U.S. patent application number 15/023543 was filed with the patent office on 2016-07-28 for non quenched and tempered steel and manufacturing process thereof.
The applicant listed for this patent is JIANGSU SUZHOU STEEL GROUP CO., LTD., PEKING UNIVERSITY FOUNDER GROUP CO., LTD., SUZHOU SUXIN SPECIAL STEEL CO., LTD.. Invention is credited to Donglin LIU, Yifeng XU, Jie YU, Xu ZHOU, Zhiwei ZHOU.
Application Number | 20160215358 15/023543 |
Document ID | / |
Family ID | 51463205 |
Filed Date | 2016-07-28 |
United States Patent
Application |
20160215358 |
Kind Code |
A1 |
LIU; Donglin ; et
al. |
July 28, 2016 |
NON QUENCHED AND TEMPERED STEEL AND MANUFACTURING PROCESS
THEREOF
Abstract
A non quenched and tempered steel and manufacturing process
thereof. The manufacturing process of the non quenched and tempered
steel has a cooling step after the finish rolling step, wherein
intense cooling and moderate cooling are carried out alternatively.
The intense cooling can ensure the surface temperature of the steel
to decrease rapidly; and the moderate cooling allows the core
temperature of the steel to dissipate gradually to the surface; a
further intense cooling is carried out to allow rapid heat
dissipation. The intense cooling and the moderate cooling can be
carried out alternately several times according to practical
requirement. A water cooling mode combining intense cooling and
moderate cooling allows the core temperature and the surface
temperature of the steel to become the same within a short time,
and thus ensures the uniformity of the mechanical properties of the
steel and improves production efficiency.
Inventors: |
LIU; Donglin; (Suzhou,
CN) ; ZHOU; Xu; (Suzhou, CN) ; XU; Yifeng;
(Suzhou, CN) ; ZHOU; Zhiwei; (Suzhou, CN) ;
YU; Jie; (Suzhou, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PEKING UNIVERSITY FOUNDER GROUP CO., LTD.
SUZHOU SUXIN SPECIAL STEEL CO., LTD.
JIANGSU SUZHOU STEEL GROUP CO., LTD. |
Beijing
Jiangsu
Jiangsu |
|
CN
CN
CN |
|
|
Family ID: |
51463205 |
Appl. No.: |
15/023543 |
Filed: |
December 3, 2013 |
PCT Filed: |
December 3, 2013 |
PCT NO: |
PCT/CN2013/088382 |
371 Date: |
March 21, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/60 20130101; C21D
8/005 20130101; C21D 9/0081 20130101; B22D 11/001 20130101; C21D
6/004 20130101; C22C 38/06 20130101; C21D 2211/009 20130101; C22C
38/04 20130101; C21D 11/005 20130101; C22C 38/002 20130101; C21D
8/065 20130101; C21D 8/0231 20130101; C22C 38/58 20130101; C21D
9/0062 20130101; C21D 1/667 20130101; C21D 6/005 20130101; C22C
33/04 20130101; C22C 38/46 20130101; C22C 38/16 20130101; C21D
6/008 20130101; C21D 2211/005 20130101; C22C 38/02 20130101; C22C
38/42 20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C22C 38/46 20060101 C22C038/46; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C21D 1/60 20060101 C21D001/60; C22C 38/00 20060101
C22C038/00; C22C 33/04 20060101 C22C033/04; C21D 8/00 20060101
C21D008/00; C21D 6/00 20060101 C21D006/00; C21D 1/667 20060101
C21D001/667; B22D 11/00 20060101 B22D011/00; C22C 38/02 20060101
C22C038/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2013 |
CN |
201310443677.8 |
Claims
1. A non quenched and tempered steel, comprising following chemical
components by weight percent: carbon 0.42 to 0.50, silicon 0.20 to
0.40, manganese 0.60 to 1.00, chromium 0.00 to 0.30, aluminum 0.010
to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus
0.000 to 0.025, sulfur 0.00 to 0.025, vanadium 0.06 to 0.25, and
balance iron.
2. The non quenched and tempered steel according to claim 1,
comprising following chemical components by weight percent: carbon
0.45 to 0.48, silicon 0.20 to 0.30, manganese 0.60 to 0.8, chromium
0.20 to 0.30, aluminum 0.020 to 0.030, nickel 0.005 to 0.10, copper
0 to 0.20, phosphorus 0 to 0.015, sulfur 0 to 0.015, vanadium 0.10
to 0.25, and balance iron.
3. A process for producing the non quenched and tempered steel of
claim 1, comprising a cooling step carried out at least after a
finishing step, characterized in that: in said cooling step, an
intense cooling and a moderate cooling are carried out alternately
to allow the steel to undergo at least two stages of water cooling,
so that the core temperature and the surface temperature of the
steel become the same within a specified time.
4. the process for producing non quenched and tempered steel
according to claim 3, characterized in that: in said cooling step,
the steel is subjected to three stages of water cooling, wherein
the first stage of water cooling uses intense cooling, the second
stage of water cooling uses moderate cooling, and the third stage
of water cooling uses intense cooling.
5. The process for producing non quenched and tempered steel
according to claim 3, characterized in that: in said cooling step,
the cooling intensity is controlled by controlling the opening
degree of valve(s) of a water cooling unit.
6. The process for producing non quenched and tempered steel
according to claim 3, characterized in that: in said cooling step,
the temperature of the steel decreases 150.degree. C.-300.degree.
C. in 5-7 seconds after water cooling, and the temperature of the
steel further decreases 50.degree. C.-100.degree. C. after
temperature reversion.
7. The process for producing non quenched and tempered steel
according to claim 6, characterized in that: in said cooling step,
the first stage valve opening degree is controlled to be 30%-40%,
the second stage valve opening degree is controlled to be 20%, and
the third stage valve opening degree is controlled to be 30%-40%,
to ensure the surface temperature of the steel to decrease
150.degree. C.-300.degree. C. in 5-7 seconds.
8. The process for producing non quenched and tempered steel
according to claim 6, characterized in that: in said cooling step,
the steel is subjected to temperature-dropping cooling by means of
spray cooling after temperature reversion
9. The process for producing non quenched and tempered steel
according to claim 6, characterized in that: after said
temperature-dropping cooling, the steel is separately disposed on a
cold bed and subjected to air cooling for 12-14 minutes.
10. The process for producing non quenched and tempered steel
according to claim 9, characterized in that: after said air
cooling, the steel is stacked and subjected to shield cooling.
11. The process for producing non quenched and tempered steel
according to claim 3, characterized in that: the steel billet is
subjected to low temperature rolling when the steel billet
temperature is 850.degree. C.-900.degree. C.; the process further
comprises a finish rolling step before the cooling step, in said
finish rolling step, the temperature of the steel billet is
controlled at .ltoreq.850.degree. C. at the entry into the finish
rolling step.
12. The process for producing non quenched and tempered steel
according to claim 3, characterized in that: the process further
comprises a smelting step before the finish rolling step, the
smelting step comprises electric furnace smelting step, ladle
furnace smelting and refining step carried out sequentially.
13. The process for producing non quenched and tempered steel
according to claim 12, characterized in that: a molten iron
smelting is utilized in the electric furnace smelting, the final
phosphorus content is .ltoreq.0.010%, the final carbon content is
0.03% to 0.08%, and the final temperature is 1620-1680.degree.
C.
14. The process for producing non quenched and tempered steel
according to claim 12, characterized in that: in the ladle furnace
smelting step and/or the refining step, silicon carbide,
ferrosilicon powder are used to deoxidize.
15. The process for producing non quenched and tempered steel
according to claim 12, characterized in that: the ladle furnace
smelting step comprises making white slag, and holding the white
slag for 20 minutes or more.
16. The process for producing non quenched and tempered steel
according to claim 12, characterized in that: in the refining step,
the refining time is 45 minutes or more, and the hydrogen content
is controlled at 1.3 ppm or less.
17. The process for producing non quenched and tempered steel
according to claim 16, characterized in that: the process further
comprises a continuous casting step after the refining step, in
said continuous casting step, a overheat is controlled at
20-35.degree. C., a pulling speed is controlled at 0.5 m/min-0.6
m/min.
18. The process for producing non quenched and tempered steel
according to claim 17, characterized in that: the process further
comprises a heating step after the continuous casting step, in said
heating step, the steel billet is placed in a heating furnace to be
heated, wherein the preheating stage temperature is controlled at
850.+-.30.degree. C., the heating stage temperature is controlled
at 1100.+-.30.degree. C., the soaking stage temperature is
controlled at 1130.+-.30.degree. C., and the total time of the
soaking stage is not less than 2 hours.
19. A process for producing a non quenched and tempered steel, the
process comprises following steps, successively: (1) an electric
furnace smelting step: providing iron raw materials having desired
steel composition, a molten iron smelting is utilized in the
electric furnace smelting, wherein final phosphorus content is
.ltoreq.0.010%, final carbon content is 0.03% to 0.08%, and final
temperature is 1620.degree. C.-1680.degree. C.; (2) a ladle furnace
smelting step: wherein deoxidation is carried out using silicon
carbide and/or ferrosilicon powder, white slag is made by adding
lime, the white slag is held for 20 minutes or more; (3) a refining
step: wherein deaeration is carried out, the refining time is
controlled at 45 minutes or more, and the hydrogen content is
controlled at 1.3 ppm or less; (4) a continuous casting step: the
steel melt obtained in the refining step is subjected to continuous
casting, the overheat of the steel melt is controlled at
20-35.degree. C., the pulling speed is controlled at 0.5 m/min-0.6
m/min; (5) a heating step: the steel billet produced in the
continuous casting step is placed in a furnace to be heated,
wherein the preheating stage temperature is controlled at
850.+-.30.degree. C., the heating stage temperature is controlled
at 1100.+-.30.degree. C., the soaking stage temperature is
controlled at 1130.+-.30.degree. C., the total time of the soaking
stage is not less than 2 hours; (6) a finish rolling step: wherein
the temperature of the steel is controlled at 850.degree. C. at the
entry into the finish rolling step; the steel is subjected to low
temperature rolling when the steel temperature is 850-900.degree.
C.; and (7) a cooling step: wherein intense cooling and moderate
cooling are carried out alternately to allow the steel to undergo
at least two stages of water cooling, so that the core temperature
and the surface temperature become the same during the cooling.
Description
TECHNICAL FIELD
[0001] The present invention relates to a non quenched and tempered
steel and its manufacturing process, and belongs to the technical
field of ferrous metallurgy.
BACKGROUND
[0002] Currently, bars for machine cutting in China generally use
ordinary steels, such as 45, 40Cr and 42CrMo steels. These bars
need quenching and tempering heat treatment when used as raw
materials for machine cutting. Quenching and tempering processes
are high in cost. Since quenching and tempering processes increase
energy consumption, pollute the environment, and bring some waste
product loss, they do not meet the energy-saving and environmental
protection requirements at present. Therefore, non quenched and
tempered steels that do not require quenching and tempering
processes and can be directly used for cutting will gradually
replace ordinary steels and become a trend in the future. Non
quenched and tempered steels means mechanical structural steels
that can achieve performance requirements without quenching and
tempering treatment. Using such steels to manufacture parts can
omit quenching and tempering heat treatment process, and has
advantages such as saving energy and materials, simple technology,
etc., and can decrease environmental pollution and prevent
oxidation, decarbonization, deformation and cracking.
[0003] The domestic traditional process for producing non quenched
and tempered steels for cutting includes: electric furnace
smelting.about.refining.about.mold casting.about.controlled rolling
and controlled cooling. The difficulty of such process in
production is the control of steel performance. At present, the
domestic and oversea manufacturers generally achieve the control of
steel performance by improving chemical composition of non quenched
and tempered steels. However, as shown by studies, it is difficult
to make non quenched and tempered steels achieve performance
requirements only by component design.
[0004] Therefore, the Shougang Corporation has proposed a novel
process for producing non quenched and tempered steels, which
process comprises: converter smelting, slag cutoff tapping,
deoxidation alloying in ladle, LF ladle refining, feeding S line,
ladle bottom blowing argon to achieve full protection casting, slab
temperature control, controlled cooling and rolling, etc. In the
rolling step, the heating temperature is 1100-1180.degree. C.,
initial rolling temperature is 1020-1100.degree. C., finishing
rolling temperature is 850-920.degree. C., the relative deformation
is 15-35%. After rolling, the temperature is decreased to
600.degree. C. and then slowly cooled to room temperature. The non
quenched and tempered steels produced by above process are
difficult to ensure the core temperature and the surface
temperature of the steels to become the same by slow cooling within
a short time, and tend to cause the surface and core of the steels
have large differences in strength and toughness, and seriously
nonuniform mechanical properties. When the above process is used to
produce large-sized non quenched and tempered steels (e.g. .phi.70
to .phi.145 mm bars), the phenomenon of nonuniform mechanical
properties of the surface and the core of the bars is even more
obvious.
SUMMARY OF THE INVENTION
[0005] The technical problem to be solved by the present invention
is to overcome the defects that the surface mechanical properties
and core mechanical properties of the steels produced by existing
processes for producing non quenched and tempered steels are
nonuniform. Therefore, the present invention provides a non
quenched and tempered steel and its production process to ensure
the uniformity of the surface mechanical properties and core
mechanical properties of final products.
[0006] For this purpose, the present invention provides a non
quenched and tempered steel consisting of following chemical
components by weight percent: carbon 0.42 to 0.50, silicon 0.20 to
0.40, manganese 0.60 to 1.00, chromium 0.00 to 0.30, aluminum 0.010
to 0.030, nickel 0.00 to 0.10, copper 0.00 to 0.20, phosphorus
0.000 to 0.030, sulfur 0.00 to 0.035, vanadium 0.06 to 0.25, and
balance iron.
[0007] In a preferred embodiment, the non quenched and tempered
steel according to present invention consists of following chemical
components by weight percent: carbon 0.45 to 0.48, silicon 0.20 to
0.30, manganese 0.60 to 0.8, chromium 0.20 to 0.30, aluminum 0.020
to 0.030, nickel 0.005 to 0.10, copper 0 to 0.20, phosphorus 0 to
0.030, sulfur 0 to 0.035, vanadium 0.10 to 0.25, and balance
iron.
[0008] The present invention provides a process for producing non
quenched and tempered steel, comprising a cooling step at least
carried out after a finishing step, characterized in that: in said
cooling step, an intense cooling and a moderate cooling are carried
out alternately to allow the steel to undergo at least two stages
of water cooling, so that the core temperature and the surface
temperature of the steel become the same within a specified
time.
[0009] According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
steel is subjected to three stages of water cooling, wherein the
first stage of water cooling uses intense cooling, the second stage
of water cooling uses moderate cooling, and the third stage of
water cooling uses intense cooling.
[0010] According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
cooling intensity is controlled by controlling the opening degree
of valve(s) of a water cooling unit.
[0011] According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
temperature of the steel decreases 150.degree. C.-300.degree. C. in
5-7 seconds after water cooling, and the temperature of the steel
further decreases 50.degree. C.-100.degree. C. after temperature
reversion.
[0012] According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
first stage valve opening degree is controlled to be 30%-40%, the
second stage valve opening degree is controlled to be 20%, and the
third stage valve opening degree is controlled to be 30%-40%, to
ensure the surface temperature of the steel to decrease 150.degree.
C.-300.degree. C. in 5-7 seconds.
[0013] According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
steel is subjected to temperature-dropping cooling by means of
spray cooling after temperature reversion.
[0014] According to the process for producing non quenched and
tempered steel of present invention, after said
temperature-dropping cooling, the steel is separately disposed on a
cold bed and subjected to air cooling for 12-14 minutes.
[0015] According to the process for producing non quenched and
tempered steel of present invention, after said air cooling, the
steel is stacked and subjected to shield cooling.
[0016] In the process for producing non quenched and tempered steel
according to present invention, the steel billet is subjected to
low temperature rolling when the steel billet temperature is
850.degree. C.-900.degree. C., the process further comprises a
finish rolling step before the cooling step, in said finish rolling
step, the temperature of the steel billet is controlled at
.ltoreq.850.degree. C. at the entry into the finish rolling
step.
[0017] The process for producing non quenched and tempered steel
according to present invention further comprises a smelting step
before the finish rolling step, the smelting step comprises
electric furnace smelting step, ladle furnace smelting step and
refining step carried out sequentially.
[0018] According to the process for producing non quenched and
tempered steel of present invention, a molten iron smelting is
utilized in the electric furnace smelting, wherein the final
phosphorus content is .ltoreq.0.010%, the final carbon content is
0.03% to 0.08%, and the final temperature is 1620-1680.degree.
C.
[0019] According to the process for producing non quenched and
tempered steel of present invention, in the ladle furnace smelting
step and/or the refining step, silicon carbide, ferrosilicon powder
are used to deoxidize.
[0020] According to the process for producing non quenched and
tempered steel of present invention, the ladle furnace smelting
step comprises making white slag, and holding the white slag for 20
minutes or more.
[0021] According to the process for producing non quenched and
tempered steel of present invention, in the refining step, the
refining time is 45 minutes or more, and the hydrogen content is
controlled at 1.3 ppm or less.
[0022] The process for producing non quenched and tempered steel of
present invention further comprises a continuous casting step after
the refining step, in said continuous casting step, a overheat is
controlled at 20-35.degree. C., a pulling speed is controlled at
0.5 m/min-0.6 m/min.
[0023] The process for producing non quenched and tempered steel of
present invention further comprises a heating step after the
continuous casting step, in said heating step, the steel billet is
placed in a heating furnace to be heated, wherein the preheating
stage temperature is controlled at 850.+-.30.degree. C., the
heating stage temperature is controlled at 1100.+-.30.degree. C.,
the soaking stage temperature is controlled at 1130.+-.30.degree.
C., and the total time of the soaking stage is not less than 2
hours.
[0024] In a preferred embodiment, the present invention provides a
process for producing a non quenched and tempered steel, the
process comprises following steps, successively: [0025] (1) an
electric furnace smelting step: providing iron raw materials having
desired steel composition, a molten iron smelting is utilized in
the electric furnace smelting, wherein final phosphorus content is
.ltoreq.0.010%, final carbon content is 0.03% to 0.08%, and final
temperature is 1620.degree. C.-1680.degree. C.; [0026] (2) a ladle
furnace smelting step: wherein deoxidation is carried out using
silicon carbide and/or ferrosilicon powder, white slag is made by
adding lime, the white slag is held for 20 minutes or more; [0027]
(3) a refining step: wherein deaeration is carried out, the
refining time is controlled at 45 minutes or more, and the hydrogen
content is controlled at 1.3 ppm or less; [0028] (4) a continuous
casting step: the steel melt obtained in the refining step is
subjected to continuous casting, the overheat of the steel melt is
controlled at 20-35.degree. C., the pulling speed is controlled at
0.5 m/min-0.6 m/min; [0029] (5) a heating step: the steel billet
produced in the continuous casting step is placed in a furnace to
be heated, wherein the preheating stage temperature is controlled
at 850.+-.30.degree. C., the heating stage temperature is
controlled at 1100.+-.30.degree. C., the soaking stage temperature
is controlled at 1130.+-.30.degree. C., the total time of the
soaking stage is not less than 2 hours; [0030] (6) a finish rolling
step: wherein the temperature of the steel is controlled at
.ltoreq.850.degree. C. at the entry into the finish rolling step;
the steel is subjected to low temperature rolling when the steel
temperature is 850-900.degree. C., and [0031] (7) a cooling step:
wherein intense cooling and moderate cooling are carried out
alternately to allow the steel to undergo at least two stages of
water cooling, so that the core temperature and the surface
temperature become the same during the cooling.
[0032] The non quenched and tempered steels according to present
invention have following advantages:
[0033] 1. The process for the production of non quenched and
tempered steel according to present invention alters the cooling
mode before finish rolling in previous production of non quenched
and tempered steel; the process at least has a cooling step after
the finish rolling step; in contrast to the cooling mode in the
prior art which employs the cooling mode of single water cooling or
consistent air cooling, the present process utilizes alternate
intense cooling and moderate cooling. The intense cooling can
ensure the surface temperature of the steel to decrease rapidly;
and the moderate cooling allows the core temperature of the steel
to dissipate gradually to the surface; a further intense cooling is
carried out to allow rapid heat dissipation. The intense cooling
and the moderate cooling can be carried out alternately several
times according to practical requirement. A water cooling mode
combining intense cooling and moderate cooling allows the core
temperature and the surface temperature of the steel to become the
same within a short time, and thus ensures the uniformity of the
mechanical properties of the steel billets and improves the
production efficiency.
[0034] 2. According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
steel is subjected to three stages of water cooling, wherein the
first stage of water cooling uses intense cooling, the second stage
of water cooling uses moderate cooling, and the third stage of
water cooling uses intense cooling. Upon the completion of the
finish rolling, the steel has a relatively high temperature. An
intense cooling is utilized in the first stage of water cooling so
that the surface temperature of the steel decreases rapidly. Due to
the heat transfer effect, after the surface temperature decreases,
the heat of the core gradually transfers to the surface. In order
to make the heat of the core transfers to the surface as much as
possible, a moderate cooling is utilized in the second stage of
water cooling. After the moderate cooling, heat transfer allows the
surface temperature to increase, and then the surface is cooled
rapidly again by intense cooling mode so that the surface heat is
removed quickly. At this time, the heat transfer allows the surface
temperature and the core temperature become the same, thus ensuring
the uniformity of mechanical properties.
[0035] 3. According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
cooling is controlled by controlling the opening degree of valves
of a water cooling unit, in particular, the first stage valve
opening degree is controlled to be 30%-40%, the second stage valve
opening degree is controlled to be 20%, and the third stage valve
opening degree is controlled to be 30%-40% to ensure the surface
temperature of the steel to decrease 150.degree. C.-300.degree. C.
in 5-7 seconds. The flow rate of water can be controlled by
controlling the opening degree of valve, such that the intensity of
water cooling can be controlled. This control manner is very
simple. After the valve opens a certain length, the steel is
introduced into water to perform water treatment. During the water
treatment, the surface of the steel is cooled in all directions to
ensure the uniformity of surface cooling.
[0036] 4. According to the process for producing non quenched and
tempered steel of present invention, in said cooling step, the
steel is subjected to temperature-dropping cooling by means of
spray cooling after temperature reversion. Spray cooling is an
advantageous supplement of water cooling. By spray cooling, the
heat of the core further spreads to the surface, which further
ensures the core temperature and the surface temperature become the
same.
[0037] 5. According to the process for producing non quenched and
tempered steel of present invention, after said
temperature-dropping cooling, the steel is separately disposed on a
cold bed and subjected to air cooling for 12-14 minutes. After
spray cooling, the steel is separately disposed on a cold bed to
perform air cooling. A spray cooling can be additionally provided
to further dissipate surface heat.
[0038] 6. According to the process for producing non quenched and
tempered steel of present invention, after said air cooling, the
steel is stacked to perform shield cooling. Shield cooling is a
manner of slow cooling. In order to avoid the above cooling
procedure too rapid and impairing the structure and property of the
steel, the steel is stacked and subjected to shield cooling. After
the cooling scheme including water cooling, spray cooling and air
cooling, the surface temperature and the core temperature of the
steel has essentially become the same. At this time, shield cooling
is used to slow down the cooling speed, thereby improving the
structure and property of the steel billets.
[0039] 7. According to the process for producing non quenched and
tempered steel of present invention, in said ladle furnace smelting
step, white slag is made and held for 20 minutes or more. The white
slag holding time is controlled strictly to make the effect of
deoxidation, desulfurization and inclusion removal of the white
slag more obvious, thereby improving the purity of steel.
[0040] 8. According to the process for producing non quenched and
tempered steel of present invention, in the refining step, the
refining time is not less than 45 minutes, and the hydrogen content
is controlled at 1.3 ppm or less. The refining process effectively
controls the hydrogen content and can better solve the risk of
subsequent hydrogen induced cracking of steel; there is an adequate
time to make a more uniform composition; and the process provides
an adequate time for inclusions to float upward, thereby
effectively solving the problem of controlling the inclusions, such
that the finished product will be purer.
[0041] 9. According to the process for producing non quenched and
tempered steel of present invention, in the continuous casting
step, the overheat is strictly controlled at 20-35.degree. C.; the
pulling speed is controlled at 0.5 m/min-0.6 m/min. In the
continuous casting, a low overheat and a low pulling speed ensure
the quality of casting blanks.
[0042] 10. The metallographic structure at a magnification of
500.times. of the non quenched and tempered steel produced by above
manufacturing process of present invention comprises ferrite and
pearlite, and has an actual grain size (100.times.) rated to 10 to
11 according to GB/T6394; the grains are fine and uniform; and the
rate difference from the core to the edge is not greater than 1.5.
The mechanical properties of the surface and the core of steel are
uniform. The strength and the toughness from the core to the edge
change very little, thus effectively avoiding the defects that the
mechanical properties of general materials cannot satisfy
application demands after subjecting to large surface processing.
The hardness difference from the core to the edge is less than 30
HB, thus effectively avoiding the disadvantageous effect of large
hardness changes on cutting tools and processing. Moreover, the
content of inclusions is low, and the purity of the steel is high.
The non quenched and tempered steel of present invention can take
the place of high-demanding quenched and tempered steels for use in
cutting application.
[0043] The keypoint of present invention is to make the performance
of the surface and core of steels substantially consistent by
controlling the rolling and by controlling the cooling step after
the rolling, thereby improving the quality of steels. Specific
cooling control includes:
[0044] (1) After finish rolling, the steel is subjected to at least
two stages of water cooling by alternating intense cooling and
moderate cooling, such that the core temperature and the surface
temperature of the steel become the same within a certain time. In
particular, after finish rolling, said steel is subjected to three
stages of water cooling, wherein the first stage of water cooling
uses intense cooling, the second stage of water cooling uses
moderate cooling, and the third stage of water cooling uses intense
cooling. In the specific water cooling, the intensity of cooling is
controlled by controlling the opening degree of valve(s) of the
water cooling unit. In present invention, the intense cooling
generally means a cooling at a cooling rate of .gtoreq.7.degree.
C./s, whereas the moderate cooling means a cooling at a cooling
rate of 2-4.degree. C./s.
[0045] (2) After water cooling, the steel is annealed and then
subjected to temperature-dropping cooling by means of spray cooling
after temperature reversion.
[0046] (3) After said temperature-dropping cooling, the steel is
separately disposed on a cold bed and subjected to air cooling for
12-14 minutes.
[0047] (4) After said air cooling, the steel is stacked and
subjected to shield cooling.
[0048] After finish rolling, the steel is subjected to cooling
control through above manner (especially water cooling), which
alters the cooling mode before finish rolling in previous
production of non quenched and tempered steel; the process at least
has a cooling step after the finish rolling step; in contrast to
the cooling mode in the prior art which employs the cooling mode of
single water cooling or consistent air cooling, the present process
utilizes alternate intense cooling and moderate cooling. The
intense cooling can ensure the surface temperature of the steel to
decrease rapidly; and the moderate cooling allows the core
temperature of the steel to dissipate gradually to the surface; a
further intense cooling is carried out to allow rapid heat
dissipation. The intense cooling and the moderate cooling can be
carried out alternately several times according to practical
requirement. A water cooling mode combining intense cooling and
moderate cooling allows the core temperature and the surface
temperature of the steel to become the same within a short time,
and thus ensures the uniformity of the mechanical properties of the
steel and improves the production efficiency. On such a basis, by a
combination control of subsequent spray cooling, air cooling and
shield cooling allows the core temperature to continuously
dissipate to the surface and the surface temperature to be taken
away constantly; moreover, the combination of said cooling manners
results in an appropriate cooling speed. Use of shield cooling
after air cooling allows the cooling speed not so fast in the case
that the surface temperature and the core temperature of steel are
consistent, thus improving overall mechanical properties.
DESCRIPTION OF THE DRAWINGS
[0049] In order to make present invention to be clearly understood
more easily, the present invention will be further described in
detail according to the embodiments with reference to the drawings,
wherein:
[0050] FIG. 1 is a metallograph at a magnification of 500.times. of
a non quenched and tempered steel produced by the manufacturing
process of present invention;
[0051] FIG. 2 is an image showing the grain size of a non quenched
and tempered steel produced by the manufacturing process of present
invention;
[0052] FIG. 3 is an image showing inclusions of a non quenched and
tempered steel produced by the manufacturing process of present
invention;
DETAILED DESCRIPTION
Example 1
[0053] This example provides a process of producing non quenched
and tempered steel, comprising a finish rolling step and a cooling
step after the finish rolling, wherein, in the finish rolling step,
the temperature of rods at the entry into the finish rolling step
was controlled at .ltoreq.850.degree. C., the rods were subjected
to low temperature rolling when the rods temperature is 850.degree.
C.-900.degree. C. After the rolling, the steel was subjected to
three stages of water cooling by means of a specialized
controllable water cooling unit, wherein the first stage of water
cooling employed intense cooling, the second stage of water cooling
employed moderate cooling, and the third stage of water cooling
employed intense cooling.
[0054] Here, it should be noted that there are many ways to control
water cooling intensity. In this example, the water flow was
controlled by controlling the opening degree of the valve(s) of
water cooling unit so as to control water cooling strength.
Specifically, the first stage valve opening degree was controlled
to be 30%-40%, the second stage valve opening degree was controlled
to be 20%, and the third stage valve opening degree was controlled
to be 30%-40% to ensure the surface temperature of the rods to
decrease 150.degree. C.-300.degree. C. in 5 seconds. After the
temperature of the rods was reversed, the rods were subjected to
spray cooling to decrease the rods temperature by 50-100.degree.
C., such that the heat quickly dissipates, then the rods were
separately disposed on a cold bed and subjected to air cooling for
12-14 minutes, finally the rods were removed from the cold bed and
stacked to undergo shield cooling.
[0055] In the process for producing non quenched and tempered steel
of this example, the rods were subjected to three stages of water
cooling, wherein the first stage of water cooling employed intense
cooling, the second stage of water cooling employed moderate
cooling, and the third stage of water cooling employed intense
cooling. Upon the completion of finish rolling, the rods had a
relatively high temperature. The rods were subjected to the first
stage of water cooling using intense cooling so that the surface
temperature of the steel decreased rapidly. After the surface
temperature decreased, the heat of the core gradually transferred
to the surface, due to the heat transfer effect. In order to make
the heat of the core transfer to the surface as much as possible,
the second stage of water cooling employed moderate cooling so that
more time was left for heat transfer of the core during cooling.
After the moderate cooling, the heat transfer allowed the surface
temperature increasing, and then the surface was cooled rapidly
again by intense cooling manner so that the heat of the surface was
removed quickly. At this time, the heat transfer allowed the
surface temperature and the core temperature become the same, thus
ensuring the uniformity of mechanical properties.
Example 2
[0056] This example provides a process of producing non quenched
and tempered steel, which was a further improvement of Example 1.
As compared with Example 1, this process further comprises a
smelting step before the finish rolling step, the smelting step
comprising electric furnace smelting step, ladle furnace smelting
and refining step carried out sequentially.
[0057] In the electric furnace smelting step, molten iron smelting
was employed, the phosphorus content before steel tapping was
strictly controlled at 0.010%, the final carbon content was 0.03%
to 0.08%, and the final temperature was 1620.degree.
C.-1680.degree. C. The electric furnace smelting can better control
deslagging operation than traditional converter smelting.
[0058] In the ladle furnace (LF) smelting step, deoxidization was
performed using silicon carbide, ferrosilicon powder; lime was
added to make white slag; the white slag was held for 20 minutes or
more so that the inclusions can be thoroughly removed by the white
slag.
[0059] In the refining furnace (VD) smelting step, degassing
treatment was carried out to ensure that hydrogen content was
controlled at 1.3 ppm or less, and the refining time was not less
than 45 minutes.
[0060] Advantages of using LF furnace+VD furnace refining: as
compared with traditional process using LF furnace refining only,
this refining process effectively controls hydrogen content, and
can better solve the risk of subsequent hydrogen induced cracking
of bars; an adequate time allows to obtain a more uniform
composition; and an adequate time is provided for inclusions to
float upward, thus effectively solving the problem of inclusion
control.
Example 3
[0061] This example provides a process for producing non quenched
and tempered steel, which was a further improvement of Example 2.
In this example, the continuous casting step and heating step were
improved. The continuous casting step and the heating step were
arranged after the refining step, and before the rolling step and
the water cooling step.
[0062] In the continuous casting step, molten iron in a tundish was
introduced into a crystallizer by a submerged nozzle, thus the
problem that air tends to be brought in when using traditional
nozzles is avoided. Furthermore, argon gas was blow to the joint
site of the submerged nozzle and the tundish, thus preventing air
from entering into the tundish. The overheat was strictly
controlled at 20-35.degree. C., and the pulling speed was
controlled at 0.5 m/min-0.6 m/min. In the continuous casting, low
overheat and low casting speed ensured the quality of casting
blank. After cutting, the surface of the casting blank needed to be
manually checked to ensure no obvious defects. A macrostructure
sample of the casting blank was taken to ensure that the casting
blank had no cracks or shrinkage. The central looseness was not
more than level three; this requirement was to ensure the quality
of the surface and macrostructure of the rods that are subsequently
rolled. After passing inspection, the casting blank was sent to a
heating furnace to be heated, wherein the preheating stage
temperature was 850.+-.30.degree. C., the heating stage temperature
was 1100.+-.30.degree. C., the soaking stage temperature was
1130.+-.30.degree. C., and the total time of the soaking stage was
not less than 2 hours.
Example 4
[0063] This example provides a non quenched and tempered steel
produced by the process described in above Example 1, which
consists of following chemical components by weight percent: carbon
0.42, silicon 0.20, manganese 0.60, chromium 0.30, aluminum 0.030,
nickel 0.10, copper 0.05, phosphorus 0.010, sulfur 0.015, vanadium
0.06, and balance iron.
Example 5
[0064] This example provides a non quenched and tempered steel
produced by the process described in above Example 1, which
consists of following chemical components by weight percent: carbon
0.49, silicon 0.40, manganese 0.60, chromium 0.20, aluminum 0.010,
nickel 0.05, copper 0.05, phosphorus 0.010, sulfur 0.020, vanadium
0.25, and balance iron.
Example 6
[0065] This example provides a non quenched and tempered steel
produced by the process described in above Example 1, which
consists of following chemical components by weight percent: carbon
0.48, silicon 0.30, manganese 0.80, chromium 0.20, aluminum 0.020,
nickel 0.08, copper 0.06, phosphorus 0.015, sulfur 0.025, vanadium
0.15, and balance iron.
Example 7
[0066] This example provides a non quenched and tempered steel
produced by the process described in above Example 2, which
consists of following chemical components by weight percent: carbon
0.47, silicon 0.25, manganese 0.70, chromium 0.25, aluminum 0.025,
nickel 0.025, copper 0.05, phosphorus 0.010, sulfur 0.020, vanadium
0.50, and balance iron.
Example 8
[0067] This example provides a non quenched and tempered steel
produced by the process described in above Example 2, which
consists of following chemical components by weight percent: carbon
0.49, silicon 0.35, manganese 0.9, chromium 0.30, aluminum 0.030,
nickel 0.075, copper 0.06, phosphorus 0.025, sulfur 0.020, vanadium
0.80, and balance iron.
Example 9
[0068] This example provides a non quenched and tempered steel
produced by the process described in above Example 2, which
consists of following chemical components by weight percent: carbon
0.48, silicon 0.28, manganese 0.95, chromium 0.30, aluminum 0.030,
copper 0.05, phosphorus 0.012, sulfur 0.012, vanadium 0.85, and
balance iron.
Example 10
[0069] This example provides a non quenched and tempered steel
produced by the process described in above Example 3, which
consists of following chemical components by weight percent: carbon
0.43, silicon 0.20, manganese 0.70, chromium 0.30, aluminum 0.030,
nickel 0.10, copper 0.08, sulfur 0.020, vanadium 0.25, and balance
iron.
Example 11
[0070] This example provides a non quenched and tempered steel
produced by the process described in above Example 3, which
consists of following chemical components by weight percent: carbon
0.44, silicon 0.23, manganese 0.50, chromium 0.35, aluminum 0.030,
nickel 0.10, phosphorus 0.015, sulfur 0.012, vanadium 0.25, and
balance iron.
Example 12
[0071] This example provides a non quenched and tempered steel
produced by the process described in above Example 3, which
consists of following chemical components by weight percent: carbon
0.46, silicon 0.20, manganese 1.0, chromium 0.30, aluminum 0.030,
nickel 0.005, copper 0.05, phosphorus 0.015, sulfur 0.015, vanadium
0.25, and balance iron.
[0072] The metallographic structures at a magnification of
500.times. of the core of the non quenched and tempered steel
produced in Examples 4-12 comprise ferrite and pearlite (as shown
in FIG. 1), and have actual grain size (100.times.) rated to 10 to
11 according to GB/T6394 (as shown in FIG. 2); the grains are fine
and uniform; and the rate difference from the core to the edge is
not greater than 1.5. The mechanical properties of the surface and
the core of steel are uniform. The strength and the toughness from
the core to the edge change very little, thus effectively avoiding
the defects that the mechanical properties of general materials
cannot satisfy application demands after subjecting to large
surface processing. The hardness difference from the core to the
edge is less than 30 HB, thus effectively avoiding the
disadvantageous effect of large hardness changes on cutting tools
and processing. Moreover, the content of inclusions is low, and the
purity of the steel is high (as shown in FIG. 3).
[0073] The mechanical property data of the steel produced in
Examples 4-12 were as shown in Table 1. It is clear from Table 1
that the non quenched and tempered steels produced by the process
of present invention have excellent overall mechanical properties
in terms of yield strength, tensile strength, elongation, reduction
of area, impact absorbing energy, and so on. Moreover, it can be
seen from the performance data in Table 1 that, the Example in
which the process of Example 3 is used, and the steel consisting of
carbon 0.48, silicon 0.28, manganese 0.95, chromium 0.30, aluminum
0.030, copper 0.05, phosphorus 0.012, sulfur 0.012, vanadium 0.85,
and balance iron, exhibited the best overall mechanical properties;
i.e. Example 9 exhibited the best overall mechanical
properties.
[0074] As can be seen from the performance data provided in
following table, the non quenched and tempered steels produced by
the process provided in present invention can completely replace
common quenched and tempered 45 steel directly for cutting
processing, and exhibit excellent overall mechanical
properties.
TABLE-US-00001 TABLE 1 mechanical performance data of Examples 4-12
Re- Impact Yield Tensile Elon- duction absorbing strength strength
gation of area energy Hardness Example (N/mm.sup.2) (N/mm.sup.2) %
% J HB 4 532 783 26 46 79 191 5 538 788 24 43 76 190 6 532 784 26
46 79 192 7 533 786 27 47 80 194 8 558 796 28 48 82 202 9 560 810
28 53 86 210 10 554 799 27 46 82 202 11 534 788 26 45 78 191 12 540
802 27 45 80 194
Example 13
[0075] This example provides a general process for producing non
quenched and tempered steels, the process begins from smelting
step. The smelting step comprising electric furnace smelting step,
ladle furnace smelting and refining step carried out sequentially.
In the electric furnace smelting step, molten iron smelting was
employed, the phosphorus content before steel tapping was strictly
controlled at 0.010%, the final carbon content was 0.03% to 0.08%,
and the final temperature was 1620.degree. C.-1680.degree. C. The
electric furnace smelting can better control deslagging operation
than traditional converter smelting. In the ladle furnace (LF)
smelting step, deoxidization was performed using silicon carbide,
ferrosilicon powder; lime was added to make white slag; the white
slag was held for 20 minutes or more so that the inclusions can be
thoroughly removed by the white slag. In the refining furnace (VD)
smelting step, degassing treatment was carried out to ensure that
hydrogen content was controlled at 1.3 ppm or less, and the
refining time was not less than 45 minutes.
[0076] Advantages of using LF furnace+VD furnace refining: as
compared with traditional process using LF furnace refining only,
this refining process effectively controls hydrogen content, and
can better solve the risk of subsequent hydrogen induced cracking
of bars; an adequate time allows to obtain a more uniform
composition; and an adequate time is provided for inclusions to
float upward, thus effectively solving the problem of inclusion
control.
[0077] The refining step was followed by a continuous casting step.
In the continuous casting step, molten iron in a tundish was
introduced into a crystallizer by a submerged nozzle, thus the
problem that air tends to be brought in when using traditional
nozzles is avoided. Furthermore, argon gas was blow to the joint
site of the submerged nozzle and the tundish, thus preventing air
from entering into the tundish. The overheat was strictly
controlled at 20-35.degree. C., and the pulling speed was
controlled at 0.5 m/min-0.6 m/min. In the continuous casting, low
overheat and low casting speed ensured the quality of casting
blank. After cutting, the surface of the casting blank needed to be
manually checked to ensure no obvious defects. A macrostructure
sample of the casting blank was taken to ensure that the casting
blank had no cracks or shrinkage. The central looseness was not
more than level three; this requirement was to ensure the quality
of the surface and macrostructure of the rods that are subsequently
rolled. After passing inspection, the casting blank was sent to a
heating furnace to be heated, wherein the preheating stage
temperature was 850.+-.30.degree. C., the heating stage temperature
was 1100.+-.30.degree. C., the soaking stage temperature was
1130.+-.30.degree. C., and the total time of the soaking stage was
not less than 2 hours.
[0078] The heating step was followed by a finish rolling step and a
cooling step. In the finish rolling step, the temperature of rods
at the entry into the finish rolling step was controlled at
.ltoreq.850.degree. C., the rods were subjected to low temperature
rolling when the rods temperature is 850.degree. C.-900.degree. C.
After the rolling, the steel was subjected to three stages of water
cooling by means of a specialized controllable water cooling unit,
wherein the first stage of water cooling employed intense cooling,
the second stage of water cooling employed moderate cooling, and
the third stage of water cooling employed intense cooling.
[0079] In this example, the water flow was controlled by
controlling the opening degree of the valve(s) of water cooling
unit so as to control water cooling strength. Specifically, the
first stage valve opening degree was controlled to be 30%-40%, the
second stage valve opening degree was controlled to be 20%, and the
third stage valve opening degree was controlled to be 30%-40% to
ensure the surface temperature of the rods to decrease 150.degree.
C.-300.degree. C. in 5 seconds. After the temperature of the rods
was reversed, the rods were subjected to spray cooling to decrease
the rods temperature by 50-100.degree. C., such that the heat
quickly dissipates, then the rods were separately disposed on a
cold bed and subjected to air cooling for 12-14 minutes, finally
the rods were removed from the cold bed and stacked to undergo
shield cooling.
[0080] In the process for producing non quenched and tempered
steels of this example, the rods were subjected to three stages of
water cooling, wherein the first stage of water cooling employed
intense cooling, the second stage of water cooling employed
moderate cooling, and the third stage of water cooling employed
intense cooling. Upon the completion of finish rolling, the rods
had a relatively high temperature. The rods were subjected to the
first stage of water cooling using intense cooling so that the
surface temperature of the steel decreased rapidly. After the
surface temperature decreased, the heat of the core gradually
transferred to the surface, due to the heat transfer effect. In
order to make the heat of the core transfer to the surface as much
as possible, the second stage of water cooling employed moderate
cooling so that more time was left for heat transfer of the core
during cooling. After the moderate cooling, the heat transfer
allowed the surface temperature increasing, and then the surface
was cooled rapidly again by intense cooling manner so that the heat
of the surface was removed quickly. At this time, the heat transfer
allowed the surface temperature and the core temperature become the
same, thus ensuring the uniformity of mechanical properties.
[0081] Obviously, the above examples are merely exemplary examples
for clear description, but not a limitation to the embodiments.
Those skilled in the art can make a change or modification in other
forms on the basis of above descriptions. It is unnecessary and
impossible to list all the embodiments. Obvious changes or
modifications derived therefrom are still within the protection
scope of the present invention.
* * * * *