ORIGINAL
0 DESCRIPTION
TITLE OF INVENTION
STEEL FOR STEEL TUBE WITH EXCELLENT SULFIDE STRESS CRACKING
RESISTANCE
TECHNICAL FIELD
[0001]
The present invention relates to a steel for steel tube with excellent sulfide
stress cracking resistance (hereinafter referred also to as "SSC resistance"), which is
excellent in cleanliness with fewer harmful coarse inclusions, particularly, a steel for
steel tube with excellent SSC resistance, which is suitable for application to steel tubes,
and casings, tubing, excavating drill pipes, drill collars and the like for oil well or
natural gas well.
BACKGROUND ART
[0002]
Non-metallic inclusions in steel (hereinafter referred simply to as "inclusions")
lead to, as well as causing defective or flaws of steel product, the deterioration of
weldability or strength/ductility and further the deterioration of corrosion resistance and,
particularly, the larger the size thereof, the more serious such adverse effects.
Therefore, a number of methods are developed for reducing the number of or reforming
the inclusions, and particularly large-size inclusions.
[0003]
At the threshold of the development, techniques such as reforming of an
oxygen contamination source such as slag, optimization of deoxidation conditions or the
like, and moreover removal of inclusions by a secondary refining apparatus such as RH
were rigorously developed, and these techniques are being used even now. However,
since these techniques cannot meet the required performance of steel product that has
been escalated, a control technique of inclusions morphology such as Ca treatment has
been developed to respond to such a demand in combination with the existing
techniques.
0 [0004]
In recent years, the required performance of steel product is further escalated,
and a number of new techniques have been proposed to respond to this demand.
[0005]
For example, Patent Literature 1 discloses a technique for improving bore
expandability by use of MgO or MgO-containing inclusions, and Patent Literature 2
discloses a technique for dispersing harmful oxygen as fine MgO by controlling the
content of Mg in steel in a specific range.
[0006]
The present applicant also proposes, in Patent Literature 3, a technique for
reducing harmful coarse carbonitride inclusion constituents by generating carbonitrides
using a Ca-Al-based oxysulfide inclusion constituent as nuclei.
[0007]
In this way, the latest techniques utilize the inclusions rather than simple
removal or reduction of inclusions that has been performed in the related prior art.
[0008]
On the other hand, there are various types of inclusions which primarily have
constituents such as sulfides, oxysulfides or carbonitrides other than oxides, singly or
otherwise in combination. In the past, it was at most one or two of these types of
inclusions that hinder efforts to obtain the characteristics required for steel product.
For example, surface defects in a cold-rolled steel sheet are principally caused by the
coarse oxide type, and the deterioration of the weldability in a structural material such
as a steel beam is caused by the sulfide type, so that a desired effect could be attained by
taking specific measures against specific inclusion types as described above.
[0009]
In recent years, however, it has been demanded also to simultaneously satisfy
a plurality of characteristics, in addition to the escalated required performance of steel
product. For example, a combination of high strength and high corrosion resistance, a
combination of high strength and high workability or the like is sought after.
[0010]
When two kinds of characteristics, let's say, characteristic A and characteristic
B, are simultaneously required, for example, two measures against the relevant
A inclusions such as a measure "a" for satisfying the characteristic A and a measure "b"
for satisfying the characteristic B must be taken at the same time according to the
conventional point of view.
[0011]
However, taking a plurality of measures simultaneously may create problems in
performance, besides cost and productivity.
[0012]
For example, although the sulfides can be reduced by reducing the content of S
in steel, the decrease in content of S can lead to increase in the number of the oxide type
inclusions since the interfacial tension between molten iron and inclusions reduces
according to the decrease in content of S to thereby deteriorate the floatation
separability of inclusions. Further, the reduction in content of S in steel leads to a
change in content of N in steel which results from an increased rate of denitrification or
nitride absorption of molten iron, and as a result, the number of nitrides can likely vary.
[0013]
Namely, the decrease of a specific type of inclusions can create problems such
as the increase of other types of inclusions and the deterioration of inclusions
controllability.
[0014]
Further, when a plurality of characteristics are simultaneously required with
particularly high performance, what matters is not the number of specific types of
inclusions such as oxides or sulfides that affects other characteristics, but the total
number of two or more types of inclusions such as oxides, sulfides, oxysulfides and
carbonitrides. For example, even if MnS is reformed with Ca or the like to be made
harmless for the purpose of improving the corrosion resistance of steel product,
Ca-based inclusions after the reformation may degrade the surface quality of the steel
product. In such a case, it is necessary to reduce the total number of inclusions after
the reformation, in addition to render MnS harmless, and the necessary measures
therefor are further complicated.
[0015]
In this way, when a plurality of different characteristics are to be satisfied at a
high level, the measures against inclusions are complicated to end up in deteriorating
ST
*1
A the stability of quality, while causing the productivity and costs of product to be
deteriorated. Since this deterioration of stability causes the reduction of product yield,
further efforts for commercial industrial production are needed while the supply of the
product is possible.
CITATION LIST
PATENT LITERATURE
[0016]
Patent Literature 1: Japanese Patent Application Publication No. 2001-342543
Patent Literature 2: Japanese Patent Application Publication No. 5-302112
Patent Literature 3: WO 03/083152
Patent Literature 4: Japanese Patent Application Publication No. 2003-160838
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0017]
As described above, it is difficult for the related prior art to stably satisfy a
plurality of performances or characteristics at the same time. From the viewpoint of
this problem, the present invention has an object to provide a steel for steel tubes with
excellent SSC resistance, which can simultaneously satisfy a plurality of characteristics.
SOLUTION TO PROBLEM
[0018]
To simultaneously secure a plurality of characteristics, as described above, it is
necessary to reduce the number of coarse inclusions while controlling a specific type of
inclusions which affects a specific characteristic after settling the composition of steel
product in a predetermined range. As a result of studies and investigations on the
composition of steel and the composition of inclusions from this point of view with
respect to the steel for steel tubes, the present inventors found that the steel for steel
tubes having predetermined strength and toughness as well as excellent SSC resistance
can be obtained by setting the content of Mg in a specific range, as described later, after
settling the composition of steel product in a predetermined range, so as to control the
A morphology of inclusions contained in the steel product, thereby reducing the number of
coarse inclusions. The present invention is achieved based on this knowledge, and the
gist of the invention consists in the steel for steel tubes with excellent SSC resistance
described in the following (1) and (2).
[0019]
(1) A steel for steel tubes with excellent SSC resistance, including, by mass%:
C: 0.2 to 0.7%; Si: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: not more than 0.005%; P: not
more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: not
more than 0.007%; Cr: 0. 1 to 1.5%; and Mo: 0.2 to 1.0%; the balance being Fe, Mg and
impurities, being characterized in that: the content of Mg in the steel is not less than 1.0
ppm and not more than 5.0 ppm; and non-metallic inclusions of not less than 50% of the
total number of those in steel each having the maximum bulk size of not less than 1 am
and comprising two or more elements of Ca, Al, Mg, Ti and Nb and two or more
elements of O, S and N have such a morphology that Mg-Al-O-based oxides exist at the
central part of the inclusion, Ca-Al-based oxides and/or Ca-Al-based oxysulfides
enclose the Mg-Al-O-based oxides, and Ti-containing-carbonitrides or -carbides further
exist on a complete or partial periphery of the Ca-Al-based oxides and/or Ca-Al-based
oxysulfides (hereinafter referred to as "first inventive steel").
[0020]
(2) A steel for steel tubes with excellent SSC resistance, including, by mass%:
C: 0.2 to 0.7%; Si: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: not more than 0.005%; P: not
more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: not
more than 0.007%; Cr: 0. 1 to 1.5%; Mo: 0.2 to 1.0%; and one or more of Nb: 0.005 to
0.1%, Zr: 0.005 to 0.1%, V: 0.005 to 0.5% and B: 0.0003 to 0.005%; the balance being
Fe, Mg and impurities, being characterized in that: the content of Mg in the steel is not
less than 1.0 ppm and not more than 5.0 ppm; and non-metallic inclusions of not less
than 50% of the total number of those in steel each having the maximum bulk size of
not less than 1 urn and comprising two or more elements of Ca, Al, Mg, Ti and Nb and
two or more elements of O, S and N have such a morphology that Mg-Al-O-based
oxides exist at the central part of the inclusion, Ca-Al-based oxides and/or Ca-Al-based
oxysulfides enclose the Mg-Al-O-based oxides, and Ti-containing-carbonitrides or
-carbides further exist on a complete or partial periphery of the Ca-Al-based oxides
AT
A and/or Ca-Al-based oxysulfides (hereinafter referred to as "second inventive steel").
[0021]
In the following, with respect to component compositions of steel and slag,
"mass%" and "mass ppm" will be simply referred to as "%" and "ppm".
[0022]
In the descriptions herein and the claims, the composition of steel is used in the
sense of "content in steel tube product" unless otherwise noted.
[0023]
Various types of inclusions recited in the claims are defined as follows.
"Non-metallic inclusions in steel comprising two or more elements of Ca, Al,
Mg, Ti and Nb and two or more elements of O, S and N": Among coarse inclusions each
having the maximum bulk size of not less than 1 urn in steel tube products, defined is
the one in which each content of at least two elements selected from Ca, Al, Mg, Ti and
Nb, and each content of at least two elements selected from O, S and N are 5% or more,
respectively, and the total content of Ca, Al, Mg, Ti, Nb, O, S and N is not less than 80%.
In addition, the inclusion defined here is an aggregation of plural non-metallic inclusion
constituents (inclusion phases): "Mg-Al-O-based oxides", "Ca-Al-based oxides" and/or
"Ca-Al-based oxysulfides" and "Ti-containing-carbonitrides or -carbides" which are
defined below.
"Mg-Al-O-based oxides": defined is a constituent of the abovementioned
aggregate in which each content of Mg, Al, O is 2.5% or more, and the total content of
Mg, Al and O in the constituent is not less than 8%.
"Ca-Al-based oxides": defined is a constituent of the abovementioned
aggregate in which each content of Ca, Al and O is 3.0% or more, and the total content
of Ca, Al and O in the constituent is not less than 15%.
"Ca-Al-based oxysulfides": defined is a constituent of the abovementioned
aggregate in which each content of Ca, Al, O and S is 2.0% or more, and the total
content of Ca, Al, O and S in the constituent is not less than 15%.
"Ti-containing-carbonitrides or -carbides": defined is a constituent of the
abovementioned aggregate in which each content of Ti, N and C is 1.2% or more, and
the total content of Ti, N and C in the constituent is not less than 5%.
1
ADVANTAGEOUS EFFECTS OF INVENTION
[0024]
The steel for steel tubes according to the present invention is excellent in
cleanliness with fewer harmful coarse inclusions, usable as a steel material for steel
tubes, and casings, tubing, excavating drill pipes, drill collars, etc. for oil well or natural
gas well, excellent particularly in SSC resistance while having predetermined strength
and toughness, and easy to be produced and controlled.
BRIEF DESCRIPTION OF DRAWINGS
[0025]
[FIG 1 ] Fig. 1 is a graph showing a relation between a Mg content in steel and an
inclusion total quantity index; and
[FIG. 2] Fig. 2 is a schematic view illustrating a morphology of an inclusion of not less
than 1 jxm in size which exists in steel when a Mg content in steel is not less than 1.0
ppm and not more than 5.0 ppm.
DESCRIPTION OF EMBODIMENTS
[0026]
The steel for steel tubes of the present invention will be then described in detail
with respect to the reasons of specifying the steel of the present invention as described
above and preferred embodiments for producing the steel of the present invention.
[0027]
1. Ranges of Chemical Composition of the Steel of the Invention, and Reasons for
Limitation
1-1. Basic Elements
C: 0.2 to 0.7%
C is an important element for securing the strength of a steel tube, and its
content needs to be not less than 0.2%. However, an excessively high content of C not
only leads to the saturation of the effect, but also causes a change in generated
morphology of non-metallic inclusions to thereby deteriorate the toughness of steel and
lead to a high susceptibility to quenching crack. Therefore, the upper limit of the
4 | content of C is set to 0.7%. A preferable C content is 0.22 to 0.65%; more preferably
0.24 to 0.40%.
[0028]
Si: 0.01 to 0.8%
Si is added for the purpose of deoxidizing steel or improving the strength of
steel. When the content of Si is below 0.01%, the effect of deoxidizing the steel or
improving the strength is not exerted. On the other hand, a content of Si exceeding
0.8% causes reduction in activity of Ca or S, which affects the morphology of inclusions.
Therefore, the content of Si is set in the range of 0.01 to 0.8%. The Si content is
preferably 0.10 to 0.85%.
[0029]
Mn: 0.1 to 1.5%
Mn is added with a content of not less than 0.1% for the purpose of enhancing
the strength of steel through improvement in quenching-hardenability of the steel.
However, since an excessively high content may cause deterioration in toughness, the
upper limit of the content of Mn is set to 1.5 %. The Mn content is preferably 0.20 to
1.40%, more preferably 0.25 to 0.80%.
[0030]
S: Not more than 0.005%
S is an impurity which forms sulfide-based inclusions, and when the content of
S is increased, the deterioration in toughness or corrosion resistance of steel becomes
serious. Therefore, the content of S is set to not more than 0.005%. A lower S in
content is more desirable.
[0031]
P: Not more than 0.03%
P is an element included in steel as an impurity, and causes deterioration in
toughness or corrosion resistance of steel. Therefore, the upper limit of the content of
P is set to 0.03%. The P content is preferably at most 0.02%, more preferably 0.012%.
It is desirable that the content of P is as least as possible.
[0032]
Al: 0.0005 to 0.1%
Al is an element to be added for deoxidizing molten steel. When the content
4A of Al is less than 0.0005%, coarse composite oxides of Al-Si type, Al-Ti type, Al-Ti-Si
type and the like can be generated due to insufficient deoxidation. On the other hand,
an excessively increased content of Al only leads to saturation of the effect, ending up in
the increase of useless solid-soluble Al. Therefore, the upper limit of the content of Al
is set to 0.1%.
[0033]
1-2. Additive Elements for Improving SSC Resistance
Further, the SSC resistance of steel can be improved by setting each content of
Ti, Ca, N, Cr and Mo to the range described below.
[0034]
Ti: 0.005 to 0.05%
Ti has the effect of improving the strength of steel by action such as grain
refining or precipitation hardening. Further, when B is added to improve the
quenching-hardenability of steel, Ti can inhibit nitridation of B so that the effect of
improving the quenching-hardenability can be exerted. To secure these effects, the
content of Ti must be not less than 0.005%. However, since an excessively high
content of Ti increases carbide-based precipitates to deteriorate the toughness of steel,
the upper limit of the content of Ti is set to 0.05%. A preferable Ti content is 0.008 to
0.035%.
[0035]
Ca: 0.0004 to 0.005%
Ca is an important element which reforms sulfides and oxides at the same time
to improve the SSC resistance of steel. To secure this effect, the content of Ca must be
not less than 0.0004%. However, since an excessively high content of Ca causes
coarsening of inclusions or deterioration in corrosion resistance of steel, the upper limit
of the content of Ca is set to 0.005%.
[0036]
N: Not more than 0.007%
N is an impurity element which tends to be mixed to raw materials or mixed
during melting processes. An increased content of N leads to deterioration in
toughness, corrosion resistance and SSC resistance of steel, inhibition of the effect of
improving the quenching-hardenability by addition of B, or the like. Therefore, a
^r
/0
4 * lower N in content is more desirable. Although an element such as Ti which forms
nitrides is added to suppress this adverse effect of N, this follows generation of
nitride-based inclusions. Accordingly, since an excessively high content of N disables
the control of inclusions, the upper limit of the content of N is set to 0.007%.
[0037]
Cr: 0.1 to 1.5%
Cr has the effect of improving the corrosion resistance of steel, and further has
the effect of improving the SSC resistance of steel since it improves the
quenching-hardenability to improve the strength of steel and also enhances the
resistance to softening by tempering of steel to thereby enable high-temperature
tempering. To secure these effects, the content of Cr must be not less than 0.1 %.
However, since an excessively increased content of Cr only leads to saturation of the
effect of improving tempering softening resistance, and can cause deterioration in
toughness of steel, the upper limit of the content of Cr is set to 1.5%. A preferable Cr
content is 0.5 to 1.2%.
[0038]
Mo: 0.2 to 1.0%
Mo improves the quenching-hardenability to improve the strength of steel, and
also improves the SSC resistance of steel since it enhances the resistance to softening by
tempering to enable high-temperature tempering. To secure these effects, the content
of Mo must be not less than 0.2%. However, since an excessively increased content of
Mo only leads to saturation of the effect of improving the resistance to softening by
tempering, and can cause deterioration in toughness of steel, the upper limit of the
content of Mo is set to 1.0%. A preferable Mo content is 0.25 to 0.85%.
[0039]
1-3. Additive Elements for Further Improving SSC Resistance
The SSC resistance of steel can be further improved by controlling, besides the
above, the contents of Nb, Zr, V and B to the following ranges.
[0040]
Nb: 0.005 to 0.1%, Zr: 0.005 to 0.1%
Nb and/or Zr may not need to be added. However, if added, these elements
exert an effect such as grain refining or precipitation hardening to effectively improve
ti
A the strength of steel. Such an effect cannot be secured with a content of less than
0.005% of each element, and when the content of each element exceeds 0.1%, the
toughness of steel is deteriorated. Therefore, if Nb and/or Zr is added, the content of
each element is preferably set to 0.005 to 0.1%. More preferably the content of each
element is set in the range of 0.008 to 0.05%.
[0041]
V: 0.005 to 0.5%
V may not need to be added. However, V has effects such as precipitation
hardening, improvement in quenching-hardenability, and increase in resistance to
softening by tempering, and if added, the effect of improving the strength and the SSC
resistance can be expected. To secure this effect, the content of V is preferably set to
not less than 0.005%. However, since an excessively increased content of V causes
deterioration in toughness or corrosion resistance of steel, the upper limit of the content
of V is preferably set to 0.5%. More preferably the V content is set in the range of
0.01 to 0.25%.
[0042]
B: 0.0003 to 0.005%
B may not need to be added. However, a slight addition of B has the effect of
improving the quenching-hardenability of steel. When the content of B is below
0.0003%, such an effect cannot be obtained, and when the content exceeds 0.005%, the
toughness of steel is deteriorated. Therefore, if B is added, the content is preferably set
to 0.0003 to 0.005%.
[0043]
1-4. Addition of Mg
1-4-1. Relation Between Mg Content in Steel and Total Number of Inclusions
In the present invention, the Mg content in the steel is set in the range of 1.0 to
5.0 ppm. The Mg content is preferably 1.2 to 4.8 ppm, more preferably 1.4 to 4.6 ppm.
Next, Mg will be described in detail. As described above, a plurality of characteristics
can be simultaneously secured by simultaneously controlling two or more types of
inclusions in order to control a plurality of elements and by taking remedies to prevent
the total number of the inclusions from increasing. Further, it is desirable that factors
to be controlled or managed are as least as possible.
)T
£ [0044]
From such a point of view, the relation between the inclusion morphology, the
number of inclusions and steel compositions were investigated in detail. Namely, 300
kg of each molten steel with steel compositions variously varies within the
above-mentioned ranges was solidified in a mold, a test piece was cut from the resulting
steel ingot, and observed within a 10 mmxlO mm field of view at a magnification of
lOOOx by use of a scanning electron microscope to measure the number of inclusions
each being not less than 1 urn in size. The total of all the number of oxides,
oxysulfides and carbonitrides was defined as "the total number of inclusions". The
evaluation was performed using an inclusion total quantity index with 1 indicating the
total number of inclusions in a sample having a Mg content of 1.5 ppm in steel. The
Mg content in steel was obtained by dissolving machining swarf sampled from each
steel ingot with nitric acid, and diluting the resulting solution to a concentration of 1/10,
followed by quantitative determination by ICP-MS (Inductively Coupled Plasma Mass
Spectrometry).
[0045]
Fig. 1 is a graph showing a relation between a Mg content in steel and an
inclusion total quantity index. As a result of the above-mentioned examination, a
general tendency such that the lower the S content, the less the sulfide inclusions, and
the higher the O content, the more the oxide inclusions was obtained, and results shown
in Fig. 1 were also obtained.
[0046]
On the surface, Fig. 1 appears to indicate that it is difficult to organize the total
number of inclusions of interest in the present invention only by a Mg content in steel,
and the contents of elements such as O and S also contribute to the total number of
inclusions as described above. However, paying attention to the results on the low Mg
content side in Fig. 1, it is found that the total number of inclusions is stably reduced
when the Mg content in steel is not less than 1.0 ppm (0.00010%) and not more than 5.0
ppm (0.00050%). On the other hand, when the Mg content in steel is below 1.0 ppm
or beyond 5.0 ppm, cases with the total number of inclusions being big are also obtained
while there are many cases with the total number of inclusions being small.
[0047]
A Namely, it is found that the total number of the targeted inclusions of 1 um or
more in size may be reduced by controlling the content of Mg when the Mg content in
steel is not less than 1.0 ppm and not more than 5.0 ppm; however, when the Mg
content in steel is below 1.0 ppm or beyond 5.0 ppm, the control of other elements in
addition to the Mg content is needed even under the same condition.
[0048]
1-4-2. Inclusion Morphology
Further, the inclusion morphology was observed in detail, with respect to cases
in which the Mg content in steel is not less than 1.0 ppm and not more than 5.0 ppm in
Fig. 1 and the total number of inclusions is small. As a result, an average of 78.3%
(67.3 to 95.3%) of the number of the targeted inclusions of not less than 1 jam in size
has a structure illustrated in Fig. 2 as the inclusion morphology. The remaining 21.7%
of inclusions were oxides free of carbonitrides or inclusions only composed of
oxysulfides or carbonitrides.
[0049]
Fig. 2 is a schematic view illustrating a morphology of an inclusion of not less
than 1 jam in size which exists in steel when a Mg content in steel is not less than 1.0
ppm and not more than 5.0 ppm.
As shown in Fig. 2, this inclusion has a morphology in which
Ti-containing-carbonitrides or -carbides 3 exists in a periphery part of Ca-Al-based
oxides 2a and Ca-Al-based oxysulfides 2b. Since this inclusion alone enables the
control of O, S, C and N, a treatment for controlling inclusions for each of impurity
elements is not necessary. The present applicant made clear this morphology of
inclusion in Patent Literature 3 described above.
[0050]
However, it has been clarified now that Mg-Al-O-based oxides 1 exist at the
central part of the inclusion so as to be enclosed by Ca-Al-based oxides 2a and
Ca-Al-based oxysulfides 2b. It has been ascertained that when the inclusion
morphology shown in Fig. 2 emerges, the total number of inclusions is reduced. This
inclusion may have a morphology in which the Ti-containing-carbonitrides or -carbides
3 exist on a complete periphery of the Ca-Al-based oxides 2a and the Ca-Al-based
oxysulfides 2b. The inclusion may solely include either of the Ca-Al-based oxides 2a
yr
A or the Ca-Al-based oxysulfides 2b.
[0051]
1-4-3. Mechanism of Forming Inclusions and Mechanism of Reducing Total Number of
Inclusions
The mechanisms related to the above-mentioned inclusion morphology can be
explained as follows.
[0052]
When Mg exists in steel, Mg starts deoxidation reaction prior to Al and Ca
since it is a strong deoxidizing element. The Mg-Al-O-based oxides 1 are generated
thereby prior to the Ca-Al-based oxides 2a and the Ca-Al-based oxysulfides 2b. Since
Mg starts the deoxidation reaction even at lower supersaturation than those of the other
elements due to its deoxidizing power, inclusions become small in size. Namely, when
the content of Mg is within a predetermined range, fine Mg-Al-O-based oxides 1 are
preferentially generated. Thereafter, using these fine Mg-Al-O-based oxides 1 as
generation nuclei, Ca-Al-based oxides 2a and the Ca-Al-based oxysulfides 2b are
generated on their surfaces, and again using these as generation nuclei,
Ti-containing-carbonitrides or -carbides 3 are further generated on their surfaces during
solidification. As a result, the inclusion morphology as shown in Fig. 2 is completed.
At this time, since the formation of the inclusion is originated from fine Mg-Al-O-based
oxides 1, the resulting final inclusions are also fine, and coarse inclusions are
consequently reduced.
[0053]
However, when the Mg content in steel is lower than 1.0 ppm, the final
inclusions can be enlarged since the fine Mg-Al-O-based oxides 1 as origins are not
generated. On the other hand, when the Mg content in steel is higher than 5.0 ppm, the
Mg-Al-O-based oxides 1 can grow to be large since the Mg deoxidation reaction
excessively proceeds, resulting in enlarged final inclusions.
[0054]
Namely, it is found that the inclusion morphology is changed as a result of
change in generation process of the inclusions by the control of the Mg content in steel,
whereby coarse inclusions can be reduced.
[0055]
yr
£ 2. Control Methods of Mg Content in Steel and Inclusions
2-1. Control Method of Mg Content in Steel
Control methods of Mg content in steel and inclusions will be then described.
Firstly, the control method of Mg content in steel is described.
[0056]
A first method is to directly add Mg to molten steel. In this method, metal Mg
or Mg alloy alone or a mixture of Mg or Mg alloy with a compound such as CaO or
MgO is added to molten steel.
[0057]
This addition may be carried out by blowing Mg into molten steel or by use of
an iron-coated wire, similarly to the after-mentioned case of Ca. The addition amount
(per ton of molten steel) is desirably set to 0.05 to 0.2 kg/ton in terms of pure Mg
content. When the addition amount is below 0.05 kg/ton, the Mg content in steel
cannot be increased, and the addition by the amount higher than 0.2 kg/ton can lead to
an increased Mg content in steel which exceeds 5.0 ppm.
[0058]
The addition of Mg is performed desirably at a terminal stage of secondary
refining, and further desirably just before casting. This is to minimize the change in
Mg content in steel because Mg vaporizes from the molten steel. The addition just
before casting can be performed, for example, by addition into molten steel within the
tundish of a continuous casting machine.
[0059]
A second method is to indirectly supply Mg to molten steel by use of slag and
refractory. Since the refractory or slag generally contains MgO, this MgO is used as a
Mg source to the molten steel. When the refractory contains no MgO, only the slag is
used as a Mg source.
[0060]
Based on the principle that Al, Ca and the like in molten steel exhibit the
reaction of reduction of the MgO included in the refractory or slag, the reduced Mg is
supplied to the molten steel. This reduction reaction extremely gently proceeds since
Mg has strong deoxidizing power and MgO is stable. Therefore, the second method is
suitable to control the content of a small amount of Mg in molten steel. Specifically,
A the second method is carried out in the following manner.
[0061]
In general, the refractory composition is controlled so that the content of MgO
in the slag is not less than 5% since the refractory composition is constant. Although 1
the MgO in the slag is increased also by the reaction of the slag with the refractory,
MgO may be added to the slag if the MgO in the slag is insufficient. This addition
treatment of MgO is performed desirably at an early stage of steelmaking process such
as during pouring from a converter to a ladle or before starting the secondary refining,
because the reaction of MgO with molten steel is slow as described above.
[0062]
When a deoxidizing element such as Al is then put into the molten steel, the
reaction of MgO with the molten steel is started to gradually increase the content of Mg
in the molten steel. Since the increasing rate of Mg content at this time depends on the
content of the deoxidizing element such as Al, Ca or the like or the slag composition in
the molten steel, but is constant if the content of the deoxidizing element or the slag
composition is constant, the final content of Mg in the molten steel depends on only the
treatment time. Therefore, a relation between the addition amount of the deoxidizing
element and the treatment time is acquired from temporal change records of Mg content
in the molten steel in the steelmaking process, whereby the content of Mg in the molten
steel can be controlled based on the acquired relation. This method is advantageous in
terms of both time and cost since Mg addition treatment is unnecessary, and strict
management of the treatment time, the addition of the deoxidizing element and the slag
composition suffice as the control.
[0063]
Of the above-mentioned two methods for controlling the Mg content in steel,
the second method is preferred when the controls of Mg content in steel and inclusions
are simultaneously performed.
[0064]
Since Mg-based inclusion constituents are used as nuclei of relevant inclusions
in the steel of the present invention, it is important that the inclusion constituents that
form the nuclei are uniformly and homogeneously distributed in the steel. In order to
have the inclusion constituents uniformly and homogeneously in steel, it is necessary to
, > ~
i7
4 | equilibrate the reaction between molten steel and inclusion constituent. Although the
equilibration of the reaction can be attained by extending the treatment time, this is not
viable commercially. Further, when the deoxidizing element such as metal Mg is
added to molten steel by adopting the first method, attaining uniform and homogeneous
inclusion constituents can be impaired since various types of inclusions are formed due
to the distribution of concentration which occurs until the added Mg is uniformly mixed
to the molten steel.
[0065]
On the other hand, since the molten steel-slag reaction is used, the second
method does not cause such distribution of concentration which should occur due to the
delay of uniform mixing of Mg. Further, since the slag is the same as Mg-Al-O-based
oxides that form nuclei, the relevant inclusion constituents can be prevented from being
heterogeneous by using the equilibration in molten steel-slag-inclusions/constituents
reaction.
[0066]
2-2. Specific Factors in Second Method
Specific Factors in the second method include slag factors and deoxidation
factors as described below.
[0067]
2-2-1. Slag Factors
Firstly, the slag factors in the second method will be described. The slag to be
used is required to have a composition such that the content of CaO is not less than 40%,
the content of MgO is not less than 5%, and a total content of Fe oxides and Mn oxides
is not more than 3% in the slag. Further, by controlling the content of MgO in the slag
to not more than 15% and the content of CaO in the slag to not more than 70%, the
accuracy of the control of Mg content in steel is improved.
[0068]
When the content of MgO in the slag is below 5%, the content of Mg in molten
steel cannot be increased, and when it is higher than 15%, the controllability of the Mg
content in steel is deteriorated since the fluidity of the slag is deteriorated to reduce the
reaction rate of the molten steel-slag reaction.
[0069]
I*"
A When the content of CaO in the slag is below 40%, the MgO in the slag cannot
be subjected to reducing reaction to be supplied to the molten steel since the oxygen
activity at the slag-metal interface cannot be sufficiently decreased. When the content
of CaO in the slag is higher than 70%, the controllability of Mg content in steel is
deteriorated due to deterioration of the fluidity of the slag.
[0070]
When the total content of Fe oxides and Mn oxides in the slag is higher than
3%, the MgO in the slag cannot be subjected to reducing reaction to be supplied to
molten steel since the oxygen activity at the slag-metal interface cannot be sufficiently
decreased.
[0071]
Further, the amount of slag in use (per ton of molten steel) is desirably set to
not less than 10 kg/ton and not more than 20 kg/ton. When the amount of slag is
below 10 kg/ton, the absolute amount of MgO is insufficient, and when the amount is
larger than 20 kg/ton, the time required for equalizing the slag composition is extended.
[0072]
2-2-2. Deoxidation Factors
Next, deoxidation factors in the second method are described. The relevant
inclusions can be further accurately controlled, in addition to the Mg content in molten
steel, by satisfying the deoxidation factors of the molten steel after satisfying the
above-mentioned slag factors. The deoxidizing elements used in controlling are Al
and Ca.
[0073]
2-2-2-1. Factors for Al
Firstly, factors for Al are described. In general, since deoxidation is
sufficiently performed when the content of Al in molten steel is not less than 0.01%,
refining is usually performed with a content of Al in molten steel in the range of about
0.01 to 0.05%. Although Mg can be controlled if the content of Al in the molten steel
is continuously controlled to a narrow range within such a content range, this causes
extension of the refining time and deterioration of the accuracy in the inclusions
morphology control. Therefore, as a method to avoid them, it can be adopted to
enhance the content of Al in the molten steel to 0.05% or more for not less than 1
n
4k minute in the secondary refining such as RH.
[0074]
It is extremely effective for reduction of MgO in the slag and decrease of Fe
oxide and Mn oxide in the slag to enhance the content of Al in the molten steel even in a
time as short as 1 minute, and the control accuracy of Mg and inclusions in steel is
consequently improved.
[0075]
2-2-2-2. Factors for Ca
Finally, factors for Ca are described. Ca is an important element which forms
inclusions, similarly to Mg, and the following method is effectively used to cause
Mg-based inclusions to be nuclei.
[0076]
For causing the Mg-based inclusions to be nuclei, it goes without saying that
the addition of Ca must be performed after the Mg content in molten steel is sufficiently
stabilized. However, it is more necessary to inhibit Ca from promoting the reaction of
reducing the MgO in the slag by its reaction with the slag, and further to inhibit
excessive progress of the reaction of Ca with the Mg-based inclusions lest even the
nuclei of the inclusions should be reduced by Ca.
[0077]
To satisfy this factors, it is necessary to add Ca in the absence of the slag, and
stop the reaction by rapidly casting and solidifying as soon as Ca is added. For
satisfying these conditions, it is most desirable to perform the addition of Ca within the
tundish of the continuous casting machine.
[0078]
The addition amount of Ca (per ton of molten steel) must be not less than 0.02
kg/ton and not more than 0.05 kg/ton. This addition amount of Ca is extremely low,
compared with a general addition amount of Ca. The reason is that Ca can reduce the
nuclei if the addition amount of Ca is more than 0.05 kg/ton. On the other hand, when
the addition amount of Ca is below 0.02 kg/ton, sufficient Ca-based inclusions for
enclosing the nuclei are not formed.
[0079]
As described above, to control relevant non-metal inclusions in steel intended
4k by the present intention, which has an Mg content in steel of not less than 1.0 ppm and
not more than 5.0 ppm, and is composed of two or more elements of Ca, Al, Mg, Ti and
Nb and two or more elements of O, S and N into a morphology in which an
Mg-Al-O-based oxide exists at the central part of the inclusion, a Ca-Al-based oxide or
a Ca-Al-based oxysulfide encloses the Mg-Al-O-based oxide, and a
Ti-containing-carbonitrides or -carbides further exists on a complete or partial periphery
of the Ca-Al-based oxide or Ca-Al-based oxysulfide, it is important to temporarily
increase the content of Al in molten steel to 0.05% or more after controlling the slag
composition into a proper range, and further add not less than 0.02 kg/ton and not more
than 0.05 kg/ton of Ca within the tundish of the continuous casing machine.
[0080]
3. Preferable Production Conditions for Attaining Inclusion Morphology
Preferable steel production conditions for achieving such an inclusion
morphology will be described with examples of general production processes such as
converter, secondary refining, and continuous casting.
[0081]
3-1. Control of Sulfides
Firstly, the control of sulfides will be described. When the content of S in
steel is lowered, the amount of formed sulfides or oxysulfides is reduced, and inclusions
thereof become smaller in size and fewer in number. For having smaller and fewer
inclusions, the content of S in steel is preferably not more than 0.002%, and further
preferably not more than 0.001%.
[0082]
To attain such an S content in steel, desulfurization treatment in secondary
refining may be needed in addition to desulfurization treatment in hot pig iron
preliminary treatment. The desulfurization in secondary refining is performed by
blowing gas to molten steel after producing a slag having desulfurizing capability on the
molten steel, or by blowing a desulfurizing flux into molten steel or spraying it onto the
surface of molten steel. In the treatment using the desulfurizing flux, each of a method
of performing the treatment under the atmosphere and a method of performing the
treatment under reduced pressure by use of RH or the like can be applied.
[0083]
A 3-2. Control of Oxides
With respect to oxides, also, the effect of having fewer inclusions can be
developed by lowering the content of O in steel, similarly to the control of sulfide
inclusions by lowering the content of S in steel. To secure this effect, the content of O
in steel is preferably not more than 0.0015%, and further preferably not more than
0.0010%.
[0084]
For lowering the content of O in steel, two methods represented by the
intensified deoxidation and the inclusions removal in molten steel, are effective.
[0085]
Although it is effective to set the content of Al to not less than 0.01% for the
intensified deoxidation, the deoxidation may be performed further by the
above-mentioned slag refining method of setting the content of CaO in slag to not less
than 40%, a method of setting the total content of Fe oxides and Mn oxides in slag to
not more than 3%, or the like.
[0086]
The removal of inclusions may be performed by blowing inert gas into molten
steel, by circulating molten steel by use of a vacuum treatment device such as RH, or
the like.
[0087]
The addition of Ca may be performed by blowing metal Ca or Ca alloy or a
material containing them into molten steel, by performing the addition by use of
iron-coated wire, or the like, and any other methods are also applicable. The addition
of Ca is desirably performed after the desulfurization in secondary refining. This is to
inhibit the reaction of Ca with S. The content of Ca is preferably not more than
0.002%, and further preferably not more than 0.0012%. The reason is that an
increased content of Ca intensifies the deoxidation effect but leads to activation of
forming CaS or the like.
[0088]
3-3. Control of Carbonitrides
Although the amount of formed carbonitrides can be reduced by lowering the
content of C or Ti, the contents of these elements cannot be lowered since they
?r
9>-
A contribute to improve the strength of base metal as described above. Therefore,
lowering the content of N is effective for the control of carbonitrides. In particular, the
content of N is preferably not more than 0.004%, and further preferably not more than
0.003%.
[0089]
The control technique characterized by a combination of Ca and Ti, which is
proposed in Patent Literature 4 by the present applicant, can be used in combination.
[0090]
3-4. Other Preferable Conditions
As mentioned above, the content of O in steel is desirably not more than
0.0015%, and further desirably not more than 0.0010%. The inclusion morphology
shown in Fig. 2 can be easily obtained with an O content in steel of not more than
0.0015%, and substantially all inclusions show the morphology shown in the same
figure with not more than 0.0010%.
[0091]
Lanthanoid such as La, Ce or Nd can be added to the steel of the present
invention. These elements have the effect of stabilizing the Mg content in addition to
reducing the activities of O and S. The desirable content of lanthanoid is not less than
0.001% and not more than 0.05% in total. The effect is insufficient with a content
below 0.001%, and the inclusions intended by the present invention cannot be obtained
with a content beyond 0.05% since the inclusions are changed to a lanthanoid-based
oxysulfides such as CeaC^S.
[0092]
The steel of the present invention is desirably produced using a converter, an
RH and a continuous casting machine. Gas blowing refining may be performed before
or after RH treatment. Since the control accuracy of slag composition is improved
thereby, the control accuracy of inclusion morphology can be further enhanced.
[0093]
When temperature adjustment is performed in RH, a treatment for reacting
oxygen with Al and Si in molten steel by adding oxygen gas or solid oxides to the
molten steel may be performed. This treatment is preferably performed at an initial
stage of RH, since the added oxygen interrupts the control of Mg content by the
4k slag-metal reaction.
EXAMPLES
[0094]
For confirming the effect on characteristics of the steel for steel tubes of the
present invention, the following test was carried out, and the results were evaluated.
[0095]
1. Test Conditions
After refining a low alloy steel in a converter, composition adjustment and
temperature adjustment were performed by RH vacuum treatment. MgO was poured
into a ladle during teeming from the converter to adjust the content of MgO in slag to 5
to 10%. Time between the teeming from the converter and the RH treatment was 1
hour.
[0096]
Steel compositions are as shown in Table 1. Test Nos. 1 to 3 are inventive
examples satisfying the limitation of the first inventive steel, Test Nos. 4 to 6 are
inventive examples satisfying the limitation of the second inventive steel, and Test Nos.
7 to 9 are inventive examples satisfying the limitations of the second inventive steel
with preferred production conditions. Test Nos. 10 to 15 are comparative examples
which does not satisfy any limitations of the first inventive steel and the second
inventive steel.
[0097]
[Table 1]
^W f N i n o o m t ^ - , ^ — m ON oo o fN _ ™
5 0 0 0 0 0 S 0 0 0 0 0 0 ' § §
• ^ o o o o o H p o o o o o P P
o o o o o ° o o o o o o 00
«/-> m ON ro 0© cs r~ NO
«3 > 1 1 1 1 1 O C S O 1 1 1 1 1 I O
'C O O O* O
a. >/-> v> •t r^
E u O — © —
__ [sg > ' ' © O 1 ' ' ' ' ' ' O O •
•o © 0 © ©'
S m — fi 00 CN — £9 o > n ' o O N < n > o © p < o < o < o O N p ' * i n
"c3 © © — ' © © o ' — — P ' © © © — • © ' ©
- ° O N i n c N O N O O T T r o — >n 1/1 ci — — < N CN
- C 7 © © © © © © © © © © © © © © ©
* t P P P P P P P P P P P P P P P vP* © ' © © © © ' © ' © ' © © © © © ' © © ' © l£v £ T t - o o i n » o m O N < N — O N T f o o m O s — — P © © — — C N ©
H * © © © © © © © © © © © © © © ©
c U © p © p © © © © © © © © © © ©
^ © ©' © © ©* O o' O © ©' © O ©' © O
S ' E - " © © © o © © © © p p p p © p ©
rt g © ' © © ' o ' © © ' © © ' © o ' o o © © ' o'
u 9 - — CJ «n — m t ~ - r - ( N ' * > n ^ - — • r- 00 — __ /
X > o < © © © © © © © © © © © © . © © © pi
rf2 j^ © O © © " © © © ' 0 0 0 0 ' © ' © © ' © S
o © © © © © © © © © © © © © © ©
g o - p O O O © © © © . © © , © © © © ©
§ © © ' © © © o o o ' © ' © © © © © ©
^ " / i r ^ m T i n O N — m o o m o \ r ^ < / ~ t i n \ o
U — © — © © © — © © — © — © © O
r / ) © © © © © © © © © © © © © © ©
" J 0 © © © © © 0 © © © © © © © 0
o © ' o © © o © © © o ' © ' © ' © o ' © '
• ^ o o © ' 0 © © © © © ' © © © ' © ' 0 © '
t~- — 00 — 00 ON — 00 m r-~ (N 00 — •>* CN
© © © ©' © © © ' o ' © ' o o © © ' © ' o '
r - r t o o o N — o o N O O N , ~ r ^ — ON — O O T J -
I o © ' © ' © © ' © © © ' 0 © © © ' © ' © ' ©
D a> D a> a> D
Q. Q. Q. O. O- "5.
_ a . ' o . Q . a . a . a . ' a . a . a . E E E E EE
§ E E E E E £ S E E g S S 5 S 3
•& rt «* «* «* « rt 3 w 1 rn rn m M ,n r?i
O U J [ X 1 U J U U J U J U J U J L U I U < U O < U ( U ( U
^ QJ 1* Q> 1) QJ U 0> OJ 1) . « .— .^ .^ .^ .^
'8 .> .> .> .2 .£ .i .5 .^ .2 I ff £ 1 g 2
r 1 u u < u < u ( u < u i u < u a > a . a . a . c x Q . o .
_ ^ — — — — — — — o o o o o o
U U U CJ CJ U
0
^ - c s m ^ - < n v o r - o o o < 2 - 2 2 2 : 2
£ [0098]
For Test Nos. 1 to 6, 10 to 12, 14 and 15, a metal Mg wire was added to molten
steel within the ladle after the RH treatment, and a CaSi wire was thereafter further
added.
[0099]
For Test Nos. 7 to 9, CaO and MgO were added during teeming from the
converter to control the content of CaO in slag to 55 to 65%, the content of MgO to 8 to
12%, and a total content of Fe oxides and Mn oxides in slag to not more than 1.5%, and
then, the content of Al in molten steel at the beginning of RH treatment was controlled
to 0.07%. For Test Nos. 7 to 9, Ca of 0.03 kg/ton was solely added to the tundish
without adding metal Mg.
[0100]
The molten steel was processed to yield a round billet 220 to 360 mm in
diameter by continuous casting. The following rolling and heat treatment were
performed to the cast round billet to evaluate corrosion resistance.
[0101]
The cast round billet was subjected to piercing and rolling to make a hollow
shell, followed by hot rolling and dimensional adjustment with a mandrel mill and a
stretch reducer under generally employed conditions, thereby producing seamless steel
tubes. Such steel tubes were quenched by heating at 920°C and then adjusted to a
level of yielding strength 758 MPa or more (less than 862MPa) corresponding to 110
ksi grade and a level of yielding strength 862 MPa or more corresponding to 125 ksi
grade by selecting the tempering temperature.
[0102]
2. Evaluation Conditions for Corrosion Resistance
With respect to steel tubes which were heat-treated and examined for strength
and hardness, an evaluation test for SSC resistance was performed.
[0103]
The evaluation of 110 ksi grade (yielding strength 758 to 862 MPa) was
performed for a stress corrosion test specimen comprising 2 mm in thickness, 10 mm in
width, and 75 mm in length which was sampled from each steel tube for testing.
[0104]
it,
flfe A predetermined amount of strain was given to the test specimen by four-point
bending according to a method specified in ASTM G39 to apply the stress
corresponding to 90% of yield strength of steel to the test specimen. Being immersed
in the solution comprising 5% saline water of 25°C which was saturated with 10 atm
hydrogen sulfide, the test specimen was encapsulated in an autoclave together with a
testing jig. Five percent saline water was then introduced into the autoclave while
leaving plenum to deaerate the solution, hydrogen sulfide gas of a predetermined
pressure was then introduced and sealed in the autoclave, and this pressurized hydrogen
sulfide gas was saturated to the liquid phase by stirring the liquid phase. After the
autoclave was sealed, it was held at 25°C for 720 hours while stirring the solution at a
rate of 100 revolutions per minute, and thereafter depressurized to take out the test
specimen.
[0105]
Determination of cracking was performed by visual observation and, in the 1
case where visual determination is difficult, by embedding the tested test specimen in I
resin and microscopically observing a cross section thereof. I
[0106]
The evaluation of 125 ksi grade (yielding strength 862 to 965 MPa) was
performed to a round bar tensile test piece of 6.35 mm in diameter, which was sampled
in parallel to a longitudinal direction of the steel tube. I
[0107]
The stress corresponding to 90% of actual yield strength is continuously
applied to the test piece for 720 hours in 2.5% acetic acid+0.41% Na acetate+5% saline
solution of 25°C, which was saturated with 0.1 atm hydrogen sulfide gas with the
balance carbon dioxide, by a method according to NACE-TM-0177-A-2005, and
thereafter checked for fracture.
[0108]
2. Test Results
With respect to the test pieces subjected to the test in the above-mentioned I
conditions, evaluation was performed using the inclusion morphology, the total number
of inclusions and the fracture rate as evaluation indicators. The test results are shown
in Table 2.
0 [0109]
[Table 2]
Table 2
Inclusion Quantity Fracture Rate Fracture Rate
Test No. Classification
Morphology Index (HOksi) (125ksi)
1 Inventive Example o 1 1.3 1.6
2 Inventive Example o 0.95 0.9 1.2
3 Inventive Example o 0.97 1.2 1.1
4 Inventive Example o 1.02 0.3 0.2
5 Inventive Example o 0.98 0.2 0.2
6 Inventive Example o 0.91 0.3 0.1
7 Inventive Example o 0.85 0 0
8 Inventive Example o 0.86 0 0
9 Inventive Example o 0.82 0 0
10 Comparative Example x 3.23 10.3 15.2
11 Comparative Example x 1.28 13.1 11.5
12 Comparative Example x 8.52 14.5 13.3 I
13 Comparative Example x 9.12 18.9 17.5 I
14 Comparative Example x 9.75 11.3 12.1 I
15 Comparative Example x 5.35 15.3 13.1 I
[0110]
As the evaluation indicator for corrosion resistance, the fracture rate was used.
The fracture rate was calculated, based on the test results, according to the following
expression (1) for both the 110 ksi grade and the 125 ksi grade.
Fracture Rate = (The number of fractured test pieces out of all test pieces) /
(The total number of test pieces) * 100 (1)
[0111]
The same test pieces were observed within a visual field of 10 mmx 10 mm at a
Mk magnification of lOOOx by use of a scanning electron microscope to measure the
number of inclusions of not less than 1 um in size. The total of all the number of
oxides, oxysulfides and carbonitrides was defined as the total number of inclusions as
described above. In Table 2, further, the total number of inclusions was indexed using
the total number of inclusions of Test No. 1 as a reference, and organized in terms of
quantity index.
[0112]
As a result of the SEM observation, an inclusion morphology which
corresponds to the morphology shown in Fig. 2 described above was shown by o and an
inclusion morphology other than the morphology shown in the same figure was shown
by x in the column of inclusion morphology of Table 2. More specifically, inclusion
morphology was investigated using SEM and EDS, where 30 counts of inclusions of not
less than 1 um in size are selected at random and elements analysis for the inclusions
was conducted using EDS. According to the EDS elements analysis, the sample in I
which 15 or more counts of inclusions correspond to the morphology shown in Fig. 2
was evaluated as o, and the one in which less than 15 counts of inclusions correspond to
the morphology shown in Fig. 2 was evaluated as x.
[0113]
By comparison of the test results of Test Nos. 1, 2 and 3 which satisfy the
limitation of the first inventive steel with respect to chemical compositions including
Mg content and inclusion morphology, as shown in Table 2, with the test results of Test
Nos. 10, 11 and 12 which satisfy none of the limitations of the first inventive steel and
the second inventive steel, the number of inclusions was as small as 0.95 to 1 in Test
Nos. 1, 2 and 3, compared with 1.28 to 8.52 in Test Nos. 10, 11 and 12. This could
confirm that the total number of inclusions can be reduced by satisfying the limitations
of the present invention. The fracture rate was also as low as 0.9 to 1.6 in Test Nos. 1,
2 and 3, compared with 10.3 to 15.2 in Test Nos. 10, 11 and 12.
[0114]
By comparison of the test results of Test Nos. 4, 5 and 6 which satisfy the
limitation of the second inventive steel with the test results of Test Nos. 13, 14 and 15
which satisfy none of the limitations of the first inventive steel and the second inventive
steel, the fracture rate in Test Nos. 13, 14 and 15 was 11.3 to 18.9%, which was two
Jfc digit larger than 0.1 to 0.3% of the fracture rate in Test Nos. 4, 5 and 6.
[0115]
Further, Test Nos. 4, 5 and 6 were found to be excellent in corrosion resistance,
with the fracture rate reduced to 0.1 to 0.3 by the addition of alloy elements, compared
with Test Nos. 1, 2 and 3 with less alloy elements.
[0116] I
Moreover, among the inventive examples, Test Nos. 7, 8 and 9 in which the I
molten steel treatment method was optimized were further reduced in the number of
inclusions, compared with Test Nos. 1 to 6, and the fracture rate therein was 0. Thus,
by actively controlling the steel compositions and the inclusions, the effects of the steel
of the present invention can be stabilized at high level.
[0117]
As described above, the number of inclusions can be reduced by satisfying the
• limitation of the first inventive steel, and the corrosion resistance of steel product can be
improved by satisfying the limitation of the second inventive steel.
INDUSTRIAL APPLICABILITY
[0118]
The steel for steel tubes of the present invention is excellent in cleanliness with
fewer harmful coarse inclusions, and usable as a steel material for steel tubes, and
. casings, tubing, excavating drill pipes, drill collars, etc. for oil well or natural gas well,
and can simultaneously improve various characteristics thereof. This steel is also easy I
to be produced and controlled.
REFERENCE SIGNS LIST
[0119]
1: Mg-Al-O-based oxides
2a: Ca-Al-based oxides
2b: Ca-Al-based oxysulfides
3: Ti-containing-carbonitrides or -carbides
7°

We claim:
1. A steel for steel tubes with excellent sulfide stress cracking resistance which
comprises, by mass%:
C: 0.2 to 0.7%;
Si: 0.01 to 0.8%;
Mn: 0.1 to 1.5%;
S: not more than 0.005%;
P: not more than 0.03%;
Al: 0.0005 to 0.1%;
Ti: 0.005 to 0.05%;
Ca: 0.0004 to 0.005%;
N: not more than 0.007%;
Cr:0. 1 to 1.5%; and
Mo: 0.2 to 1.0%;
the balance being Fe, Mg and impurities,
being characterized in that:
the content of Mg in the steel is not less than 1.0 ppm and not more than 5.0
ppm;
and non-metallic inclusions of not less than 50% of the total number of those in
steel each having the maximum bulk size of not less than 1 urn and comprising two or
more elements of Ca, Al, Mg, Ti and Nb and two or more elements of O, S and N have
such a morphology that Mg-Al-O-based oxides exist at the central part of the inclusion,
Ca-Al-based oxides and/or Ca-Al-based oxysulfides enclose the Mg-Al-O-based oxides,
and Ti-containing-carbonitrides or -carbides further exist on a complete or partial
periphery of the Ca-Al-based oxides and/or Ca-Al-based oxysulfides.
2. A steel for steel tubes with excellent sulfide stress cracking resistance which
comprises, by mass%:
C: 0.2 to 0.7%;
Si: 0.01 to 0.8%;
Mn:0.1 to 1.5%;
S: not more than 0.005%;
P: not more than 0.03%;
Al: 0.0005 to 0.1%;
Ti: 0.005 to 0.05%;
Ca: 0.0004 to 0.005%;
N: not more than 0.007%;
Cr: 0.1 to 1.5%;
Mo: 0.2 to 1.0%; and
one or more of Nb: 0.005 to 0.1%, Zr: 0.005 to 0.1%, V: 0.005 to 0.5% and B:
0.0003 to 0.005%;
the balance being Fe, Mg and impurities,
being characterized in that:
the content of Mg in the steel is not less than 1.0 ppm and not more than 5.0
ppm; and
non-metallic inclusions of not less than 50% of the total number of those in
i
steel each having the maximum bulk size of not less than 1 urn and comprising two or
more elements of Ca, Al, Mg, Ti and Nb and two or more elements of O, S and N have
such a morphology that Mg-Al-O-based oxides exist at the central part of the inclusion,
Ca-Al-based oxides and/or Ca-Al-based oxysulfides enclose the Mg-Al-O-based oxides,
and Ti-containing-carbonitrides or -carbides further exist on a complete or partial
periphery of the Ca-Al-based oxides and/or Ca-Al-based oxysulfides.