U.S. patent application number 17/261457 was filed with the patent office on 2021-06-03 for spring steel having superior fatigue life, and manufacturing method for same.
This patent application is currently assigned to BAOSHAN IRON & STEEL CO., LTD.. The applicant listed for this patent is BAOSHAN IRON & STEEL CO., LTD.. Invention is credited to Feng JIN, Yanfeng QI, Genjie WAN, Zhenping WU, Zan YAO.
Application Number | 20210164078 17/261457 |
Document ID | / |
Family ID | 1000005403231 |
Filed Date | 2021-06-03 |
United States Patent
Application |
20210164078 |
Kind Code |
A1 |
YAO; Zan ; et al. |
June 3, 2021 |
SPRING STEEL HAVING SUPERIOR FATIGUE LIFE, AND MANUFACTURING METHOD
FOR SAME
Abstract
A spring steel having a superior fatigue life, and a
manufacturing method for the same. The chemical components thereof
are as follows in weight percentage: C: 0.52-0.62%, Si: 1.20-1.45%,
Mn: 0.25-0.75%, Cr: 0.30-0.80%, V: 0.01-0.15%, Nb: 0.001-0.05%, N:
0.001-0.009%, O: 0.0005-0.0040%, P: .ltoreq.0.015%, S:
.ltoreq.0.015%, and Al: .ltoreq.0.0045%, with the remainder being
Fe and incidental impurities, wherein the following condition is
also met 0.02.ltoreq.(2Nb+V)/(20N+C).ltoreq.0.40. The spring steel
of the present invention has a microstructure of tempered
troostite+tempered sorbite, a prior austenite grain size less than
80 um, a size of alloy nitride and carbide precipitates being 5-60
nm, and a maximum width of single-grain inclusions being less than
30 pm. The spring steel has a handling strength greater than 2020
MPa, superior ductility and toughness (the reduction of
area.gtoreq.40%), and a fatigue life.gtoreq.800,000 times, thereby
meeting application requirements of high-stress springs in
industries, such as automobiles, machinery, and the like.
Inventors: |
YAO; Zan; (Shanghai, CN)
; JIN; Feng; (Shanghai, CN) ; WAN; Genjie;
(Shanghai, CN) ; QI; Yanfeng; (Shanghai, CN)
; WU; Zhenping; (Shanghai, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BAOSHAN IRON & STEEL CO., LTD. |
Shanghai |
|
CN |
|
|
Assignee: |
BAOSHAN IRON & STEEL CO.,
LTD.
Shanghai
CN
|
Family ID: |
1000005403231 |
Appl. No.: |
17/261457 |
Filed: |
July 19, 2019 |
PCT Filed: |
July 19, 2019 |
PCT NO: |
PCT/CN2019/096726 |
371 Date: |
January 19, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/26 20130101;
C22C 38/06 20130101; C21D 9/02 20130101; C22C 38/04 20130101; C21D
9/562 20130101; C22C 38/24 20130101; C22C 38/02 20130101; C21D
2211/001 20130101; C21D 9/5737 20130101; C21D 8/065 20130101; C22C
38/002 20130101 |
International
Class: |
C22C 38/26 20060101
C22C038/26; C21D 9/02 20060101 C21D009/02; C21D 9/573 20060101
C21D009/573; C22C 38/24 20060101 C22C038/24; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 8/06 20060101
C21D008/06; C22C 38/06 20060101 C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 27, 2018 |
CN |
201810842312.5 |
Claims
1. A spring steel having a superior fatigue life, wherein its
chemical composition based on weight percentage is: C: 0.52-0.62%;
Si: 1.20-1.45%; Mn: 0.25-0.75%; Cr: 0.30-0.80%; V: 0.01-0.15%; Nb:
0.001-0.05%; N: 0.001-0.009%; O: 0.0005-0.0040%; P: .ltoreq.0:015%;
S: .ltoreq.0:015%; Al: .ltoreq.0:0045%; a balance of Fe and
unavoidable impurities, wherein the following relationship is
satisfied: 0.02.ltoreq.(2Nb+V)/(20N+C).ltoreq.0.40.
2. The spring steel having a superior fatigue life according to
claim 1, wherein the spring steel has a microstructure that is a
tempered troostite+sorbite structure, an original austenite grain
size.ltoreq.80 .mu.m, a size of alloying nitride and carbide
precipitates in the range of 5-60 nm, and a maximum width of
monoparticle inclusions.ltoreq.30 um.
3. The spring steel having a superior fatigue life according to
claim 1, wherein the spring steel has a machining
strength.gtoreq.2020 MPa, an area reduction rate.gtoreq.40%, and a
fatigue life.gtoreq.800000 cycles.
4. A method for manufacturing the spring steel having a superior
fatigue life according to claim 1, comprising: smelting, continuous
casting, rough rolling, high-speed wire rolling, Stelmor controlled
cooling, wire rod drawing, and quenching and tempering treatment,
wherein an electric furnace or a converter is used for the
smelting; after the smelting, secondary refining is performed with
the use of an LF furnace plus VD or RH degassing treatment; during
the LF refining, the composition and basicity of a synthetic slag
are adjusted to control the contents of the P and S elements in the
steel to be lower than 0.015% and 0.015%; stirring in the presence
of argon is performed to allow for full reaction between a refining
slag and inclusions in the molten steel to realize denaturation and
removal of the inclusions; VD or RH vacuum degassing time is more
than 30 minutes to control a final O content at 0.0005-0.0040%, a
final N content at 0.0010-0.0090%, and a H content of less than 2
ppm; killing time of the ladle is more than 15 min at the end of
the refining to facilitate floating of large particle inclusions,
so that the size of inclusions in molten steel is smaller than 30
.mu.m; in the high-speed wire rolling, heating of a heating furnace
is controlled at 920-1150.degree. C., and holding time is 1.0-3.0
h; a rolling speed is controlled at 15-115 m/s in the high-speed
wire rod rolling process; an online temperature control scheme is
as follows: an inlet temperature of a finishing rolling unit is
880-1050.degree. C., an inlet temperature of a reducing-sizing unit
is 840-970.degree. C., and a silking temperature is 800-950.degree.
C.
5. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein a continuous casting
machine is used to cast a round or square billet having a size of
320-500 mm; during the continuous casting process, a drawing speed
is controlled in the range of 0.5-0.8 m/min, and a tail end soft
reduction is controlled to be greater than 10 mm, so as to control
carbon segregation in a core of the billet to achieve a target of
lower than 1.08.
6. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein the rough rolling adopts
a twice-heating production process, wherein a cast billet is
bloomed into a 115-170 mm square or round blank at a temperature of
1050-1270.degree. C., and a total rolling reduction is higher than
40%.
7. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein when the wire rod is
drawn, a drawing speed is not higher than 3.5 m/min.
8. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein in the quenching and
tempering treatment, a heating temperature prior to the quenching
and tempering treatment of the drawn steel wire is controlled in
the range of 850-1100.degree. C.; oil or water is used as a
quenching medium; a temperature of the quenching medium is
controlled at 15-40.degree. C.; and a tempering temperature is
controlled at 370-550.degree. C., so that a size of nitride and
carbide precipitates in a finished steel wire is controlled in the
range of 5-60 nm.
9. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein in the Stelmor
controlled cooling, air volumes of 14 fans on a Stelmor line are
adjusted in the following ranges: fans F1-F7 have an air volume of
10-100%, fans F8-F12 have an air volume of 0-50%, and fans F13-F14
have an air volume of 0-50%.
10. The spring steel having a superior fatigue life according to
claim 2, wherein the spring steel has a machining
strength.gtoreq.2020 MPa, an area reduction rate.gtoreq.40%, and a
fatigue life.gtoreq.800000 cycles.
11. The spring steel having a superior fatigue life according to
claim 1, wherein 0.045.ltoreq.(2Nb+V)/(20N+C).ltoreq.0.37.
12. The spring steel having a superior fatigue life according to
claim 11, wherein 0.15.ltoreq.(2Nb+V)/(20N+C).ltoreq.0.37.
13. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein the spring steel has a
microstructure that is a tempered troostite+sorbite structure, an
original austenite grain size.ltoreq.80 .mu.m, a size of alloying
nitride and carbide precipitates in the range of 5-60 nm, and a
maximum width of monoparticle inclusions.ltoreq.30 um.
14. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein the spring steel has a
machining strength.gtoreq.2020 MPa, an area reduction
rate.gtoreq.40%, and a fatigue life.gtoreq.800000 cycles.
15. The method for manufacturing the spring steel having a superior
fatigue life according to claim 13, wherein the spring steel has a
machining strength.gtoreq.2020 MPa, an area reduction
rate.gtoreq.40%, and a fatigue life.gtoreq.800000 cycles.
16. The method for manufacturing the spring steel having a superior
fatigue life according to claim 4, wherein in the chemical
composition of the spring steel,
0.045.ltoreq.(2Nb+V)/(20N+C).ltoreq.0.37.
17. The method for manufacturing the spring steel having a superior
fatigue life according to claim 16, wherein
0.15.ltoreq.(2Nb+V)/(20N+C).ltoreq.0.37.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a spring steel and a
method for manufacturing the same, in particular to a spring steel
having a superior fatigue life and a method for manufacturing the
same, wherein the steel may be used to manufacture automotive
springs having a machining strength of at least 2020 MPa, an area
reduction rate.gtoreq.40%, a fine structure, a high steel purity,
as well as a low cost and a superior fatigue life.
BACKGROUND ART
[0002] As an important shock absorption and functional component,
springs are widely used in various aspects of social production and
people's lives, such as transportation, machinery manufacturing,
automobile industry, military industry and daily life. The spring
is used within its elastic range and should return to its original
state after unloading. It is desired to have plastic deformation as
small as possible. Hence, steel wire should have high elastic
limit, yield strength and tensile strength. The higher the yield
ratio, the closer the elastic limit is to the tensile strength, and
thus the higher the strength utilization rate, resulting in a
spring having higher elastic force. The spring relies on elastic
deformation to absorb impact energy. Hence, the spring steel wire
does not have to have high plasticity, but it must at least have
plasticity that can endure spring forming and sufficient toughness
that can endure impact energy. The spring usually works for a long
time under alternating stress, so they must have high fatigue
limit, and good creep and relaxation resistance.
[0003] Along with the progress of the technologies in the
automobile and machinery industries, higher requirements are
imposed on the strength and fatigue life of spring parts.
Development of materials with high strength, good plasticity, and
high fatigue resistance for manufacturing springs has become the
focus of advanced steel companies in various countries.
[0004] At present, the conventional Cr-V family, Cr-Mn family, and
Si-Mn family spring steel materials cannot meet the requirements of
the high-strength spring production. On the other hand, the
commonly used Si-Cr family spring steel with higher strength and
better yield ratio has already reached the limits of the strength
and fatigue life.
[0005] Chinese Patent No. CN101787493B discloses a high-strength
spring steel alloy comprising: 0.56%-0.64% C, 0.80%-1.10% Si,
0.80%-1.20% Mn, P.ltoreq.0.035%, S.ltoreq.0.03%, 0.80%-1.20% Cr,
0.60%-1.00% Mo, 0.20%-0.30% V, 0.05%-0.12% Nb, 0.01%-0.060% N,
0.02%-0.07% RE, and a balance of Fe. Mn, Cr, Mo alloying elements
are added to the above designed material in relatively high
amounts, wherein Mo is mainly used to improve the tempering
stability, long-lasting creep resistance and heat resistance of the
steel.
[0006] Chinese Patent No. CN100455691C discloses a spring steel
alloy comprising 0.4-0.6% C, 1.7-2.5% Si, 0.1-0.4 Mn, 0.5-2.0% Cr,
0-0.006% N, and 0.021-0.07% Al. A design route featuring a
high-carbon, high-silicon and low-manganese alloy is adopted. A
main consideration is to control the amount, size and shape of the
residual austenite to enhance the hydrogen embrittlement resistance
of the steel. High requirements are imposed on the quenching and
tempering process for the material. At the same time, the high
content of alloying AL increases the difficulty in controlling the
inclusions in smelting, and the hard and brittle alumina can easily
lead to reduction of the fatigue life of the spring.
[0007] Chinese Patent No. CN1279204C discloses a spring steel alloy
having a compositional design as follows: 0.30-0.50% C, 0.80-2.0%
Si, 0.50-1.0% Mn, 0.40-1.0% Cr, 0.01-0.5% W, 0.08-0.30% V,
0.005-0.25% of rare earth elements, and optional 0.001-0.10% of B.
A design featuring low carbon is mainly adopted for this alloy. The
content of the Si element is increased to enhance the strength. At
the same time, the W element is used to improve the hardenability
of the steel, improve deformation resistance and prevent
decarburization. However, it is difficult to control smelting and
heat treatment in the presence of W and the rare earth
elements.
[0008] Chinese patent CN1039725C discloses a low-decarburized,
high-toughness spring steel for automobile suspension springs. In
this kind of steel, the content of the Si element is increased
without reducing the C content. The steel comprises 0.5-0.7% C,
1.0-3.5% Si, 0.3-1.5% Mn, 0.3-1.0% Cr, 0.05-0.5% V and/or Nb, less
than 0.02% of P, less than 0.02% of S, 0.5-5.0% Ni and other
unavoidable impurities, the remainder being Fe. In order to solve
the decarburization problem and improve the toughness of the
material, a relatively large amount of the Ni element is added, and
thus the alloy cost is high.
[0009] The existing technical solutions involving alloying mainly
increase material strength by adjustment of the C, Si and Mn
elements. If the Si content is too low, the elastic limit of the
material will be reduced, and the elasticity attenuation resistance
will become poor. If the Si content is too high, the plasticity of
the material will be deteriorated; at the same time, the difficulty
in controlling decarburization will be increased, affecting the
fatigue life of the spring. The addition of alloying elements in
excessively high amounts will lead to higher material costs and
affect the precipitation size at the same time, resulting in
degraded fatigue performance of the material. The designed strength
of the material is still low, and the fatigue life of the spring is
not considered much.
[0010] The lightweight development of automobiles and the
technological progress in the machinery industry have prompted the
continuous improvement of the strength of spring materials. At
present, the commonly used Cr-V family, Cr-Mn family, Si-Mn family,
and Cr-Si family spring steels have reached the limits of the
materials.
Summary
[0011] One object of the present disclosure is to provide a spring
steel having a superior fatigue life and a method for manufacturing
the same. The spring steel has a machining strength.gtoreq.2020
MPa, good plastic toughness (an area reduction rate.gtoreq.40%) and
a fatigue life.gtoreq.800000 cycles. It can meet the application
requirements of high-stress springs in the industries such as
automobiles and machinery.
[0012] To achieve the above object, the technical solution of the
present disclosure is as follows:
[0013] A spring steel having a superior fatigue life, wherein its
chemical composition based on weight percentage is:
[0014] C: 0.52-0.62%;
[0015] Si: 1.20-1.45%;
[0016] Mn: 0.25-0.75%;
[0017] Cr: 0.30-0.80%;
[0018] V: 0.01-0.15%;
[0019] Nb: 0.001-0.05%;
[0020] N: 0.001-0.009%;
[0021] O: 0.0005-0.0040%;
[0022] P: .ltoreq.0.015%;
[0023] S: .ltoreq.0.015%;
[0024] Al: .ltoreq.0.0045%;
[0025] a balance of Fe and unavoidable impurities, wherein the
following relationship is satisfied:
0.02.ltoreq.(2Nb+V)/(20N+C).ltoreq.0.40.
[0026] The microstructure of the spring steel according to the
present disclosure is a tempered troostite+sorbite structure. The
original austenite grain size is .ltoreq.80 .mu.m; the size of
alloying nitride and carbide precipitates is 5-60 nm; and the
maximum width of a monoparticle inclusion is .ltoreq.30 pm.
[0027] In the compositional design of the spring steel according to
the present disclosure:
[0028] C is an essential component for ensuring the room
temperature strength and hardenability of the spring steel, and it
is also an element for the spring steel to achieve a high elastic
limit and good elasticity attenuation resistance. When the C
content is less than 0.52%, the strength of the alloy spring steel
cannot be guaranteed to achieve 2020 MPa or higher, and it is also
undesirable for precipitation of carbides/nitrides of microalloying
elements. However, when the C content is too high, the carbide size
will be too large in the tempering process, and the plasticity of
the material deteriorates, which is undesirable for maintaining
good plastic toughness under high strength, and thus affects the
fatigue life of the material. Hence, the content of the C element
must be less than 0.62%.
[0029] Si is a non-carbide forming element. It is mainly
solid-dissolved in the ferrite phase to play a strengthening role.
Increasing the alloying silicon content is desirable for improving
the elastic limit and elasticity attenuation resistance of the
material, thereby optimizing the spring performances. However, if
the Si content is too high, the plasticity of the material will be
deteriorated, which is undesirable for spring forming, and affects
the life of the finished spring. At the same time, the high content
of Si will increase the tendency of decarburization during the
production and heat treatment of the material, resulting in
increased processing cost. Upon comprehensive consideration, the Si
content in the present material is controlled in the range of
1.2-1.45%.
[0030] Mn is an additive element commonly used in steel. It can
effectively improve hardenability and strength while having little
influence on the plasticity of the steel. To ensure the strength
and hardenability of the alloy, the Mn content cannot be less than
0.25%. When the Mn content is too high, it will cause serious
segregation, and at the same time, it will cause grain growth.
Hence, Mn in the steel needs to be controlled, and the allowable
range is 0.25-0.75%.
[0031] Cr has the effect of improving the hardenability of the
spring steel, and allows for precipitation of alloy cementite in
the tempering process to increase the strength of the material. The
Cr element also has the effect of refining the structure.
Therefore, in order to utilize the effect of Cr on solid solution
strengthening and precipitation strengthening while improving the
material structure, the Cr content should be controlled within
0.30-0.80% in the design of the present material.
[0032] The V and Nb elements are commonly added to steel as
microalloying elements. These two types of elements have a strong
tendency to form nitrides and carbides, thereby increasing the
precipitation and nucleation rate of carbides/nitrides during
tempering, and refining the structure. The carbides/nitrides of V
and Nb are precipitated during the wire rod rolling process, which
is desirable for reducing the austenite grain size in the material,
and improving the strength and plasticity of the material.
Nano-sized precipitates are beneficial to the improvement of the
material strength, plasticity and fatigue life. When the contents
of V and Nb in the alloy are too high, the size of the precipitates
will increase. At the same time, with the mutual influence of these
two elements taken into account, after several runs of testing,
it's verified that desirable effects can be resulted when the V
content is controlled at 0.01-0.15%, and the Nb content is
0.001-0.05%. An increased N content will increase the brittleness
of the material. At the same time, with the effect of N on the
precipitation of alloying elements taken into consideration, it is
necessary to control the N content in the steel in the range of
0.001-0.009%. At the same time, in order to fulfill the purpose of
refining precipitates, (2Nb+V)/(20N+C) in the steel is controlled
in the range of 0.02-0.40, preferably in the range of 0.045-0.37.
In some embodiments, (2Nb+V)/(20N+C) in the steel is in the range
of 0.15-0.37. In order to achieve high strength, good plasticity
and long fatigue life of the finished spring, the original
austenite grain size in the material is .ltoreq.80 pm after
quenching and tempering treatment, and the size of the precipitates
in the steel is controlled in the range of 5-60 nm.
[0033] Al mainly has a deoxygenation effect in the steel. However,
alumina formed by deoxygenation with Al is a hard and brittle phase
which has a significant influence on the fatigue life of the
spring. Large brittle inclusions are one of the main factors that
cause abnormal spring fracture. In order to have an effective
control over the alumina inclusions in the steel, the Al content is
controlled to be .ltoreq.0.0045% in the steel, and the oxygen
content is controlled in the range of 0.0005-0.0040%. In order to
prolong the fatigue life of the spring under high strength, the
width of a monoparticle inclusion in the steel needs to be
controlled at .ltoreq.30 .mu.m.
[0034] In order to ensure the toughness of the material and prevent
defects such as hot brittleness and cold brittleness in the
production process, the contents of the harmful P and S elements in
the steel are controlled at 0.015% or less and 0.015% or less
respectively to increase the purity of the steel.
[0035] The method for manufacturing the spring steel having a
superior fatigue life according to the present disclosure includes:
smelting, continuous casting, rough rolling, high-speed wire
rolling, Stelmor controlled cooling, wire rod drawing, and
quenching and tempering treatment, wherein,
[0036] an electric furnace or a converter is used for the smelting;
after the smelting, secondary refining is performed with the use of
an LF furnace plus VD or RH degassing treatment; during the LF
refining, the composition and basicity of a synthetic slag are
adjusted to control the contents of the P and S elements in the
steel to be lower than 0.015% and 0.015%; stirring in the presence
of argon is performed to allow for full reaction between a refining
slag and inclusions in the molten steel to realize denaturation and
removal of the inclusions; the VD or RH vacuum degassing time
should be more than 30 minutes to ensure sufficient gas removal and
control a final O content at 0.0005-0.0040%, a final N content at
0.0010-0.0090%, and a H content of less than 2 ppm; killing time of
the ladle is more than 15 min at the end of the refining to
facilitate floating of large particle inclusions, so that the size
of inclusions in molten steel can be controlled at .ltoreq.30
um.
[0037] In the high-speed wire rolling, the heating of the heating
furnace is controlled at 920-1150.degree. C., and the holding time
is 1.0-3.0 h. The rolling speed is controlled at 15-115 m/s in the
high-speed wire rod rolling process. A preferred scheme for online
temperature control is as follows: an inlet temperature of a
finishing rolling unit is 880-1050.degree. C., an inlet temperature
of a reducing-sizing unit is 840-970.degree. C., and a silking
temperature is 800-950.degree. C.
[0038] Preferably, a continuous casting machine is used to cast a
round or square billet having a size of 320-500 mm. During the
continuous casting process, the drawing speed range is controlled
in the range of 0.5-0.8 m/min, and the tail end soft reduction is
controlled to be greater than 10 mm, so as to control the carbon
segregation in the core of the billet to achieve the target of
lower than 1.08, prevent secondary oxidation during the casting
process of the molten steel, and at the same time, facilitate
floating and removal of inclusions larger than 30 .mu.m.
[0039] Preferably, the rough rolling adopts a twice-heating
production process, wherein the cast billet is bloomed into a
115-170 mm square or round blank at a temperature of
1050-1270.degree. C., and the total rolling reduction is higher
than 40%. Preferably, when the wire rod is drawn, the drawing speed
is not higher than 3.5 m/min.
[0040] Preferably, in the quenching and tempering treatment, the
heating temperature before the quenching and tempering treatment of
the drawn steel wire is controlled in the range of 850-1100.degree.
C.; the quenching medium is oil or water; the temperature of the
quenching medium is controlled at 15-40.degree. C.; and the
tempering temperature is controlled at 370-550.degree. C., so that
the size of nitride and carbide precipitates in the finished steel
wire is controlled in the range of 5-60 nm.
[0041] Preferably, in the Stelmor controlled cooling, the air
volumes of 14 fans on the Stelmor line are adjusted in the
following ranges: fans F1-F7 have an air volume of 10-100%, fans
F8-F12 have an air volume of 0-50%, and fans F13-F14 have an air
volume of 0-50%.
[0042] In the method for manufacturing the spring steel according
to the disclosure:
[0043] an electric furnace or a converter is used for the smelting;
after the smelting, secondary refining is performed with the use of
an LF furnace plus VD or RH degassing treatment; during tapping of
the molten steel from the electric furnace or converter, furnace
slag is prevented from entering the steel ladle; during the LF
refining, the composition and basicity of the synthetic slag are
adjusted to control the contents of the P and S elements in the
steel to be lower than 0.015% and 0.015%; stirring in the presence
of argon is performed to allow for full reaction between the
refining slag and the inclusions in the molten steel to realize
denaturation and removal of the inclusions; the VD or RH vacuum
degassing time should be more than 30 minutes to ensure sufficient
gas removal and control a final O content at 0.0005-0.0040%, a
final N content at 0.0010-0.0090%, and a H content of less than 2
ppm. The killing time of the ladle is more than 15 min at the end
of the refining to facilitate floating of the large particle
inclusions, so that the size of the inclusions in the molten steel
is controlled at .ltoreq.30um.
[0044] The smelted alloy is cast with a continuous casting machine.
Round or square billets may be cast. The size of the round or
square billets is 320-500 mm. By adjusting the drawing speed and
the tail end soft reduction parameter during the continuous casting
process, the carbon segregation in the core of the billet can be
controlled to achieve the target of less than 1.08. Secondary
oxidation in the molten steel casting process is prevented, and at
the same time, the floating and removal of inclusions larger than
30 um are facilitated. A twice-heating production process is used,
wherein the cast billet is bloomed into a 115-170 mm square or
round blank at a temperature of 1050-1270.degree. C.; the total
rolling reduction is required to be higher than 40%; and the
structure is refined.
[0045] Heating is performed with the heating furnace, wherein the
heating is controlled at 920-1150.degree. C., and the holding time
is 1.0-3.0 h. In the high-speed wire rod rolling process, the
rolling speed is controlled to be 15-115 m/s. A preferred scheme
for online temperature control is as follows: the inlet temperature
of the finishing rolling unit is 880-1050.degree. C., the inlet
temperature of the reducing-sizing unit is 840-970.degree. C., and
the silking temperature is 800-950.degree. C. By adjusting the
rolling process temperature and the silking temperature, the
original austenite grains in the material are refined to .ltoreq.80
um, and the size of the precipitates is controlled to be 5-60
nm.
[0046] The size of the rolled wire rod is .PHI.5-28mm. After the
wire rod is rolled, the structure change of the wire rod is
controlled by adjusting the fan components of the Stelmor line. The
air volumes of 14 fans on the Stelmor line are adjusted in the
following ranges: fans F1-F7 have an air volume of 10-100%, fans
F8-F12 have an air volume of 0-50%, and fans F13-F14 have an air
volume of 0-50%.
[0047] The wire rod needs to be drawn before heat treatment, and
the drawing speed should be controlled to be not higher than 3.5
m/min during drawing. The heating temperature of the drawn steel
wire is controlled in the range of 850-1100.degree. C. before
quenching and tempering treatment. The quenching medium may be oil
or water, and its temperature is controlled at 15-40.degree. C. The
tempering temperature is controlled at 370-550.degree. C., so as to
control the size of the precipitates in the finished steel wire at
5-60 nm.
[0048] The beneficial effects of the present disclosure
include:
[0049] The strength of the spring steel produced using the steel
composition and manufacturing method according to the present
disclosure can reach 2020 MPa or higher. The cost of this alloy is
low. The material strengthened by the nano-sized precipitates has
good plastic toughness and good spring formability at the same
time, and cracking during the processing is prevented. With the
refinement of the structure and the control over the composition
and size of the inclusions, the finished spring has a high fatigue
life, which can meet the requirements of automotive lightweight as
well as high strength and long service life in the machinery
industry. This is desirable for promoting the technical level of
the industry, and brings about favorable economic benefits.
DETAILED DESCRIPTION
[0050] The chemical compositions of Examples A1-10# according to
the present disclosure and three Comparative Steel Grades B1-3# are
shown in Table 1 below, and the specific manufacturing methods are
as follows:
[0051] Examples A1-5# according to the present disclosure, and
Comparative Steel Grades B1 and B2 alloys were smelted with the use
of an electric furnace, and Example A6-10190 and Comparative Steel
Grade B3 alloys were smelted with the use of a converter. Then,
secondary refining was performed, wherein Examples A1-3#, A6-8#,
and Comparative Steel Grade B1 alloys were treated with an LF
furnace plus VD refining, while Examples A4-5#, A9-10#, Comparative
Steel Grade B2, and B3 alloys were treated with LF plus RH. The
structure and basicity of a synthetic slag were optimized. A1-6#,
and B1 were vacuum degassed for 30 minutes, and A7-10#, B2, and B3
were vacuum degassed for 35 minutes. The final O content was
controlled at 0.0005-0.0040%, the N content was 0.001-0.009%, and
the H content was less than 2 ppm.
[0052] After smelting, A1-4# and B1 were cast into 300 mm round
billets, A5-6# were cast into 450 mm round billets, A7-9# and B2
were cast into 320*420 mm square billets, and A10# and B3 were cast
into 500mm square billets. A tundish covering agent and a casting
mold with good sealing performance were used to protect the slag in
the casting process. The blooming temperature for the A1-5# and B1
continuously cast billets was 1050.degree. C., and the end face
size of the rolled small square blanks was 115 mm. The heating
temperature for A6-7# and B2 square billets was 1270.degree. C.,
and the size of the rolled blanks was 125 mm. The heating
temperature for A8-10# and B3 square billets was 1100.degree. C.,
and the size of the rolled blanks was 170 mm.
[0053] The furnace temperature of the heating furnace for A1-4# and
B1 was controlled at 920.degree. C., and the holding time was 1.0
h. The temperature of the heating furnace for A5-10#, B2 and B3 was
controlled at 1150.degree. C., and the holding time was 3.0 h. In
the high-speed wire rod rolling process, the rolling speed was
controlled to be 15-115 m/s. The online temperature control scheme:
for the A1-6# and B1 alloys, the inlet temperature of the finishing
rolling unit was 880-950.degree. C., the inlet temperature of the
reducing and sizing unit was 840-950.degree. C., and the silking
temperature was 800-890.degree. C.; for the A7-10#, B2 and B3
alloys, the inlet temperature of the finishing rolling unit was
950-1050.degree. C., the inlet temperature of the reducing and
sizing unit was 940-970.degree. C., and the silking temperature was
870-950.degree. C.
[0054] The dimensions of the A1-5#, B1 and B2 alloy rolled wire
rods were .PHI.5-15mm respectively, and the rolling specifications
of the A6-10# and B3 alloy wire rods were .PHI.16-28mm. After the
rolling of A1-5# and B1 alloy wire rods, the Stelmor cooling
process was: the air volume was 40% for fans F1-F4, 10% for fans
F5-F7, 5% for fans F8-F12, and 40% for fans F13-F14. After the
rolling of A6-10#, B2 and B3 alloy wire rods, the Stelmor cooling
process was: the air volume was 50% for fans F1-F4, 20% for fans
F5-F7, 15% for fans F8-F12, and 35% for fans F13-F14. The structure
of the wire rods after the Stelmor cooling was sorbite plus a very
small amount of ferrite.
[0055] The wire rods were drawn prior to heat treatment. The
quenching and tempering treatment temperatures for the drawn steel
wires were divided into three groups, wherein the heating
temperature was 850.degree. C. and the tempering temperature was
550.degree. C. for A1-2190 and B1; the heating temperature was
980.degree. C. and the tempering temperature was 470.degree. C. for
A3-7# and B2; and the heating temperature was 1100.degree. C. and
the tempering temperature was 370.degree. C. for A8-10# and B3.
[0056] The mechanical properties of the high-strength springs of
Examples A1-A10 and the Comparative Steel Grades B1-B3 are shown in
Table 2 below. As can be seen from the table, the strength of the
alloys all reach 2020 MPa or higher, higher than that of the
samples of Comparative Examples B1-B3. At the same time, the area
reduction rate of the materials can still reach 40% or higher. A
good combination of plasticity and toughness is obtained. The
high-strength springs according to the present disclosure and the
comparative alloys were made into the same type of helical springs,
and the fatigue life of the helical springs was measured using a
spring fatigue testing machine according to GBT16947-2009 "Helical
Spring Fatigue Testing Standard". The results are shown in Table 3.
Under the same conditions, the fatigue life of the high-strength
spring steel according to the present disclosure is superior to
that of the comparative steel.
TABLE-US-00001 TABLE 1 Chemical compositions (wt %) of Examples
A1-10# according to the present disclosure and Comparative Steel
Grades B1-3# Steel No. C Si Mn Cr V Nb Al N O P S A1 0.60 1.40 0.75
0.80 0.15 0.03 0.0030 0.001 0.004 0.015 0.008 A2 0.62 1.45 0.75
0.75 0.15 0.03 0.0027 0.001 0.003 0.010 0.008 A3 0.55 1.30 0.75
0.70 0.15 0.03 0.0030 0.001 0.0015 0.010 0.008 A4 0.58 1.35 0.60
0.60 0.10 0.03 0.0045 0.001 0.0015 0.010 0.008 A5 0.54 1.35 0.55
0.70 0.05 0.05 0.0045 0.001 0.0015 0.010 0.008 A6 0.56 1.35 0.30
0.67 0.02 0.05 0.0010 0.009 0.0015 0.008 0.008 A7 0.55 1.35 0.25
0.70 0.02 0.008 0.0010 0.009 0.0005 0.008 0.008 A8 0.52 1.35 0.60
0.65 0.02 0.05 0.0010 0.009 0.001 0.008 0.015 A9 0.53 1.20 0.60
0.30 0.01 0.05 0.0020 0.005 0.001 0.008 0.004 A10 0.52 1.45 0.60
0.60 0.05 0.001 0.0020 0.005 0.001 0.008 0.004 B1 0.55 1.50 0.70
0.75 0 0 0.0020 0.005 0.001 0.008 0.004 B2 0.65 1.35 0.70 1.05 0.2
0 0.0045 0.001 0.004 0.017 0.001 B3 0.5 1.6 0.55 0.8 0.15 0.08
0.003 0.001 0.006 0.015 0.015
TABLE-US-00002 TABLE 2 Alloy steel structure according to the
present disclosure Original austenite Nitride-carbide Maximum width
of grain size precipitate size monoparticle inclusions Steel No.
(.mu.m) (nm) (.mu.m) A1 75 10-60 28 A2 60 5-55 30 A3 55 30-55 15 A4
54 5-45 19 A5 67 10-55 25 A6 80 7-45 18 A7 60 10-56 30 A8 34 23-60
10 A9 56 12-55 25 A10 77 12-60 30 B1 90 -- 45 B2 50 15-100 50 B3 45
25-145 25
TABLE-US-00003 TABLE 3 Properties of alloy steel examples according
to the present disclosure and comparative steel grades Tensile
strength Area reduction rate Fatigue life Steel No. MPa % Number of
cycles A1 2080 46 85 .times. 10.sup.4 A2 2110 42 90 .times.
10.sup.4 A3 2050 43 105 .times. 10.sup.4 A4 2090 40 99 .times.
10.sup.4 A5 2110 44 90 .times. 10.sup.4 A6 2075 41 98 .times.
10.sup.4 A7 2095 42 100 .times. 10.sup.4 A8 2130 44 110 .times.
10.sup.4 A9 2097 40 95 .times. 10.sup.4 A10 2089 45 97 .times.
10.sup.4 B1 1905 45 70 .times. 10.sup.4 B2 2080 37 57 .times.
10.sup.4 B3 2055 35 60 .times. 10.sup.4
* * * * *