amount of equivalent plastic strain of the surface layer in the
sheared portion is calculated based on a Mises yield function
represented by the following Formula (1).
d _ P = 2 3 ( d xx 2 + d yy 2 + d zz 2 + 2 d xy 2 + 2 d yz 2 + 2 d
zx 2 ) ( 1 ) ##EQU00001##
[0020] A sixth aspect of the invention is to provide the surface
layer grain refining hot-shearing method according to the first or
second aspect of the invention, wherein the shearing of the steel
sheet starts within three seconds after the steel sheet comes in
contact with the die.
[0021] A seventh aspect of the invention is to provide the surface
layer grain refining hot-shearing method according to the first or
second aspect of the invention, wherein the rapid cooling is
performed when the steel sheet comes in contact with the die.
[0022] An eighth aspect of the invention is to provide the surface
layer grain refining hot-shearing method according to the first or
second aspect of the invention, wherein the rapid cooling is
performed when water jetting from a puncture formed in a contacting
portion of the steel sheet with the die passes through a groove
provided in the contacting portion of the steel sheet.
[0023] A ninth aspect of the invention is to provide the surface
layer grain refining hot-shearing method according to the first or
second aspect of the invention, wherein press forming not
accompanying fracture of the steel sheet is performed between the
heating and the shearing of the steel sheet.
[0024] A tenth aspect of the invention is to provide a workpiece
obtained by surface layer grain refining hot-shearing, including: a
steel sheet having a carbon content of 0.15% or more by mass, a
surface layer of a sheared portion of the steel sheet having a
carbon content of 0.15% or more by mass including a ferrite phase
and a remainder, the surface layer being defined as a range up to
100 .mu.m inside of the steel sheet in a normal direction of a
shear plane from a fracture plane of the sheared portion; wherein
the remainder includes at least one phase of a bainite phase, a
martensite phase, or a residual austenite phase which have a
crystal grain diameter of 3 .mu.m or less, and includes cementite
and inevitably generated inclusions; wherein the ferrite phase has
an average grain size of 3 .mu.m or less; wherein the surface layer
contains 5% or more grains by number having an aspect ratio of 3 or
more; and wherein a region out of the range of 100 .mu.m includes:
martensite and inevitably generated inclusions; or bainite,
martensite, and inevitably generated inclusions.
[0025] An eleventh aspect of the invention is to provide the
workpiece obtained by surface layer grain refining hot-shearing
according to the tenth aspect of the invention, wherein, in the
surface layer, the cementite has a number density of 0.8
pieces/.mu.m.sup.3 or less and the cementite has a maximum length
of 3 .mu.m or less.
[0026] A twelfth aspect of the invention is to provide the
workpiece obtained by surface layer grain refining hot-shearing
according to the tenth or eleventh aspect of the invention, wherein
a total area ratio of the bainite phase, the martensite phase, and
the residual austenite phase, which are measured by an
electron-beam backscattering diffraction (EBSD) method, is from 10%
to 50% in the surface layer.
[0027] A thirteenth aspect of the invention is to provide a
workpiece obtained by surface layer grain refining hot-shearing,
the workpiece produced by: heating and keeping a steel sheet having
a carbon content of 0.15% or more by mass in a temperature range of
from Ac3 to 1400.degree. C. to austenitize the steel sheet;
subsequently shearing the steel sheet in a state in which the steel
sheet is placed on a die; and quenching by rapidly cooling the
sheared steel sheet, wherein a start temperature of the shearing is
set to be a temperature (.degree. C.) obtained by adding a
temperature of from 30.degree. C. to 140.degree. C. to a previously
measured Ar3 of the steel sheet.
[0028] A fourteenth aspect of the invention is to provide a
workpiece obtained by surface layer grain refining hot-shearing,
the workpiece produced by: heating and keeping a steel sheet having
a carbon content of 0.15% or more by mass in a temperature range of
from Ac3 to 1400.degree. C. to austenitize the steel sheet;
subsequently shearing the steel sheet in a state wherein the steel
sheet is placed on a die; and quenching by rapidly cooling the
sheared steel sheet, wherein a start temperature of the shearing is
set to be a temperature (.degree. C.) obtained by adding a value,
which is calculated by multiplying an amount of equivalent plastic
strain of a surface layer in a sheared portion by a coefficient
from 40 to 60, to a previously measured Ar3 of the steel sheet.
Advantageous Effects of Invention
[0029] According to a surface layer grain refining hot-shearing
method and a workpiece obtained by surface layer grain refining
hot-shearing of the invention, it is possible to suppress delayed
fracture in a sheared portion and to provide a workpiece having
excellent dimension accuracy without increasing the shearing time
and new steps.
BRIEF DESCRIPTION OF DRAWINGS
[0030] FIG. 1A is a schematic diagram illustrating an example of
punching-shearing by a punch and a die.
[0031] FIG. 1B is a schematic diagram illustrating an example of
trimming-shearing by a punch and a die.
[0032] FIG. 2 is a diagram illustrating an example of a sheared
portion of a steel sheet.
[0033] FIG. 3 is a diagram illustrating a relation between a
temperature history and an Ar3 point.
[0034] FIG. 4A is a diagram illustrating a state of a hot-shearing
apparatus used in Test A before shearing.
[0035] FIG. 4B is a diagram illustrating a state of the
hot-shearing apparatus used in Test A during shearing.
[0036] FIG. 4C is a diagram illustrating a state of the
hot-shearing apparatus used in Test A after shearing.
[0037] FIG. 5 is a diagram illustrating inclusions (a transmission
electron microscope image observed by a replica method), which are
observed by a replica method using a transmission electron
microscope in Comparative Example, in a surface layer of a sheared
portion.
[0038] FIG. 6A is a diagram illustrating a region in which
equivalent plastic strain is averaged.
[0039] FIG. 6B is a diagram illustrating a region in which a fine
structure in an actually hot-sheared portion is formed.
[0040] FIG. 7 is an example of metal structure (EBSD image)
obtained by Example 1.
[0041] FIG. 8 is an example of inclusions (a transmission electron
microscope image observed by a replica method) of a metal structure
obtained by Example 1.
[0042] FIG. 9A is a diagram illustrating a bending state of a
hot-shearing apparatus used in Test B.
[0043] FIG. 9B is a diagram illustrating a shearing state of a
hot-shearing apparatus used in Test B.
DESCRIPTION OF EMBODIMENTS
First Embodiment
[0044] A surface layer grain refining hot-shearing method and a
workpiece obtained by surface layer grain refining hot-shearing
according to a first embodiment of the invention will be described
below.
[0045] First, general shearing will be described and a sheared
portion of the sheared workpiece which is subjected to the shearing
will be then described.
[0046] As illustrated in FIGS. 1A and 1B, punching-shearing or
trimming-shearing is performed on a steel sheet 1 placed on a die 3
by lowering of a punch 2. At this time, as illustrated in FIG. 2, a
sheared portion 8 of the steel sheet 1 is configured by (a) a shear
drop 4 that is formed in such a manner that the steel sheet 1 is
totally pressed by the punch 2, (b) a shear plane 5 that is formed
in such a manner that the steel sheet 1 is drawn into a clearance
between the punch 2 and the die 3 (a gap between the punch 2 and
the die 3) and is then locally stretched, (c) a fracture plane 6
that is formed in such a manner that the steel sheet 1 drawn into
the clearance between the punch 2 and the die 3 is fractured, and
(d) a burr 7 that is generated on the back surface of the steel
sheet 1.
[0047] In the following description of the embodiment, the same
components are also denoted by the same reference numerals and the
detailed description thereof will be not presented.
[0048] In this embodiment, a term of "surface layer of the sheared
portion" is used, and this refers to a region from the surface of
the sheared portion up to 100 .mu.m in a normal direction of the
shear plane.
[0049] Hereinafter, first, the findings of the inventors on the hot
shearing are described, the surface layer grain refining
hot-shearing method found based on the findings is then described,
and the workpiece obtained by surface layer grain refining
hot-shearing formed by such a shearing method is finally described
together with the operation of the shearing method.
[0050] In the hot shearing according to this embodiment, a steel
sheet of high-carbon region of 0.15% or more by mass is used. A
transformation start temperature (Ae3 point) in a state diagram
from austenite to ferrite of the steel sheet is from 800.degree. C.
to 900.degree. C. A portion, which is subjected to large plastic
deformation in the austenite state, is transformed to ferrite
without an occurrence of martensite transformation even when being
rapidly cooled. Therefore, when being rapidly cooled after being
sheared at a temperature range of an austenite single phase based
on the state diagram, almost entirely of the surface layer of the
sheared portion having large plastic deformation is transformed
into ferrite and other portions, which are not plastically
deformed, are transformed into martensite. However, when the
shearing temperature is high, dimension accuracy becomes poor due
to thermal strain. In addition, there was a problem that variation
in occurrence of delayed fracture results from the
plastically-deformed ferrite at the time of the shearing at a
temperature range in which the austenite and the ferrite are mixed
based on the state diagram.
[0051] Then, the inventors have experimented to perform the
shearing on the steel sheet which is subjected to a soaking
treatment followed by changing a temperature for starting the
shearing (shearing start temperature). With respect to the shearing
start temperature, a thermocouple was embedded at the center in a
thickness direction of the sheet at a position spaced apart by 3 to
5 mm from a shearing position of the steel sheet to measure the
temperature at the start of shearing. Since the steel sheet is
heat-released and thus lowered in the temperature when coming in
contact with a die, the shearing of the steel sheet started within
three seconds after the steel sheet comes in contact with the
die.
[0052] In this embodiment, the "die" refers to the die 3 and a pad
9 (see FIG. 4A) to be used during the shearing. Furthermore, the
meaning of "after the steel sheet comes in contact with the die"
refers to the time after the steel sheet 1comes in contact with
either of the die 3 or the pad 9.
[0053] As a result, the inventors found that there is a temperature
range in which the delayed fracture does not occur on the sheared
portion (fracture plane) of the steel sheet and the dimension
accuracy is improved and that this temperature range varies
depending on shearing conditions or components of the steel sheet.
The inventors also found that cooling control of the steel sheet
before the shearing also affects the delayed fracture of the
sheared portion (fracture plane) or the dimension accuracy of the
workpiece.
[0054] The inventors found that fine bainite or fine martensite and
fine residual austenite are added in addition to fine ferrite and
that cementite reduces when the shearing start temperature is set
to be an appropriate temperature as will be described below.
[0055] In general, the fine ferrite structure has toughness higher
than the martensite structure. Therefore, when the fine ferrite
structure having high toughness is present in the surface layer of
the sheared portion, the delayed fracture is suppressed.
[0056] The shearing start temperature having an appropriate
temperature range was obtained by considering temperature changes
in the hot shearing and further calculating the size of shearing
strain.
[0057] The steel sheet was first heated to 950.degree. C. and after
keeping it for 90 seconds and then cooling it in a state being
placed on four pointed needles (hereinafter, sometimes referred to
as a "pin support"), the transformation temperature of the steel
sheet was measured. The temperature was measured by the
thermocouple embedded in the steel sheet.
[0058] The measured Ar3 point is a temperature that starts to
transform to a BBC crystalline structure such as ferrite from the
austenite structure of an FCC crystal at a finite cooling rate
rather than the assumption that the cooling rate is zero as in the
state diagram.
[0059] The measured Ar3 point was significantly different in the
range of from 200 to 300.degree. C. from a transformation
temperature (Ae3 point) at which austenite was changed to ferrite
as illustrated in the state diagram. Further, the Ar3 point
measured in a surface contact state with the die (quenching is
inadequate, but the cooling rate is faster compared to the case of
the pin support) was as low as about 400.degree. C. compared to the
Ae3 point, that is, was as low as about 100.degree. C. compared to
the case of the pin support.
[0060] The fact that the Ar3 point is lower than the Ae3 point is
common technical knowledge in the field of metallic materials.
However, a quantitative difference between the Ar3 point and the
Ae3 point is not clear. By testing of the inventors, it was clear
that the significant difference between the Ar3 point and the Ae3
point is present in the hot shearing as described above.
[0061] For reference, results of measurement of the Ar3 point by
the above measuring method (pin support) are illustrated in FIG. 3.
The steel sheet to be mainly used had a sheet thickness of 1.5 mm.
The range of the thickness of the steel sheet to be used in the
shearing is of about from 0.5 mm to 3.0 mm. Since the Ar3 point is
the transformation start temperature at which the austenite is
changed to the ferrite, it is not necessary to include shearing and
a quenching (rapid cooling) process on the measurement of the Ar3
point. Accordingly, the quenching process is not included in the
graph of FIG. 3.
[0062] In FIG. 3, initially, the cooling rate was 7.degree. C./s,
and the cooling rate has sharply declined when the time has elapsed
for 50 seconds from a cooling start. A temperature (about
680.degree. C.) of the steel sheet at which the cooling rate of the
steel sheet is equal to or less than 1.degree. C./s is identified
as the transformation temperature (Ar3 point). At the time of the
measurement of the Ar3 point, the steel sheet is cooled to room
temperature as it is, but, in actual fact, the shearing starts at a
temperature higher than the Ar3 point and the quenching process is
then performed.
[0063] In this embodiment, an Ar3 temperature measured using the
same method as in the case of the above pin support under placing
conditions of a sheet to be actually sheared is defined as the
"measured Ar3 (of the steel sheet)". The cooling rate is generally
about from 5.degree. C./s to 30.degree. C./s (state of cooling by
leaving) at the time of the measurement in many cases.
[0064] As long as appropriate hot-shearing conditions are
ascertained by performing the above experiment as a preliminary
test, when performing appropriate soaking temperature management of
the steel sheet and time management up to the shearing start after
placing the steel sheet in the die at steps of an actual mass
production process, it is not necessary to perform the operation
after preparing the die in which the thermocouple is embedded and
measuring a surface temperature of the steel sheet to be sheared at
the time of the shearing start for every shearing. In the case of
performing the operation by measuring the surface temperature of
the steel sheet in the mass production process, the surface
temperature of the steel sheet may be measured immediately before
the hot-shearing using a radiation thermometer.
[0065] From the fact that the plastic deformation caused by the
shearing is related to the structure of the sheared portion as
described above, the inventors derived plastic strain in the
vicinity of the sheared portion by numerical calculation. Here, the
plastic strain was evaluated as equivalent plastic strain.
[0066] From the fact that the actual shearing is performed at a
range higher than the measured Ar3 temperature, as a premise of the
calculation, the numerical value of mechanical characteristics such
as deformation resistance of the steel sheet was defined as a value
of austenite. In addition, the temperature dependence of the
mechanical characteristics of austenite was obtained using an
actual measurement value in a hot tensile test (after heating the
steel sheet to a temperature higher than or equal to the Ac3 point,
the steel sheet is cooled by leaving to a predetermined
temperature, and then a tensile test is performed) of 22MnB5
equivalent steel which is widely used for hot stamping. Such a
temperature dependence is disclosed in, for example, "Hongsheng
Liu, Jun Bao, Zhongwen Xing, Dejin Zhang, Baoyu Song, and Chengxi
Lei; "Modeling and FE Simulation of Quenchable High Strength Steels
Sheet Metal Hot Forming Process", Journal of Materials Engineering
and Performance, Vol. 20(6), 2011, pp. 894 to pp. 902"
(hereinafter, sometimes referred to as "Non-Patent Literature 3"),
and practitioners may use values disclosed in this Literature
without actually measuring the values.
[0067] The plastic strain obtained by the numerical calculation is
largest at the surface of the shearing surface, and becomes smaller
moving away from the surface. Furthermore, it was found that an
occurrence region of the equivalent plastic strain of 100% or more
at the sheared portion coincides with an actual occurrence region
of the fine structure in a predetermined temperature range.
[0068] With respect to the values obtained by the numerical
calculation, it is concerned that variation is caused by analysts.
Therefore, the inventors performed the numerical calculation using
steel grades, analyst, and software in plural ways. As a result of
the numerical calculation, the inventors obtained the result that
the temperature range at which the occurrence region (distance) of
the equivalent plastic strain of 100% or more in the normal
direction of the shear plane at the sheared portion coincides with
the occurrence region of the fine structure in the normal direction
of the shear plane is a temperature range higher by approximately
30 to 140.degree. C. than the measured Ar3.
[0069] Here, at a temperature range higher than a temperature
obtained by adding 140.degree. C. to the measured Ar3 (hereinafter,
sometimes referred to as "higher than Ar3+140.degree. C."), the
occurrence region of the equivalent plastic strain of about 100% in
the normal direction of the shear plane on the sheared portion
which is obtained by calculation becomes larger than the actual
fine region on the sheared portion of the workpiece. As a result of
analysis of the fine structure region, the region was mainly
configured by ferrite and carbide. On the other hand, other regions
except the surface layer are configured by a martensite
structure.
[0070] The ferrite and the martensite have a different volume,
respectively, from the difference of a crystal structure and a
solid-solution state of element. Therefore, when the fine structure
region is widely formed on the surface layer of the sheared portion
and most of the fine structure is configured by ferrite, the
boundary area between the fine ferrite and the fine martensite
increases. As a result, the dimension accuracy of the workpiece
deteriorates. In consideration of the thermal strain, the dimension
accuracy of the workpiece deteriorates as the shearing start
temperature becomes higher.
[0071] Furthermore, when the shearing start temperature is lower
than a temperature obtained by adding 30.degree. C. to the measured
Ar3 (hereinafter, sometimes referred to as "lower than
Ar3+30.degree. C."), the actual fine region is smaller than the
occurrence region of the equivalent plastic strain of 100% or more.
Since the occurrence region of the equivalent plastic strain of
100% or more becomes smaller, the actual fine structure region
smaller than such a region becomes further smaller. At the
temperature lower than "Ar3+30.degree. C." which is measured, a
part of austenite starts to transform into ferrite by the influence
of internal heat distribution, and such ferrite is plastically
deformed by the shearing. Consequently, the inventors found that
residual stress is excessively large on the surface of the sheared
portion of the workpiece and thus the risk of the delayed fracture
increases.
[0072] On the other hand, when the shearing start temperature is
higher than "Ar3+30.degree. C.", the steel sheet is subjected to
the shearing before austenite starts to transform into ferrite, so
excessive residual stress on the sheared portion due to ferrite is
avoided.
[0073] Based on the above findings, the surface layer grain
refining hot-shearing method according to this embodiment was
configured as follows.
[0074] First, a shearing machine used in the test will be briefly
described. As illustrated in FIG. 4A, a shearing machine 10
includes the die 3 on which the steel sheet 1 is placed, a pad 12
that is disposed on the die 3 to press the steel sheet 1 placed on
the die 3, and a punch 2 that is disposed inside the pad 12 and is
inserted into a puncture 14 of the die 3 to punch a predetermined
range of the steel sheet 1.
[0075] First, the steel sheet 1 having the carbon content of 0.15%
or more by mass is placed on the die 3 after being heated to the
range of from Ac3 to 1400.degree. C. higher than the shearing start
temperature in the range of from Ar3+30.degree. C. to
Ar3+140.degree. C. and being subjected to a soaking treatment (see
FIG. 4A).
[0076] Then, as illustrated in FIG. 4B, after the steel sheet 1 on
the die 3 is pressed by the pad 12, the steel sheet 1 is subjected
to the shearing by the punch 2. After the steel sheet 1 is placed
on the die 3, the shearing of the steel sheet 1 starts within three
seconds. By control of the time (shearing start time) until the
shearing starts after the steel sheet 1 is placed on the die 3, the
temperature of the steel sheet 1 during the shearing is controlled
in the range of from Ar3+30.degree. C. to Ar3+140.degree. C.
[0077] As illustrated in FIG. 4C, a predetermined range of the
steel sheet 1 is punched by the punch 2, the punched steel sheet 1
is rapidly cooled and quenched by the die 3 and the pad 12, and
thus a shearing-workpiece is formed.
[0078] Operation of the surface layer grain refining hot-shearing
method according to this embodiment as described above and the
workpiece obtained by surface layer grain refining hot-shearing
(hereinafter, sometimes referred to as a "workpiece") formed by
this shearing method will be described.
[0079] In the sheared portion 8 of the workpiece (steel sheet)
formed in this manner, the surface layer of the sheared portion 8
defined as the range up to 100 .mu.m inside of the steel sheet in a
normal direction of the shear plane 5 includes a ferrite phase
forming at least a portion of the fracture plane and the remainder,
and the remainder has a bainite phase, a martensite phase, a
residual austenite phase, and cementite and inevitably generated
inclusions. The ferrite phase, the bainite phase, the martensite
phase, and the residual austenite phase which are formed in the
surface layer of the sheared portion 8 have an average grain size
of 3 .mu.m or less, respectively. The surface layer of the sheared
portion 8 contains 5% or more grains by number having an aspect
ratio of 3 or more. In addition, other regions except the surface
layer of the sheared portion 8 includes a mixed structure of an
inevitably generated inclusion and martensite or a mixed structure
of martensite, bainite, and an inevitably generated inclusion.
[0080] That is, since the workpiece is formed by the shearing of
the steel sheet 1 heated to the temperature of from Ar3
point+30.degree. C. to Ar3 point+140.degree. C., a fine ferrite
structure, a fine martensite structure, a fine bainite structure,
and a fine residual austenite structure are formed in the surface
layer of the sheared portion 8 (fracture plane 6) (see FIG. 2).
FIG. 6B illustrates the steel sheet 1 which has actually been
subjected to the shearing. As illustrated in FIG. 6B, a fine
structure 11 is formed from the fracture plane 6 toward the shear
plane 5 in the sheared portion 8 in the surface layer, but the fine
structure is formed particularly up to a depth of about 100 .mu.m
from the surface in the fracture plane 6.
[0081] The fine ferrite structure has generally higher toughness
than the martensite structure. Accordingly, since the fine ferrite
structure of the high toughness is present in the surface layer of
the sheared portion 8 (fracture plane 6), occurrence of the delayed
fracture in the sheared portion 8 (fracture plane 6) due to the
delayed fracture is suppressed.
[0082] As will be described below, in the workpiece according to
this embodiment, the occurrence of the delayed fracture in the
sheared portion 8 (fracture plane 6) can be suppressed by the fine
martensite structure, the fine bainite structure, and the fine
residual austenite structure which are formed in the surface layer
of the sheared portion 8 (fracture plane 6).
[0083] For reference, FIG. 7 illustrates a structure photograph of
the surface layer of the sheared portion obtained by an EBSD of
this embodiment.
[0084] In FIG. 7, a black part indicates a bainite phase, a
martensite phase, or a residual austenite phase. As in the
photograph, although crystal grains having the aspect ratio of 3 or
more are present, the delayed fracture does not occur for reasons
which will be described below.
[0085] The "grain size" used herein means a circle diameter, that
is, a circle conversion diameter (circle equivalent diameter) when
an area of each ferrite crystal grain, which is observed in a cross
section along the thickness direction of the steel sheet in the
normal direction of the shear plane, is replaced by a circle of the
same area.
[0086] The bainite phase, the martensite phase, or the residual
austenite phase rather than the single phase of the fine ferrite
phase is present in the surface layer of the sheared portion 8.
Generally, the bainite phase, the martensite phase, or the residual
austenite phase present in the ferrite phase traps diffusible
hydrogen that causes the delayed fracture. Therefore, when these
phases are present in the fine ferrite phase, it is possible to
obtain an effect of suppressing the delayed fracture.
[0087] In addition, when the bainite phase, the martensite phase,
or the residual austenite phase becomes finer to be 3 .mu.m or
less, sites for trapping the diffusible hydrogen further increase,
and thus the delayed fracture is further suppressed.
[0088] On the other hand, the cementite has a small effect of
trapping the diffusible hydrogen and can be a start point of the
occurrence of the delayed fracture, so it is preferable that the
cementite becomes smaller.
[0089] In order for the remainder to have the fine bainite phase,
martensite phase, and/or residual austenite phase having the grain
size of 3 .mu.m or less, ferrite having an aspect ratio of more
than 3 inevitably appeared. As a result of analysis using a
transmission electron microscope, the ferrite having the aspect
ratio of more than 3 is in a state where plastic deformation little
occurs or is small, but is not in a state of being plastically
deformed and stretched as described in Patent Literature 6, so the
ferrite did not adversely affect resistance to the delayed
fracture. While the details of the operation is not clear, in order
for the remainder to have the bainite phase, the martensite phase,
or the residual austenite phase described above, the ferrite
structure having the aspect ratio of more than 3 is essentially
present.
[0090] In order to also make these structures, it is necessary to
adjust the shearing temperature to a temperature range of from
Ar3+30.degree. C. to Ar3+140.degree. C. It is considered that since
the steel sheet is cooled at a certain cooling rate, the austenite
structure remains at the shearing temperature, but the appropriate
amount of shearing strain is added and transformation nuclei to
transform into other phases other than the martensite is already
generated. In this case, the cooling rate contributes to any phase
transformation.
[0091] The cooling rate is fast when the temperature exceeds
Ar3+140.degree. C., and the austenite becomes a supercooled state
during cooling (temperature is lower than a temperature range at
which structure morphology can be present) when the shearing strain
is applied to the extent in which transformation to martensite
cannot occur. In such a case, austenite is easily transformed into
a fine ferrite structure.
[0092] On the other hand, when the temperature is equal to or lower
than Ar3+140.degree. C., grains are formed in which transformation
to ferrite does not occur and transformation to martensite also
does not occur under the influence of shearing strain. Such grains
become a bainite phase. In addition, grains are also present in
which shearing strain is small and transformation to martensite
occurs. Additionally, the transformation to the non-uniform three
phases partially induces enrichment of carbon to austenite, and
such austenite becomes residual austenite in order to be stable
even at room temperature. Since these phases occur between the fine
ferrite grains, the phases themselves also become finer to be 3
.mu.m or less.
[0093] In order to stably form these structures, the shearing of
the steel sheet preferably starts within three seconds after the
steel sheet comes in contact with the die. When the shearing starts
after three seconds, scale occurs on the surface of the steel sheet
and the contact of the die with the steel sheet becomes
non-uniform. When heat irregularity occurs due to the non-uniform
contact, variation in cooling condition of the sheared portion is
caused.
[0094] In addition, FIG. 5 illustrates cementite distribution in
the surface layer of the fracture plane when the steel sheet
disclosed in Patent Literature 6 is subjected to the shearing at a
temperature higher than Ar3 point+140.degree. C. In Patent
Literature 6, since the shearing start temperature is simply set to
only a temperature range of from 400.degree. C. to 900.degree. C.,
the shearing start temperature also includes the case of being
higher than Ar3+140.degree. C. In this case, for example, as
illustrated in FIG. 5, cementite C (black parts excluding circles)
has a number density of 0.8 pieces/.mu.m.sup.3 or more and the
maximum length of 3 .mu.m or more.
[0095] On the other hand, in the case of this embodiment, cementite
(black parts excluding circles) in the surface layer of the
fracture plane of the steel sheet has a number density of 0.8
pieces/.mu.m.sup.3 or less and the maximum length of 3 .mu.m or
less as indicated in test results (FIG. 8) to be described below.
According to the experience of the inventors, when the number of
cementite is small to this extent and the size of cementite is also
small, the cementite itself does not almost cause a problem of
being a start point of the occurrence of the delayed fracture.
[0096] As illustrated in FIG. 7, a total area ratio of the bainite
phase, the martensite phase, or the residual austenite phase, which
is measured by observation in the range up to 100 .mu.m inside of
the steel sheet in the normal direction of the shear plane from the
fracture plane in the sheared portion of the steel sheet using an
electron-beam backscattering diffraction (EBSD) method, is from 10%
to 50%.
[0097] As for this, according to the experience of the inventors,
when the total area ratio of these phases is less than 10%, it is
not possible to sufficiently perform the storage of the diffusible
hydrogen and the risk of the delayed fracture increases. On the
other hand, when the total area ratio of these phases exceeds 50%,
the ratio of the fine ferrite in the surface layer of the fracture
plane reduces, whereby the effect of toughness improvement due to
the fine ferrite decreases and the risk of the delayed fracture
increases. Although the effect of the invention does not
immediately disappear when the total area ratio of these phases is
out of such a range, the total area ratio of these phases is more
preferably within such a range.
[0098] A method of rapidly cooling the steel sheet 1 after the
shearing is not limited to rapid cooling by the contact of the die
(die 3 and pad 12) with the steel sheet 1 as in this embodiment
and, for example, the steel sheet 1 may be rapidly cooled by
allowing the steel sheet 1 to come in directly contact with water.
Examples of the method of allowing the steel sheet 1 to come in
contact with water may include a method of passing cooling water
through a groove formed in a contacting portion of the steel sheet
with the die.
[0099] Even in the case of performing the shearing after press
forming, as in the workpiece of this embodiment, it is possible to
suppress the delayed fracture of the sheared portion to form a
workpiece with dimension accuracy.
Second Embodiment
[0100] A surface layer grain refining hot-shearing method according
to a second embodiment of the invention will be described. The same
components as in the first embodiment are denoted by the same
reference numerals, and the detailed description thereof will not
be presented. In addition, a workpiece obtained by surface layer
grain refining shearing formed by the surface layer grain refining
hot-shearing method according to this embodiment is the same as in
the first embodiment, so operational effects thereof will not be
described.
[0101] The inventors found that the temperature range at which the
occurrence region of about 100% equivalent plastic strain in the
normal direction of the shear plane in the sheared portion
coincides with the occurrence region (distance) of the fine ferrite
structure, the fine martensite structure, the fine bainite
structure, or the fine residual austenite structure in the normal
direction of the shear plane is obtained when a temperature
(.degree. C.) obtained by adding a value, which is calculated by
multiplying the amount of equivalent plastic strain of the surface
layer in the sheared portion by a coefficient from 40 to 60, to the
measured Ar3 is set as a shearing start temperature.
[0102] In this embodiment, it was considered that the following
value was appropriate to use as the amount of equivalent plastic
strain of the surface layer in the sheared portion.
[0103] As illustrated in FIG. 6A, an average value-of the amounts
of plastic strain obtained by calculation at a region A (within a
thick line frame) in the range of from 5 to 20% of a thickness H of
the steel sheet 1 from the shear plane 5 of the sheared portion 8
to the inside of the steel sheet 1 in the normal direction of the
shear plane 5 and in the range of from 20% to 50% of the thickness
H of the steel sheet 1 in the thickness direction of the steel
sheet 1 from a bottom 12 on the burr 7 side of the sheared portion
8 was used as the amount of equivalent plastic strain of the
surface layer in the sheared portion.
[0104] By setting the region A in this way, the inventors found
that the amount of equivalent plastic strain having a small
influence by differences in analyst or analysis condition was
obtained. This value is considered to be a reasonable numerical
value as the amount of equivalent plastic strain as will be
described below, but other values of correction strain may be used
according to a calculation unit.
[0105] The amount of equivalent plastic strain of the surface layer
in the sheared portion used a value obtained by the calculation at
a temperature range of from 500.degree. C. to 800.degree. C. It was
confirmed that the amount of equivalent plastic strain of the
surface layer becomes approximately constant at this range.
[0106] the reason that a lower limit of 40 is set for the
coefficient to be multiplied by the amount of equivalent plastic
strain is due to consideration of differences in the coefficient
due to a steel grade and errors in numerical calculation. By
repetitive experiment and numerical calculation, the fine ferrite
structure, the fine martensite structure, the fine bainite
structure, or the fine residual austenite structure appeared even
in the case of being out of this coefficient range, but the
inventors obtained 40 as the lower limit of the coefficient in
which appearance probability becomes higher.
[0107] In addition, the reason why the upper limit of the
coefficient to be multiplied by the amount of equivalent plastic
strain is set to 60 is that the dimension accuracy of the workpiece
deteriorates when the shearing temperature is too high. This reason
is considered that the region of the fine structure in the surface
layer becomes wider as the temperature becomes higher, but the
dimension accuracy deteriorates after cooling because a difference
in density between the surface layer and another region adjacent to
the surface layer is large and the thermal strain also
increases.
[0108] In a case in which the difference between a workpiece
dimension and a design dimension of the workpiece generally falls
within the range of -0% +5% of the design dimension, the defective
rate of product is lowered to the extent of being economically
acceptable and thus problems substantially disappear. Thus, as a
result of trial and error, such an upper limit was determined.
[0109] The measured Ar3 point of the steel sheet should be
previously measured by a temperature drop history at the
thermocouple or the like in a state in which the steel sheet is
placed on the die to be actually used. The thermocouple is embedded
in the die, and it is preferable to cause a thermocouple sensor to
come in directly contact with the steel sheet which is a member to
be sheared. This reason is that the measured Ar3 point varies
depending on the cooling rate of the steel sheet. As illustrated in
FIG. 3, it is widely known that the measured Ar3 point is measured
as a point at which a temperature lowering rate varies. This
technique is also used in Tests A and B to be described below.
[0110] In this embodiment, it is important to calculate the
equivalent plastic strain of the sheared portion. In the
hot-shearing, the metal-structure transformation inevitably occurs
during or immediately after the shearing, and thus it is not
possible to measure the equivalent plastic strain. Therefore, a
shearing simulation is performed by analysis using a finite element
method (FEM), and thus the equivalent plastic strain is
calculated.
[0111] In the shearing simulation, the plastic strain is steeply
changed. For this reason, calculation results of the plastic strain
of the surface layer in the sheared portion are likely to differ
depending on analysts or analysis conditions. In order to reduce
the influence of these analysts or analysis conditions, it is
preferable to set a constant FEM analysis region and to average and
calculate the equivalent plastic strain within the region.
[0112] The inventors have set the region as a result of trial and
error. FIG. 6A illustrates the region in which the equivalent
plastic strain is averaged. As illustrated in FIG. 6A, the region A
(within the thick line frame), in which the equivalent plastic
strain is averaged, was set in the range of from 5 to 20% of the
thickness H (see FIG. 4) of the steel sheet 1 from the shear plane
5 of the sheared portion 8 to the inside of the steel sheet 1 in
the normal direction of the shear plane 5 and in the range of from
20% to 50% of the thickness H of the steel sheet 1 in the thickness
direction of the steel sheet 1 from the bottom 12 on the burr 7
side of the sheared portion.
[0113] During the simulation, since the temperature change
sequentially occurs, it is necessary to perform repetitive
calculation in such a manner that: a tentative shearing start
temperature is set; the equivalent plastic strain is calculated
based on the tentative shearing start temperature; and a true
shearing start temperature is determined based on the calculated
equivalent plastic strain. Such calculation requires costs.
[0114] As a result of the calculation with several levels by the
inventors, it was found that approximation can be performed when a
numerical simulation is once performed based on stress-strain
diagram at any of the steel sheet temperature of from 500.degree.
C. to 800.degree. C.
[0115] As a premise of the calculation, when the shearing is
performed at the range higher than the measured Ar3 temperature,
numerical values of mechanical characteristics such as rigidity of
the steel sheet at that time were defined as values of
austenite.
[0116] During the simulation, the shearing start temperature can be
calculated without any problem when the equivalent plastic strain
is calculated by a Mises yield function on the supposition of an
isotropy without considering an anisotropy in particular.
[0117] An increment in equivalent plastic strain "d.epsilon.-P" by
the Mises yield function is represented by the following formula
when a material coordinate system is defined as x, y, and z, and
the equivalent plastic strain is given as an integral of this
increment.
d _ P = 2 3 ( d xx 2 + d yy 2 + d zz 2 + 2 d xy 2 + 2 d yz 2 + 2 d
zx 2 ) ( 1 ) ##EQU00002##
[0118] As described above, in the shearing method according to this
embodiment, the structures such as the fine ferrite are formed in
the surface layer in the sheared portion and the occurrence of the
delayed fracture in the sheared portion (fracture plane) is
suppressed when the steel sheet is subjected to the shearing at the
calculated shearing start temperature, and it is possible to
suppress the thermal strain or the like and ensure the dimension
accuracy of the workpiece by allowing the shearing start
temperature to be within the predetermined range.
[0119] In particular, since the predetermined range region A is set
to calculate the amount of equivalent plastic strain in the sheared
portion, it is possible to calculate the amount of equivalent
plastic strain having a small error.
[0120] During the FEM simulation for calculating the equivalent
plastic strain, since the temperature change sequentially occurs,
it was necessary to perform repetitive calculation in such a manner
that: the equivalent plastic strain was calculated based on the
tentative shearing start temperature; and the true shearing start
temperature was determined based on the calculated equivalent
plastic strain. In this embodiment, however, since the
approximation can be performed when a numerical simulation is only
once performed based on stress-strain diagram at any of the steel
sheet temperature of from 500.degree. C. to 800.degree. C., the
calculation is simplified.
[0121] Since the equivalent plastic strain is calculated by the
Mises yield function on the supposition of an isotropy, the
calculation is further simplified.
[0122] The method of calculating the amount of equivalent plastic
strain disclosed in the surface layer grain refining hot-shearing
method according to the second embodiment is applicable to the
calculation of the amount of equivalent plastic strain in the
surface layer grain refining hot-shearing method according to the
first embodiment.
EXAMPLES
[0123] Next, Examples of the invention will be described. However,
shearing conditions in Examples are examples adopted to confirm
feasibility and an effect of the invention and the invention is not
limited to these shearing conditions. The invention can adopt
various shearing conditions as long as the object of the invention
is achieved within a range of not departing from the gist of the
invention.
[0124] (Test A)
[0125] Using the shearing machine 10 illustrated in FIGS. 4A to 4C,
after the high-strength steel sheet 1 (200 mm.times.150 mm) of
steel grades A to C having compositions indicated in Table 1 is
placed on the die 3, the punch 2 together with the pad 12 approach
the top of the steel sheet 1 from the above. The steel sheet 1 is
pressed by the pad 12 and the steel sheet 1 is subjected to the
shearing by the punch 2 (width of 65 mm) at the same time. The
sheared steel sheet 1 is rapidly cooled by the die (die 3 and pad
12). Shearing conditions are as indicated in Table 2. A clearance
between the punch 2 and the die 3 was set to be 0.15 mm.
[0126] Except for Comparative Examples, the keeping time until the
shearing of the steel sheet 1 starts after coming in contact with
the die 3 was set to be from 0.5 seconds to 3 seconds. The shearing
start temperatures in Table 2 are temperatures obtained within the
range of the keeping time.
[0127] The thickness of the steel sheets used in Examples was set
to be 1.5 mm. The thickness of the steel sheet applicable to the
invention has the range of from about 0.5 mm to 3 mm.
[0128] The measured Ar3 point of each steel sheet was obtained by
the measurement of the temperature history at the time when the
steel sheet heated to 950.degree. C. is cooled in contact with the
top of the die on the shearing machine (a temperature at which the
cooling rate of the steel sheet was 1.degree. C./sec. or less
before the temperature of the steel sheet was lowered to the room
temperature was regarded as the Ar3 point).
[0129] For estimation of the equivalent plastic strain, shearing
simulation, in which deformation resistance was input when the
steel sheet is 750.degree. C., was performed by a finite element
method using Abaqus/Standard made by Dassault Systemes Co, which is
commercial software. In this case, the Mises yield function was
used, and the analysis region in the vicinity of a tool cutting
edge was defined as a quadrilateral complete integration element of
0.02 mm.times.0.04 mm. In addition, remeshing was performed every
0.05 mm punching press. The fracture was defined by a ductile
fracture model of Hancock & Mackenzie, and the rigidity of
elements satisfying conditions was zero. Parameters of the ductile
fracture model were fitted based on a shear plane ratio which was
actually observed in certain conditions. The equivalent plastic
strain was used which was averaged in the region A set in the range
of 10% of the thickness H of the steel sheet 1 from the shear plane
5 of the sheared portion 8 in the normal direction of the shear
plane 5 and in the range of 30% of the thickness H of the steel
sheet 1 in the thickness direction of the steel sheet 1 from the
bottom 12 on the burr 7 side of the sheared portion 8 (see FIG.
6A).
[0130] A length of a scrap 16 (see FIG. 4C) punched out after the
shearing was evaluated as the dimension accuracy. Unless a
dimensional error occurs, the length of the scrap 16 after the
shearing should be 65 mm. Thus, values are obtained in such a
manner that the error in length of the scrap 16 after the shearing
is divided by 65 and is then converted into percentage (.times.100)
are disclosed as the dimensional error in Table 2.
TABLE-US-00001 TABLE 1 (% by mass) Steel grade C Si Mn B Cr A 0.22
0.22 1.20 0.002 0.16 B 0.16 0.40 1.00 0.001 0.23 C 0.25 0.21 1.24
0.002 0.34
TABLE-US-00002 TABLE 2 Steel sheet Steel-sheet heating Amount of
Presence or Steel Time Rapidly equivalent Shearing start absence of
Dimensional grade Ar3(.degree. C.) Temperature(.degree. C.) (min.)
cooling plastic strain Coefficient temperature(.degree. C.) cracks
error (%) Example 1 A 580 950 1.5 Water 2.0 50 680 Absence 2.0
Example 2 A 580 950 1.5 Die 2.0 60 700 Absence 4.2 Example 3 A 580
1000 1.0 Die 2.0 50 680 Absence 1.1 Example 4 B 620 950 1.5 Water
2.5 50 745 Absence 3.0 Example 5 B 620 950 1.5 Water 2.5 40 720
Absence 2.7 Example 6 C 570 950 1.5 Water 1.8 50 660 Absence 2.3
Comparative A 580 950 1.5 Water 2.0 10 600 Presence 1.8 Example 1
Comparative B 620 950 1.5 Water 2.5 -10 595 Presence 1.8 Example 2
Comparative A 580 950 1.5 Water 2.0 85 750 Absence 5.1 Example 3
Comparative B 620 950 1.5 Water 2.5 80 820 Absence 6.3 Example 4
Comparative C 570 950 1.5 Water 1.5 100 720 Absence 5.1 Example
5
[0131] The test was performed three times for each Examples and
Comparative Examples. With respect to the presence or absence of
the delayed fracture, it was evaluated that the delayed fracture
was present when delayed fracture occurs even once. In addition,
the dimensional error was an average value of three measured
values.
[0132] In Examples 1 to 6, it can be understood that the occurrence
of the delayed fracture in the sheared portion (fracture plane) is
suppressed and the dimension accuracy of the workpiece is
improved.
[0133] A microstructure in the range of 100 .mu.m from the fracture
plane of the sheared portion in Example 1 will be described with
reference to FIG. 7 (EBSD, microstructure image) and FIG. 8 (image
of an extraction replica sample observed by the transmission
electron microscope).
[0134] As illustrated in FIG. 7, it was confirmed that the
microstructure includes ferrite, bainite, martensite, residual
austenite, cementite, and inclusions derived from alloy elements
other than iron as a result the EBSD analysis, EDS (characteristic
energy dispersion type X-ray analysis), and electron diffraction
analysis of the transmission electron microscope.
[0135] Specifically, FIG. 7 illustrates the microstructure image
observed by the EBSD in a state where a cross-section sample of
Example 1 along the thickness direction of the steel sheet in the
normal direction of the shear plane in the sheared portion is
embedded in a hard resin and is then subjected to polishing and
electropolishing. In addition, FIG. 8 illustrates the image
observed by the transmission electron microscope of the sample of
Example 1 which is prepared by an extraction replica method using
an SPEED method (Potentiostatic Etching by Electrolytic
Dissolution: potentiostatic electrolysis method in nonaqueous
solvent).
[0136] As illustrated in FIG. 7 (EBSD microstructure image), in the
surface layer of the fracture plane in the range of 100 .mu.m in
the normal direction of the shear plane from the fracture plane,
the grain size of ferrite (parts excluding black in FIG. 7) F was
as very small as 3 .mu.m or less and the grain size of BMA (black
part in FIG. 7) including martensite, bainite, or residual
austenite was also 3 .mu.m or less. The crystal grain having the
aspect ratio exceeding 3 was also seen in this range and the ratio
was about 6% by number.
[0137] The same microstructure was observed in any of Examples 2 to
6. During the identification of the microstructure, five points of
field-of-view of 8.0.times.20 .mu.m were randomly photographed for
each Example, in the range of 100 .mu.m from the surface of the
fracture plane.
[0138] Furthermore, as illustrated in FIG. 8, it can be seen that
the ratio of cementite (black parts excluding circles) C in Example
1 is very small. In Example 1, the number density of the cementite
was 0.8 pieces/.mu.m.sup.3, and the maximum length of the observed
cementite was 3 .mu.m or less. In order to determine a state of
cementite distribution, five points of field-of-view of
9.5.times.7.5 .mu.m from the surface layer of the sheared portion
were randomly photographed for each condition. This was the same in
any of Examples 2 to 6.
[0139] In Comparative Examples 1 to 5, on the other hand, a mixed
structure (Comparative Examples 1 and 2) of bainite and martensite
not including ferrite or a single phase of ferrite (Comparative
Examples 3 to 5) was observed. In Comparative Examples 1 and 2,
cementite and inclusion was hardly observed in almost same manner
as illustrated in FIG. 8. In Comparative Examples 3 to 5, however,
the cementite (see FIG. 5, black part excluding circles) C having
very high number density greatly exceeding 0.8 pieces/.mu.m.sup.3
as illustrated in FIG. 5 was observed.
[0140] An experiment was performed in a state where other
conditions except for the shearing start temperature were the same
as in Example 1, and the keeping time until the shearing of the
steel sheet starts after being cooled in contact with the die 3 and
the pad 9 (also referred to as a die) was set to be 3.5 seconds. In
this case, the shearing start temperature was also (Ar3+30.degree.
C.) or higher, the delayed fracture occurred once in three
repetitive experiments. As a result of observation the surface of
the shearing surface of the resulting workpiece, in the range of
100 .mu.m from the shear plane, the structure of the surface layer
of the sheared portion in the workpiece without an occurrence of
the delayed fracture was configured to include: ferrite of which
the grain size was as very small as 3 .mu.m or less; and
martensite, bainite, or residual austenite of which the grain size
was also 3 .mu.m or less. The crystal grain having the aspect ratio
exceeding 3 was also seen and the ratio was about 7% by number.
[0141] In the range of 100 .mu.m from the shear plane, however, the
structure of the surface layer of the sheared portion in the
workpiece with occurrence of the delayed fracture was configured to
include: ferrite of which the grain size was about 5 .mu.m; and
martensite, bainite, or residual austenite of which the grain size
was also 5 .mu.m. In the surface layer of the sheared portion, the
crystal grain having the aspect ratio exceeding 3 was also seen and
the ratio was about 7% by number.
[0142] (Test B)
[0143] A shearing machine 20 includes: a die 3 which is formed with
a hole 22 for bending and forming and a puncture 24 for punching
deformation on the bottom of the hole 22 and in which the steel
sheet 1 is placed; a punch 2 which is inserted into the hole 22 to
cause bending deformation of the steel sheet 1; and a movable die
26 which is incorporated into the punch 2 and is inserted into the
puncture 24 after the bending deformation to form a puncture
(shearing) in a predetermined range of the steel sheet 1.
[0144] By simulating press forming not accompanying fracture of the
steel sheet, the shearing machine 20 formed the heated steel sheet
1 in a hat shape by initially driving the punch 2 after the steel
sheet 1 was placed on the die 3 (see FIG. 9A). Thereafter, a test
of punching the steel sheet 1 using a movable die 13 to have a
diameter of 20 mm was performed (see FIG. 9B).
[0145] Except for Comparative Examples, the time until the shearing
of the steel sheet 1 starts after coming in contact with the
movable die 26 was from about 0.1 seconds to about 0.5 seconds.
[0146] A clearance between the punch 2 and the die 3 was set to be
0.15 mm and the measured Ar3 was identified from a thermal history
after the hat forming. The equivalent plastic strain was calculated
in the same way as in Test A. Shearing conditions indicated in
Table 3 were adopted.
[0147] An evaluation method in Test B is also the same as that in
Test A.
[0148] By the way, the dimension accuracy in Test B was evaluated
by a diameter of a punch hole after the shearing. When the
dimensional error does not occur, the diameter of the punch hole of
the steel sheet 1 after the shearing should be 20 mm. Thus, values
are obtained in such a manner that the error in diameter of the
punch hole after the shearing is divided by 20 and is then
converted into percentage (.times.100) and the values are disclosed
as the dimensional error in Table 3 which indicates an
implementation result of this test.
TABLE-US-00003 TABLE 3 Steel sheet Steel-sheet heating Amount of
Presence or Steel Time Rapidly equivalent Shearing start absence of
Dimensional grade Ar3(.degree. C.) Temperature(.degree. C.) (min.)
cooling plastic strain Coefficient temperature(.degree. C.) cracks
error (%) Example 7 A 420 950 1.5 Water 2.0 40 500 Absence 1.1
Example 8 A 420 950 1.5 Die 2.0 60 560 Absence 1.1 Example 9 B 480
950 1.5 Water 2.5 40 580 Absence 1.2 Example 10 C 460 950 1.5 Water
1.8 40 532 Absence 1.1 Comparative A 420 950 1.5 Water 2.0 10 440
Presence 0.7 Example 6 Comparative B 480 950 1.5 Water 2.5 10 475
Presence 0.8 Example 7 Comparative C 460 950 1.5 Water 1.8 10 478
Presence 0.5 Example 8 Comparative A 420 950 1.5 Water 2.0 90 600
Absence 2.3 Example 9 Comparative B 450 950 1.5 Water 2.5 80 650
Absence 2.8 Example 10 Comparative C 460 950 1.5 Water 1.8 100 640
Absence 2.8 Example 11
[0149] In Examples 7 to 10, it can be understood that the
occurrence of the delayed fracture in the sheared portion (fracture
plane) is suppressed.
[0150] In Examples 7 to 10 indicated in Table 3, the microstructure
in the surface layer of the sheared portion (in the range of 100
.mu.m from the surface) included ferrite, bainite, martensite,
residual austenite, cementite, and inclusions derived from alloy
elements other than iron as in Examples 1 to 6 (FIG. 7
(microstructure) and FIG. 8 (inclusion)). The microstructure and
inclusions in Examples 7 to 10 are the same as those in Examples 1
to 6.
[0151] The microstructure and inclusions in Comparative Examples 6
to 11 are the same as those in Comparative Examples 1 to 5. That
is, a mixed structure of bainite and martensite not including
ferrite was observed in Comparative Examples 6 to 8, and a single
phase of ferrite was observed in Comparative Examples 9 to 11. In
Comparative Examples 6 to 8, the cementite was hardly observed. In
Comparative Examples 9 to 11, however, the cementite having very
high number density greatly exceeding 0.8 pieces/.mu.m.sup.3 was
observed.
[0152] This application is based upon and claims the benefit of
priority of the prior Japanese Patent application No. 2013-099243,
filed on May 9, 2013, the entire contents of which are incorporated
herein by reference.
INDUSTRIAL APPLICABILITY
[0153] As described above, according to the invention, it is
possible to prevent the delayed fracture occurring in the
hot-sheared portion without increasing the shearing time or new
steps during the hot shearing of the steel sheet. Accordingly, the
invention has high applicability in a steel sheet working
technology industry.
* * * * *