SURFACE LAYER GRAIN REFIMNG HOT-SHEARING METHOD AND WORKPIECE
OBTAINED BY SURFACE LAYER GRAIN REFINING HOT-SHEARING
Technical Field
[0001] The present invention relates to a surface layer grain refining hot-shearing tnethod of
a steel sheet, which bas a carbon content of 0.15% or more by mass and is used in
automobiles, ships, bridges, co~istructione quipment, various plants, or the like, and a
workpiece obtained by surface layer grain refining hot-shearing.
Background At
[0002] From the past, a metal material (steel sheet) to be used in autoniobiles, ships, bridges,
cot~structione quipment, various plants, or the like has beet1 often subjected to shearing by a
puncli 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 I"), a quenching press is
performed in which press forming atid heat treatment are alrl~osts imultat~eouslyp erformed to
form a high-strength t~iember
[0003] Ageneral cold-pressed workpiece is subjected to shearing such as punching and
trimmitlg after being subjected to press fornling. However, when the quenching-pressed
workpiece is subjected to shearing after being subjected to forming, a service life of a
shearing tool becomes sig~iificantlys horter due to high hardness of the t~ie~~ibe11n1 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 niethods
have been proposed so far: a method of perforniit~ga heat treatment after shearing (for
example, see Japanese Patent Application Laid-Open (JP-A) No. 2009-197253 (hereinafter,
referred to as "Patent Literature 1")); t~iethodso f reducing residual stress in a sheared portion
by sitnultat~eouslyp erforming sliearing and hot pressing before quenching (for exatiiple, see
P-ANo. 2005-138111 (hereinafter, referred to as "Patent Literature 2"), JP-ANo.
2006-104526 (hereinafter, referred to as "Patent Literature 3"), and JP-ANo. 2006-83419
(hereinaftel; referred to as "Patent Literature 4")); a method of reducing quenching hardness
by gradually lowering a cooling rate of a sheared pottiotl (for exa~u~plsee,e JP-ANo.
2003-32803 1 (hereinafter, referred to as "Patent Literature 5")); a method of working to soften
1
only a shearing scheduled portion by perfor~iiinglo cal electric-heating (for example, see
"CLRP Aiaals-Ma~iufacturingT echnology" 57 (2008), p. 321-324.(hereinafter, referred to as
"Non-Patent Literature 2")); and a shearing-related tecli~iologyf or controlling structures in a
surface layer of a shear plane in a high-strength steel sheet to improve delayed fracture
resista~~c(see e JP-ANo. 2012-237041 (liereinaftel; 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 atid
the method disclosed in Non-Patent Literature 2. According to tlie method disclosed in
Patent Literature 1, since the method can be used for only a specific material and is used to
perform shearing 011 a quenched material, the problem such as deterioration in service life of
tlie tool is not solved.
[0006] According to tlie methods disclosed in Patent Literatures 2 to 4, the residual stress in
tl~css hcared portion caused by deforniation resistance of tlie steel sheet can be reduced, but it
is not possible to reduce thermal stress caused by seizure of tlie 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, wlieti ductility of tlie hot-sheared portion is low, the pt'obleti~s uch
as occurrence of tlie delayed fracture is not solved. A method of i~iiprovingtl ie ductility of
tlie hot-sheared portion is not disclosed in Patent Literatures 2 to 4.
[0007] According to tlie method disclosed in Patent Literature 5, it is considered that
ductility can be improved because tlie sheared portion of tlie steel sheet is not hardened, but a
shearing time becomes longer and thus costs increase as tlie coolihg 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.
[OOOS] Accorditig 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 frotii 400°C to
900°C is defined regardless of a tnaterial of a member to be sheared or a cooling rate. For
this reason, the shearing tilay occur at a temperature range (low-temperature side), at \vliich
the delayed fracture occurs, depe~idingo n the materials of the member to be sheared or
shearing conditiotis. Conversely, when the shearing is perfor~iled at a high temperature more
than necessary such that the delayed fracture does not occur, tlie amount of thermal expansion
becomes larger and a dimensional change becotiies larger at tlie time of returning to an
ambient temperature. As a result, tlie dimensional error of tlie workpiece becotiies greater.
Therefore, in a case in which tlie shearing temperature is precisely controlled at the lower
2
temperature according to actual sl~earingc onditions, there still retilains a possibility of
suppressing the delayed fracture while further improving shearing accuracy of the work piece:^
[0009] Patent Literature 6 discloses that the delayed fi-acture 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 AS indicated in Table 5 obtained by steel sheet
cotnponetit 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 satlie 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 sllearing 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, wl~ichm eets these requirements, for the purpose of achieving of
these tasks.
Solution to Problern
[OOll] 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 at1 appropriate range based on the amount of
equivalent plastic strain of a surface layer of a sheared portion, delayed fiacture 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, cornpositions 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 it1 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 405 1 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,
3
and S45C defined in JIS G 4051 when au actually ~ileasuredA r3 point is approximately
500°C or lower at the time of coolit~gb y leaving.
[0014] The it~ventioht~as bee11 made based on the above findings and the gist thereof is as
follows.
[0015] A first aspect of the i~~ver~itsi oto~ pl rovide a surface layer grain refining hot-sl~earing
method i~~cludinhge:a ting and keeping a steel sheet having a carbon content of 0.15% or
tnore by mass in a temperature range of fi-on1 Ac3 to 1400°C to austenitize the steel sheet;
subseque~itly shearing the steel sheet in a state in whic11 the steel slieet 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 (OC) obtained by adding a ten~peratureo f from 30°C to
140°C to a previously measured Ar3 of the steel sheet.
[0016] Asecond aspect of the invet~tioi~si t o provide a surface layer grain refining
hot-shearit~gm ethod includit~gh: eating and keeping a steel sheet havcng a carbon content of
0.15% or tnore by mass in a temperature range of from Ac3 to 1400°C to austenitize the steel
sheet; subsequently shearing the steel sheet in a state it1 which the steel sheet is placed on a
die; and quenchii~gb y rapidly cooling the sheared steel sheet, wherein a start temperature of
the shearing is set to be a temperature (OC) obtained by adding a value, which is calculated by
t~iultiplyi~an~ gam outlt of equivalent plastic strain of a surface layer in a sheared portion by a
coeficie~f~rot m 40 to 60, to a previously measured Ar3 of the steel sheet.
[0017] Athird 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 at1 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 ~~ornidailre ction of the shear plane and in a range of from 20% to 50% of the
thickness of the steel sheet in a thickness directiot~o f the steel slieet from a bottom on a burr
side of the sheared portion.
[0018] A fourth aspect of the i~~vet~tiiso tno provide the surface layer grain refining
hot-shearing method accordit~gto the secot~do r third aspect of the i~ivet~tiown,h erein the
amount of equivalent plastic strain of the surface layer in the sheared polti011 is calculated by
a numerical sitnulation that is perfort~ied based or1 a stress-strain diagram at a steel sheet
temperature of from 500°C to 800°C.
[0019] A fifth aspect of the inve~~tiios tt~o provide the surface layer grain refirling
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
poi-tioil is calculated based on a Mises yield function represented by the followi~~Fgor mula
(1).
[0020]
[0021] A sixth aspect of the invention is to provide the surface layer grain refining
hot-shearing method accordittg to the first or secot~da spect of the invention, wherein the
shearing of the steel sheet starts within three seconds after the steel sheet comes in contact
wit21 the die.
[0022] A seventh aspect of the invetltion is to provide the surface layer grain refilling
hot-shearir~gm ethod 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.
[0023] An eighth aspect of the invention is to provide the surface layer grain refining
hot-shearing method according to the first or secotld aspect of the invention, wherein the rapid
cooli~igis perfonlied when water jetting fi.0111 a puttcture formed in a contacting portion of the
steel sheet with the die passes through a groove provided in the contacting portiot~o f the steel
sheet
[0024] 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 perfornied between the heating and
the shearing of the steel sheet.
[0025] A tenth aspect of the invention is to provide a workpiece obtained by surface layer
grain refilling hot-shearing, including: a steel sheet having a carbon content of 0.15% or rliore
by mass, a surface layer of a sheared portion of the steel sheet having a carbott content of
0.15% or more by mass i~lcludinga ferrite phase and a remainder, the surface layer being
defined as a range up to 100 pln inside of the steel sheet itt a normal directiot~o f a shear plane
from a fracture plane of the sheared portion; wherein the re~i~aindienrc ludes at least one phase
of a bainite phase, a martensite phase, or a residual austenite pl~asew hich have a crystal grain
diati~etero f 3 pnl or less, and includes cementite and inevitably generated iticlusions; wherein
the ferrite phase has an average grain size of 3 pm or less; wherein the surface layer contains
5% or more grains by ~lutllberh aving an aspect ratio of 3 or more; aud wherein a region out of
the range of 100 ptn includes: martensite and ir~evitablyg enerated inclusions; or bainite,
inartensite, and inevitably generated inclusions.
100261 Ateleventh 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, tlie cenientite lias a number density of 0.8 pie~es/~~otri ?le ss and tlie celnentite
lias a maximuni length of 3 prn or less.
[0027] 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 pliase, and the residual austenite
phase, which are nieasured by an electron-beam backscattering diffraction (EBSD) tnethod, is
from 10% to 50% in the surface layer.
[0028] A tliirteetdh aspect of tlie 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 tnore by mass in a temperature range of froni Ac3
to 1400°C to austenitize the steel slieet; subsequently shearing the steel sheet in a state in
which the steel slieet is placed on a die; and quetiching by rapidly cooling the sheared steel
slieet, wherein a start temperature of the shearing is set to be a temperature PC) obtained by
adding a temperature of from 30°C to 140°C to a previously measured Ar3 of the steel sheet.
[0029] Afourteentli aspect of the invention is to provide a workpiece obtained by surface
layer grain refining hot-shearing, the workpiece produced by: heating atid keeping a steel
sheet having a carbon content of 0.15% or more by Inass in a tetnperature range of from Ac3
to 1400°C to austenitize the steel sheet; subsequently shearing the steel sheet in a state
wherein the steel sheet is placed on a die; and quencl~ingb y rapidly cooling the sheared steel
slieet, wlierein a start teniperature of the shearing is set to be a temperature (oC) obtained by
adding a value, which is calculated by tilultiplying an amount of equivalent plastic strain of a
surface layer in a slieared portion by a coefficient from 40 to 60, to a previously rileasured Ar3
of the steel sheet.
Advantageous Effects of Invention
[0030] According to a surface layer grain refining hot-shearing method and a workpiece
obtained by surface layer grain refining hot-shearing of tlie invention, it is possible to
suppress delayed fracture in a sheared portion and to provide a workpiece having excellent
dimension accuracy without increasing tlie shearing time and new steps.
BREF DESCRIPTION OF DRAWINGS
[0031] Fig. 1Ais a schematic diagram illustrating an example of putlching-shearing by a
pui~clai nd a die.
- Fig. 1B is a schematic diagram illustrating an example of tritnming-sheari~igb y a
punch and a die.
Fig. 2 is a diagram illustratit~ga n exatiiple of a sheared poltion of a steel sheet.
Fig. 3 is a diagram illustrating a relation between a telnperature liisto~ya nd an Ar3
point.
Fig. 4A is a diagram illustratitlg a state of a hot-shearing apparatus used in Test A
before shearing.
Fig. 48 is a diagram illustrating a state of the hot-shearing apparatus used in Test A
during shearing
Fig. 4C is a diagram illustrating a state of tlie hot-shearing apparatus used in Test A
after shearing.
Fig. 5 is a diagram illustratit~gin clusions (a trat~smissione lectron microscope itnage
observed by a replica method), which are observed by a replica method using a tratismission
electron n~icroscopein Comparative Exatnple, it1 a surface layer of a sheared portion
Fig. 6A is a diagram illustratit~ga region in which equivalent plastic strain is
averaged.
Fig. 6B is a diagram illustrating a region in which a fine structure in an actually
hot-sheared portion is fortned.
Fig. 7 is an example of metal structure (EBSD image) obtained by Example 1.
Fig. 8 is an example of il~clusions(a transmission electron microscope image
observed by a replica method) of a metal structure obtained by Exatnple 1.
Fig. 9Ais a diagram illustrating a bending state of a Iiot-shearing apparatus used in
Test B.
Fig. 9B is a diagratn illustrating a shearing state of a hot-shearing apparatus used in
Test B.
DESCRIPTION OF EMBODIMENTS
[0032] [First Embodiment]
[0033] 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 it~ventiotwl ill
be described below.
[0034] First, get~erals hearing will be described and a sheared pottion of the sheared
workpiece which is subjected to the shearing will be then described.
[0035] As illustrated in Figs. 1A and lB, punching-sl~earitigo r trimming-shearing is
7
performed on a steel sheet 1 placed on a die 3 by lowering of a punch 2. At this tinte, 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 fortiled in sucli 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 fornled in sucli a nlaliller 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.
[0036] In the following description of the etnbodiment, the same components are also
denoted by the satne reference nunlerals and the detailed description thereof will be not
presented.
[0037] In this enlbodinle~~at ,te rm of "surface layer of the sheared portion" is used, and this
refers to a region fiom the surface of the sheared pokion up to 100 pm in a normal direction
of the shear plane.
[0038] 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 filldings 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.
[0039] In the hot shearing according totl~iesm bodiment, a steel sheet of high-carbon region
of 0.15% or more by mass is used. Atransfortnation start temperature (Ae3 point) in a state
diagranl from austenite to ferrite ofthe steel sheet is fiom 800°C to 90OoC. Aportiotl, which
is subjected to large plastic defonnation in the austenite state, is transfortned 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, aln~oset ntirely of the surface layer of the sheared pottion
having large plastic deformation is transforrlled into ferrite and other portions, which are not
plastically deformed, are tratlsforrned into martensite. However, when the shearing
temperature is high, dimension accuracy becon~esp oor due to thermal strain. In addition,
there was a problem that variation in occurrence of delayed fracture results fro111 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.
[0040] Then, the inventors have experimented to perform the shearing on the steel sheet
which is subjected to a soaking treatment followed by changing a tetnperature for sta~tingth e
sheari~lg( shearit~gs tart 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
8
spaced apart by 3 to 5 nnn from a shearing position of the steel sheet to measure the
temperature at the start of shearing. Since the steel sheet is heat-released atid thus lowered in
the temperature when conling in contact with a die, the shearit~go f the steel sheet started
within three seconds after the steel sheet comes in contact with the die.
[0041] In this enibodiment, 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 conies it1 contact
with the die" refers to the titlie after the steel sheet 1 comes in co~~tawcitt h either of the die 3
or the pad 9.
[0042] 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 poltion
(fracture plane) or the dimensiot~a ccuracy of the workpiece.
[0043] 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.
[0044] In general, the fine ferrite stlucture has tought~essh igher than the martensite structure.
Therefore, when the fine ferrite structure having high tought~essis present in the surface layer
of the sheared portion, the delayed fracture is suppressed.
[0045] The shearing start tetnperature having an appropriate temperature range was obtained
by considering tenlperature changes in the hot shearing and further calculating the size of
shearing strain.
[0046] The steel sheet was first heated t'o 950°C and after keeping it for 90 seconds and then
cooling it in a state being placed on four pointed needles (hereinafter, sonletitnes referred to as
a "pin support"), the transfor~nationte mperature of the steel sheet was measured. The
tetnperature was measured by the thermocouple embedded in the steel sheet.
[0047] 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.
[0048] The measured Ar3 point was significantly different in the range of from 200 to'300'~
fiotn a tratisforn~ationt enlperature (Ae3 point) at which austenite was changed to ferrite as
illustrated in the state diagratn. Furthel; 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°C compared to the Ae3 point, that is, was as low as
9
about 100°C compared to the case of the pin support.
[0049] The fact that the Ar3 point is lower than the Ae3 point is cotnlnoti technical
knowledge in the field of n~etallicm aterials. However, a quantitative difference between the
Ar3 point and theAe3 point is not clear. By testing of the inventors, it was clear that the
significant difference between the Ar3 point and tlie Ae3 point is present in the hot sheari~iga s
described above.
[0050] For reference, results of ~i~easure~noefn tth e Ar3 point by the above measuring
method (pin support) are illustrated in Fig. 3. The steel sheet to be mainly used had a sheet
tliicktiess of 1.5 mm. The range of the thickness of tlie steel sheet to be used in tlie shearing
is of about from 0.5 mtn 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 measuremeIlt of the Ar3 point.
Accordingly, the quenching process is not included in tlie graph of Fig. 3.
[0051] InFig. 3, initially, the cooling rate was 7OC/s, and the cooling rate has sharply
declined when tlie time has elapsed for 50 seconds from a cooling start. A temperature
(about 680°C) of the steel sheet at which the cooling rate of the steel sheet is equal to or less
than I0C/s is identified as the transforniation temperature (Ar3 point). At the time of the
measurement of tlie Ar3 point, tlie steel sheet is cooled to room temperature as it is, but, in
actual fact, the shearing stalts at a temperature higher tliat~th e Ar3 poitit and the quenching
process is then performed.
[0052] In this embodiment, an Ar3 temperature measured using the same nietliod as in the
case of the above pin support under placing conditions of a sheet to be actually sheared is
defined as tlie "measured Ar3 (of the steel sheet)". The cooling rate is generally about from
5"C/s to 30°C/s (state of cooling by leaving) at tlie time of tlie measurement in many cases.
[0053] As long as appropriate hot-shearing conditions are ascertained by perforniing the
above experiment as a preliminary test, wlieti perforniing appropriate soaking temperature
management of the steel sheet and time managemeut 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
perfor111 the operation after preparing tlie die in which the thennocouple is embedded and
~ileasuringa surface tetiiperature of the steel sheet to be sheared at tlie titile of the shearing
start for every shearing. In the case of performing the operation by measuring the surface
temperature of tlie steel sheet in the mass productioi~p rocess, the surface temperature of tlie
steel sheet may be measured immediately before tlie hot-shearing using a radiation
[0054] 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 plasticstrain in the
vicinity of the sheared portion by numerical calculation. Here, the plastic strain was
evaluated as equivalent plastic strain.
[0055] From the fact that the actual shearing is perfonned at a range higher than the
measured Ar3 tetnperature, as a premise of the calculation, the numerical value of mechanical
characteristics such as ?eforniation resistance of the steel sheet was defined as a value of
austenite. 111 addition, the temperature dependence of the mechanical characteristics of
austetiite 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 22Mt~B5
equivalent steel which is widely used for hot stamping. Such a temperature dependence is
disclosed in, for example, "Ho~lgshe~Llgiu , Jut1 Bao, Zhongwen Xing, Dejin Zl~angB, aoyu
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, sometitnes referred to as "Non-Patent Literature
3"), and practitioners may use values disclosed in this Literature without actually measuring
the values.
[0056] The plastic strain obtained by the numerical calculatiot~is largest at the sulface of the
shearing surface, and becomes smaller ~novitlga way from the surface. Furthermore, it was
found that an occurrence region of the equivalent plastic strain of 100% or tnore at the
sheared portion coirlcides with an actual occurrence region of the fine structure in a
predetermined tetnperature range.
[0057] With respect to the values obtained by the ~tlumericalc alculation, it is concer~~ethda t
variation is caused by analysts. Therefore, the itlvetltors perfor~nedt he numerical
calculatiot~u sing steel grades, analyst, and software in plural ways. As a result of the
~lumericacl alculation, 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 nonnal direction of the shear plane is a tetnperature range higher by
approxitnately 30 to 140°C than the measured Ar3.
[0058] Here, at a temperature range higher than a tetnperature obtained by adding 140°C to
the measured Ar3 (hereinafter, so~netitnerse ferred to as "higher than Ar3 + 140°C"), the
occurrence region of the equivalent plastic strain of about 100% in the tlortnal direction of the
shear plane on the sheared portion which is obtained by calculation becornes larger that1 the
11
actual fine region on tlte sheared portion of the workpiece. As a result of analysis of the fine
structure regioqtlie region was mainly configured by ferrite and carbide. On the other I~and,
other regions except the surface layer are configured by a martensite structure.
[0059] The ferrite and the ~nartensiteh ave a different volume, respectively, from the
difference of a crystal structure and a solid-solution state of element. Tl~ereforew, hen the
fine structure region is widely formed on the surface layer of the sheared portion and nlost of
the fine structure is configured by ferrite, the boundary area between the fine ferrite and the
fine mattensite increases. As a result, the dimension accuracy of the workpiece deteriorates.
111 consideration of the thermal strain, the dimension accuracy of the workpiece deteriorates as
the shearing start temperature beco~nesh igher.
[0060] Furtllermore, when the shearitig start temperature is lower than a temperature
obtained by adding 30°C to tlie measured Ar3 (hereinafter, sonletimes referred to as "lower
than Ar3 + 30°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 tnore becomes smaller, tlie actual fine structure region smaller than
such a region becomes further snialler. At the temperature lower than "Ar3 + 30°C" which
is measured, a part of austenite starts to transfonn into ferrite by the influence of inter~iahl eat
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.
[0061] On the other hand, when the shearing start teniperature is higher than "Ar3 + 30°C",
the steel sheet is subjected to the shearing before austenite starts to transfortn into ferrite, so
excessive residual stress on the sheared portion due to ferrite is avoided.
[0062] Based on the above findings, the surface layer grain refining hot-shearing method
according to this embodiment was configured as follows.
[0063] First, a shearing nlachitle used in the test will be briefly described. As illustrated in
Fig. 4A, a shearing machine 10 includes tlie die 3 on which the steel sheet 1 is placed, a pad
12 that is disposed OII the die 3 to press the steel sheet 1 placed 011 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
predeternlined range of the steel sheet 1.
[0064] First, the steel sheet 1 having the catboll 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°C higher than the shearing
start teniperature in the range of from A1-3 + 30°C to Ar3 + 140°C and being subjected to a
soaking treatment (see Fig. 4A).
[0065] Then, as illustrated in Fig. 4B, after the steel slieet 1 on the die 3 is pressed by tlie
pad 12, the steel slieet 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 slieet 1 starts within three seconds. By control
of the time (shearing statt time) until the shearing statts after the steel slieet 1 is placed on the
die 3, the temperature of the steel sheet 1 during the shearing is controlled in tlie range of
from Ar3 + 30°C to Ar3 + 140°C.
[0066] 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 tlie pad 12,
and thus a shearing-workpiece is formed.
[0067] Operatiotlof the surfacelayer 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.
100681 hi the sheared portion 8 of tlie workpiece (steel sheet) formed in this manner, the
surface layer of the sheared portion 8 defined as the range up to 100 pm inside of tlie steel
sheet in a normal direction of the shear plane 5 includes a ferrite phase forming at least a
portion of tlie 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 pni or less, respectively. The surface layer of the sheared portion 8 cotltains
5% or more grains by number having an aspect ratio of 3 or more. 111 addition, other regions
except tlie 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.
[0069] That is, since the workpiece is formed by the shearing of the steel sheet 1 heated to
the temperature of fiomAr3 point + 30°C to Ar3 point + 14OoC, a fine ferrite structure, a fine
maltensite structure, a fine bainite structure, and a fine residual austenite sttucture are formed
in the surface layer of the sheared portion 8 (fracture plane 6) (see Fig. 2). Fig. 6B illustrates
tlie steel sheet 1 which has actually been subjected to the shearing. As illustrated in Fig. 6B,
a fine structure 11 is formed from tlie fracture plane 6 toward tlie shear plane 5 in the sheared
portion 8 in the surface layer, but the fine stlucture is fortiled particularly up to a depth of
about 100 ptn fro111 tlie surface in the fracture plane 6.
[0070] The fine ferrite structure has generally higher toughness than the tiiartensite structure.
Accordingly, sitice the fine ferrite structure of tlie high toughness is present in the surface
layer of the sheared portion 8 (fracture plane 6), occurrence of the delayed fracture in the
sheared portio118 (fracture plane 6) due to tlie delayed fracture is suppressed.
[0071] As will be described below, in the workpiece according to this embodiment, the
occurrence of the delayed fracture in the sheared potti011 8 (fracture plane 6) can be
suppressed by tlie fitie tnattensite structure, the fine bainite structure, and tlie fine residual
austenite structure which are formed in the surface layer of the sheared pottion 8 (fracture
plane 6).
[0072] For reference, Fig. 7 illustrates a structure photograph of tlie surface layer of tlie
sheared portion obtained by an EBSD of this embodiment.
[0073] hl Fig. 7, a black pact indicates a bainite pliase, a martensite phase, or a residual
austenite phase. As in tlie photograph, although crystal grains having tlie aspect ratio of 3 or
tiiore are present, tlie delayed fracture does not occur for reasons which will be described
below.
[0074] Tlie "grain size" used herein nieatis a circle dialnetel; that is, a circle conversion
diameter (circle equivalent diameter) when an area of each ferrite c~ystagl rain, which is
observed it1 a cross section along tlie thickt~essd irection of tlie steel sheet it1 tlie normal
direction of the shear plane, is replaced by a circle of the same area.
[0075] Tlie bai~iitep hase, tlie martensite phase, or the residual austetiite pliase rather tllau
the single pliase of tlie fitie ferrite pliase is present in tlie surface layer of the sheared portion 8
Generally, tlie baitiite phase, tlie martensite phase, or tlie residual austenite pliase present it1
the ferrite phase traps diffusible hydrogen that causes the delayed fracture. Therefore, when
these phases are present in tlie fine ferrite pliase, it is possible to obtain an effect of
suppressing tlie delayed fracture.
[0076] I11 addition, when the bainite phase, the martensite phase, or the residual austetiite
pliase becomes finer to be 3 ptn or less, sites for trapping the diffusible liydrogen further
increase, and thus the delayed fracture is filrther suppressed.
[0077] On the other hand, tlie cementite has a stiiall effect oftrapping tlie diffisible
hydrogen atid car1 be a start poitit of the occurrence of the delayed fracture, so it is preferable
that the ceriietitite beconies smaller.
[0078] It1 order for tlie remainder to have the fine bainite phase, niattensite phase, and/or
residual austenite phase having tlie grain size of 3 pm or less, ferrite having an aspect ratio of
more than 3 inevitably appeared. As a result of atialysis using a tratistiiission electron
microscope, tlie ferrite having tlie aspect ratio of more than 3 is in a state where plastic
defortiiation little occurs or is small, but is not in a state of being plastically defornied and
14
stretched as described in Patent Literature 6, so the ferrite did not adversely affect resistance
to the delayed fracture. While the details offhe operation is not clear, in order for the
remainder to have tlie bainite phase, the martensite phase, or the residual austenite phase
described above, the ferrite structure having tlie aspect ratio of more than 3 is essentially
present.
[0079] In order to also titake these structures, it is necessary to adjust tlte shearing
temperature to a tetilperature range of from Ar3 + 30°C to Ar3 + 140°C. It is considered that
since the steel sheet is cooled at a certain cooling rate, the austenite structure remains at tlie
shearing temperature, but the appropriate atuount of shearing strain is added and
tt-a~isforrnatio~n iuclei'totr ansform into other phases other than the niartensite is already
generated. In this case, the cooling rate contributes to any phase transforniation.
[0080] The cooli~tgra te is fast when the temperature exceeds Ar3 + 140°C, and the austenite
becomes a supercooled state during coolitlg (temperature is lower than a temperature range at
which structure ntorphology can be present) when the shearing strait1 is applied to the extent
in which transformation to martensite cannot occur 111 such a case, austenite is easily
transformed into a fine ferrite structure.
[0081] On the other hand, when the temperature is equal to or lower than Ar3 + 140°C,
grains are for111ed in which transfortnatioti to ferrite does not occur and transforn~ationto
maltensite also does not occur under the influence of sltearing 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-utliform
tluee phases partially induces e~trichnlenot f carbon to austenite, and such austenite becomes
residual austenite in order to be stable even at room temperature. Since these phases occur
betweeti the fine ferrite grains, the phases theinselves also becotlle finer to be 3 ptn or less.
[0082] In order to stably fortn these stnictures, the shearing of tlie steel sheet preferably
starts within three secotids after the steel sheet comes in contact with the die. When tlte
shearing starts after three seconds, scale occurs on the surface of the steel sheet and tlie
contact of the die with tlie steel sheet becomes not]-unifortn. When heat irregularity occurs
due to the non-unifor~nc ontact, variation in cooling cottdition of the sheared portion is
caused.
[0083] In addition, Fig. 5 illustrates cementite distribution it1 the surface layer of the fracture
plane when the steel sheet disclosed in Pateitt Literature 6 is subjected to the shearing at a
temperature higher than Ar3 point + 140°C. In Patent Literature 6, since the sliearittg start
tetnperature is simply set to only a temperature range of from 400°C to 900°C, the shearing
start temperature also includes the case of being higher than Ar3 + 140°C. In tliis case, for
example, as illustrated in Fig. 5, cementite C (black patts excludi~lgc ircles) has a number
density of 0.8 pie~es/~no?r more and the maxin~umle ngth of 3 pm or more.
[0084] On the other hand, in the case of this embodiment, cementite (black parts excluding
circles) in tlie surface layer of the fiacture plane of the steel sheet has a iiumber density of 0.8
p i e c e ~ / ~otr~ lnes~s and the maxi~numle ngth of 3 pm or less as indicated in test results (Fig. 8)
to be described below. According to tlie experience of tlie itiventors, when the number of
cementite is small to this extent and the size of cementite is also small, the cementite itself
does not allnost cause a problem of being a start point of the occurrence of the delayed
fracture.
[0085] 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 pm
inside of the steel sheet in tlie nornlal 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 fro111 10% to 50%.
[0086] As for this, according to tlie experience of the inventors, when the total area ratio of
these phases is less than lo%, it is not possible to sufficiently perfor111 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 SO%, the ratio of the fine ferrite in tlie surface layer
of the fracture plane reduces, whereby the effect of tougl~nessim provetnent due to the fine
ferrite decreases and the risk of the delayed fracture increases. Although the effect of the
invention does not itinnediately disappear when the total area ratio of these phases is out of
such a range, the total area ratio of these phases is Inore preferably within such a range.
[0087] Amethod 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
embodiniet~ta nd, for example, tlie steel sheet 1 may be rapidly cooled by allowing the steel
sheet 1 to conie in directly contact with water Examples of tlie method of allowing the steel
sheet 1 to come in contact with water may include a method of passing cooling water tl~oug11
a groove formed in a contacting portion of the steel sheet with the die.
[0088] Even in the case of performing tlie shearing after press forming, as in tlie workpiece
of tliis embodiment, it is possible to suppress the delayed fiacture of the sheared portion to
form a workpiece wit11 dimension accuracy.
[0089] [Second Embodiment]
[0090] A surface layer grain refining hot-shearing metl~od according to a second
e~nbodimenot f the i~iventionw ill be described. The same conipotlents as in the first
16
emboditneut 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
etl~bodimenti s the same as in the first embodiment, so operational effects thereof will not be
described.
[0091] The inventors found that the temperature range at which the occurrence region of
about 100Yo equivalent plastic strail? in the normal direction of the shear plane in the sheared
portion coincides with the occun-rence region (distance) of the fine ferrite structure, the fine
martensite structure, the fine bainite sttucture, or the fine residual austenite structure in the
normal direction of the shear plane is obtained when a tenlperature ("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 tneasured Ar3 is set as a
shearing start temperature.
[0092] I11 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.
[0093] As illustrated in Fig. 6 4 an average value of the amounts of plastic strain obtained
by calculation at a regionA(within a thick line fratne) in the range of fiom 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 nornial direction of the shear plane 5 and in the range of fiom 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 it1 the sheared portion.
[0094] By setting the region A in this way, the inventors found that the anloutit 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
[0095] 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°C to 800°C. It
was confirmed that the amount of equivalent plastic strain of the surface layer becomes
approximately constant at this range.
[0096] the reason that a lower limit of 40 is set for the coefficient to be n~ultipliedb y the
atllount of equivalent plastic strain is due to consideration of differences in the coefficient due
to a steel grade and errors in numerical calculatiot~. By repetitive experiment and nun~erical
calculation, the fine ferrite structure, the fine martensite sttucture, the fine bainite structure, or
17
the fine residual austenite stlucture appeared even in the case of being out of this coefficient
range, but the iilventors obtained 40 as the tower limit of the coefficient in which appearance
probability becomes l~igl~er.
[0097] In addition, the reason why the upper limit of the coefficient to be n~ultipliedb y 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 lugh. This reason is considered that tlie
region of the fine structure in tlie surface layer becomes wider as the temperature becotlies
higher, but the di~ne~~saicocnu racy deteriorates after coolit~gb ecause a difference in density
between the surface layer and another region adjacent to the surface layer is large and the
tliernial strain also increases.
[0098] In a case in which tlie difference between a workpiece diniension and a design
dimension of the workpiece generally falls within the range of -0% +5% of the design
dimet~siont,h e 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 litnit was determined.
[0099] The tneasured Ar3 point of the steel sheet sl~ouldb e previously measured by a
temperature drop history at the thern~ocoupleo r the like in a state in which the steel sheet is
placed on the die to be actually used. The tliertnocouple is embedded in the die, and it is
preferable to cause a thet-mocouple sensor to come in directly contact with the steel sheet
which is a tnetnber 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.
[0100] 111 this embodiment, it is itnpo~tatt~ot calculate the equivalent plastic strain of the
sheared portion. In the hot-shearing, the metal-sttucture transfortnation inevitably occurs
during or in~~nediatealyft er the shearing, and thus it is not possible to measure the equivalent
plastic strain. Therefore, a shearing sitnulation is performed by analysis usit~ga finite
eletnent method (FEM), and thus the equivalent plastic strain is calculated.
[0101] In tlie shearing simulation, the plastic strain is steeply clianged. For this reason,
calculatio~ri esults of the plastic strain of the surface layer in the sheared poltion 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.
[0102] The inventors have set the region as a result of trial and error. Fig. 6A illustrates the
region in which tlie equivalent plastic strain is averaged. As illustrated in Fig. 6A, the region
1s
A(withi11 the thick line frame), in wliicl~tl ie 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 tl~esh eared portion 8 to the inside of the steel sheet 1 in the nortnal direction of the
shear plane 5 and in the range of fiom 20% to 50% of the thickness H of tlie steel sheet 1 in
the thickness direction of the steel sheet 1 from the bottoni 12 on the burr 7 side of the
sheared portion.
[0103] During the siniulation, since the temperature change sequentially occurs, it is
necessary to perform repetitive calculation in such a tnatiner 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.
[0104] As a result of the calculation with several levels by the inventors, it was found that
approxin~ationc an be perfortlied when a numerical siniulation is once perforn~edb ased on
stress-strain diagram at any of the steel sheet temperature of from 500°C to 80OoC.
[0105] As a premise of the calculation, when the shearing is performed at the range higher
than the measured Ar3 temperature, nu~neric~vall ues of mechanical characteristics sucl~a s
rigidity of the steel sheet at that time were defined as values of austenite.
[0106] 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.
[0107] An increment in equivalent plastic strain "de-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 incren~ent.
[O 1081
[0109] As described above, in the shearing tuetllod 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 ten~peratureto be within the predetermined range.
[0110] In particular, since the predetermined range region A is set to calculate the atnount of
equivalent plastic strain in the sheared portion, it is possible to calculate the amount of
equivalent plastic strain having a small error.
[0111] During tlie FEM siniulation for calculatilig the equivalent plastic strain, since the
teniperature change sequentially occurs, it was necessary to perfon11 repetitive calculation in
such a tnanner that: the equivaletit plastic strain was calculated based on the tentative shearitig
start temperature; and the true shearing start temperature was determined based on the
calculated equivalent plastic strain. In this etnbodinletlt, however, since the approximation
can be performed whet1 a nu~nericals imulation is only once performed based on stress-strain
diagram at any of tlie steel sheet teniperature of from 500°C to 800°C, the calculatio~ils
simplified.
[0112] Sitice the equivalent plastic strain is calculated by the Mises yield htiction on the
supposition of an isotropy, the calculation is further simplified.
[0113] The niethod of calculating the amount of equivalent plastic strain disclosed in the
surface layer grain refining hot-shearing method according to the second etnboditnet~its
applicable to the calculatiorl of the amount of equivalent plastic strain in the surface layer
grain refining hot-shearing method according to the first etnbodimetlt.
[Examples]
[0114] Next, Examples of the invention will be described. However, shearing conditions in
Exanlples are exalnples adopted to confirm feasibility and an effect of the itivention and tlie
invention is not limited to these shearing conditions. The invention call 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 itlvention.
[0115] (Test A)
[0116] Using tlie shearing machine 10 illustrated in Figs. 4A to 4C, after the high-strength
steel sheet 1 (200 t1lm x 150 mni) of steel grades A to C having cotnpositions indicated in
Table 1 is placed on the die 3, the punch 2 together with tlie pad 12 approach tlie 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 nin~)a t tlie same time. The sheared
steel sheet 1 is rapidly cooled by tlie die (die 3 and pad 12). Shearing conditions are as
indicated in Table 2. A clearance between the punch 2 and tlie die 3 was set to be 0.15 tnm.
[0117] Except for Cor~~parativEex amples, the keeping tinie 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 teniperatures obtained within tlie
range of tlie keeping time.
[0118] The thickness of the steel sheets used in Examples was set to be 1.5 mm. The
20
thickness of the steel sheet applicable to the invention has tlie range of from about 0.5 tnni to
3 mm. --
[0119] The measured Ar3 point of each steel sheet was obtained by the measurement of the
tetnperature histoly at the time when the steel sheet heated to 950°C is cooled in contact with
tlie top of tlie die on the sliearing machine (a teniperature at which the cooling rate of tlie steel
sheet was I0C/sec. or less before tlie temperature of the steel sleet was lowered to the room
temperature was regarded as the Ar3 point).
[0120] For estitnation of the equivalent plastic strain, shearing simulation, in which
deformation resistance was input when the steel sheet is 750°C, was perfornled by a fiuite
element tnethod using AbaqusIStandard made by Dassault Systemes Co, which is comn~ercial
software. h~ this case, the Mises yield fi~nctionw as used, and the atialysis region in the
vicinity of a tool cutting edge was defined as a quadrilateral complete integration element of
0.02 nim x 0.04 mm. In addition, remeshing was perfornled every 0.05 nlm punching press.
The fiacture was defined by a ductile fracture model of Hancock & Mackeluie, and the
rigidity of elenients satisfying conditions was zero. Paratiieters of the ductile fracture model
were fitted based on a shear plane ratio which was actually observed in certain conditiotis.
The equivalent plastic strain was used which was averaged in the region Aset in tlie range of
10% of tlie thickness H of the steel sheet 1 from tlie shear plane 5 of tlie sheared portion 8 in
the tiormal 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).
[0121] A letigth of a scrap 16 (see Fig. 4C) punched out after tlie shearing was evaluated as
the dimension accuracy. Unless a ditilensiot~ale rror occurs, tlie length of tlie scrap 16 after
the shearing should be 65 tnni. Thus, values are obtained in such a manner that tlie error in
length of tlie scrap 16 after the sliearing is divided by 65 and is then converted into percentage
(x 100) are disclosed as the diniet~sionael rror in Table 2.
[0122] [Table 11
(% by mass)
[0123] [Table 21
Steel grade
A
-B -
C
C
0.22
0.16
-0.25
Si
0.22
0.40
0.21
Mn
1.20
1.00
1.24
B
0.002
0,001
0.002
Cr
0.16
0.23
0.34
Comparative
Example 4
Comparative
Example 5
620
570
950
950
1.5
1.5
Water
Water
2.5
1.5
80
100
820
720
Absence
Absence
6.3
5.1
[0124] The test was performed thee titnes for each Exa~ilplesa nd Comparative Exatiiples.
With respect to the presence or absence of the delayed fracture, it was evaluated that tlie
delayed ftacture was present when delayed fracture occurs even once. In addition, the
ditiiensional error was an average value of three ~ileasuredv alues.
[0125] I11 Exanlples 1 to 6, it can be understood that the occurrence of the delayed fiacture
in the sheared portion (fracture plane) is suppressed and the diniensioti accuracy ofthe
workpiece is improved.
[0126] A~ilicrostructurein the range of 100 ptn from tlie fracture plane of tlie sheared
portion in Exanlple 1 will be described with reference to Fig. 7 (EBSD, n~icrostructureim age)
and Fig. 8 (image of an extraction replica sample obsetved by tlie transnlission electron
microscope).
[0127] As illustrated in Fig. 7, it was cotifir~nedth at the niicrostructure includes ferrite,
bainite, martensite, residual austenite, cementite, and inclusions derived fiom alloy elements
other than iron as a result tlie EBSD analysis, EDS (characteristic energy dispersion type
X-ray analysis), and electron diffraction analysis of the trans~nissione lectron mnicroscope.'
[0128] Specifically, Fig. 7 illustrates the nlicrost~uctureit nage observed by the EBSD in a
state where a cross-section sample of Example 1 along the thick~iessd irection of the steel
sheet in the nornial direction of the shear plane in tlie sheared portion is embedded in a hard
resin and is then subjected to polishing and electropolishing. In addition, Fig. 8 illustrates
tlie image observed by the transnlission electron microscope of the sample of Exaniple 1
which is prepared by an extraction replica method using an SPEED method (Potentiostatic
Etching by Electrolytic Dissolution: potentiostatic electrolysis method in nonaqueous
solvetit).
[0129] As illustrated in Fig. 7 (EBSD microstructure image), in the surface layer of tlie
fracture plane in tlie range of 100 pm in tlie normal direction of tlie sliear platie from tlie
ftacture plane, the grain size of ferrite (parts excluding black in Fig. 7) F was as very stiiall as
3 pm or less and the grain size of BMA (black part in Fig. 7) including martensite, bainite, or
residual austenite was also 3 pm 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.
[0130] The same rnicrost~ucturew as obsetved in any of Examples 2 to 6. During tlie
identification of tlie microstructure, five points of field-of-view of 8.0 x 20 pm were
ratidotnly pliotographed for each Example, in the range of 100 pm fro111 the surface of the
fracture plane.
[0131] Furthermore, as illustrated in Fig. 8, it can be seen that tlie ratio of ce~iientite(b lack
parts excluding circles) C in Example 1 is very small. In Example 1, tlie number density of
tlie cementite was 0.8 pieces/pm3, and the maxitii~ttili engtl~o f the observed cetnetitite was 3
p n o~r less. In order to deter~ninea state of cenietitite distribution, five points of
field-of-view of 9.5 x 7.5 ym from the surface layer of tlie sheared portion were randonily
photographed for each condition. Tliis was tlie same in any of Examples 2 to 6.
[0132] In Conlparative Examples 1 to 5, on tlie other hand, a mixed structure (Comparative
Examples 1 and 2) of bainite and niartensite not including ferrite or a single phase of ferrite
(Comparative Examples 3 to 5) was observed. In Comparative Exanlples 1 and 2, cetiientite
and iilclusioli was hardly observed in alnlost satne iilanner as illustrated in Fig. 8. In
Cotnparative Examples 3 to 5, however, tlie cementite (see Fig. 5, black part excluding
circles) C having very high number density greatly exceeding 0.8 pieceslpti? as illustrated in
Fig. 5 was observed.
[0133] An experinletit 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 tlus case, the shearing start
temperature was also (Ar3 + 30°C) or highel; the delayed fracture occurred once in three
repetitive experiments. As aresult of observation tlie surface of the shearing surface of the '
resulting workpiece, in the range of 100 pm from the shear plane, the stlucture of the surface
layer of the sheared portion in the workpiece without an occurrence of tlie delayed fracture
was configured to iticlude: ferrite of wliich the grain size was as very small as 3 In1 or less;
and martensite, bainite, or residual austenite of which the graiti size was also 3 pm or less
The crystal grain liaving tlie aspect ratio exceeding 3 was also seen and the ratio was about
7% by tiuniber.
[0134] In the range of I00 pm fro111 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 pm; and martensite, bainite,
or residual austetiite of which the grain size was also 5 pm. hi the surface layer of tlie
sheared portion, the crystal graiti having tlie aspect ratio exceeditig 3 was also seeti arid tlie
ratio was about 7% by tiumber.
[0135] (Test B)
[0136] A shearing machine 20 includes: a die 3 which is fortiled with a hole 22 for bending
alid forming and a puncture 24 for punching deforliiation on the bottoni of tlie hole 22 and in
wliich the steel sheet 1 is placed; a punch 2 which is insetted into tlie 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 deforniation to form a puncture
(shearing) in a predetern~inedr ange of the steel slieet 1.
[0137] By siniulating press fortning not accompanying fiacture of the steel sheet, tlie
shearing n~achine2 0 fornled 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 slieet 1 using a movable die 13 to have a diameter of 20 tnm was
performed (see Fig. 9B).
[0138] Except for Comparative Examples, the tinie until the shearing of the steel sheet 1
starts after conling in contact wit11 tlie movable die 26 was from about 0.1 seconds to about
0.5 seconds.
[0139] A clearance between the punch 2 and the die 3 was set to be 0:lS mm and the
measured Ar3 was identified from a thermal history after tlie hat forming. The equivalent
plastic strain was calculated in the sanie way as in Test A. Shearing conditions indicated in
Table 3 were adopted.
[0140] An evaluation method in Test B is also the same as that in Test A.
[0141] By the way, the dinlension 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 tlie error in diameter of the punch hole after the shearing is
divided by 20 and is then converted into percentage (x 100) and the values are disclosed as
the dimensional error in Table 3 which indicates an impletnentation result of this test.
[O 1421 [Table 31
Comparative
Example 10
Comparative
Example 11
450
460
950
950
1.5
1.5
Water
Water
2.5
1.8
80
100
650
640
Absence
Absence
2.8
2.8
[0143] In Examples 7 to 10, it can be understood that the occurrence of the delayed kacture
in the sheared portion (fracture plane) is suppressed.
[0144] In Examples 7 to 10 indicated in Table 3, the microstructure in the surface layer of
the sheared portio~(i~n the range of 100 p n ~fro m 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
n~icrostructurea nd inclusions in Examples 7 to 10 are the same as those in Exa~nples1 to 6.
[0145] The microstlucture and inclusions in Comparative Examples 6 to 11 are the same as
those in Comparative Examples 1 to 5. That is, a mixed stlucture of bainite and martensite
not including ferrite was observed in Comparative Examples 6 to 8, and a single phase of
fetrite was observed in Comparative Examples 9 to 11. In Comparative Examples 6 to 8, the
cementite was liardly observed. In Conlparative Examples 9 to 11, however, the cementite
having very high nun~berd ensity greatly exceeding 0.8 pieceslL~mw3a s observed.
[0146] This applicatiot~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
[0147] 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 itlventiot~h as high
applicability in a steel sheet working technology industry.

we claims:-
1. A surface layer grain refining hot-shearing method co~nprisi~lg:
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°C to austetiitize the steel sheet;
subsequently shearing the steel sheet in a state in which tlie steel sheet is placed on a
die; and
quenching by rapidly cooling the sheared steel slieet,
wherein a start temperature of tlie shearing is set to be a temperature (OC) obtained by
adding a tenlperature of from 30°C to 140°C to a previously measured Ar3 of the steel slieet.
2. A surface layer grain refining hot-shearink method comprising:
heating and keeping a steel slieet having a carbon content of 0.15% or more by niass
in a temperature range of from Ac3 to 1400°C to auste~~ititzhee steel sheet;
subsequently shearing tlie 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 (OC) obtained by
adding a value, which is calculated by lnultiplying 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 tlie amount of equivalent plastic strain of tlie 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 froni 5% to 20% of a thickness of the steel sheet from a shear plane of the sheared portion
to an inside of the steel slieet in a nornial 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 &om a
bottom on a burr side of the sheared portion.
4. The surface layer grain refining hot-shearing metliod according to clainl2 or 3,
wherein the aatnount of equivalent plastic strain of the surface layer in the sheared portion is
calculated by a nunierical simulation that is performed based on a stress-strain diagram at a
steel slieet temperature of from 500°C to 800°C.
5. The surface layer grain refining hot-shearing method according to any one of
clainis 2 to 4, wherein tlie amount of equivalent plastic strain of the surface layer in tlie
sheared portion is calculated based on a Mises yield fUnction represented by the following
Forniula (1)
6. The surface layer grain refining hot-shearing n~ethoda ccording to claini 1 or 2,
wherein the shearing of the steel sheet starts within three seconds after the steel sheet conies
in contact with the die.
7. The surface layer grain refining hot-shearing nietliod according to claim 1 or 2,
wherein the rapid cooling is perfornled when the steel sheet comes in contact with tlie die.
8. The surface layer grain refining hot-shearing tuethod according to claim 1 or 2,
wherein the rapid cooling is performed wlien 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 clain~1 or 2,
wherein press forniitig not accompanying fracture of the steel sheet is performed between the
Iieatitig and the shearing of the steel slieet.
10. Aworkpiece obtained by surface layer grain refining hot-shearing, comprising:
a steel sheet having a carbon content of 0.15% or tilore by mass, a surface layer of a
sheared portion of a steel sheet having a carbon content of 0.15% or Inore by tiiass including a
ferrite phase and a remainder, tlie surface layer being defined as a range up to 100 ptn inside
of the steel sheet in a tior~nald irection of a shear plane from a ft-acture 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 pm or less, and
29
includes cenientite and inevitably generated inclusions,
wlierein tlie ferrite phase has an average grain size of 3 p111 or less,
wlierein the surface layer contains 5% or tiiore grains by number having an aspect
ratio of 3 or more, and
wherein a region out of the range of 100 pln includes: mattensite and inevitably
generated inclusions; or bainite, martensite, and inevitably generated inclusions.
11. The workpiece obtained by surface layer grain refining hot-shearing according
to clain~1 0, wherein, in the surface layer, the cementite has a number density of 0.8
p i e c e s ~o~r ~leiss~ a~nd the cenie~ititeh as a maxiniut~lie ngth of 3 pm or less.
12. The workpiece obtained by surface layer grain refining hot-sl~earinga ccording
to claim 10 or 11, wherein a total area ratio of the baitiite phase, the martensite phase, and the
residual austenite phase, \vhicli are measured by an electron-beam backscattering diffraction
(EBSD) method, is froni 10% to 50% in the surface layer.
13. Aworkpiece 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 tnore by mass in a temperature range of from Ac3 to 1400°C to austenitize the steel sheet;
subsequently shearing tlie steel sheet in a state in which the steel sheet is placed 011 a die; and
quenching by rapidly cooling the sheared steel sheet,
~vl~ereain s tart temperature of the shearing is set to be a teniperature ("C) obtained by
adding a temperature of from 30°C to 140°C to a previously tneasured Ar3 of the steel sheet.
14 Aworkpiece obtained by surface layer grain refining hot-shearing, the
workpiece produced by heating and keeping a steel slieet having a carbon content of 0.15% or
tnore by mass in a tetnperature range of from Ac3 to 1400°C to austenitize the steel sheet;
subsequet~tlys hearing the steel sheet in a state in which the steel sheet is placed on a die; at>d
quenchit~gb y rapidly cooling the sheared steel sheet, and
a start temperature of the shearing is set to be a ten~petature(" C) obtained by adding
a value, which is calculated by nlultiplying an amount of equivalent plastic strain of a surface
layer in a sheared portion by a coefficient fi-om 40 to 60, to a previously measured Ar3 of the
steel sheet.