[Docu~iienTt ype] Specilication
[Titlc of the Invention] STEEL AND METHOD OF MANUFACTURNG THE SAME
[Technical Field of the Invention]
5 [OOOI]
The present invention relates to ultrahigh-strength steel such as steel for a
vehicle, steel for an oil well pipe, and steel for building construction which are suitable
for use when ductility is indispensable, and a method of manufacturing the steel.
Specifically, the present invention relates to ultrahigh-strenglh steel in which a tensile
10 strength is 900 MPa or greater, and which has excellent ductility and excellent impact
characteristics, and a method of manufacturing the steel.
[Related Art]
[0002]
15 Recently, development of a material, which contributes to energy saving, has
been required from the viewpoint of global environment protection. In fields of steel
for a vehicle, steel for an oil well pipe, steel for building construction, and the like, a
demand for reduction 111 weight of steel and a demand for ultrahigh-strength steel, which
can be applied to a reduction in weight of steel and a harsh usage environment, have
20 increased, and thus an application range thereof has been expanded. As a result, it is
important for the ultrahigh-strength steel that is used in the fields to secure not only
strength characteristics but also safety in a usage environment. Specifically, it is
important to increase the tolerance with respect to an external plastic deformation by
increasing the ductility of steel.
25 [0003]
2
For example, in a case where a vehicle collides with a structure body, it is
necessary tliat the teusile strength of steel is 900 MPa or greater, and a value (TSxEL) of
the product of the teusile strength (TS) and the total elougatio~(lE L) is 24000 MPa% or
greater in order to sufficiently miligate an impact by using an anti-collision member of
5 the vehicle. However, along with an increase in the tensile strength, the ductility
significantly decreases, and thus there is no ultrahigh-strength steel which satisfies the
above-described characteristics and of which industrial mass production is possible
Accordingly, various kinds of research and development have been conducted so as to
improve the ductility of the ultrahigh-strength steel, and suggested microstructure control
10 methods for realization of the improvement have been suggested
[0004]
For example, Patent Document 1 discloses that with respect to steel which
contains 1.2% to 1.6% of ~i (in this specification, % relating to a chemical composition
of steel represents mass%), and approximately 2% of Mn, a metallographic structure is
15 controlled by optimizing a heating temperature and a retention condition of austempering
so that approximately 10% of austenite is contained in steel, and thus steel having a
tensile strength of 80 kg/mm2 (784 MPa) or greater and excellent ductility is obtained.
[00051
Patent Document 2 discloses that steel, which contains 0.17% or greater of C,
20 and 1 .O% to 2.0% of Si and A1 in a total amount, and approximately 2% of Mn, is heated
to a temperature region of an a~istenites ingle phase, is rapidly cooled down to a
temperature range of 50°C to 300°C, and is heated again to control a metallographic
structure of steel so tliat both martensite and austenite are contained in steel, aud thus
steel having a tensile strcngtl~o f 980 MPa or greater a~ide xcellent ductility is obtained.
25 [0006]
3
Patent Docutiient 3 discloses that steel, wlticli contains 0.10% of C, 0.1% of Si,
and 5% of Mn, is heat-treated at a temperature of A1 point or lower, and thus steel, in
which the value of the product of the tensile strength and the elongation is significantly
high, is obtained.
5
[Prior Art Docu~nent]
[Patent Documnent]
[0007]
Patent Document 1: Japanese Unexamined Patent Application, First Publication
10 NO. 2004-269920
Patent Document 2: Japanese Unexamined Patent Application, First Publication
NO. 2010-90475
PatentDocument 3: Japanese Unexamined Patent Application, First Publication
NO. 2003-138345
15
[Disclosure of the Invention]
[Probletns to be Solved by the Invention]
[0008]
As described above, several technologies whiclt provide ultrahigll-strength steel
20 having excellent ductility are suggested. However, as described below, none of the
technologies can be said to be sufficient.
[0009]
In the technology disclosed ia Patent Document 1, the tensile strength of steel
cannot be set to 900 MPa or greater. The reason for this is as follows. In the
25 techlology disclosed in Patent Document 1, getleration of ferrite is promoted during
4
heating and cooling down to 600°C so as to enhance stability of austcnilile that is
contained in steel. If ferrite is generated, the tensile strength of steel significantly
decreases. Accordingly, the technology disclosed in Patent Docu~nent1 cannot be
applied to steel in which a tensile strength of 900 MPa or greater is required.
5 [OO I O]
In the technology disclosed in Patent Document 2, material stability with respect
to the manufacturing method is deficient, and thus safety of a structure body, to which the
obtained steel is applied, is not secured. That is, in the technology disclosed in Patent
Document 2, the tensile strength is controlled in accordance with heat treatment
10 conditions after rapid cooling, specifically, a cooling rate, a cooling stopping temperature
(a temperature at which cooling is stopped), and reheating conditions. However, similar
to Patent Document 2, in a case where the cooling rate is set to 8 'Clsecond or faster, and
steel, which is heated, is cooled down to a temperature range of 50°C to 300°C, a
teinperature distribution in steel becomes extremely non-uniform due to transfor~nation
15 heat generation and the like. That is, the technology disclosed in Patent Document 2
has a problem in that control of the cooling rate and the cooling stopping temperature is
very difficult. In a case where the temperature distribution during cooling is
non-uniform, the strength distribution of steel becomes extremely non-uniform, and thus
safety of a structure body,' to which steel is applied, is not secured due to early fracture of
20 a weak low-strength portion. According to this, the technology disclosed in Patent
Docu~nen2t is deficient in material stability, and cannot be applied to steel in which
safety is necessary.
[OOll]
Aproduct (steel), which is obtained by the technology disclosed in Patent
25 Docunie~l3t , is deficient in impact characteristics, and thus safety of a slructure body, to
5
which steel is applied, is not secured. That is, in the technology disclosed in Patent
Document 3, Mn segregation is used, and thus a large amount of austenite is generated
during heat in a temperature region O ~ ApoIi nt or lower. On the other haud, a large
amount of coarse cementite precipitates due to heating at a temperature of A1 point or
5 lower, and thus local stress concentration is likely to occur during deformation. Due to
the stress concentration, austenite, which is contained in steel, is transformed into
martensite at an early time of impact deformation, and thus voids are generated at the
periphery of martensite. As a result, impact characteristics of steel decrease.
Accordingly, steel, which is obtained by the technology disclosed in Patent Document 3,
10 is deficient in the impact characteristics, and cannot be used as steel in which safety is
necessaly.
[0012]
As desctibed'above, several technologies which provide ultrahigh-strength steel
which has a tensile strength of 900 MPa or greater, and is excellent in ductility arc
15 suggested. However, steel in the technologies is deficient in material stability or impact
characteristics, and thus it cannot be said that the material stability and the impact
characteristics are sufficient.
[0013]
The present invention has been made to solve the above-described problem, and
20 an object thereof is to provide ultrahigh-strength steel that has excellent ductility and
excellent impact characteristics while having a tensile strength of 900 MPa or greater,
and a method of manufacturing the steel.
[0014]
Here, the "excellent ductility" represents that a value of the product of the
25 tensile strength and the total elongation is 24000 MPa% or greater. In addition, the
6
"excellent impact characteristics" represent that an impact value in a Charpy test at O°C
is 20 J / C ~oIr ~gr eater.
5 The present inventors have extensively studied to solve the above-described
problem. As a result, the following new findings are obtained. Specifically, with
regard to a chemical composition of steel, it is important to contain a large amount of Si
and Mn. In addition, with regard to a manufacturing method, it is important to apply
heat treatment conditions which are optimal to base steel having the chemical
10 composition. In addition, with regard to the base steel that is subjected to a heat
treatment, it is important to make the structure thereof be composed of a fine martensite
single phase. As described above, by controlling the material and the heat treatment
conditions, it is possible to stably manufacture ultrahigh-strength steel which cannot be
manufactured in the related art and which has excellent ductility and excellent impact
15 characteristics while having a tensile strength of 900 MPa or greater. The present
invention has been made on the basis of the finding, and the gist of the present invelltion
is as follows.
[00 161
(1) An aspect of the present invention is a steel that has a chemical
20 conlposition, by mass %, 0.050% to 0.40% of C, 0.50% to 3.0% of Si, 3.0% to 8.0% of
MII, 0.001% to 3.0% of sol. Al, 0.05% or less of P, 0.01% or less of S, 0.01% or less of N,
0% to I .O% of Ti, 0% to 1 .O% of Nb, 0% to 1 .O% of V, 0% to 1 .O% of Cr, 0% to 1 .O% of
Mo, 0% to 1.0% of Cu, 0% to 1.0% of Ni, 0% to 0.01% of Ca, 0% to 0.01% of Mg, 0%
to 0.01% of REM, 0% to 0.01% of Zr, 0% to 0.01% of B, 0% to 0.01% of Bi, and the
25 remainder includillg Fe and impurities, wherein a metallographic structure contains 10%
7
to 40% of austenite in terms of % by volume, an average concentl.ati.on of C in the
austenite is 0.30% to 0.60%, by inass %, structure uniforn~ityw, hich is represented by a
value obtained by subtracting the minimum value from the maximum value of Viclccrs
hardness that is nieasured, in the nietallographic structure is 30 I-Tv or less, and a tensile
5 strength is 900 MPa to 1800 MPa.
[OO 1 71
(2) In the steel according to (I), the chemical composition nlay contain one or
two or more selected from the group consisting of 0.003% to 1.0% of Ti, 0.003% to 1.0%
ofNb, 0.003% to 1.0% ofV, 0.01% to 1.0% of Cr, 0.01% to 1.0% ofMo, 0.01% to 1.0%
10 of Cu, and 0.01% to 1.0% of, by mass %.
[0018]
(3) In the steel according to (1) or (2), the chemical composition may contain
one or two or more selected from the group consisting of 0.0003% to 0.01% of Ca,
0.0003% to 0.01% of Mg, 0.0003% to 0.01% of REM, 0.0003% to 0.01% of Zr, and
15 0.0003%to 0.01% of B, by mass %.
[0019]
(4) In the steel according to any one of ( I ) to (3), the chemical composition may
contain 0.0003% to 0.01% of Bi, by Inass %.
[0020]
20 (5) In the steel according to any one of (1) to (4), the chemical co~npositionn lay
contain 4.0% to 8.0% of Mn, by Inass %.
[0021]
(6) Another aspect of the present invention provides a method of manufacturing
a steel, the method includes performing a heat treatment with respect to base steel having
25 the cl~emicalc omposition according to any one of (1) to (5), and a metallograpl~ic
8
structure in which an average grain size of a prior austenite is 20 pnl or less and which is
conlposed of a martensite single phase, wherein the heat treatnlenl includes a retention
process of retaining the base steel at a temperature that is equal to or higher than 670°C
and lower than 78OoC, and is lower than an Ac3 point for 5 seconds to 120 seconds, and a
5 cooling process of cooling the base steel in such a manner that an average cooling rate
from the temperature region to 150°C is 5 "Clsecond to 500 'Clsecond after the retention
process.
[Effects of the Invention]
[0022]
10 According to the present invention, it is possible to manufacture
ultrahigh-strength steel that is excellent in ductility and impact characteristics while
having a high tensile strength of 900 MPa or greater. The ultrahigh-strength steel
according to the present invention can be widely used in an industrial field, particularly, a
vehicle field, an energy field, a building field, and the like. Furthermore, in a case
15 where the tensile strength is too high, low-temperature toughness may deteriorate, and
thus it is preferable that the tensile strength of steel is 1800 MPa or less.
[Embodiment of the Invention]
[0023]
20 Hereinafter, steel according to an embodiment of the present invention will be
described in detail.
1. Chemical Composition
A cheniical composition of steel (ultrahigh-strength steel having excellent
ductility and excellent impact characteristics) according to this embodiment is as follows.
9
As described above, "%", which represe~itsth e amount of each cle~ncnitn this
embodiment, is mass%.
[0024]
C: 0.050% to 0.40%
5 C is an element that promotes generation of austenite, and contributes an
increase in strength and an improvement in ductility. The lower limit of the amount of
C is set to 0.050% in order to set the tensile strength of steel to 900 MPa or greater, and
in order to set a value (TSxEL) of the product of the tensile strength and the elongation
of steel to 24000 MPa% or greater. When the amount of C is set to 0.080% or greater
10 while controlli~lgo ther elements in an appropriate range, the tensile strength becomes
1000 MPa or greater. Accordingly, it is preferable that the amount ofC is set to 0.080%
or greater. However, when the amount of C is greater than 0.40%, impact
characteristics deteriorate. According to this, the upper limit of the amount of C is set
to 0.40%. The upper limit of the amount of C is preferably 0.25%.
15 [0025]
Si: 0.50% to 3.0%
Si is art element that promotes generation of austenite, and contributes to an
improveme~lt in ductility. The lower limit of the amount of Si is set to 0.50% in order to
set the value of the product of the tensile strength and the total elongation of steel to
20 24000 MPa% or greater. When the amount of Si is set to 1.0% or greater, weldability
is improved. Accordingly, it is preferable that the lower limit of the amount of Si is set
to 1.0%. However, when the amount of Si is greater than 3.096, the impact
characteristics deteriorate. Accordingly, the upper limit of the amount of Si is set to
3.0%.
25 [0026]
10
Mn: 3.0% to 8.0%
Mn is an elenlent that promotes generation of austenite, and contributes to all
increase in strength and an improvement in ductility. When the amount of Mn is set to
3.0% or greater, non-uniformity of a struclure, which is caused by Mn micro-segregation,
5 decreases, and thus austenite is nniforu~ly distributed. As a result, it is possible to set
the tensile strength of steel to 900 MPa or greater, and it is possible to set the value of the
product of the tensile strength and the total elongation of steel to 24000 MPa% or greater.
Accordingly, the lower limit of the amount of MI is set to 3.0%. Furthennore, in a case
where the amount of C is 0.40% or less, when the amount of Mu is set to 4.0% or greater,
10 stability of austenite increases and work hardening persists, and thus the tensile strength
becomes 1000 MPa or greater. Accordingly, it is preferable that the lower limit of the
amount of Mn is set to 4.0%. However, when the amount of Mn is greater than 8.0%,
refining and castiligin a converter becomes significantly difficult. According to this,
the upper limit of the amount of Mn is set to 8.0%. The upper limit of the amount of
15 Mn is preferably 6.5%.
[0027]
P: 0.05% or less
P is an element that is contained as an impurity. However, P is also an element
that contributes to an increase in strength, and thus P may be positively contained.
20 However, when the amount of P is greater than 0.05%, casting becomes significantly
difficult. According to this, the upper limit of the amount of P is set to 0.05%. The
upper limit of the amount of P is preferably 0.02%.
The lower the amount of P is, the nlore preferable. Accordingly, the lower
linlit of the anlount of P is 0%. However, the lower limit of the amount of P may be set
25 to 0.003% fro111 the viewpoints of manufacturing cost and the like.
[0028]
S: 0.01% or less
S is an element that is contained as an impurity, and significantly deteriorates the
impact characteristics of steel. According to this, the upper litnit of the amount of S is
5 set to 0.01%. The upper limit of the amount of S is preferably 0.005%, and more
preferably 0.001 5%.
The lower the amount of S is, the more preferable. Accordingly, the lower
limit of the amount of S is 0%. However, the lower liinit of the amount of S may be set
to 0.0003% from the viewpoints of manufacturing cost and the like.
10 [0029]
sol. Al: 0.001% to 3.0%
A1 is an element that has an effect on deoxidizing steel. The lower limit of the
amount of sol. A1 is set to 0.001% for soundness of steel. The lower limit of the amount
of sol. A1 is preferably 0 010%. On the other hand, when the amount of sol. Al is
15 greater than 3.0%, casting becomes significantly difficult. According to this, the upper
limit of the amount of sol. Al is set to 3.0%. The upper limit of the amount of sol. A1 is
preferably 1.2%. The amount of sol. Al represents the amount ofAl that is soluble to
acid in steel.
[0030]
20 N: 0.01% or less
N is an element that is contained as an impurity, and significantly deteriorates
aging resistance of steel. Accordingly, the upper limit of the amount of N is set to
0.01%. The upper limit of the amount of N is preferably 0.006%, and more preferably
0.003%. The lower the amount ofN is, the more preferable. Accordingly, the lower
25 limit of the amount of N is 0%. However, the lower limit of the amount of N may be set
12
to 0.001% from the viewpoints of manufacturing cost and the lilte.
[003 11
One or Two or More Selectedfrom Group Consisting of Ti: 1.0% or Less, Nb:
1 .O% or Less, V: 1 .O% or Less, Cr: 1 .O% or Less, Mo: 1 .O% or Less, Cu: 1 .O% or Less,
5 and Ni: 1 .O% or Less
The elements are elements which are effective to stably secure the strength of
steel. Accordingly, one or two or more of the elements may be contained. However,
when the amount of any of the element is greater than 1.0%, it is difficult to perform hot
working of steel. According to this, the amount of each of the elements in the case of
10 being contained is set as described above. It is not necessary for the elements to be
contained. Accordingly, it is not necessary to particularly limit the lower limit of the
amount of the elements, and the lower limit is 0%.
Furthermote, it is preferable to satisfy at least one of Ti: 0.003% or greater, Nb:
0.003% or greater, V: 0.003% or greater, Cr: 0.01% or greater, Mo: 0.01% or greater, Cu:
15 0.01% or greater, and Ni: 0.01% or greater so as to more reliably obtain the effect of the
elements.
[0032]
One or Two or More Selected fro111 Group Consisting of Ca: 0.01% or Less, Mg:
0.01% or Less, REM: 0.01% or Less, Zr: 0.01% or Less, and B: 0.01% or Less
20 The ele~nentsa re elements having an effect on increasing low-temperature
toughness. Accordingly, one or two or more of the elements may be contained
However, when any of the elements is contained in an amount of greater than 0.01%, a
surrace quality of steel deteriorates. According to this, the amount of each of the
elements in a case of being contained is set as described above. lt is not necessary for
25 the elements to be contained. According to this, it is not necessary to particularly limit
13
the lower limit of the amount, and the lower limit of the amount is 0%
Furthennore, it is preferable to set the amount of at least one of the elements to
0.0003% or greater so as to more reliably obtain the effect of the elements. Here, REM
represents total 17 elements including Sc, Y, and lantlianoids, and the amount of REM
5 represents the total amount of these elements. Industrially, the lauthanoids are added in
a type of a misch metal.
[0033]
Bi: 0.01% or less
Bi is an element that reduces segregation of Mn, and mitigates anisotropy of
10 mechanical properties. Accordingly, Bi may be contained to obtain this effect.
However, the amount of Bi is greater than 0.01%, it is difficult to perform hot-working of
steel. According to this, the upper limit of the amount of Bi in a case of being contained
is set to 0.01%: 'lt is not necessary for Bi to be contained. According to this, it is not
necessary to particularly limit the lower limit of the amount, and the lower limit is 0%.
15 Furthermore, it is preferable to set the amount of Bi to 0.0003% or greater so as
to more reliably obtain the effect due to containing of Bi.
[0034]
2. Metallographic structure
The steel according to this embodiment has the chemical comnposition, and has a
20 metallographic structure it1 which. 10% to 40% of austenite is contained in terms of % by
volume, and the average concentration of C in the austenite is 0.30% to 0.60%, by
mass%. The mctallographic structure can be obtained by applying the following
manufacturing nlethod to base steel having the above-described chemical composition.
[0035]
25 Volu~ncR atio ofAustenite: 10% to 40%
14
In a metallographic structure of steel having the above-described chemical
composition, when the volume ratio of austenite is 10% or greater, a tensile strength of
900 MPa or greater and excellent ductility are obtained. When the volume ratio of
austenite is less than lo%, an improvement in ductility is not sufficient. Accordingly,
5 the lower limit of the volume ratio of anstenite of the stcel according to this embodiment
is set to 10%. On the other hand, when the volume ratio of austenite is greater than
40%, delayed fracture resistance deteriorates. According to this, the upper limit of the
volume ratio of austenite of the steel according to this embodiment is set to 40%.
Furthermore, it is preferable that a remaining structure other than austenite is
10 martensite and ferrite is not contained in order to secure a tensile strength of 900 MPa or
greater.
[0036]
Average Concentration of C in Austenite: 0.30 Mass% to 0.60 Mass%
When the average concentration of C in austenite of steel having the
15 above-described chemical composition is 0.30 mass% or greater, the impact
characteristics of steel are improved. When the average conce~ltrationo f C is less than
0.30 mass%, an improvement in the impact characteristics becotnes not sufficient.
Accordingly, the lower limit of the average concentration of C in austenite of the steel
according to this embodiment is set to 0.30 mass%. On the other haad, in a case where
20 the average concentration of C is greater than 0.60%, martensite, which is generated in
accordance with a TRlP phenomenon, becomes full hard, and micro-cracks are likely to
generate in the vicinity of the martensite, and thus impact characteristics deteriorate.
According to this, the upper limit of the average concentration of C in austenite of the
steel according to this embodiment is sct to 0.60 mass%
25 [0037]
15
Structure Uniformity
In the metallograpliic structure of steel having the above-described chei~lical
composition, when structure uniformity, which is represented by a difference (the
maximum value-the minimum value) between the minimum value and the maximum
5 value of the Vickers hardness that is measured, is 30 Hv or less, non-uniform
deformation is suppressed, and thus good ductility is stably secured. Accordingly, the
structure uniformity of steel according to this einboditnent is set to 30 Hv or less. The
smaller the difference between the maximum value and tlie minimum value of Vickers
hardness is, the more preferable it is. Accordingly, tlie lower limit of the structure
10 uniformity is 0.
Furthermore, the structure unifort~iityc an be obtained as follows. Specifically,
the hardness at five points is measured under a load of 1 kg by using a Vickers tester, and
the difference between the maximum value and the minimum value of the Vickers
hardness at that time is obtained as the structure uniformity.
15 [0038]
3. Manufacturing Method
A description of a method (manufacturing method according to this
embodiment) of niaiiufacturiilg the steel according to this embodiment will be given.
[0039]
20 As described above, in order to obtain ultrahigh-strength steel having a tensile
strength of 900 MPa orgreater and excellent ductility and excellent impact
characteristics, it is itliportatit that in the metallographic structure after a heat treatment,
10% to 40% of austcnite is contained in terms of % by volume, and the average
concentration of C in austenite is set to 0.30% to 0.60%, by mass%. The
25 above-described iiietallographic structure is obtained by performing the following heat
16
treatment to steel, which has a cl~en~iccaol nlposition in the abovc-described range, and
has a metallographic structure in which an average grain size of prior austenite is 20 pm
or less and which is composed of a martensite single phase, as a material (base steel).
Specifically, the metallographic structure is obtained by heating the base steel to a
5 temperahue region which is equal to or higher than 670°C and lower than 780, and is
lower than the Ac3 point, by retaining the base steel in the temperature region for 5
seconds to 120 seconds (retention process), a ~bdy cooling down the base steel in such a
manner that the average cooling rate from the temperature region to 150°C is 5
"C/second to 500 "C/second (cooling process)
10 Furthermore, even when performing the heat treatment, the chemical
composition of steel does not vary. That is, the chemical cotllposition is not different
,between the steel (base steel) before the heat treatment and the steel according to this
embodiment.
15 Metallographic structure of Steel (Base Steel, that is, Steel before Heat
Treatment) Used in Heat Treatment.
As the steel that is subjected to the heat treatment, steel, which has the
above-described chemical composition, and has the metallograpl~ics tructure in which the
average grain size of prior austenite is 20 pm or less and which is composed of a
20 , martensite single phase, is used. When the steel having the metallographic structure is
subjected to a heat treatment under the followiug conditions, ultrahig11-strengtl steel,
which has a high strength such as a tensile strength of 900 MPa or greater and is
excellent in ductility and impact characteristics, is obtained.
In a case where the structure of steel that is subjected to the heat treatment is not
17
composed of a marlensite single phase, growth of austenitc during the hcat treatment is
delayed, and thus the volu~nera tio of austenite after the heat treatment decreases. In
addition, in a case where the structure of steel that is subjected to the heat treatment is not
composed of a martensite single phase, in steel after the heat treatment, TSxEL decreases,
5 and thus early fracture occurs during collision.
I11 a case where the average grain size of prior austenite is greater than 20 pnl,
localization of C in austenite becomes significant at an early period of reaction, and thus
there is a concern that the average concentration of C in austenite exceeds 0.60 mass%.
[0041]
10 For example, the steel (base steel), which has the above-described
metallographic structure and is used in the heat treatment, can be manufactured by
performing hot working with respect to steel such as a steel piece having the
above-described chemical composition at a temperature of 850°C or lower, and by
rapidly cooling the steel to room temperature at a cooling rate of 20 "Clsecond or faster,
15 or by heating the steel at a temperature at which the metallographic structure becomes an
austenite single phase after cold-working, and by rapidly cooling the steel to room
temperature at a cooling rate of 20 "C/second or faster. In a case where the average
grain size of prior austenite is 20 pm or less, the steel may be sub.ject to tempering.
Furthermore, relention may be performed at a steel piece stage at 1150°C to
20 1350°C for 0.5 hours to 10 hours in order to enhance the structure uniformity of the steel
after the heat treat~uent.
[0042]
I-Ieating and Retention Conditions (Heat Treatment Conditions): Retellti011 in
Temperature Region That is Equal to or Nigher than 670°C and is Lower than 780°C, and
18
is Lowcr than Ac3 Point for 5 Seconds to 120 Seconds
Tlie base steel, which has the metallographic structure in which the average
grain size of prior austc~litcis 20 pn or less and which is composed of a nlartensite
single phase, is heated to a temperature region that is equal to or higher than 670°C and is
5 lower than 780°C, and is lower than the Ac3 point (OC), whic11 is defined by the following
Expression (I) and at which an austenite single phase is obtained, and is retained in the
temperature region for 5 seconds to 120 seconds.
[0043]
Here, theAc3 point is calculated with the following Expression (1) by using the
10 amount of each element.
~c~=910-203x(~~-~)-15.2x~i+441.047xxV~+i3+1 .5xMo-30xMn-l1 xCr-20x
Cu+700xP+400xAl+50xTi ... (1)
In Expression (I), each of the element symbols represents the amount of the
element (unit: mass%) in the chemical composition of steel.
15 [0044]
When the retention temperature is lower than 670°C, the average concentration
of C in austenite, which is contained in steel after the heat treatment, becomes excessive.
As a result, in steel after the heat treatment, impact characteristics deteriorate, and it is
difficult to secure a tensile strength of 900 MPa or greater. Accordingly, the lower limit
20 of the retention temperature is set to 670°C. On the other hand, when the retention
temperature becomes 780°C or higher, or the Ac3 point or higher, an appropriate amount
of austenite is not contained in steel after the heat treatment, and ductility significantly
deteriorates. Accordingly, the retention lenlperature is set to be lower than 780°C and
be lower than the Ac3 point. Here, the temperature, which is lower than 780°C and is
19
lower tlian the A c ~po int represents a te~nperaturel ower than thc Ac3 pomt in a case
where theAc3 point is lower than 780°C, and represents a temperature that is lower than
780°C in a case where the Ac3 point is 780°C or higher.
On the other hand, when the retention time is shorter than 5 seconds, a
5 temperature distribution remains in steel, and thus it is difficult to stably secure tensile
strength after the heat treatment. Accordingly, the lower limit ofthe retention time is
set to 5 seconds. On the other hand, when the retention time is longer than 120 seconds,
the average concentration of C ill austenite that is contained in steel after the heat
treatment becomes excessively small, and thus impact characteristics deteriorate.
10 Accordingly, the upper limit of the retention time is set to 120 seconds. Furthermore,
when the steel is heated to a temperature that is equal to or higher than 670°C and is
lower than 780°C, and is lower than the Ac3 point, and is retained in the temperature
region for 5 seconds to 120 seconds, it is preferable to set the average heating rate to 0.2
"C/second to 100 OC/second. When the average heating rate is slower than 0.2
15 "C/second, productivity deteriorates. On the other hand, in a case of using a typical
furnace, when the average heating rate is faster than 100 "C/second, it is difficult to
control the retention temperature. However, in a case of using high-frequency heating,
even when performing heating at a temperature-increasing rate that is faster than
10O0C/second, the above-described effect can be obtained.
20 [0045]
Average Cooling Rate (Heat Treatment Condition) from Retelltion Temperature
Region During Heating to 150°C: 5 "C/sccond to 500 OClsecond
After the above-described heating aud retention, cooling is performed in such a
manlier that an avcragc cooling rate from the heating and retention teniperaturc region to
20
150°C becomes 5 "Clsecond to 500 OC/second. When the average cooling rate is slower
than 5 OC/seco~~sodf,t ferrite or pearlite is excessively generated, and thus it is difficult to
secure a tensile strength of 900 MPa or greater in steel after the heat treatment.
Accordingly, the lower limit of the average cooling rate is set to 5 OClsecond. On the
5 other hand, when the average cooling rate is faster than 500 "C/seco~~ad ,q uenching
crack is likely to occur. Accordingly, the upper limit of the average cooling rate is set to
500 OCIsecond. Furtherillore, as long as the average cooling rate up to 150°C is set to 5
"C/second to 500 oC/seco~~tdhe, cooling rate at a temperature of 150°C or lower may be
the same as the range, or may be different from the range.
10 [0046]
According to the ~nanufacturingm ethod according to this embodiment, it is
possible to manufacture ultrahigh-strength steel having a rnetallographic structure which
contains 10% to 40% of austenite in terms of % by volume and in which an average
concentration of C in austenite is 0.30% to 0.60%, by mass%, and having a tensile
15 strength of 900 MPa or greater and having excellent ductility and impact characteristics.
[Examples]
[0047]
Base steel having a chemical composition shown in Table 1 and a
metallographic structure shown in Table 2 is used in a heat treatment under conditions
20 shown in Table 3.
[0048]
The base steel, which was used, was prepared by subjecting slab that was
obtained through melting in a laboratory to hot working. The base steel was cut into
dimensions of 3 mm (thickness), 100 lnln (width), and 200 mnt (length), and was heated,
retained, and cooled under conditions in Table 3. A tliertnocouple was attached to a
surface of tlie steel to perforin temperature measurement during a heat treatment. In
Table 3, the average heating rate represents a value in a temperature region from room
temperature to a heating temperature, a retention time represents time take11 for retention
5 at the heating temperature, and the average cooling rate represents a value in a
temperature region from a retention temperature to 150°C. As described below, a
metallographic structure of metal that was used in the heat treatment, and the
metallographic structure and the mechanical properties of steel that was obtained through
the heat treatment were investigated through metallographic structure obseivation, X-ray
10 diffraction measurement, a tensile test, and a Charpy test. Test results are shown in
Table 4.
[0049]
(Metallographic structure of Steel (Base steel) That is subjected to Heat
Treatment)
15 A cross-section of steel, which was used in the heat treatment, was observed and
photographed with an electron microscope, and a total region of 0.04 mm2 was analyzed
to identify a metallographic structure and to measure an average grain size of prior
austenite. The average grain size of prior austenite was obtained by measuring tlie
average slice length in the observed image that was obtained, and by nl~~ltiplyiltlhge
20 length by 1.78.
An observation position was set to a position that avoids the central segregation
portion at a position (position of 1/2t) of approsimately 112 times tlie sheet thickness.
The reason for avoiding the central segregation portion is as follows. The central
segregation portion may have a metallographic structure that is locally different from a
25 representative metallographic structure of steel. I-Iowever, the central segregation
LL
portion is a nlinute region with respect to the entirety of the sheet thiclil-tcss, and hardly
has an effect on the characteristics of steel. That is, it camlot be said that the
metallographic structure of the central segregation portion represents a n~etallographic
structure of steel. According to this, it is preferable to avoid the central segregation
5 pollion in identification of the rnetallographic structure.
[OOSO]
(Volume Ratio ofAustenite in Steel after Heat Treatment)
A test specimen having a width of 25 mm and a length of 25 mm was cut out
from the steel after the heat treatment, the test specimen was subjected to chemical
10 polishing so as to reduce the thickness by 0.3 mnl, and X-ray diffraction was performed
three times with respect to a surface of the test specimen after the chemical polishing
Profiles, which were obtained, were analyzed, and were averaged to calculate the volume
ratio of austenite.
[005 I]
15 (Average Concentration of C in Auslenite in Steel after Heat Treatment)
The profiles, which were obtained in the X-ray diffraction, were analyzed to
calculate a lattice constant (a: unit is A) of austenite, and the average concentration (c:
unit is mass%) of C in austenite was determined on the basis of the following Expression
(2).
20 c=(a-3.572)/0.033 ... (2)
[0052]
(Structure Unifom~ity)
The hardness at five points under a load of I kg was measured by using a
Vickers tester, and evaluation was made by setting a difference between the ~naximu~u
25 . value and the lninilnunl value of the Vickers hardness as the structure uniformity.
23
[0053]
(Tensile Test)
A tensile test specinlen of No. JIS 5 having a thickness of 2.0 mm was collected
from steel after the heat treatment, and a tensile test was performed in conformity to JIS
5 22241 to measure TS (tensile strength) and EL (total elongation). In addition, TSxEL
was calculated from TS and EL.
[0054]
(Impact Characteristics)
Front and rear surfaces of the steel after the heat treatment were grinded to have
10 a thickness of 1.2 mm, and a V-notched test specimen was prepared. Four sheets of the
test specimen were laminated and were fixed with a screw, and the resultant laminated
sheets were provided to a Charpy impact test in conformity to JIS 22242. With regard
to impact characteristics, a case where an impact value at 0°C became 20 j/cm2 or greater
was regarded as "Good", and a case where an impact value at O°C was less than 20 ~ / c r n ~
15 was regarded as "Poor".
[OOSS]
[Table 11
1 ill1 i111(e r lllc rcpl'csellL5 1 it11 a va ilC Is not in a i-angc o thc invallic>n
[0056]
[Table 21
(1tcmat.l.) an ~cntle~lilre~]e~ ~eseiil~?:itt s: i \:iluc ih not in a range orthc i~~\ention
[0057]
[Table 31
(ICemnrk) an underline represenls ihal a v:~lue is no( in a range ol'lhe i11~:eniion
[OOSS]
[Table 41
[0059]
As shown in Table 4, sanlpleNos. 1,3,4,8, 10, 12, 14, 18,20,23,24,26,27,
and 28 according to the present invention had a tensile strength of 900 MPa or greater,
5 and the value of the product of the tensile strength and the total elongation (TSxEL) was
28
24000 MPa.% or greater. According to this, it could beseen that the ductility was
excellent. In addition, an impact value in the Charpy test at O°C was 20 JICIII~o r greater,
and thus it could be seen that impact characteristics were also good. Particularly, in
Sanlple Nos. 4, 10, 12, 14, 18,20,23,24,26,27, and 28, the amount of C and the
5 amount of Mn were in a preferable range, and the tensile strength was very high as 1000
MPa or greater.
Furthermore, a structure other than austenite was composed of mariensite.
[0060]
On the other hand, in sample No. 2, the metallographic structure of steel, which
10 was used in the heat treatment, was not appropriate, and thus the volume ratio of
austenite was low and the ductility was low after the heat treatment. In sample No. 5,
the grain size of prior austenite of the steel (base steel), which was used in the heat
treatment, was'not appropriate, and thus the average co~icentrationo f C in austenite in the
steel after the heat treatment was high, and the impact characteristics were poor. In
15 Sample Nos. 6,22, and 25, the chemical composition was not appropriate, and thus the
ductility was poor, Accordingly, a target tensile strength was not obtained. In addition,
in Sample Nos. 22 and 25, the structure uniformity did not satisfy a target value. In
Sample Nos. 7, 11, and 17, the chemical composition was not appropriate, and thus the
impact characteristics were poor. In Sample No. 9, the cooling rate after the heat
20 treatment was too slow, and thus a required tensile strength was not obtained. In
Saniple Nos. 13 and 15, the retention temperature during the heat treatment was too high,
and thus a desired structure was not obtained. Accot-dingly, the ductility was inferior.
I11 Sample No. 16, the chemical conlposition was not appropriate, and thus the ductility
was inferior. In Sample No. 19, the retention temperature during the heat treatment was
25 too low, and thus a desired structure was not obtained. Accordingly, the intpact
29
cliaracteristics were poor, and a required tensile strength was not obtained. In Sample
No. 21, the retention time during the heat treatment was too long, aud thus a desired
structure \&!as not obtained. Accordingly, the impact characteristics were poor
Industrial Applicability
5 [006 11
According to the present invention, it is possible to manufacture
ultrahigh-strength steel excellent in ductility and impact characteristics while having a
high strength such as a tensile strength of 900 MPa or greater. For example, the
ultrahigh-strength steel according to the present invention can be widely used in a vehicle
10 field, an energy field, and a building field, atid thus an illdustrial use value thereof is
high.
3 0
CLAIMS
What is clainled is:
1. A steel that has a chen~icalc omposition comprising, by mass%:
0.050% to 0.40% of C,
0.50% to 3.0% of Si,
3.0% to 8.0% of Mn,
0.001% to 3.0% of sol. Al,
0.05% or less of P,
0.01% or less of S,
0.01% or less ofN,
0% to 1 .O% of Ti,
0% to 1 .O% of Nb,
0% to 1.0% of V,
0% to 1.0% of Cr,
15 0% to 1 .O% of Mo,
0% to 1 .O% of Cu,
0% to 1 .O% of Ni,
0% to 0.01% of Ca,
0% to 0.01% of Mg,
20 0% to 0.01% of REM,
0% to 0.01% of Zr,
0% to 0.01% of B,
0% to 0.01% of Bi, and
the remainder including Fe and impurities,
25 rvherei~ia metallographic structure contains 10% to 40% of ausienite in terms
3 1
of % by volume;
an average co~lce~ltratioof~ Cl in the austenite is 0.30% to 0.60% by ~uass% ;
a structure uniformity, which is represented by a value obtained by subtracting
the minimum value from the maximum value of Vickers hardness that is measured, in the
5 metallographic structure is 30 Hv or less; and
a tensile strength is 900 MPa to 1800 MPa.
2. The steel according to claim I,
wherein the chemical composition contains one or two or more selected from the
10 group co~~sistinogf 0.003% to 1.0% of Ti, 0.003% to 1.0% of Nb, 0.003% to 1.0% of V,
0.01% to 1.0% of Cr, 0.01% to 1.0%of Mo, 0.01% to 1.0% of Cu, and 0.01% to 1.0% of
Ni, by mass %.
3. The steel according to claim 1 or 2,
15 wherein the chemical composition contains one or two or more selected from the
group consisting of 0.0003% to 0.01% of Ca, 0.0003% to 0.01% of Mg, 0.0003% to
0.01% of REM, 0.0003% to 0.01% ofZr, and 0.0003% to 0.01% of B, by mass %.
4. The steel according to any oue of claims 1 to 3,
wherein the chemical co~upositionc ontains 0.0003% to 0.01% of Bi, by mass %.
5. The steel according to any one of claims 1 to 4,
wherein the chemical composition contains 4.0% to 8.0% of Mn, by mass %.
6. A method of manufacturing a steel, comprising:
32
performing a heat treatment with respect to base steel having the chemical
composition according to any one of claims 1 to 5, and a lnetallographic structure in
which an average grain size of a prior austenite is 20 pm or less and which is composed
of a martensite single phase,
5 wherein the heat treatment includes:
a retention process of retaining the base steel at a temperature that is equal to or
higher than 670°C and lower than 780°C, and is lower than an Ac3 point for 5 seconds to
120 seconds; and
a cooling process of cooling the base steel in such a manner that an average
10 cooling rate from the temperature region to 150°C is 5 "Clsecond to 500 "Clsecond after
the retention process.