U.S. patent application number 17/021404 was filed with the patent office on 2021-11-25 for hot-work die steel and a preparation method thereof.
This patent application is currently assigned to University of Science and Technology Beijing. The applicant listed for this patent is University of Science and Technology Beijing. Invention is credited to Jinfeng HUANG, Jianqiang LI, Yong LIAN, Cheng ZHANG, Cheng ZHANG, Jin ZHANG, Chao ZHAO.
Application Number | 20210363603 17/021404 |
Document ID | / |
Family ID | 1000005955200 |
Filed Date | 2021-11-25 |
United States Patent
Application |
20210363603 |
Kind Code |
A1 |
HUANG; Jinfeng ; et
al. |
November 25, 2021 |
HOT-WORK DIE STEEL AND A PREPARATION METHOD THEREOF
Abstract
The present application provides a hot-work die steel and a
preparation method thereof, wherein the chemical constituents of
the hot-work die steel in mass percentage are as follows: C:
0.20-0.32 wt %, Si: .ltoreq.0.5 wt %, Mn: .ltoreq.0.5 wt %, Cr:
1.5-2.8 wt %, Mo: 1.5-2.5 wt %, W: 0.5-1.2 wt %, Ni: 0.5-1.6 wt %,
V: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %, and a balance of iron, wherein
an alloying degree is 5-7%; a tensile strength of the hot-work die
steel at 700.degree. C. is 560-700 MPa; a value of hardness of the
hot-work die steel at room temperature is 32-38 HRC after holding
at 700.degree. C. for 3-5 h; and the hot-work die steel has an
elongation of 14% to 16% at room temperature, a percentage
reduction of area of 48% to 65%, and an impact toughness of 52-63 J
at room temperature. The hot-work die steel of the present
application has an excellent thermal stability as well as a good
plasticity and a toughness at room temperature.
Inventors: |
HUANG; Jinfeng; (Beijing,
CN) ; ZHANG; Jin; (Beijing, CN) ; ZHANG;
Cheng; (Beijing, CN) ; ZHAO; Chao; (Beijing,
CN) ; LIAN; Yong; (Beijing, CN) ; LI;
Jianqiang; (Beijing, CN) ; ZHANG; Cheng;
(Luoyang City, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Science and Technology Beijing |
Beijing |
|
CN |
|
|
Assignee: |
University of Science and
Technology Beijing
Beijing
CN
|
Family ID: |
1000005955200 |
Appl. No.: |
17/021404 |
Filed: |
September 15, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/CN2020/091225 |
May 20, 2020 |
|
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17021404 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/005 20130101;
C21D 6/005 20130101; C22C 38/04 20130101; C22C 38/54 20130101; C22C
38/52 20130101; C21D 1/28 20130101; C22C 38/002 20130101; C22C
38/50 20130101; C22C 1/02 20130101; C21D 6/004 20130101; C22C 38/02
20130101; B21D 37/10 20130101; C21D 9/0068 20130101; C22C 38/46
20130101; B22C 9/061 20130101; C22C 38/48 20130101; C22C 38/44
20130101; C21D 6/008 20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C21D 8/00 20060101 C21D008/00; C21D 6/00 20060101
C21D006/00; C21D 1/28 20060101 C21D001/28; C22C 1/02 20060101
C22C001/02; C22C 38/54 20060101 C22C038/54; C22C 38/52 20060101
C22C038/52; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; B22C 9/06 20060101
B22C009/06; B21D 37/10 20060101 B21D037/10 |
Claims
1. A hot-work die steel, comprising the following chemical
constituents: C: 0.20-0.32 wt %, Si: .ltoreq.0.5 wt %, Mn:
.ltoreq.0.5 wt %, Cr: 1.5-2.8 wt %, Mo: 1.5-2.5 wt %, W: 0.5-1.2 wt
%, Ni: 0.5-1.6 wt %, V: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %, and a
balance of iron, wherein an alloying degree is 5-7 wt %; wherein a
tensile strength of the hot-work die steel at 700.degree. C. is
560-700 MPa; wherein a value of hardness of the hot-work die steel
at room temperature is 32-38 HRC after holding at 700.degree. C.
for 3-5 h; and wherein the hot-work die steel has an elongation of
14% to 16% at room temperature, a percentage reduction of area of
48% to 65% at room temperature, and an impact toughness of 52-63 J
at room temperature.
2. The hot-work die steel according to claim 1, wherein the
hot-work die steel further comprises at least one of the following
chemical constituents: Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %, B:
0.001-0.005 wt %, Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y:
0.01-0.1 wt %.
3. The hot-work die steel according to claim 1, wherein the
hot-work die steel comprises less than 0.02 wt % of S and less than
0.02 wt % of P.
4. The hot-work die steel according to claim 1, wherein the
hot-work die steel comprises a tempered sorbite structure that
retains lath characteristics after the hot-work die steel is
stretched at 700.degree. C.
5. The hot-work die steel according to claim 1, wherein the
hot-work die steel comprises a nanoscale acicular alloy carbide
after the hot-work die steel is stretched at 700.degree. C.
6. The hot-work die steel according to claim 5, wherein the
nanoscale acicular alloy carbide is:
V.sub.0.5-0.8Mo.sub.0.5-0.6Cr.sub.0.15-0.3W.sub.06-0.14Nb.sub.0.01-0.02C.
7. (canceled)
8. A method for producing the hot-work die steel according to claim
1, comprising the following steps: a smelting step: preparing a raw
material according to the following mass percentages: C: 0.20-0.32
wt %, Si: .ltoreq.0.5 wt %, Mn: .ltoreq.0.5 wt %, Cr: 1.5-2.8 wt %,
Mo: 1.5-2.5 wt %, W: 0.5-1.2 wt %, Ni: 0.5-1.6 wt %, V: 0.15-0.7 wt
%, Nb: 0.01-0.1 wt %, and a balance of iron, processing the raw
material into an electrode rod by arc smelting, secondary refining,
vacuum degassing, and forging in a forging furnace; an electroslag
remelting step: removing an oxidized layer of the electrode rod,
then introducing the electrode rod into a vacuum electroslag
remelting device for secondary refining, keeping a temperature of
water in the water cooling system of the electroslag remelting
device not higher than 70.degree. C., and obtaining an electroslag
ingot by electroslag remelting from the electrode rod, wherein a
melting rate is 7-12 kg/min, and a temperature of a cooling water
of a crystallizer is held at 40-50.degree. C.; a homogenizing
annealing step: heating the electroslag ingot to 1200-1250.degree.
C. and holding for 15-23 h; a forging step: cooling the electroslag
ingot to a forging heating temperature of 1150-1200.degree. C. and
then forging to obtain an ingot, wherein an initial forging
temperature is 1130 to 1160.degree. C., and a final forging
temperature is .gtoreq.850.degree. C.; an annealing after forging
step: introducing the ingot into an annealing furnace after the
temperature of the ingot is lower than 500.degree. C., heating to
830-890.degree. C. at a heating rate not more than 100.degree.
C./h, holding for [120 min+r (mm).times.2 min/mm] or [120 min+d
(mm)/2.times.2 min/mm], lowering the temperature to below
500.degree. C. at a cooling rate of 20-40.degree. C./h, taking the
ingot out from the annealing furnace and air-cooling to obtain an
annealed ingot; a heat treatment of fine grain step: heating the
annealed ingot to 930-1150.degree. C. and performing a first
holding for a first holding time of [(15-40) min+r (mm).times.2
min/mm] or [(15-40) min+d (mm)/2.times.2 min/mm], water cooling to
400-500.degree. C. within 1-2 min, then air cooling to
250-280.degree. C. and performing a second holding for a second
holding time of 5-10 h; and then holding at a temperature of
660-700.degree. C. for 5-10 h; a tempering treatment step: heating
the held ingot to 980-1100.degree. C. and holding for [(15-40)
min+r (mm).times.2 min/mm] or [(15-40) min+d (mm)/2.times.2
min/mm], then quenching to 50-150.degree. C., and then tempering at
580-660.degree. C. for 6-16 h to obtain the hot-work die steel;
wherein r is a radius of the material and d is a thickness of the
material.
9. The method for producing the hot-work die steel according to
claim 8, wherein the raw material further comprises at least one of
the following constituents: Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %,
B: 0.001-0.005 wt %, Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y:
0.01-0.1 wt %.
10. The method for producing the hot-work die steel according to
claim 8, wherein the forging step includes: forming and forging by
means of a precision forging machine, wherein the forging heating
temperature is 900-1050.degree. C., the initial forging temperature
is 850-950.degree. C., and the final forging temperature is
.gtoreq.800.degree. C.; alternatively, forming and forging by a
hydraulic hammer or oil hydraulic press, wherein the forging
heating temperature is 1150-1200.degree. C., the initial forging
temperature is 1130-1160.degree. C., and the final forging
temperature is .gtoreq.850.degree. C.
11. The method for producing the hot-work die steel according to
claim 8, wherein the holding time of the annealing after forging
step is 6-8 h.
12. The hot-work die steel according to claim 1, wherein the
tensile strength of the hot-work die steel at 700.degree. C. is
600-700 MPa.
Description
FIELD OF THE INVENTION
[0001] This application relates to the field of hot-work die steel,
in particular to a hot-work die steel and a preparation method
thereof.
BACKGROUND OF THE INVENTION
[0002] Hot-work die steel is a die mainly used for pressing a solid
or liquid metal above the recrystallization temperature into a
workpiece, such as hot forging die, hot extruding die, die casting
mold, etc. The working conditions of hot-work die steel are harsh.
The mold cavity thereof is in direct contact with workpieces under
high temperature, in which the local temperature can reach
600-700.degree. C. At the meantime, the workpieces also suffer from
various effects such as heavy loads at high temperature, high
temperature strain fatigue, and cold-hot fatigue. Insufficient
strength at high temperature can cause softening, deformation, and
collapse of the die, and insufficient performances of thermal
strain fatigue resistance and cold-hot fatigue will lead to the
cracking and spalling of die. Therefore, the core and key
indicators to improve the life of the hot-work die steel are the
overall enhanced performances of the strength at high temperature,
high temperature fatigue, cold-hot fatigue and other properties of
the hot-work die steel.
[0003] The available hot-work die steel widely used is the medium
alloy chromium type H13 steel (4Cr5MoSiV1). H13 steel has a good
strength-toughness coordination and a thermal fatigue resistance
below 550.degree. C. However, the strength and the thermal
stability of H13 steel decline sharply above 600.degree. C. The
tensile strength at 700.degree. C. is only 260-320 MPa. The
decrease in strength at high temperature also leads to a
deterioration of its thermal fatigue resistance, and an increase in
the tendency to hot crack at high temperature, which is impossible
to satisfy the requirements for the working conditions of the
hot-work die steel at high temperature.
[0004] In order to improve the operating temperature and the
strength at high temperature of the hot-work die steel, it is
common to increase the contents of carbon and alloy to produce
hot-work die steel, for example the high alloy tungsten molybdenum
type hot-work die steel (3Cr2W8V). The alloy content can be raised
to above 10%, and the strength at a high temperature of 700.degree.
C. can be raised to 300-400 MPa. However, its toughness at room
temperature is only 11-13 J, and the cold-hot fatigue resistance is
poor, so that early failure often occurs due to cracking of the
die. In view of the use safety or the cost of processing, its
application range is limited.
[0005] Therefore, a hot-work die steel with sufficient strength at
high temperature, and good performances of plasticity, toughness
and fatigue resistance at room temperature is desired.
SUMMARY OF THE INVENTION
[0006] The present application aims at providing a hot-work die
steel and a preparation method thereof, so that the hot-work die
steel has satisfactory plasticity and toughness, and stability
during operation under high temperature. The specific technical
solutions are as follows.
[0007] The first aspect of the present application is to provide a
hot-work die steel, comprising the following chemical
constituents:
[0008] C: 0.20-0.32 wt %, Si: .ltoreq.0.5 wt %, Mn: .ltoreq.0.5 wt
%, Cr: L5-2.8 wt %, Mo: L5-2.5 wt %, W: 0.5-1.2 wt %, Ni: 0.54.6 wt
%, V: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %, and a balance of iron, and
an alloying degree is 5-7%;
[0009] wherein a tensile strength of the hot-work die steel at
700.degree. C. is 560-700 MPa; wherein a value of hardness of the
hot-work die steel at room temperature is 32-38 HRC after
maintaining at 700.degree. C. for 3-5 h; and
[0010] wherein the hot-work die steel has an elongation of 14% to
16% at room temperature, a percentage reduction of area of 48% to
65%, and an impact toughness of 52-63 J at room temperature.
[0011] In an embodiment of the present application; the hot-work
die steel further comprises at least one of the following chemical
constituents:
[0012] Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %, B: 0.001-0.005 wt %,
Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y: 0.01-0.1 wt %.
[0013] In an embodiment of the present application, the hot-work
die steel comprises less than 0.02 wt % of S and less than 0.02 wt
% of P.
[0014] In an embodiment of the present application, the tempered
sorbite structure still retains the lath characteristic after the
hot-work die steel is stretched at 700.degree. C.
[0015] In an embodiment of the present application, the carbide in
the hot-work die steel is a nanoscale acicular MC type alloy
carbide after the hot-work die steel is stretched at 700.degree.
C.
[0016] In an embodiment of the present application; the nanoscale
acicular MC type alloy carbide is:
V.sub.0.5-0.8Mo.sub.0.5-0.6Cr.sub.0.15-0.3W.sub.0.06-0.14Nb.sub.0.01-0.02-
C.
[0017] In an embodiment of the present application, the tensile
strength of the hot-work die steel at 700.degree. C. is 600-700
MPa.
[0018] The second aspect of the present application is to provide a
method for producing the hot-work die steel according to any one of
the above aspects, comprising: a smelting step: preparing a raw
material according to the following mass percentages:
[0019] C: 0.20-0.32 wt %, Si: .ltoreq.0.5 wt %, Mn: .ltoreq.0.5 wt
%, Cr: 1.5-2.8 wt Mo: 1.5-2.5 wt %, W: 0.5-1.2 wt %, Ni: 0.5-1.6 wt
%: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %;
[0020] and a balance of iron,
[0021] processing the raw material into an electrode rod by arc
inciting, secondary refining, vacuum degassing, and forging in a
forging furnace;
[0022] an electroslag remelting step: removing an oxidized layer of
the electrode rod, then introducing the electrode rod into a vacuum
electroslag remelting device for secondary refining, keeping a
temperature of water in the water cooling system of the electroslag
remelting device not higher than 70.degree. C., and obtaining an
electroslag ingot by electroslag remelting from the electrode rod;
wherein the melting rate is 7-12 kg/min, and the temperature of a
cooling water of a crystallizer is held at 40-50.degree. C.;
[0023] a homogenizing annealing step: heating the electroslag ingot
to 1200-1250.degree. C. and holding for 15-23 h;
[0024] a forging step: cooling the electroslag ingot to a forging
heating temperature of 1150-1200.degree. C. and then forging to
obtain an ingot, wherein the initial forging temperature is 1130 to
1160.degree. C., and the final forging temperature is
.gtoreq.850.degree. C.; an annealing after forging step:
introducing the ingot into an annealing furnace after the
temperature of the ingot is lower than 500.degree. C., heating to
830-890.degree. C. at a heating rate of not more than 100 holding
for [120 min+r (mm).times.2 min/min] or [120 min+d (mm)/2.times.2
min/mm], lowering the temperature to below 500.degree. C. at a
cooling rate of 20-40.degree. C./h, taking the ingot out from
annealing furnace, and air-cooling to obtain an annealed ingot;
[0025] a heat treatment of fine grain step: heating the annealed
ingot to 930-1150.degree. C. and performing a first holding for a
first holding time of [(15-40) min+r (mm).times.2 min/mm] or
[(15-40) min+d (mm)/2.times.2 min/mm], water cooling to
400-500.degree. C. within 1-2 min, then air cooling to
250-280.degree. C. and performing a second holding for a second
holding time of 5-10 h; and then holding at a temperature of
660-700.degree. C. for 5-10 h;
[0026] a tempering treatment step: heating the held ingot to
980-1100.degree. C. and holding for [(15-40) min+r (mm).times.2
min/mm] or [(15-40) min+d (mm)/2.times.2 min/mm], then cooling to
50-150.degree. C., and then tempering at 580-660.degree. C. and
holding for 6-16 h to obtain the hot-work die steel;
[0027] wherein r is a radius of the material and d is a thickness
of the material.
[0028] In an embodiment of the present application, the raw
material further comprises at least one of the following
constituents: Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %, B:
0.001-0.005 wt %, Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y:
0.01-0.1 wt %.
[0029] In an embodiment of the present application, the forging
step specifically includes: forming and forging by means of a
precision forging machine, wherein the forging heating temperature
is 900-1050.degree. C., the initial forging temperature is
850-950.degree. C., and the final forging temperature is
.gtoreq.800.degree. C.;
[0030] alternatively, forming and forging by a hydraulic hammer or
oil hydraulic press, wherein the forging heating temperature is
1150-1200.degree. C., the initial forging temperature is
1130-1160.degree. C., and the final forging temperature is
.gtoreq.850.degree. C.
[0031] In an embodiment of the present application, the holding
time of the annealing after forging step is 6-8 h.
[0032] In the present application, the term "alloying degree"
refers to the total content of other elements in addition to iron
and carbon in the steel.
[0033] The present application provides a hot-work die steel with a
tensile strength of 560-700 MPa at 700.degree. C., which is twice
more than H13 steel, and about 1.5 times more than 3Cr2W8V. The
operating temperature is increased from 600.degree. C. (for
available H13 steel) to 700.degree. C., and the increase range is
up to 100.degree. C. Therefore, the stability of the hot-work die
steel is enhanced during operation at much higher temperature,
compared with conventional hot-work die steel. In addition, the
hot-work die steel of the present application has good plasticity
and toughness at room temperature as well as fatigue resistance at
high temperature, thus expanding the application range of the
hot-work die steel.
[0034] The present application provides a heat treatment process
for the hot-work die steel, wherein the hot-work die steel is
allowed to have a tensile strength of 560-700 MPa at 700.degree. C.
and a value of hardness of 32-38 HRC at room temperature after
holding for 3-5 h at 700.degree. C. by controlling the addition
proportions of each raw material and reasonable forging and heat
treatment process. Moreover, the hot-work die steel of the present
application has good plasticity and toughness at room temperature,
which is superior than that of the available H13 steel, and is
equivalent to low-carbon and low-alloy hot-work die steel. It also
has good high temperature strain fatigue resistance, thus expanding
the application range of the hot-work die steel.
[0035] Indeed, it is not necessary to achieve all of the above
benefits at the same time when implementing any one of product or
method of the present application.
DESCRIPTION OF THE DRAWINGS
[0036] In order to further explicitly explain the technical
solutions in the present application and in the art, accompany
figures regarding the examples and the prior art are briefly
introduced as follows. These figures are only some examples of the
present application and it is obvious for those skilled in the art
to obtain other technical solutions based on these figures without
inventive efforts.
[0037] FIG. 1 is a process chart of the heat treatment process for
the hot-work die steel of the present application.
[0038] FIG. 2 is a schematic diagram of the tensile strength of the
hot-work die steel in Example 5 of the present application and H13
steel in Comparative Example 1 as a function of the
temperature.
[0039] FIG. 3a is an electron microscope photo of the hot-work die
steel in Example 5 of the present application at room
temperature.
[0040] FIG. 3h is an electron microscope photo of the hot-work die
steel in Example 5 of the present application after stretching at
700.degree. C.
[0041] FIG. 3c is a partial enlargement of FIG. 3b.
[0042] FIG. 4a is an electron microscope photo of H13 steel in
Comparative Example 1 at room temperature.
[0043] FIG. 41 is an electron microscope photo of H13 steel in
Comparative Example 1 after stretching at 700.degree. C.
[0044] FIG. 4c is a partial enlargement of FIG. 4b.
[0045] FIG. 5a is a micro topography of the carbide obtained from
the hot-work die steel in Example 5 of the present application
after stretching at 700.degree. C.
[0046] FIG. 5b is an electron diffraction pattern of the selected
area of the hot-work die steel in Example 5 of the present
application after stretching at 700.degree. C.
[0047] FIG. 5c is a high-resolution photo of the MC type alloy
carbide obtained from the hot-work die steel in Example 5 of the
present application after stretching at 700.degree. C.
[0048] FIG. 6 is an analysis diagram of the constitution of the
carbide obtained from the hot-work die steel in Example 5 of the
present application.
DETAILED DESCRIPTION OF THE INVENTION
[0049] The object, technical solution and advantages of the
invention will be described in detail below with reference to the
accompany figures and the examples in order to further illustrate
the present application. It is apparent that the described examples
are only a part of the examples of the present application, not all
of them. All of the examples obtained based on the examples of the
invention without inventive effort made by those skilled in the art
are within the protection scope of the present application.
[0050] In the prior art, H13 steel is improved by raising the
content of carbon and alloy to promote the formation of carbide
with high melting point to enhance the high temperature strength by
solution strengthening and dispersion strengthening of the carbide,
so that the low temperature toughness and the high temperature
strength at room temperature of this hot-work die steel are
enhanced. Although this process has certain enhancing effect on the
high temperature strength of the steel at about 600.degree. C., the
enhancing effect on the steel at higher temperatures, such as at
700.degree. C., is limited. This is mainly because the coherent
relationship between M.sub.2C or MC carbide and matrix is damaged
when the temperature exceeds 600.degree. C., and the carbide
transforms into incoherent M.sub.6C or M.sub.23C.sub.6 carbide
which is easy to grow up and will lead to a significantly weakened
strengthen effect. Therefore, the existing design principles and
methods for increasing the carbon content and high alloying to
increase the high temperature strength have increased the high
temperature strength of hot work die steel to the hunt, and will
lead to a sharp decline in plastic toughness, high temperature
fatigue and cold-thermal fatigue.
[0051] In view of this, the application provides a hot-work die
steel and a preparation method thereof. The inventor found that the
stability of coherent relationship between carbide and matrix at
high temperature is decisive to the strength at high temperature.
On this basis, carbon and alloy elements are selected, and the heat
treatment parameters of the thermal process are decided.
Multi-element alloying design of W, Mn, Mo, V, Cr, Ni and Nb and
optimization of heat treatment process are performed, thereby
degree of mismatch in the carbide/matrix interface is regulated to
obtain a nanoscale MC type alloy carbide with low degree of
mismatch which is distributed dispersedly. The coherent
relationship between carbide and matrix allows stability at
700.degree. C. by hindering dislocation motion and
recrystallization of lath sorbite, thereby allowing high strength
at high temperature. At the same tittle, low content design (C
content of 0.20-0.32%) is included in the application, and a
quenched fine-grain structure of dislocation martensite is obtained
through the heat treatment of fine grain step to ensure the
toughness and fatigue resistance of the tempered material.
Therefore, the service life of the novel steel is promised due to
the organized structure.
[0052] The present application provides a hot-work die steel,
comprising following chemical constituents:
[0053] C: 0.20-0.32 wt %, Si: .ltoreq.0.5 wt %, Mn: .ltoreq.0.5 wt
%, Cr: L5-2.8 wt %, Mo: L5-2.5 wt %, W: 0.5-1.2 wt %, Ni: 0.5-1.6
wt %, V: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %, and a balance of iron,
wherein an alloying degree is 5-7%;
[0054] wherein, a tensile strength of the hot-work die steel at
700.degree. C. is 560-700 MPa, preferably 600-700 MPa, and more
preferrably 650-690 MPa;
[0055] wherein a value of hardness of the hot-work die steel at
room temperature is 32-38 HRC after holding at 700.degree. C. for
3-5 h; wherein the holding time is not specifically defined, for
example, 3-5 h; specifically, the holding time can be 3 h, 4 h, or
5 h, preferably 4 h; and
[0056] wherein, the hot-work die steel has an elongation of 14% to
16% at room temperature, a percentage reduction of area of 48% to
65%, and an impact toughness of 52-63 J at room temperature.
[0057] The inventor found through research that carbon (C) is an
important element in the hot-work die steel, which is decisive with
regard to the hardness and strength of the martensite formed by
quenching, plays a key role of secondary hardening (fining
tempering, and has important influence on the strength and
toughness of the hot-work die steel. Not limited by any theory, the
quenched structure of low carbon steel is usually dislocation
martensite, which has not only high toughness, but also certain
ability of plastic deformation, so that the formation of quenching
cracks can be avoided and reduced. However, acicular martensite
formed from high carbon steel in an explosive manner, which has
great stress, and a twin martensite has a low toughness, thus,
plastic deformation is impossible, and microscopic cracks often
appear during quenching.
[0058] Based on the above research, the carbon content needs to be
designed at low carbon level. If the carbon content in the matrix
is under 0.25 wt %, structure of full lath martensite can be
obtained after quenching. In view of carbon consumption during the
formation of a first carbide from strong carbide forming elements,
such as Mo, W, V and the like, the carbon content of the hot-work
die steel of the present application is controlled in the range of
0.20-0.32 wt %. Accordingly, it will meet the requirement to
facilitate the mass production of hot-work die steel, while
improving the toughness and fatigue performance of the
material.
[0059] The inventors also found through research that both silicon
(Si) and manganese (Mn) are mainly used for deoxidation in the
steel, and have certain effects of solution strengthening and
improving the hardenability. Si exhibits good solution
strengthening effect. A small amount of Si allows good solution
strengthening effect. However, too much Si can reduce the toughness
of the material sharply. Mn is an austenitizing forming element.
However, too much Mn can lead to residual austenite in the material
after quenching. Since excessive residual austenite material is
harmful to the performance of the material at high temperature, the
contents of Si and Mn in this application are controlled to:
Si.ltoreq.0.5 wt %, Mn.ltoreq.0.5 wt %.
[0060] The main effect of chromium (Cr) is to increase the
strength, hardenability and oxidation resistance of steel. In
addition, Cr is a carbide forming element, which can form a variety
of carbides with carbon, such as Cr.sub.7C.sub.3, Cr.sub.23C.sub.6,
etc. However, high Cr content is not conducive to improve the high
temperature strength of the steel, since high degree of mismatch is
between those carbides and the matrix, in which the coherent
relationship is impossible to maintain at high temperature, and
those carbides are easy to grow up and become coarsened. Therefore,
the content of Cr in the present application is controlled in the
range of 1.5-2.8 wt %.
[0061] Tungsten (W) and molybdenum (Mo) can not only improve the
hardenability of materials, but also form a large amount of
W.sub.2C and Mo.sub.2C carbides with high melting point in the
material. They can even dissolve in carbide VC to form an alloy
carbide, which shows the secondary hardening effect, and can
suppress aggregation and growing up of the carbide, so as to
improve the high temperature strength. However, too much W and Mo
will lead to high degree of mismatch between carbide and matrix at
high temperature, so that the coherent relationship no longer
exists. In this case, the formation of carbides, such as M.sub.6C,
which is easy to grow up and become coarsened is promoted, leading
to failure of strengthening effect at high temperature. In this
application, the Mo, W, and V contents are coordinated through
adjusting the Mo content to 1.5-2.5 wt %, and adjusting the W
content to 0.5-1.2 wt % to form a MC type alloy carbide which can
maintain coherent relationship with the matrix with low degree of
mismatch at high temperature, thereby improving the high
temperature strength of the hot-work die steel.
[0062] Vanadium (V) is a strong carbide forming element. The small
carbide particles formed from V are distributed dispersedly and
require a temperature above 1200.degree. C. to completely dissolve
in austenite, and thus reducing the grain size of the austenite,
resulting in a MC type alloy carbide with proper degree of mismatch
between the carbide and the matrix. However, high vanadium content
will lead to formation of a coarsened first carbide, which will
significantly decrease the plasticity and toughness of the steel.
The inventor accidentally found that it is beneficial to control
the V content to 0.15-0.7 wt % that the coherent relationship
between carbide and matrix at high temperature can be maintained at
700.degree. C. with the coordinated W, Mo and V elements, and
thereby significantly enhance the high temperature strength and
thermal stability of the hot-work die steel, and that the
plasticity and toughness of the hot-work die steel can also be
improved.
[0063] Nickel (Ni) can effectively increase the hardenability of
steel, and improve the low temperature toughness of steel. It will
increase the cost and decrease the critical point Ac1 of the
hot-work die steel by adding excessive Ni, which is adverse to the
red hardness. Therefore, the Ni content is controlled in the range
of 0.5-1.6% wt in this application.
[0064] Niobium (Nb) is preferred to combine with C to form a strong
carbide, which controls the growth of grain during austenitizing at
high temperature, and reduces the grain size. However, if the
content is too high, too many first carbides are formed and the
size is large when the material is solidified, which is not
conducive to the improvement of the impact toughness and fatigue
performance of the hot work die steel. Therefore, the content of Nb
is controlled in the range of 0.01-0.1 wt % in the present
application to take full advantage of the reduced grain size.
[0065] In an embodiment of the present application, the hot-work
die steel further comprises at least one of the following chemical
constituents:
[0066] Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %, B: 0.001-0.005 wt %,
Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y: 0.01-0.1 wt %.
[0067] The inventor also found through research that, without
limited by any theory, the high temperature stability, purity and
grain size of the hot-work die steel can be further improved, when
at least one of Zr, Co, B, Re, Ti and Y mentioned above is
comprised in the hot-work die steel. It may be due to the following
reasons:
[0068] Zirconium (Zr) has strong effects of deoxidizing and
denitrogenation in steelmaking process. Therefore, it is possible
to add a small amount of Zr to be combined with oxygen and nitrogen
to obtain tiny dispersed oxides and nitrides in the matrix, which
is favorable for reducing the grain size and minimizing the
structure in the smelting process. In addition, Zr element can also
combine with impurity element S to generate a sulfide, avoiding hot
brittle of the steel. Therefore, in order to obtain a steel with
smaller grain size for the structure and better purity, Zr content
is controlled in the range of 0.01-0.03% wt.
[0069] Similar to Ni and Mn, cobalt (Co) is able to form continuous
solid solution with iron, which may obstacle and delay the
precipitation and accumulation of other alloy carbides in tempering
process. Therefore, the hot strength of the material is
significantly enhanced. However, since cobalt element reduces the
hardenability of martensite steel, it should not be added too much.
Therefore, the cobalt content is controlled in the range of
0.10-0.50 wt % in this application.
[0070] Boron (B) within a certain content range has significantly
strong ability to improve hardenability of the steel. However, the
hardenability is not greatly improved when boron exceeds 0.005 wt %
in steel. In addition, B has the effect of strengthening grain
boundary in the steel, and can significantly improve the high
temperature strength of the material. Therefore, B content is
controlled in the range of 0.001-0.005 wt % in the present
application.
[0071] Rhenium (Re), which is a rare earth element, has the ability
of controlling the morphology of sulphide in the steel, and also
has effects of deoxidization, desulphurization, and improving the
lateral performance and low temperature toughness, and the effects
of dispersion and hardening in low-sulfur steel. Therefore, the Re
content is controlled in the range of 0.01-0.10 wt % in the present
application in order to deoxidize and desulfurize steel and purify
liquid steel, and improve the strength and toughness of the
steel.
[0072] Titanium (Ti) is preferred to combine with C to form a
strong carbide, which controls the growth of grain during
austenitizing at high temperature, and reduces the grain size.
However, if the content is too high, too many first carbides are
formed and the size is large when the material is solidified, which
is not conducive to the improvement of the impact toughness and
fatigue performance of the hot work die steel. Therefore, the
content of Ti is controlled in the range of 0.02-0.06 wt % in the
present application to take advantage of the reduced grain
size.
[0073] Traces of yttrium (Y) content in the steel at high
temperatures may be clustering in the grain boundary; which can
strengthen the grain boundary at high temperature, improve the high
temperature strength. Therefore, the Y content is controlled in
0.001-0.1 wt % in the present application.
[0074] Sulphur (S) and phosphorus (P) are the impurity elements,
which are adverse to toughness of the material. This may be due to
S reduces plasticity by forming a sulfide inclusion and leads to
crack phenomenon by forming (Fe+FeS) cocrystal in Sulfur-containing
atmosphere. Therefore, the S content should be reduced as much as
possible. High P content can result in reduction of toughness at
low temperature and high ductile-brittle transition temperature.
Therefore, the P content should also be reduced to the most extent
in order to avoid or mitigate adverse impacts on the plasticity of
the steel. However, the lower the content of S and P in the steel,
the higher the cost of removing these elements. The contents of S
and P in the application are controlled to be less than 0.02 wt %
and less than 0.02 wt %, respectively, in order to ensure the
excellent performance of hot-work die steel and to reduce the
production cost thereof as much as possible to facilitate
large-scale production.
[0075] In an embodiment of the present application, the tempered
sorbite structure still retains the lath characteristic after the
hot-work die steel is stretched at 700.degree. C. High density of
nanoscale MC type alloy carbide is distributed inside the lath,
which indicates that the nanoscale carbide has higher thermal
stability in the hot-work die steel of the present application.
[0076] In an embodiment of the present application, the carbide in
the hot-work die steel is a nanoscale acicular MC type alloy
carbide at 700.degree. C. The carbide is identified as
V.sub.0.5-0.8Mo.sub.0.5-0.6Cr.sub.0.15-0.3W.sub.0.06-0.14Nb.sub.0.01-0.02-
C multi-alloyed carbide through atomic probe analysis. Not limited
to any theory, the carbide can keep a coherent relationship with
the matrix at high temperature, so as to achieve high strength at
high temperature of low alloyed hot-work die steel.
[0077] The present application provides a hot-work die steel, which
has a tensile strength of 560-700 MPa at 700.degree. C., and a
hardness of 32-38 HRC after holding at 700.degree. C. for 3-5 h,
and thereby improving the operating temperature of the hot-work die
steel by 100.degree. C. to about 700.degree. C., compared to that
of existing hot-work die steel of 600.degree. C. Therefore, the
stability of the hot-work die steel is enhanced during operation at
much higher temperature. In addition, the hot-work die steel in the
present application has good plasticity and toughness at room
temperature, thus expanding the application range of the hot-work
die steel.
[0078] The present application also provides a method for producing
the hot-work die steel according to any one of the above
embodiments, comprising the following steps:
[0079] Smelting Step:
[0080] preparing the raw material according to following: C:
0.20-0.32 wt %, Si: .ltoreq.0.5 wt %, Mn: .ltoreq.0.5 wt %, Cr:
1.5-2.8 wt %, Mo: 1.5-2.5 wt %, W: 0.5-1.2 wt %, Ni: 0.5-1.6 wt %,
V: 0.15-0.7 wt %, Nb: 0.01-0.1 wt %, and a balance of iron, and
then processing the raw material into an electrode rod by arc
smelting, secondary refining, vacuum degassing, and forging.
[0081] The preparation process of the electrode rod is well known
to those skilled in the art, and there is no specific limitation in
this application. For example, the electrode rod can be prepared by
mixing the above raw materials, and forging into the electrode rod
in turn by arc smelting (EAF), secondary refining (LF), vacuum
degassing (VD) and forging in forging furnace. There is no specific
limitation to the above arc smelting, secondary refining, vacuum
degassing and forging in the present application, provided that the
objects of the present application can be achieved. For example,
the discharge temperature of arc smelting can be equal to or higher
than 1690.degree. C., and the gas content and impurity element
content in liquid steel shall be controlled to be: [nitrogen
(N)]+[hydrogen (H)]+[oxygen (O)].ltoreq.150 ppm. The heating
temperature of the secondary refining is 1600-1700.degree. C. High
basicity reductive slag can be produced in the refining process,
and desulfurization can be enhanced by controlling the temperature.
The vacuum degassing time is 15-20 min. The heating temperature is
15604675.degree. C. The absolute vacuum degree is 50400 Pa.
[0082] Electroslag Remelting Step:
[0083] removing an oxidized layer of the electrode rod, then
introducing the electrode rod into a vacuum electroslag remelting
device for secondary refining, holding the temperature of water in
the water cooling system of the electroslag remelting device not
higher than 70.degree. C. to obtain an electroslag ingot by
electroslag remelting from the electrode rod. There is no specific
limitation to electroslag remelting in the present application, as
long as the object of the application can be achieved. For example,
the melting rate can be 7-12 kg/min; the water temperature of
cooling water in the crystallizer is held at 40-50.degree. C.; the
deoxidizer can be at least one of aluminum particles or calcium
silicate powder; and inert gas, such as argon, is filled throughout
the electroslag remelting process.
[0084] The inventor found through research that the obtained
electroslag ingot structure is more uniform and finer with higher
purity when the temperature of the cooling water of the
crystallizer of the electroslag remelting device is not higher than
70.degree. C.
[0085] Homogenization and Annealing Step:
[0086] heating the electroslag ingot to 1200-1250.degree. C., and
holding for 15-23 h; Forging step:
[0087] cooling the electroslag ingot to a forging heating
temperature of 1150-1200.degree. C. and then forging to obtain an
ingot, wherein the initial forging temperature is 1130 to
1160.degree. C., and the final forging temperature is
.gtoreq.850.degree. C.
[0088] The forging heating temperature of the present application
is increased by about 50.degree. C. compared with that of the
existing die steel, so as to improve the high-temperature solid
solubility of carbon and alloy elements, and to make grains and
structure fine after forging.
[0089] Annealing after Forging Step:
[0090] introducing the ingot into an annealing furnace after the
temperature of the ingot is lower than 500.degree. C., heating to
830-890.degree. C. at a heating rate of not more than 100.degree.
C./h, holding for [120 min+r (mm).times.2 min/mm] or [120 min d
(mm)/2.times.2 min/mm], wherein the specific holding time can be
determined according to the size of the material, preferably 6-8 h,
lowering the temperature to below 500.degree. C. at a cooling rate
of 20-40.degree. C./h, taking the ingot out from annealing furnace,
and air-cooling to obtain an annealed ingot;
[0091] wherein r is a radius of the material and d is a thickness
of the material. When the ingot is a cylinder, the above r can be
used to calculate the holding time. When the ingot is a cube, the
above d can be used to calculate the holding time, wherein the
specific calculation method is determined according to the actual
shape of the material. Moreover, cooling the ingot to a lower
temperature (such as lower than 500.degree. C.) and then annealing
may avoid the grain from coarsening caused by holding too long at
high temperature.
[0092] Heat Treatment of Fine Grain Step:
[0093] Referring to FIG. 1, which is a process chart of the heat
treatment process for the hot-work die steel in the present
application, heating the annealed ingot to 930-1150.degree. C. and
performing a first holding for a first holding time of [(15-40)
rain r (mm).times.2 min/mm] or [(15-40) min+d (mm)/2.times.2
min/mm], wherein the specific first holding time can be determined
according to the size of the material, and the above process is a
normalizing process, after that, water cooling to 400-500.degree.
C. within 1-2 min, then air cooling to 250-280.degree. C. and
performing a second holding for a second holding time of 5-10 h;
and then holding at a temperature of 660-700.degree. C. for 5-10
h;
[0094] wherein r is a radius of the material and d is a thickness
of the material. When the ingot is a cylinder, the above r can be
used to calculate the holding time. When the ingot is a cube, the
above d can be used to calculate the holding time, wherein the
specific calculation method is determined according to the actual
shape of the material.
[0095] In the present application, the process of water cooling
after normalizing to 400-500.degree. C. and air cooling to
250-280.degree. C. for 5-10 h is adopted to refine grains by
forming B/M (Bainite/martensite) duplex structure, and then
dispersive secondary carbides are formed by holding at
660-700.degree. C. to hinder the growth of austenite grain during
subsequent tempering heating. The inventor unexpectedly found that
the high temperature tensile strength of the material is higher
compared with that obtained by the present heat treatment methods.
This may be due to the fact that the fine grain heat treatment
method of the present application can refine the grain while
improving the solid solubility of the material.
[0096] Tempering Treatment Step:
[0097] heating the held ingot to 980-1100.degree. C., holding for
[(15-40) min+r (mm).times.2 min/mm] or [(15-40) min+d
(mm)/2.times.2 min/mm], then cooling to 50-150.degree. C.; and then
tempering and holding at 580-660.degree. C. for 6-16 h to obtain
the hot-work die steel.
[0098] The heating temperature in the tempering treatment step of
the application is increased by 30-50.degree. C. compared with that
of the existing hot-work die steel, so as to improve the solid
solubility of alloy elements. In addition, there is no specific
limitation to the cooling method of the tempering treatment step in
the present application. It can be such as air cooling, water
cooling or oil cooling.
[0099] In the tempering and holding step of the application,
tempering at 580-660.degree. C. allows the hot-work die steel to
form a nanoscale MC type alloy carbide with low mismatch degree,
and further improves the thermal stability of the material.
[0100] In an embodiment of the present application, the raw
material further comprises at least one of the following
constituents:
[0101] Zr: 0.01-0.03 wt %, Co: 0.10-0.50 wt %, B: 0.001-0.005 wt %,
Re: 0.01-0.10 wt %, Ti: 0.02-0.06 wt %, and Y: 0.01-0.1 wt %.
[0102] In an embodiment of the present application, the forging
step may include:
[0103] using a precision forging machine for forming and forging,
with the forging heating temperature of 900-1050.degree. C., the
initial forging temperature of 850-950.degree. C., and the final
forging temperature .gtoreq.800.degree. C.; alternatively, the
forging heating temperature of 1150-1200.degree. C., the initial
forging temperature of 1130-1160.degree. C., and the final forging
temperature .gtoreq.850.degree. C., so as to obtain the forgings
with appropriate shape and size.
[0104] There is no specific limitation to the model of the
precision forging machine, hydraulic hammer or hydraulic press, so
long as the purpose of the application can be achieved, for
example, the precision forging machine produced by GFM company in
Austria can be used.
[0105] The present application provides a heat treatment process
for the hot-work die steel, wherein the hot-work die steel is
allowed to have the tensile strength of 560-700 MPa at 700.degree.
C. and the value of hardness of 32-38 HRC at room temperature after
holding for 3-5 h at 700.degree. C. by controlling the addition
proportion of each raw material and reasonable forging and heat
treatment process. Moreover, the hot-work die steel in the present
application has good plasticity and toughness at room temperature,
which expands the application range of the hot-work die steel.
[0106] In the following, examples and comparative examples are
illustrated to explain the implementation mode of the application
more specifically. Various tests and evaluations are carried out
according to the following methods. In addition, "parts" and "%"
are the weight basis unless otherwise indicated.
Example 1
[0107] <Smelting>
[0108] The raw material was prepared according to the following
mass percentages:
[0109] C: 0.19 wt %, Si: 0.20 wt %, Mn: 0.30 wt %, Cr: 2.22 wt %,
Mo: 2.30 wt %, W: 0.50 wt %, Ni: 0.50 wt %, V: 0.22 wt %, Nb: 0.20
wt %, and a balance of iron, and the raw material was processed
into an electrode rod by arc smelting, refining, vacuum degassing,
and forging in forging furnace.
[0110] <Electroslag Remelting>
[0111] The oxidized layer of the electrode rod was removed, then
the electrode rod was introduced into a vacuum electroslag
remelting device. The temperature of water in the water cooling
system of the electroslag remelting device was held at 70.degree.
C. to obtain an electroslag ingot by electroslag remelting from the
electrode rod.
[0112] <Homogenization Annealing>
[0113] The electroslag ingot was heated to 1200.degree. C. for 23
h.
[0114] <Forging>
[0115] The electroslag ingot was cooled to a forging heating
temperature of 1150.degree. C. and then forged to obtain an ingot.
The initial forging temperature is 1130.degree. C., and the final
forging temperature is 850.degree. C. The ingot had a radius of 40
min and a length of 100 min.
[0116] <Annealing after Forging>
[0117] The ingot was introduced into an annealing furnace under the
temperature of lower than 500.degree. C., heated to 830.degree. C.
at a heating rate of 80.degree. C./h, held for 200 min. Then,
lowering the temperature to below 450.degree. C. at a cooling rate
of 20.degree. C./h, taking the ingot out from annealing furnace,
and air-cooling to obtain an annealed ingot.
[0118] <Heat Treatment of Fine Grain>
[0119] The annealed ingot was heated to 930.degree. C. for a first
holding, wherein a first holding time was 2 h, water cooled to
400.degree. C. within 1 min, then air cooled to 250.degree. C. for
a second holding, wherein a second holding time was 10 h; and then
held at a temperature of 660.degree. C. for 10 h.
[0120] <Tempering Treatment>
[0121] The held ingot was heated to 1000.degree. C., held for 2 h,
then quenched to 50.degree. C., and then tempered at 600.degree. C.
for 16 h to obtain the hot-work die steel.
Example 2
[0122] <Smelting>
[0123] The raw material was prepared according to the following
mass percentages:
[0124] C: 0.23 wt %, Si: 0.20 wt %, Mn: 0.30 wt %, Cr: 2.48 wt %,
Mo: 2.15 wt %, W: 0.50 wt %, Ni: 0.50 wt %, V: 0.28 wt %, Nb: 0.10
wt %, and a balance of iron, and the raw material was processed
into an electrode rod by arc smelting, refining, vacuum degassing,
and forging in forging furnace.
[0125] <Electroslag Remelting>
[0126] The oxidized layer of the electrode rod was removed, then
the electrode rod was introduced into a vacuum electroslag
remelting device. The temperature of water in the water cooling
system of the electroslag remelting device was held at 65.degree.
C. to obtain an electroslag ingot by electroslag remelting from the
electrode rod.
[0127] <Homogenization Annealing>
[0128] The electroslag ingot was heated to 1230.degree. C. for 20
h.
[0129] <Forging>
[0130] The electroslag ingot was cooled to a forging heating
temperature of 1170.degree. C. and then forged to obtain an ingot.
The initial forging temperature is 1150.degree. C., and the final
forging temperature is 860.degree. C. The ingot had a radius of 40
mm and a length of 100 mm.
[0131] <Annealing after Forging>
[0132] The ingot was introduced into an annealing furnace under the
temperature of lower than 500.degree. C., heated to 850.degree. C.
at a heating rate of 90.degree. C./h, held for 200 min. Then,
lowering the temperature to below 480.degree. C. at a cooling rate
of 30.degree. C./h, taking the ingot out from annealing furnace,
and air-cooling to obtain an annealed ingot.
[0133] <Heat Treatment of Fine Grain>
[0134] The annealed ingot was heated to 980.degree. C. for a first
holding, wherein a first holding time was 2 h, water cooled to
450.degree. C. within 1.5 min, then air cooled to 260.degree. C.
for a second holding, wherein a second holding time was 6 h; and
then held at the temperature of 660.degree. C. for 5 h.
[0135] <Tempering Treatment>
[0136] The held ingot was heated to 1020.degree. C., held for 1.5
h, then quenched to 100.degree. C., and then tempered at
620.degree. C. for 10 h to obtain the hot-work die steel.
Example 3
[0137] <Smelting>
[0138] The raw material was prepared according to the following
mass percentages:
[0139] C: 0.27 wt %, Si: 0.04 wt %, Mn: 0.07 wt %, Cr: 2.72 wt %,
Mo: 1.90 wt %, W: 0.95 wt %, Ni: 1.22 wt %, V: 0.40 wt %, Nb: 0.10
wt %, Y: 0.02 wt % and a balance of iron, and the raw material was
processed into an electrode rod by arc smelting, refining, vacuum
degassing, and forging in forging furnace.
[0140] <Electroslag Remelting>
[0141] The oxidized layer of the electrode rod was removed, then
the electrode rod was introduced into a vacuum electroslag
remelting device. The temperature of water in the water cooling
system of the electroslag remelting device was held at 68.degree.
C. to obtain an electroslag ingot by electroslag remelting from the
electrode rod.
[0142] <Homogenization Annealing>
[0143] The electroslag ingot was heated to 1250.degree. C. for 15
h.
[0144] <Forging>
[0145] The electroslag ingot was cooled to a forging heating
temperature of 1200.degree. C. and then forged to obtain an ingot.
The initial forging temperature is 1160.degree. C., and the final
forging temperature is 870.degree. C. The ingot had a radius of 40
mm and a length of 100 mm.
[0146] <Annealing after Forging>
[0147] The ingot was introduced into an annealing furnace under the
temperature of lower than 500.degree. C., heated to 900.degree. C.
at a heating rate of 100.degree. C./h, held for 200 min. Then,
lowering the temperature to below 490.degree. C. at a cooling rate
of 40.degree. C./h, taking the ingot out from annealing furnace,
and air-cooling to obtain an annealed ingot.
[0148] <Heat Treatment of Fine Grain>
[0149] The annealed ingot was heated to 1000.degree. C. for a first
holding, wherein a first holding time was 2 h, water cooled to
500.degree. C. within 2 min, then air cooled to 280.degree. C. for
a second holding, wherein a second holding time was 6 h; and then
held at a temperature of 680.degree. C. for 5 h.
[0150] <Tempering Treatment>
[0151] The held ingot was heated to 1020.degree. C., held for 1.5
h; then quenched to 150.degree. C., and then tempered at
635.degree. C. for 6 h to obtain the hot-work die steel.
Example 4
[0152] <Smelting>
[0153] The raw material was prepared according to the following
mass percentages:
[0154] C: 0.30 wt %, Si: 0.12 wt %, Mn: 0.02 wt %, Cr: 2.00 wt %,
Mo: 1.65 wt %, W: 1.10 wt %, Ni: 1.42 wt %, V: 0.42 wt %, Nb: 0.02
wt %, Zr: 0.02 wt %, Co: 0.10 wt %, B: 0.003 wt %, Re: 0.012 wt %,
Ti: 0.03 wt %, Y: 0.02 wt % and a balance of iron, and the raw
material was processed into an electrode rod by arc smelting,
refining, vacuum degassing, and forging in forging furnace.
[0155] <Electroslag Remelting>
[0156] The oxidized layer of the electrode rod was removed, then
the electrode rod was introduced into a vacuum electroslag
remelting device. The temperature of water in the water cooling
system of the electroslag remelting device was held at 69.degree.
C. to obtain an electroslag ingot by electroslag remelting from the
electrode rod.
[0157] <Homogenization Annealing>
[0158] The electroslag ingot was heated to 1250.degree. C. for 15
h.
[0159] <Forging>
[0160] The electroslag ingot was cooled to a forging heating
temperature of 1200.degree. C. and then forged to obtain an ingot.
The initial forging temperature is 1160.degree. C., and the final
forging temperature is 870.degree. C. The ingot had a radius of 40
mm and a length of 100 mm.
[0161] <Annealing after Forging>
[0162] The ingot was introduced into an annealing furnace under the
temperature of lower than 500.degree. C., heated to 900.degree. C.
at a heating rate of 100.degree. C./h, held for 200 min. Then,
lowering the temperature to below 490.degree. C. at a cooling rate
of 40.degree. C./h, taking the ingot out from annealing furnace,
and air-cooling to obtain an annealed ingot.
[0163] <Heat Treatment of Fine Grain>
[0164] The annealed ingot was heated to 1100.degree. C. for a first
holding, wherein a first holding time was 2 h, water cooled to
500.degree. C. within 2 min, then air cooled to 270.degree. C. for
a second holding, wherein a second holding time was 6 h; and then
held at the temperature of 700.degree. C. for 5 h.
[0165] <Tempering Treatment>
[0166] The held ingot was heated to 1050.degree. C., held for 1 h,
then quenched to 100.degree. C., and then tempered at 640.degree.
C. for 6 h to obtain the hot-work die steel.
Example 5
[0167] <Smelting>
[0168] The raw material was prepared according to the following
mass percentages:
[0169] C: 0.32 wt %, Si: 0.30 wt %, Mn: 0.15 wt %, Cr: 2.75 wt %,
Mo: 2.30 wt %, W: 0.65 wt %, Ni: 0.63 wt %, V: 0.70 wt %, Ni): 0.04
wt %, Y: 0.01 wt % and a balance of iron, and the raw material was
processed into an electrode rod by arc smelting, refining, vacuum
degassing, and forging in forging furnace.
[0170] <Electroslag Remelting>
[0171] The oxidized layer of the electrode rod was removed, then
the electrode rod was introduced into a vacuum electroslag
remelting device. The temperature of water in the water cooling
system of the electroslag remelting device was held at 66.degree.
C. to obtain an electroslag ingot by electroslag remelting from the
electrode rod.
[0172] <Homogenization Annealing>
[0173] The electroslag ingot was heated to 1230.degree. C. for 20
h.
[0174] <Forging>
[0175] The electroslag ingot was cooled to a forging heating
temperature of 1180.degree. C. and then forged to obtain an ingot.
The initial forging temperature is 1140.degree. C., and the final
forging temperature is 870.degree. C. The ingot had a radius of 40
mm and a length of 100 mm.
[0176] <Annealing after Forging>
[0177] The ingot was introduced into an annealing furnace under the
temperature of lower than 500.degree. C., heated to 850.degree. C.
at a heating rate of 95.degree. C./h, held for 200 min. Then,
lowering the temperature to below 485.degree. C. at a cooling rate
of 35.degree. C./h, taking the ingot out from annealing furnace,
and air-cooling to obtain an annealed ingot.
[0178] <Heat Treatment of Fine Grain>
[0179] The annealed ingot was heated to 1140.degree. C. for a first
holding, wherein a first holding time was 2 h, water cooled to
430.degree. C. within 1 min, then air cooled to 270.degree. C. for
a second holding, wherein a second holding time is 6 h; and then
held at the temperature of 680.degree. C. for 5 h.
[0180] <Tempering Treatment>
[0181] The held ingot was heated to 1050.degree. C., held for 1 h,
then quenched to 70.degree. C., and tempered at 580.degree. C. for
4 h and then secondly tempered at 64.degree. C. for 2 h to obtain
the hot-work die steel.
Example 6
[0182] The raw material comprised W of 1.00 wt %, Ni of 1.22 wt %,
V of 0.60 wt %, of 0.02 wt %, Zr of 0.01 wt Co of 0.20 wt %, B of
0.001 wt %, Re of 0.05 wt %, Ti of 0.04 wt %, and Y of 0.02 wt %,
other constituents were the same as that of Example 5.
Example 7
[0183] The raw material comprised Cr of 1.5 wt %, W of 1.00 wt %,
Ni of 1.22 wt %, V of 0.60 wt %, Nb of 0.02 wt %, Zr of 0.03 wt %,
Co of 0.40 wt %, B of 0.005 wt %, Re of 0.10 wt %, Ti of 0.06 wt Y
of 0.10 wt %, other constituents were the same as that of Example
5.
Comparative Example 1
[0184] This Comparative Example provided a H13 hot-work die steel,
of which the specification was 40 mm in radius and 100 mm in
length. The heat treatment process thereof included the following
steps:
[0185] quenching: the forged ingot was heated to 1050.degree. C.,
held for 1 h, and water cooled; tempering: the quenched ingot was
heated to 590.degree. C., held for 2 h, then heated to 620.degree.
C. and then held for 2 h.
Comparative Example 2
[0186] This Comparative Example provided a 3Cr2W8V hot-work die
steel, of which the specification was 40 mm in radius and 100 mm in
length. The heat treatment process thereof included the following
steps:
[0187] quenching: the forged ingot was heated to 1130.degree. C.,
held for 1 h, and water cooled; tempering: the quenched ingot was
heated to 610.degree. C., held for 2 h, then heated to 630.degree.
C. and then held for 2 h.
[0188] <Performance Test>
[0189] High Temperature Strength Test:
[0190] The hot-work die steels in Examples 1-7 and Comparative
Examples 1-2 were tested for the high temperature tensile strength
at 700.degree. C. according to GB/T4338-2006, High temperature
tensile test method for metallic materials. The test results are
shown in Table 2.
[0191] Thermal Stability Test:
[0192] The hot-work die steels in Examples 1 and 5 and Comparative
Examples 1-2 were tested for the room temperature Rockwell hardness
(HRC) after holding at different temperatures for 4 h. The test
results are shown in Table 3.
[0193] Room Temperature Performance Test:
[0194] The hot-work die steels in Examples 1 and 5 and Comparative
Examples 1-2 were tested for the room temperature tensile
performances and impact toughness (U-shape notch). The test results
include elongation (A), percentage of reduction of area (z) and
room temperature impact toughness (A.sub.ku), as shown Table 4.
[0195] Fracture Toughness Test:
[0196] The compact tensile samples of Examples 1 and 5 and
Comparative Examples 1-2 were selected and tested on the fatigue
test platform (MTS810) according to GB/T 4161-2007, Experimental
method for plane strain fracture toughness K.sub.IC of metallic
materials. The test results are shown in Table 5.
[0197] High Temperature Strain Fatigue Life Test:
[0198] Example 5 and Comparative Example 1 were selected for the
fatigue life test carried out on MTS NEW810 electro-hydraulic servo
fatigue testing machine according to GB/T15248-2002, Axial constant
amplitude low cycle fatigue test method for metallic materials. The
results are shown in Table 6.
TABLE-US-00001 TABLE 1 Constitutions of the hot-work die steel in
each Example or Comparative Example of the application Element
Comparative Comparative content/ Example Example Example Example
Example Example Example Example 1 Example 2 % 1 2 3 4 5 6 7 (H13)
(3Cr2W8V) C 0.19 0.23 0.27 0.30 0.32 0.32 0.32 0.40 0.36 Si 0.20
0.20 0.04 0.12 0.30 0.30 0.30 1.0 0.21 Mn 0.30 0.30 0.07 0.02 0.15
0.15 0.15 0.3 0.28 Cr 2.22 2.48 2.72 2.00 2.75 2.75 1.50 5.00 2.52
Mo 2.30 2.15 1.90 1.65 2.30 2.30 2.30 0.46 -- W 0.50 0.50 0.95 1.10
0.65 1.00 1.00 -- 8.18 Ni 0.50 0.50 1.22 1.42 0.63 1.22 1.22 --
0.06 V 0.22 0.28 0.40 0.42 0.70 0.60 0.60 0.19 0.32 Nb 0.20 0.10
0.10 0.02 0.04 0.02 0.02 -- -- Zr -- -- -- 0.02 -- 0.01 0.03 -- --
Co -- -- -- 0.10 -- 0.20 0.40 -- -- B -- -- -- 0.003 -- 0.001 0.005
-- -- Re -- -- -- 0.012 -- 0.05 0.10 -- -- Ti -- -- -- 0.03 -- 0.04
0.06 -- -- Y -- -- 0.02 0.02 0.01 0.02 0.10 -- -- Fe Balance
Balance Balance Balance Balance Balance Balance Balance Balance
TABLE-US-00002 TABLE 2 Test results of high temperature strength of
the hot- work die steel in each Example or Comparative Example
Example R.sub.m (MPa) R.sub.p0.2 (MPa) Example 1 560 345 Example 2
621 405 Example 3 634 410 Example 4 642 420 Example 5 678 450
Example 6 687 466 Example 7 694 483 Comparative 292 255 Example 1
Comparative 415 364 Example 2
TABLE-US-00003 TABLE 3 Hardness (unit HRC) of Examples 1 and 5 and
Comparative Examples 1-2 Steel Grade 600.degree. C. 620.degree. C.
660.degree. C. 700.degree. C. Example 1 45 43.5 39 32 Example 5 47
45.1 41.3 37.2 Comparative 47 40.2 31 24 Example 1 Comparative 48
46 38.2 29.8 Example 2
TABLE-US-00004 TABLE 4 Room temperature tensile performance of
Examples 1 and 5 and Comparative Examples 1-2 Steel Grade R.sub.m
(Mpa) R.sub.p0.2 (Mpa) A (%) Z (%) A.sub.kn (J) Example 1 1310 1020
16 62 63 Example 5 1350 1050 14 48.3 52 Comparative 1389 1189 11.2
43.7 21.0 Example 1 Comparative 1647 1449 10 30.8 13 Example 2
TABLE-US-00005 TABLE 5 Test results of fracture toughness of
Examples 1 and 5 and Comparative Examples 1-2 Steel Grade Hardness
(HRC) K.sub.IC (MPa m.sup.0.5) Example 1 41 144.2 Example 5 46
107.8 Comparative 44 83.2 Example 1 Comparative 49 32.7 Example
2
TABLE-US-00006 TABLE 6 Test results of high temperature strain
fatigue life of Example 5 and Comparative Example 1 Diameter of
Total strain Frequency Load Loading Service life Steel Grade sample
(mm) amplitude (%) (Hz) (KN) (GPa) (Times) Example 5 6.50 0.2 0.5
1.0 127 12236 6.48 0.4 0.25 1.0 126 990 6.50 0.6 0.167 1.0 130 469
Comparative 6.50 0.2 0.5 1.0 113 9302 Example 1 6.47 0.4 0.25 1.0
113 817 6.50 0.6 0.167 1.0 119 417
[0199] It can be seen from Table 2 that the high-temperature
strength at 700.degree. C. of Examples 1-5 are higher than that of
H13 steel and 3Cr2W8V steel of Comparative Example 1 and
Comparative Example 2. Specifically, the high-temperature strength
at 700.degree. C. of Examples 1 increased by nearly 1 time and the
high-temperature strength at 700.degree. C., of Examples 2-5
increased by more than 1 time compared with that of Comparative
Example 1; the high-temperature strength at 700.degree. C. of
Examples 1 increased by nearly 0.5 times, and the high-temperature
strength at 700.degree. C. of Examples 3-5 increased more than 0.5
times compared with that of Comparative Example 2, indicating that
the hot-work die steel according to the present application has
excellent high temperature strength.
[0200] It can be seen from Table 3 that the hardness reduction at
room temperature of Examples 1 and 5 after holding for 4 h in the
temperature range of 600-700.degree. C. is less than that of H13
steel in Comparative Example 1 and 3Cr2W8V steel in Comparative
Example 2, indicating that the hot-work die steel according to the
application has high thermal stability.
[0201] It can be seen from Table 4 that the elongation (A),
percentage of reduction of area (Z) and room temperature impact
toughness (A.sub.ku) of Examples 1 and 5 are higher than that of
H13 steel in Comparative Example 1 and that of 3Cr2W8V steel in
Comparative Example 2, indicating that the hot-work die steel
according to the application has good room temperature plasticity
and toughness.
[0202] It can be seen from Table 5 that the hot-work die steels in
Examples 1 and 5 have fracture toughness K.sub.IC of 107.8-144.2
MPam.sup.0.5 under 41 HRC and 46 HRC, which increased to more than
1.3 times of that of H13 steel in Comparative Example 1 and more
than 3 times of that of 3Cr2W8V steel in Comparative Example 2,
indicating that the hot-work die steel according to the present
application has good room temperature fatigue resistance.
[0203] It can be seen from table 6 that the fatigue life of sample
with various diameter in example 5 is higher than that of H13 steel
with the same diameter in Comparative Example 1 under the strain
amplitude of 0.2%-0.6%, indicating that the hot-work die steel
according to the present application has better high temperature
low cycle fatigue resistance than H13 steel.
[0204] FIG. 2 is the schematic diagram of the tensile strength
varying with temperature of hot-work die steel produced in Example
5 of the present application and H13 steel of Comparative Example
1. In FIG. 2, the tensile strength of H13 steel rapidly decreases
after the temperature exceeds 600.degree. C., and the tensile
strength at 700.degree. C. is only 292 MPa. However, the tensile
strength of the hot-work die steel in the application decreases
slowly with the increase of temperature, and the tensile strength
at temperature above 650.degree. C. is higher than that of H13
steel. The tensile strength at 700.degree. C. of the steel in the
present application is about 700 MPa, which is about 2 times more
than that of H13 steel.
[0205] FIG. 3a is the electron microscope photo of the hot-work die
steel in Example 5 of the application at room temperature
(25.degree. C.). FIG. 3b is the electron microscope photo of the
hot-work die steel of Example 5 of the application after stretching
at 700.degree. C. FIG. 3c is the partial enlargement of FIG.
3b.
[0206] FIG. 4a is the electron microscope photo of H13 steel in
Comparative Example 1 at room temperature. FIG. 4b is the electron
microscope photo of H13 steel in Comparative Example 1 after
stretching at 700.degree. C. FIG. 4c is the partial enlargement of
FIG. 4b.
[0207] It is shown that the tempered microstructure of the hot-work
die steel in the present application and Comparative Example 1 both
retain lath characteristic at room temperature, according to the
comparison between FIG. 3a and FIG. 4a. However, after undergoing
at 700.degree. C., it is described that the hot-work die steel in
the present application retains the lath characteristic with high
density of nanoscale MC type alloy carbide distributed therein
according to the comparison between FIG. 3b and FIG. 4b and the
comparison between FIG. 3c and FIG. 4c, while the H13 steel in
Comparative Example 1 is completely depleted of the lath
characteristic, in which the carbides undergo coarsening and
spheroidizing. It indicates that the nanoscale carbides in the
present application have higher thermal stability and grow up
slowly at 700.degree. C. Therefore, the hot-work die steel in the
present application has excellent thermal stability.
[0208] FIG. 5a is a micro topography, specifically a bright field
image of TEM, of the carbide obtained from the hot-work die steel
in Example 5 of the present application after stretching at
700.degree. C. As shown FIG. 5a, the carbide is nanoscale acicular
MC type alloy carbide.
[0209] FIG. 5b is the electron diffraction pattern of the selected
area of the hot-work die steel in Example 5 of the present
application after stretching at 700.degree. C. As shown in FIG. 5b,
the (200) plane of .alpha. matrix is parallel to (200) plane of MC
carbide, while the [001] direction of .alpha. matrix is parallel to
[011] direction of MC carbide, indicating that MC carbide still
remains good B-N Orientation relationship with a matrix at the
temperature of 700.degree. C.
[0210] FIG. 5c is a high resolution photo of the MC type alloy
carbide obtained from the hot-work die steel in Example 5 of the
present application after stretching at 700.degree. C. As shown in
FIG. 5c, the interface between carbide/matrix still remains high
level coherent relationship, indicating that the hot-work die steel
according to the present application has excellent high temperature
stability.
[0211] FIG. 6 is the analysis diagram of the constitution of the
carbide obtained from the hot-work die steel in Example 5 of the
present application. The results of atom probe analysis shows that
the carbide is a multi-element alloy carbide
(V.sub.0.5-0.8Mo.sub.0.5-0.6Cr.sub.0.15-0.3W.sub.0.06-0.14Nb.sub.0.01-0.0-
2C), wherein the dotted box indicates that the constitution
analysis comes from the carbide in this area. The coherent
relationship between the specific carbide and the matrix is
remained at a higher temperature, and thereby the steel in this
application achieving high strength at high temperature under a low
degree of alloying.
[0212] In conclusion, not bound to any theory, the inventor
believes that the application can maintain the high-temperature
coherent relationship between the carbide and the matrix of the
hot-work die steel through the coordination of the constituents and
the inventive heat treatment process, and achieve the adjustment
and control of the mismatch degree of the carbide/matrix interface.
The stability of the coherent relationship between the carbide and
the matrix can be retained at 700.degree. C., so as to improve the
high-temperature tensile strength of the hot-work die steel.
[0213] Above are only preferred examples of the present
application, which are not intended to limit the protection scope
of this application. Any modifications, equivalent substitutions,
improvements and the like made within the spirit and principles of
the present application are included in the scope of the present
application.
* * * * *