SURFACE LAYER GRAIN REFINING HOT-SHEARING METHOD AND WORKPIECE OBTAINED BY SURFACE LAYER GRAIN REFINING HOT-SHEARING
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° 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 (° C.) obtained by adding a temperature of from 30° C. to 140° C. to a previously measured Ar3 of the steel sheet.
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° 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 (° C.) obtained by adding a temperature of from 30° C. to 140° 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° 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 (° 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° C. to 800° 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
ɛ
_
P
=
2
3
(
d
ɛ
xx
2
+
d
ɛ
yy
2
+
d
ɛ
zz
2
+
2
d
ɛ
xy
2
+
2
d
ɛ
yz
2
+
2
d
ɛ
zx
2
)
.
(
1
)
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 μm 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 μm or less, and includes cementite and inevitably generated inclusions,
wherein the ferrite phase has an average grain size of 3 μm 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 μm 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/μm 3 or less and the cementite has a maximum length of 3 μm 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° 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 (° C.) obtained by adding a temperature of from 30° C. to 140° 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° 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 (° 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.