Description
Title of Invention: Cu-Sn COEXISTING STEEL AND METHOD FOR
MANUFACTURING THE SAME
Technical Field
[0001] The present invention relates to low-alloy steel (Cu-Sn coexisting steel) for
anticorrosive heavy gauge steel plate which contains Cu and Sn, and methods for
manufacturing the same. Particularly, the present invention relates to steel without
10 surface cracking or surface defects where neither surface cracking nor surface defects
occur even if rolled to be heavy gauge steel plate, and a method for manufacturing the
15
same.
Background Art
[0002] Cu and Sn are both tramp elements in scrap iron. They are useful because
they are elements improving the corrosion resistance of steel. However, it is known
that Cu causes what is called red embrittlement, which causes cracking to occur in hot
working on steel (hereinafter red embrittlement induced by Cu is also referred to as "Cu
embrittlement"), and Sn encourages Cu embrittlement. Thus, when a steel material
20 containing both Cu and Sn is manufactured, the ultimate object is to inhibit surface
cracking and surface defects.
[0003] Patent Literature 1 discloses a steel material having outstanding
weatherability on sea shores which contains both Cu and Sn, and a structure using the
same. However, this literature does not focus on prevention of surface embrittlement
2-
of a slab at hot temperature and of surface defects in continuous casting.
[0004] Patent Literature 2 discloses hot-rolled steel containing both Cu and Sn for
the manufacture without occurrence of surface defects in hot working. This literature
also describes that although addition of Ni to steel containing Cu makes it possible to
5 prevent cracking on the surface of the steel which is induced by Cu, the effect of
preventing cracking that Ni has diminishes on steel containing Sn in addition to Cu.
However, according to this literature, Ni is considered to be a little as resources and
invite high costs, and an object is to provide hot-rolled steel of a good surface property
without addition of Ni. There is no enough description of the effect when Ni coexists
10 with Cu and Sn.
[0005] Patent Literature 3 discloses the art an object of which is to prevent surface
defects from occurring through continuous casting with the ratios of the components,
Cu/Sn and (Cu + Ni)/Sn of anticorrosive low-alloy steel of predetermined ranges.
[0006] Each steel of Patent Literatures 2 and 3 is low-alloy steel whose content of
15 Sn is more than twice that of Cu. The upper limit of the value of the ratio of the
components, Cu/Sn (%by mass) (hereinafter referred to as "Cu/Sn ratio") of the steel of
these Literatures is 0.5. If the Cu/Sn ratio is too high, surface cracking occurs. Thus,
it is difficult to improve the Cu/Sn ratio for the purpose of improvement of a property
like corrosion resistance.
20 [0007] Non Patent Literature 1 lists the following a and b as the influences of Cu
and Sn on cracking in hot working due to red shortness (liquid embrittlement) on the
surface:
[0008] a. Scales are generated on the surface of a steel material heated to 1 OOO"C or
more because of atmospheric oxidation. In a case of steel whose content of Cu is
3
approximately 0.3% by mass, Fe that is the main component of the parent phase is
selectively oxidized, and Cu is concentrated on the surface portion of the steel material.
At this time, Cu, which has a lower melting point than Fe, is separated on the surface
portion of the steel metal as a liquid phase. This penetrates grain boundaries, to invite
5 liquid membrane embrittlement.
[0009] b. Cu, Sn and Ni are all metallic elements that are more difficult to be
oxidized than Fe that is the main component of the parent phase, that is, nobler than Fe.
Surface cracking on a steel material is conspicuous in a case of steel containing C~ and
Sn among the above elements (Cu: 0.3% by mass and Sn: 0.04% by mass) compared
10 with steel containing only Cu among the above elements (Cu: 0.3% by mass). There
occurs no surface cracking in a case of steel containing only Sn among the above
elements (Sn: 0.04% by- mass).
[0010] In Non Patent Literature 1, the effect of inhibiting embrittlement induced by
Cu and Sn that Ni has is also examined. According to this literature, it is enough for
15 inhibiting embrittlement of the above described steel containing only Cu to add Ni of
0.15% by mass; on the other hand, it is necessary for inhibiting embrittlement of the
above described steel containing Cu and Sn to add Ni of 0.3% by mass.
[0011] As described above, Non Patent Literature 1 merely describes that Sn and Ni
affect inhibition of embrittlement of the above described steel containing only Cu, and
20 that there occurs no embrittlement to the above described steel containing only Sn.
Citation List
Patent Literature
[0012] Patent Literature 1: JP 2004-360063 A
Patent Literature 2: JP H6-256904 A
Patent Literature 3: JP 2011-42859 A
Non Patent Literature
5 [0013] Non Patent Literature 1: KUNISHIGE, Kazutoshi and other three,
"Suppression of Surface Hot-shortness Induced by Cu and/or Sn", Current Advances in
Materials and Processes, the Iron and Steel Institute of Japan, Vol. 13, No. 6, pp.
1080-1083, 2000
10 Summary of Invention
Technical Problem
[0014] The present invention is in view of these problems, that is, occurrence of
surface cracking and surface defects caused by Cu embrittlement when steel containing
Cu and Sn is manufactured. An object of the present invention is to provide Cu-Sn
15 coexisting steel that makes it possible to keep a good quality of its surface even if
hot-rolled, and a method for manufacturing the same.
Solution to Problem
[0015] The inventors of the present invention select low-alloy steel containing Cu
20 and Sn which can be a material of heavy gauge steel plate of a good corrosion resistance
in order to solve the problems. Specifically, selected is Cu-Sn coexisting steel
containing C: 0.04 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, Cu: 0.20 to 1.50%
and Sn: 0.06 to 0.50% by mass. This composition makes it possible to obtain a good
corrosion resistance while satisfying mechanical characteristics as a material of heavy
5
gauge steel plate. It is preferable that the Cu/Sn ratio (mass ratio) in this steel satisfies
1.0 to 8.0 in order to improve the corrosion resistance. However, Cu embrittlement is
easy to occur conspicuously to this steel because Cu and Sn coexist in this steel.
[00 16] The inventors of the present invention examined composition that makes it
5 possible to inhibit Cu embrittlement occurring to the above described Cu-Sn coexisting
steel accompanied by selective oxidation of Fe. In this examination, influence of not
only Cu and Sn but also coexisting alloying elements is focused on, and also, an internal
oxidation layer that is formed when the surface of a slab is oxidized in the process of
cooling the slab is focused on.
10 [0017] An internal oxidation layer is a preliminary oxidation layer generated by
oxidation of alloying elements that are baser than Fe at a step before Fe of the parent
phase is oxidized. In the above Cu-Sn coexisting steel, the internal oxidation layer is a
layer where minute oxides composed of Si and Mn (the main components are Si02,
MnO and SiMnO (manganese silicate)) are dispersed. The content of Ah03 in oxides
15 in this internal oxidation layer is less than 3% by mass at most so far.
[0018] As a result of the examination, it is found out that occurrence of surface
cracking accompanied by Cu embrittlement can be inhibited by: adding Al and Ni to the
above Cu-Sn coexisting steel in a molten state, to adjust the composition of this molten
steel so that the contents of Si, Mn, Cu, Sn, AI and Ni satisfy predetermined conditions;
20 further oxidizing the surface of a slab in the process of cooling the slab to form the
internal oxidation layer; and containing Ah03 in composite oxides that are generated in
this internal oxidation layer. Al and Ni are elements having a function of improving
the solid solubility of Cu into steel. On the other hand, in a case where either Al or Ni
is contained solely, no great effect is obtained on inhibition of occurrence of surface
6
5
cracking. Examined details on conditions of adding AI and Ni will be described later.
[00 19] The present invention is based on this finding. Its summary lies in the
following method for manufacturing Cu-Sn coexisting steel and Cu-Sn coexisting steel
manufactured by this manufacturing method.
[0020] A method for manufacturing Cu-Sn coexisting steel by continuous casting
of molten steel, the method including: adjusting composition of molten steel so as to
satisfy conditions represented by the following formulas (1) to (3), the molten steel
containing, as chemical composition, C: 0.04 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to
2.50%, P: no more than 0.05%, S: no more than 0.02%, Cu: 0.20 to 1.50% and Sn: 0.06
10 to 0.50% and further contains AI: 0.06 to 1.00% and Ni: 0.05 to 1.00% by mass, and Fe
and impurities as the remainder; forming an internal oxidation layer by oxidizing a
surface of a slab in a process of cooling the slab; and making composite oxides that are
generated in the internal oxidation layer, contain Al203:
15
[Al]/(3[Si] + [Mn]) ~ 0.050 ... (1)
[Ni]/([Cu] + 5[Sn]) ~ 0.10 ... (2)
[Al]/[Ni] ~ 0.20 ... (3)
wherein [Al], [Si], [Mn], [Ni], [Cu] and [Sn] represent contents (%by mass) of
AI, Si, Mn, Ni, Cu and Sn in the molten steel respectively.
[0021] It is preferable that in the method for manufacturing Cu-Sn coexisting steel
20 of the present invention, a content of Ah03 in the composite oxides that are generated
in the internal oxidation layer is 15 to 40% by mass. It is also preferable that the
composition of the molten steel is adjusted so as to further satisfy a condition
represented by the following formula (4), that is, the Cu/Sn ratio ranges from 1.0 to 8.0:
1.0 ~ [Cu]/[Sn] ~ 8.0 ... (4).
5
[0022] In the following description, "% by mass" concerning composition of the
steel and composite oxides is also represented as "%" simply. "Steel material" in the
following description shall include cast slabs and processed goods obtained by
processing on slabs such as rolling.
[0023] "Al in an oxide" in the following description means Al as one constituent
element of an oxide. Thus, in addition to AI in simple Al203, "Al in an oxide" also
includes AI in a composite oxide, for example, Al in an oxide containing Al, Si and Mn.
[0024] "The content of Al20 3 in a composite oxide" in the present invention shall
be the content of Al203 when a composite oxide is assumed to be composed of AhOJ,
10 Si02 and MnO. Actual composite oxides include oxides of complex composition of a
ternary or more system. It is difficult to calculate the content of AhOJ in such a
composite oxide. H~re, the 0 content in a composite oxide depends on the
stoichiometric ratio based on the content and a valence of each metallic element of Al,
Si and Mn. Therefore, a composite oxide is assumed to be composed of Al203, Si02
15 and MnO, and the content of Ah03 in this composite oxide shall be calculated. A
specific calculation method will be described later.
Advantageous Effects of Invention
[0025] According to the method for manufacturing Cu-Sn coexisting steel of the
20 present invention, slabs of a good quality where surface cracking and surface defects
accompanied by Cu embrittlement are inhibited from occurring can be manufactured.
[0026] The Cu-Sn coexisting steel of the present invention has no surface cracking
or surface defects, and surface cracking does not occur thereto even in hot-rolling that is
a post process. Thus, a steel material of a good surface quality can be manufactured by
means of the_ Cu-Sn coexisting steel of the present invention as a material.
Brief Description of Drawings
[0027] FIG. 1 is a flowchart to explain a method for manufacturing Cu-Sn
5 coexisting steel according to one embodiment of the present invention.
FIG. 2 is a flowchart to explain another embodiment of the method for
manufacturing Cu-Sn coexisting steel according to one embodiment of the present
invention.
FIG. 3 is a view to explain the Cu-Sn coexisting steel according to one
10 embodiment ofthe present invention.
Description of Embodiments
[0028] Described below will be the examination done for completing the method
for manufacturing Cu-Sn coexisting steel of the present invention, and the reasons why
15 the composition of the steel is specified as described above. It is noted that as to
ranges of numerical values, expression "A to B" means "no less than A and no more
than B". If a unit is appended only to the numerical value Bin such an expression, the
unit is also applied to the numerical value A.
[0029] 1. Examination for Completing Present Invention
20 1-1. Examination of Additional Elements
Originally, it is considered that red embrittlement induced by Cu (Cu
embrittlement) in steel containing Cu occurs because a Cu liquid phase penetrates grain
boundaries of an austenite phase of Fe that is the parent phase, to weaken the grain
boundaries. Separation of a Cu liquid phase is likely to occur at temperatures around
1100"C (for example, in the temperature range about 1050 to 1150"C).
[0030] The Cu liquid phase is generated because Cu that is nobler than Fe is locally
concentrated when Fe that is the main component of the steel is selectively oxidized
since the melting point of Cu is lower than Fe, and thus, the Cu concentration exceeds
5 the solubility limit in the austenite phase of Fe that is the parent phase. That is, the
solubility limit of Cu in Fe at high temperature is one of important factors for making
Cu embrittlement appear.
[0031) It is necessary for inhibiting Cu embrittlement to inhibit separation and
accumulation of the Cu liquid phase. Thus, it is considered that the solubility limit of
10 Cu in Fe is enlarged by addition of alloying elements as a way for inhibiting red
embrittlement.
[0032] Such alloying elements are so limited that are generally used for steel,
coexist Cu, and enlarge the solubility limit of Cu in Fe. The inventors of the present
invention examine various alloying elements on computational phase diagrams, and find
15 out that only elements ofNi and Al are practically usable while added to steel.
[0033] 1-2. Examination of Effects of Elements
Ni is an element nobler than Fe as well as Cu. Ni inhibits Cu embrittlement
because Ne enlarges the solubility limit of Cu in Fe, to raise the melting point of Cu.
Thus, in general, Ni is added to steel containing Cu, to prevent occurrence of cracking
20 to a steel material.
[0034] Here, Sn that is made to coexist with Cu in the steel in the present invention
is an element nobler than Fe as well as Cu. Sn encourages Cu embrittlement because
Sn shrinks the solubility limit of Cu for Fe, to drop the melting point of Cu. Thus,
when Cu and Sn coexist in the steel, the cracking susceptibility extremely increases, and
\0
therefore, it is difficult to completely prevent occurrence of cracking even ifNi is just
added.
[0035] As a way of inhibiting Cu embrittlement, such a measure is considered as
preventing a liquid phase of a low melting point from forming, that is, limiting the
5 content of Sn. Addition of Sn lowers the melting point of Cu and encourages Cu
embrittlement. Thus, it is difficult to manufacture slabs without occurrence of surface
cracking while Cu and Sn are positively made to coexist in the steel.
[0036] On the other hand, AI is baser than Fe, which is different from Cu and Ni.
AI has the function of improving the solubility limit of Cu for Fe. However, when
10 steel is oxidized, Al is selectively oxidized prior to Fe. Because of this, it is generally
considered that AI has no effect on Cu embrittlement.
[0037] Such phenomena relating to Cu embrittlement correspond to selective
oxidation behavior of steel. That is, alloying elements that are baser than Fe are
oxidized prior to the parent phase; next, Fe of the parent phase is oxidized; and alloying
15 elements that are nobler than Fe are concentrated in the parent phase.
[0038] Cu embrittlement behavior in the Cu-Sn coexisting steel was examined,
focusing on Ni and AI in selective oxidation. Used for the examination is: Cu-Sn
coexisting steel of composition suitable for a structural material for heavy gauge steel
plate, containing C: 0.04 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, Cu: 0.20 to
20 1.50% and Sn: 0.06 to 0.50%, and Fe and impurities as the remainder. This Cu-Sn
coexisting steel is a material of an extremely high cracking susceptibility because its C
content invites a high longitudinal cracking susceptibility, and in addition, Cu
embrittlement is conspicuous therein due to the coexistence of Cu and Sn. The
following findings are obtained as a result of the examination of the inventors of the
\ present invention on this Cu-Sn coexisting steel.
[0039] 1-2-1. Effects ofNi
In a case where Ni is added so that the Ni content is 0.1 to 0.5%, to make the
AI content no more than 0.05% in the above Cu-Sn coexisting steel, the surface portion
5 of the steel material is oxidized, to form scales. As to these scales, the following
effects a to d arise.
[0040] a. The shapes of interfaces between scales and the parent phase of the
surface portion of the steel material are roughened. This roughening of the interfaces
has a function of inhibiting accumulation of a liquid phase on the interfaces, which is
10 advantageous for removing the separated Cu liquid phase to the scales and inhibiting
occurrence of Cu embrittlement.
15
[0041] b. The solubility limit of Cu in Fe is enlarged, and an amount of separation
of the Cu liquid phase decreases. The melting point of the separated Cu liquid phase
rises due to dissolution ofNi in Cu.
[0042] c. It is inhibited that oxidation of entire Fe of the parent phase of the surface
portion of the steel material progresses. Ni is concentrated on the surface portion of
the steel material, and an FeNi alloy phase is generated. When the Ni concentration in
Fe increases on the surface portion of the steel material, it gets difficult that oxidation of
Fe occurs because the solid solubility of 0 in Fe increases, and at the same time, it is
20 inhibited to form the internal oxidation layer in the parent phase of the surface portion
of the steel material.
[0043] d. The Cu liquid phase separated on the surface portion of the steel material
and the FeNi alloy phase formed on the surface portion of the steel material inhibit
oxidation of a part that is inside the alloy phase formed on the surface portion of the
\2-
steel material, and also inhibit the growth of the internal oxidation layer. However,
because the alloy phase on the surface portion of the steel material is not uniform in
thickness, the internal oxidation layer in its inside is not uniform in thickness.
[0044] 1-2-2. Effects of AI
5 In a case where AI is added so that the AI content is 0.1 to 0.5%, to make the
Ni content less than 0.05% in the above Cu-Sn coexisting steel, the surface portion of
the steel material is also oxidized, to form scales. The following effects e and f arise
due to AI.
[0045] · e. The solubility limit of Cu in Fe is enlarged. However, because AI is an
10 element baser than Fe, AI is selectively oxidized prior to Fe, which is the main
component of the parent phase when the steel material is oxidized. Thus, the effect of
inhibiting separation of the Cu liquid phase of AI is smaller than that ofNi.
[0046] f. It is promoted to form the internal oxidation layer in the vicinity of the
surface of the steel material due to the selective oxidation. In a usual steel material
15 that does not contain AI, Si and Mn that are baser than Fe, which is the main component,
are selectively oxidized early. Thus, on the surface portion of the steel material,
oxides of Si and Mn are formed first, and composite oxide particles where Si and Mn
are enriched disperse into the internal oxidation layer. Later, oxides (scales) of Fe are
formed. On the other hand, AI is easier to be oxidized than Fe, as well as Mn and Si.
20 Thus, in the steel material containing AI, it is promoted to form the internal oxidation
layer accompanied by the selective oxidation. In addition, oxides where Si and Mn are
enriched and oxides where AI is enriched are generated independently, and in the
internal oxidation layer, oxide particles disperse more than in the usual steel material
that does not contain AI. Oxide particles in the internal oxidation layer are so minute
\3
because they are separated from a solid phase since 0 in the steel material is increased
by progress of oxidization of the surface to exceed the dissolution limit. In the early
stage of a separation process, while minute particles of no more than 0.1 J.Lm in diameter
can exist, generally oxide particles of 0.2 J..I.I11 or more in diameter can be easily
5 observed with an optical microscope or an electron microscope. In the internal
oxidation layer of 20 to 200 J..I.I11 in thickness in the surface of the steel material, oxide
particles approximately in the range of 0.2 to 1:0 J..I.I11 in diameter are dispersed. The
density of dispersion of observable oxide particles that are 0.2 J..I.I11 or more in diameter
is approximately 100,000 to 1,200,000 particles/mm2
•
10 [0047] 1-2-3. Effects of Using Ni and AI Together
As described above, it is found out that while addition of Ni and Al affects
selective oxidation behavior of Fe of the parent phase in the Cu-Sn coexisting steel, if
one of Ni and AI is lacking, the effect of inhibiting Cu embrittlement accompanied by
the Cu liquid phase is small.
15 [0048] The inventors of the present invention find out as a result of examination
time after time that the following effects g to j can be obtained and Cu embrittlement
can be inhibited by using Ni and Al together to have appropriate contents.
[0049] g. The shapes of interfaces between scales and the parent phase of the
surface portion of the steel material are roughened.
20 h. The solubility limit of Cu in Fe is enlarged.
i. The internal oxidation layer of uniform thickness is formed inside the Cu
liquid phase and the FeNi alloy phase that are on the surface portion of the steel
material.
j. Oxide particles in the internal oxidation layer are likely to be generated inside
\4
the alloy phase that is on the surface of the steel material, and the Cu liquid phase is
easy to be removed to scales.
[0050] Among these effects, g and h are due to the above described function of Ni.
In addition to these effects, the effects of i and j are obtained by the use of Ni and Al
5 together. According to these effects, Cu embrittlement can be inhibited and
occurrence of the surface cracking can be prevented by having the appropriate contents
of Ni and Al in the Cu-Sn coexisting steel. Roughening of interfaces between scales
and the parent phase of the surface portion of the steel material is, for example, about 20
to 100 J.Ull in depth (difference between a convex portion and a concave portion), and
10 the interval of the roughening (interval between a convex portion and a concave portion
that are adjacent to each other) is, for example, about 20 to 50 J.Ull.
[0051] 2. Composition of Cu-Sn Coexisting Steel of Present Invention and. Reason
why it is Limited
The Cu-Sn coexisting steel of the present invention is based on the findings
15 obtained from the results of the above examination. Its compol)ition is C: 0.04 to
0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, P: no more than 0.05%, S: no more than
0.02%, Cu: 0.20 to 1.50%, Sn: 0.06 to 0.50%, Al: 0.06 to 1.00% and Ni: 0.05 to 1.00%,
and Fe and impurities as the remainder. Examples of impurities in the present
invention include H, N, 0, Mg, Ca, Sr, As, Se, Sb and Te. Part of Fe can be
20 substituted with other alloy components. Examples of other alloy components in the
present invention include B, Ti, Zr, V, Nb, Cr, Mo and W.
[0052] C: 0.04 to 0.20%
C is an element having the effect of improving the strength of materials. In
order to obtain this effect, the C content shall be 0.04% or more. On the other hand, if
\.S
the C content exceeds 0.20%, the toughness decreases and the welding cracking
susceptibility increases. Thus, the C content shall be 0.04 to 0.20%.
[0053] Si: 0.05 to 1.00%
Si is an element effective for deoxidation. In order to obtain this effect, the Si
5 content shall be 0.05% or more. On the other hand, if the Si content exceeds 1.00%,
the toughness might decrease. Thus, the Si content shall be 0.05 to 1.00%.
[0054] Mn: 0.20 to 2.50%
Mn is an element having the effect of improving the strength of materials. In
order to obtain this effect, the Mn content shall be 0.20% or more. On the other hand,
10 if the Mn content exceeds 2.50%, the toughness might decrease. Thus, the Mn content
shall be 0.20 to 2.50%.
[0055] P: no more than 0.05%
P is an impurity element inevitably included in a steel material. The less the
better. If the P content exceeds 0.05%, the cracking susceptibility at hot temperature
15 increases. Thus, the P content shall be no more than 0.05%, and the less the more
preferable. The upper limit ofP is preferably 0.03%.
[0056] S: no more than 0.02%
S is an impurity element inevitably included in a steel material. The less the
better. If the S content exceeds 0.02%, the cracking susceptibility in hot working
20 increases. Also, an amount of MnS inclusions that are the starting points of corrosion
of the steel material increases, to break down the corrosion resistance. Thus, the S
content shall be no more than 0.02%, and the less the more preferable. The upper limit
of Sis preferably 0.010%.
[0057] Cu: 0.20 to 1.50%
\6
Cu is an element having the effect of improving the corrosion resistance of
steel. In order to obtain this effect, the Cu content shall be 0.20% or more. On the
other hand, if Cu in the steel material excessively exists, red embrittlement occurs in a
step accompanied by high temperature oxidation at high temperature in a step of
5 manufacturing the steel, for example, in a continuously casting step and a hot-rolling
step, and cracking or defects is/are generated on the surface of the steel material. Thus,
the Cu content shall be no more than 1.50%.
[0058) Sn: 0.06 to 0.50%
Sn is an element having the effect of improving the corrosion resistance of steel.
10 In order to obtain this effect, the Sn content shall be 0.06 % or more. On the other
hand, if the Sn content exceeds 0.50%, the corrosion resistance does not improve any
more. If Sn is contained by steel that contains Cu, the corrosion resistance improves
but red embrittlement is encouraged, and surface defects are easy to occur in the
manufacturing step. Thus, the Sn content shall be no more than 0.50%.
15 [0059] 2-1. Reasons why Contents of AI and Ni are Limited
AI: 0.06 to 1.00%
AI is originally an element used for deoxidizing steel. In the present
invention, AI is contained in order to inhibit Cu embrittlement. However, if the AI
content is less than 0.06%, the effect of inhibiting embrittlement is not sufficiently
20 obtained. In contrast, the AI content beyond 1.00% makes the content of AhOJ that is
generated in the internal oxidation layer formed in a step of cooling a slab excess, and
the effect of inhibiting embrittlement is ruined. According to the above, in the present
invention, the AI content shall be 0.06 to 1.00%. This AI content means the content of
acid soluble AI.
\-:::t[
0060] Ni: 0.05 to 1.00%
Ni is an element of enlarging the solubility limit of Cu in Fe, roughening the
interfaces between scales and the parent phase of the surface portion of a steel material
and promoting the removal of the separated Cu liquid phase toward the scale side. In
5 addition, Ni is an element of forming a FeNi alloy phase on the surface portion of a
steel material, and suppressing the progress of oxidation of the parent phase. However,
if the Ni content is less than 0.05%, the effect of inhibiting embrittlement is not
sufficiently obtained. If the Ni content exceeds 1.00%, not only it is not economically
preferable, but also it suppresses the growth of the internal oxidation layer in the alloy
10 phase because Ni is easy to form the FeNi alloy phase when the surface portion of a
steel material is selectively oxidized, to encourage the progress of oxidation of grain
boundaries. According to the above, in the present invention, the Ni content shall be
0.05 to 1.00%.
[0061] 2-2. Reasons why Ratios of Components are Specified
15 In the present invention, composition of molten steel is further adjusted so as to
satisfy the relationship of the following formulas (1) to (3):
K1 = [Al]/(3[Si] + [Mn]) 2:0.050 ... (1)
K2 = [Ni]/([Cu] + 5 [Sn]) 2: 0.10 ... (2)
K3 = [Al]/[Ni] 2: 0.20 ... (3)
20 where [AI], [Si], [Mn], [Ni], [Cu] and [Sn] are the contents(% by mass) of AI, Si, Mn,
Ni, Cu and Sn in the molten steel, respectively.
[0062] It is preferable that the composition of the molten steel is adjusted so as to
satisfy the relationship of the following formula (4):
1.0 ~ [Cu]/[Sn] ~ 8.0 ... (4)
\~
[0063] These formulas are found out as a result of the examination of the inventors
of the present invention on formation of scales on the surface of the steel material,
shapes of the interfaces between the scales and the parent phase of the surface portion of
the steel material, and influence of alloying elements on the formation of the internal
5 oxidation layer, in view of interaction of Cu, Sn, AI, Ni, Si and Mn. Satisfying these
formulas makes it possible to inhibit Cu embrittlement. The reasons why the above
formulas (1) to (4) are specified will be described below.
[0064] (1) Kl = [AI]/(3[Si] + [Mn]) 2: 0.050
K1 is a value represented by the contents of AI, Si and Mn. Kl is a value that
10 affects the formation of the internal oxidation layer. AI, Si and Mn are all elements
baser than Fe. AI, Si and Mn are oxidized prior to Fe when the oxidation of the steel
material is progressing, and generate large numbers of minute oxide particles on the
surface portion of the steel material. It is the internal oxidation layer that is formed by
the oxide particles of these elements.
15 [0065] Oxides generated in the internal oxidation layer are composite oxides
composed of AI, Si, Mn and 0. The composition of the composite oxides is roughly
grouped into the Si-Mn system containing Si02 and MnO as the main components and
AhOJ of less than 10%, the Si-Al system containing Si02 and AhOJ as the main
components and MnO of less than 20%, the Al-Mn system containing AhOJ and MnO
20 as the main components and Si02 of less than 10%, and so on. It is preferable that the
content of AhOJ, which is at the total amount in the composite oxides in the internal
oxidation layer, is no less than 15% and no more than 40%.
[0066] In a case where commercial steel that has a relatively low AI content is kept
at high temperature in the atmosphere, an internal oxidation layer is formed inside the
parent phase of a steel material. This internal oxidation layer is such that: Si02 and
MnO are contained as the main components; and the content of Ah03 is less than 3% at
best. On the other hand, in a case where the AI content is high, an internal oxidation
layer partially containing Ah03 is formed because a reducing power of AI is strong.
5 [0067] In a case where the value of Kl is less than 0.050, the main oxides in the
internal oxidation layer is SiMn oxides. Inside the Cu liquid phase, which is separated
partially, oxygen does not diffuse enough and does not react with Si or Mn. Thus, the
internal oxidation layer does not grow there, and is not uniform in thickness. As a
result, oxidation on grain boundaries of the steel material (grain boundary oxidation)
10 remarkably progresses, and it becomes easy that the separated Cu liquid phase
permeates the grain boundaries, to bring about Cu embrittlement.
[0068] On the other hand, in a case where the value ofKl is 0.050 or more, Ah03
is easy to be formed relatively to the case where the value is less than 0.050, the internal
oxidation layer grows inside the separated Cu liquid phase, and the internal oxidation
15 layer uniform in thickness is formed. As a result, Cu embrittlement is inhibited.
[0069] The value ofKl is preferably no more than 2.0. If the value ofKl is more
than 2.0, Ah03 is excessively formed inside the internal oxidation layer. Specifically,
oxides of each element that composes the steel material grow along the grain boundaries
of the steel material, which actually encourages oxidation of the steel material, and it
20 becomes easy that the separated Cu liquid phase permeates the grain boundaries, to
bring about Cu embrittlement.
[0070] (2) K2 = [Ni]/([Cu] + 5[Sn]) ~ 0.10
K2 is a value represented by the contents of Ni, Cu and Sn. K2 is a value that
affects selective oxidation behavior of Fe when oxidation of the steel material
2-0
progresses.
[0071] In a case where the value of K2 is less than 0.1 0, the Cu liquid phase is easy
to be formed and separated. Moreover, the shapes of the interfaces between scales and
the parent phase of the surface portion of the steel material are not roughened, but are
5 smooth. Thus, the Cu liquid phase separated on the interfaces is accumulated, and the
cracking susceptibility of the steel material is increased.
10
[0072] The value of K2 is preferably 1.2 or less. This is because K2 of a too large
value stops an effect from being increased any more, which is not economically
preferable.
[0073] (3) K3 = [Al]/[Ni] 2: 0.20
K3 is the ratio of the contents of AI and Ni. K3 is a value that affects the
uniformity of the formed internal oxidation layer in thickness.
[0074] In a case where the value of K3 is less than 0.20, the FeNi alloy phase
formed on the surface portion of the steel material inhibits oxidation of its inside. As a
15 result, the internal oxidation layer is not uniform in thickness. When the internal
oxidation layer is not uniform in thickness, the growth of oxides of each element that
composes the steel material along the grain boundaries of the steel material is promoted,
and it becomes easy that the separated Cu liquid phase permeates the grain boundaries.
Thus, Cu embrittlement is brought about.
-20 [0075] The value of K3 is preferably no more than 2.0. If the value of K3 is more
than 2.0, Ah03 is excessively formed inside the internal oxidation layer. Specifically,
oxides of each element that composes the steel material grow along the grain boundaries
of the steel material, oxidation of the steel material is encouraged, and it becomes easy
that the separated Cu liquid phase permeates the grain boundaries, which brings about
Cu embrittlement.
[0076] (4) Content of Ah03 in Composite Oxides that are Generated in Internal
Oxidation Layer: 15 to 40%
The content of Ah03 in composite oxides that are generated in the internal
5 oxidation layer is preferably 15 to 40%. In a case where the content of Ah03 in
composite oxides is less than 15%, ununiformity occurs to the internal oxidation layer
in thickness. This is because: while internal oxidation is progressing along with
oxidation of the surface portion (growth of scales), Ni is partially concentrated to form a
FeNi alloy phase; internal oxidation hardly progresses in this FeNi alloy phase, and as a
10 result, the internal oxidation layer is not uniform in thickness. In the area where
internal oxidation does not progress, only grain boundary oxidation remarkably
progresses as oxidation in crystal grains is inhibited, which becomes starting points of
cracking. The Cu liquid phase is easy to permeate the grain boundaries where
remarkable grain boundary oxidation progresses, to bring about Cu embrittlement. On
15 the other hand, if Ah03 is generated in the FeNi alloy phase as well, the internal
oxidation layer is uniform in thickness. As a result, Cu embrittlement is inhibited.
The content of Ah03 in composite oxides at this time is no less than 15%.
[0077] In contrast, if an amount of Al increases, the content of Ah03 in composite
oxides increases. If the content of Ah03 is 40% or more by mass, it causes occurrence
20 of faults in hot working because of the hardness. Therefore, it is preferable that the
content of Ah03 in composite oxides that are generated in the internal oxidation layer is
40% or less.
[0078] The content of Ah03 in composite oxides that are generated in the internal
oxidation layer can be obtained by, for example, passing the following 1) to 7) in order:
I) A specimen is taken out ofthe steel material, and its surface portion (vertical
section) is observed with a scanning electron microscope (SEM).
2) Compositional differences are observed on a backscattered electron image,
and an oxide is selected. Here, on the backscattered electron image, the heavier an
5 element is, the stronger its brightness is. Since elements forming oxides, 0, Al, Si and
Mn are all lighter than Fe, the oxides are observed as having weaker brightness than the
Fe parent phase, and are possible to be distinguished.
3) Composition of the oxide is evaluated with an energy dispersive X-ray
spectrometer (EDS). At this time, the composition is evaluated by means of an atomic
10 ratio (atomic concentration) concerning the area of the oxide.
4) From the constituent elements of the composition according to the atomic
concentration, the ratio of the atomic concentration of each metallic element, which is
except light elements C and 0, and the main component of the parent phase, Fe, is
obtained (the ratios of AI, Si and Mn as the main constituent elements of the composite
15 oxide is obtained).
5) In view of a valence in forming the oxide, the obtained ratios are converted
into the constituent oxides. Molecular weights of AhOJ, Si02 and MnO (Ah03
(AlOJ .s): 50.98, Si02: 60.10 and MnO: 70.94) and the obtained ratios ofthe constituent
oxides are converted into weight concentrations of the constituent oxides.
20 6) The content of AhOJ in the composite oxide is calculated.
7) Above 1) to 6) are carried out on at least ten composite inclusions, and a
mean value is obtained.
[0079] (5) 1.0 ~ [Cu]/[Sn] ~ 8.0
[Cu]/[Sn] is the ratio of the contents ofCu and Sn, that is, the above described
Cu/Sn ratio. The Cu/Sn ratio of 1.0 to 8.0 makes it possible to get enough corrosion
resistance under a severe environment such as a chloride environment and an oxidizing
environment.
[0080] In a case where [Cu]/[Sn] is less than 1.0, the steel is Sn-rich, and the ability
5 of corrosion resistance of the Cu-Sn coexisting steel, which is an object, cannot be
obtained. On the other hand, in a case where [Cu]/[Sn] exceeds 8.0, the steel is
Cu-rich, so-called, steel containing Cu, and the ability of corrosion resistance of the
Cu-Sn coexisting steel, which is an object, cannot be obtained. In view of the above,
the Cu/Sn ratio shall be 1.0 to 8.0 in the present invention.
10 [0081] 3. Method for Manufacturing Cu-Sn Coexisting Steel in Present Invention
The method for manufacturing Cu-Sn coexisting steel in the present invention
is a method including, when a slab is continuously cast using molten steel of the above
described composition, adjusting the composition of the molten steel so as to satisfy the
conditions represented by the above formulas (1) to (3), oxidizing the surface of the slab
15 in a process of cooling the slab to form an internal oxidation layer, and generating
Ah03 in composite oxides that are generated in this internal oxidation layer. Whereby,
a slab of a good quality can be manufactured wherein surface cracking and surface
defects accompanied by Cu embrittlement are inhibited from occurring.
FIG. 1 is a flowchart to explain a method for manufacturing the Cu-Sn
20 coexisting steel S 1 according to one embodiment of the present invention (hereinafter
may be referred to as "manufacturing method S1"). As depicted in FIG. 1, the
manufacturing method S 1 includes a step of adjusting the composition of the molten
steel S 11 (hereinafter may be abbreviated to "S 11 ") and a step of forming the internal
oxidation layer S12 (hereinafter may be abbreviated to "S12") in the order as described
2-4
above. The step of adjusting the composition of the molten steel S 11 is a step of, when
a slab is continuously cast using the molten steel of the above described composition,
adjusting the composition of the molten steel so as to satisfy the conditions represented
by the above formulas (1) to (3). The adjustment of the composition of the molten
5 steel in S 11 is carried out by addition of an alloy in a refining stage. The step of
forming the internal oxidation layer Sl2 is a step of forming the internal oxidation layer
by oxidizing the surface of the slab that is obtained by cooling the molten steel, whose
composition is adjusted in S 11, in the process of cooling the slab. In the
manufacturing method S 1, Ah03 is contained by composite oxides that are generated in
10 the internal oxidation layer formed in S 12. In the manufacturing method S 1, it is
preferable that the content of Ah03 in the composite oxides that are generated in the
internal oxidation layer formed in S 12 is 15 to 40% by mass.
In the method for manufacturing Cu-Sn coexisting steel of the present
invention, the step of adjusting the composition of the molten steel is preferably a step
15 of adjusting the composition of the molten steel so as to satisfy the conditions
represented by the above formulas (1) to (3), and the condition represented by the above
formula (4). FIG. 2 represents a flowchart to explain a method for manufacturing the
Cu-Sn coexisting steel S2 according to this embodiment (hereinafter may be referred to
as "manufacturing method S2"). As depicted in FIG. 2, the manufacturing method S2
20 includes a step of adjusting the composition of the molten steel S21 (hereinafter may be
abbreviated to "S21 ") and a step of forming the internal oxidation layer S22 (hereinafter
may be abbreviated to "S22") in the order as described above. The step of adjusting
the composition of the molten steel S21 is a step of, when a slab is continuously cast
using the molten steel of the above described composition, adjusting the composition of
·z5
the molten steel so as to satisfy the conditions represented by the above formulas ( 1) to
(4). The adjustment of the composition of the molten steel in S21 is carried out by
addition of an alloy in a refining stage. The step of forming the internal oxidation
layer S22 is a step of forming the internal oxidation layer by oxidizing the surface of the
5 slab that is obtained by cooling the molten steel, whose composition is adjusted in S21,
in the process of cooling the slab. In the manufacturing method S2, Al203 is contained
by composite oxides that are generated in the internal oxidation layer formed in S22.
In the manufacturing method S2, it is preferable that the content of Ah03 in the
composite oxides that are generated in the internal oxidation layer formed in S22 is 15
10 to 40% by mass.
[0082] According to this method, slabs of a good quality where surface cracking
and surface defects accompanied by Cu embrittlement are inhibited from occurring can
be manufactured. In addition, slabs manufactured by this method have no surface
cracking or surface defects, and surface cracking does not occur thereto even in
15 hot-rolling that is a post process. Thus, a steel material of a good surface quality can
be manufactured by means of the Cu-Sn coexisting steel of the present invention as a
material.
[0083] Cu embrittlement in heating and cooling can be also inhibited on an ingot
that is manufactured by pouring, into a mold having a bottom, the molten steel
20 satisfying the above described compositions and either formulas (1) to (3) or formulas
(1) to (4), by carrying out blooming thereon, forming an internal oxidation layer through
oxidation of the surface of a slab in a process of cooling the ingot after heating for
hot-rolling, and generating Ah03 in composite oxides that are generated in this internal
oxidation layer.
2-b
[0084] 4. Cu-Sn Coexisting Steel of Present Invention
FIG. 3 is a view to explain Cu-Sn coexisting steel 10 according to one
embodiment of the present invention. The Cu-Sn coexisting steel 10 depicted in FIG.
3 is a slab manufactured by the above described manufacturing method S 1. According
5 to the manufacturing method S 1, slabs of a good quality (Cu-Sn coexisting steel 1 0)
where surface cracking and surface defects accompanied by Cu embrittlement are
inhibited from occurring can be manufactured. Thus, the Cu-Sn coexisting steel 10 is
a steel material of a good quality where surface cracking and surface defects
accompanied by Cu embrittlement are inhibited from occurring. While FIG. 3
10 depicts the slab manufactured by the manufacturing method S1, the Cu-Sn coexisting
steel of the present invention can be manufactured by the manufacturing method S2.
According to the manufacturing method S2, slabs of a good quality where surface
cracking and surface defects accompanied by Cu embrittlement are inhibited from
occurring can be also manufactured.
15
Examples
[0085] The following preliminary and final tests were done in order to confirm the
effects of the method for manufacturing Cu-Sn coexisting steel of the present invention,
and results thereof were evaluated.
20 [0086] 1. Preliminary Test
1-1. Method of Test
Cu-Sn coexisting steels each having the composition of Nos. 1 to 22
represented in Table 1 were manufactured by melting in a vacuum melting furnace, to
obtain ingots of 50 kg each. In this table, the content of AI represents the content of
acid soluble AI. The obtained ingots were each forged, and these forged parts were
heated and rolled, to obtain specimens of steel materials. The surface of each
specimen was oxidized by being kept in an electric furnace having an atmosphere at
llOO"C for 15 minutes, to generate scales, and each specimen was cooled to room
5 temperature.
[0087] [Table 1]
Composition(% by Mass)
No. Class. Cu/Sn Kl K2
c Si Mn p s Cu Sn Ni AI*
I Ref. Ex. 0.04 0.26 1.51 0.009 0.002 0.36 0.10 0.32 0.14 3.60 0.06 0.37
2 Ref. Ex. 0.18 0.35 1.35 0.021 0.001 0.28 0.16 0.24 0.12 1.75 0.05 0.22
3 Ref. Ex. 0.09 0.30 1.05 0.015 0.001 1.50 0.19 0.28 0.12 7.89 0.06 0.11
4 Ref. Ex. 0.13 0.20 0.58 0.011 0.002 0.50 0.20 0.30 0.06 2.50 0.05 0.20
5 Ref. Ex. 0.13 0.20 0.58 0.011 0.002 0.50 0.20 0.30 0.06 2.50 0.05 0.20
6 Ref. Ex. 0.13 0.20 0.58 0.011 0.002 0.50 0.20 0.30 0.06 2.50 0.05 0.20
7 Ref. Ex. 0.09 0.25 0.95 0.011 0.002 0.48 0.06 0.18 0.18 8.00 0.11 0.23
8 Ref. Ex. 0.13 0.25 0.95 0.011 0.002 0.30 0.30 0.25 0.50 1.00 0.29 0.14
9 Ref. Ex. 0.12 0.25 0.95 0.011 0.002 0.45 0.15 0.15 0.10 3.00 0.06 0.13
10 Ref. Ex. 0.12 0.25 0.95 0.011 0.002 0.20 0.08 0.06 0.12 2.50 0.07 0.10
11 Ref. Ex. 0.20 0.31 1.00 0.011 0.002 0.40 0.18 1.00 0.25 2.22 0.13 0.77
12 Ref. Ex. 0.12 0.25 0.95 0.011 0.002 0.20 0.06 0.80 0.18 3.33 0.11 1.60
13 Comp. Ex. 0.12 0.31 1.00 0.011 0.002 0.40 0.18 0.32 0.05 2.22 0.03 0.25
14 Comp. Ex. 0.12 0.28 0.72 0.011 0.002 0.50 0.12 0.10 0.08 4.17 0.05 0.09
15 Comp. Ex. 0.12 0.25 0.95 0.011 0.002 0.45 0.15 0.05 0.10 3.00 0.06 0.04
16 Comp. Ex. 0.12 0.25 0.95 0.011 0.002 0.50 0.10 0.50 0.04 5.00 0.02 0.50
17 Comp. Ex. 0.12 0.25 0.95 0.011 0.002 0.60 0.20 0.60 1.05 3.00 0.62 0.38
18 Comp. Ex. 0.12 0.15 0.55 0.011 0.002 0.40 0.10 0.25 0.05 4.00 0.05 0.28
19 Comp. Ex. 0.12 0.26 0.98 0.011 0.002 0.20 0.06 0.04 0.11 3.33 0.06 0.08
20 Comp. Ex. 0.12 0.25 0.95 0.011 0.002 0.50 0.20 1.10 0.25 2.50 0.15 0.73
21 Comp. Ex. 0.12 0.30 0.95 0.011 0.002 0.35 0.10 0.24 0.06 3.50 0.03 0.28
22 Comp. Ex. 0.12 0.25 0.95 0.011 0.002 0.35 0.10 0.65 0.12 3.50 0.07 0.76
• AI represents the content of acid soluble AI.
[0088] Nos. 1 to 12 represent reference examples in each of which composition and
K3
0.44
0.50
0.43
0.20
0.20
0.20
1.00
2.00
0.67
2.00
0.25
0.23
0.16
0.80
2.00
0.08
1.75
0.20
2.75
0.23
0.25
0.18
values of Kl to K3 satisfied the specification of the present invention. Nos. 13 to 16
represent comparative examples in each of which at least one value of Kl to K3 did not
satisfy the specification of the present invention, and No. 17 represents a comparative
example where composition did not satisfy the specification of the present invention.
5 Nos. 18 and 20 represent comparative examples in each of which composition did not
satisfy the specification of the present invention, No. 19 represents a comparative
example where composition and the value of K2 did not satisfy the specification of the
present invention, No. 21 represents a comparative example where the value of Kl did
not satisfy the specification of the present invention, and No. 22 represents a
10 comparative example where the value of K3 did not satisfy the specification of the
present invention.
[0089] 1-2. Method ofEvaluation
Each specimen was evaluated from its cracking susceptibility. The evaluation
of the cracking susceptibility was carried out by means of structure observation of a
15 section of the surface portion of each specimen after cooling, with an optical
microscope, and structure observation and elementary analysis with a SEMIEDS.
[0090] Observed with an optical microscope were scales, internal oxidation layers,
the parent phases of the specimen and forms and colors of deposits. With a SEMIEDS,
forms and composition of the deposits were observed and analyzed. In the temperature
20 range where selective oxidation was progressing, a liquid phase whose main
composition was Cu (Cu liquid phase) was generated.
[0091] It was determined that in a case where the observed form of deposits was a
membranous accumulation between the interfaces of scales and the parent phase of the
surface portion of a specimen, and was linear spread on the grain boundaries of the
specimen, there was a strong possibility that these deposits were in a liquid phase. In a
case where, as a result of composition analysis of deposits, the main components of the
deposits were Cu and Sn, whose melting points were lower than Fe, that is, in a case
where the content of Cu or Sn was high and thus, the deposits were possible to be
5 considered as a fusible alloy phase of Cu or Sn actually, the deposits were determined to
be separated as a liquid phase.
[0092] Evaluation items were the following a to d:
a. A state of separation of the Cu liquid phase on the surface portion of a
specimen. This was because if the Cu liquid phase was separated, embrittlement was
10 easy to occur to a steel material. A liquid phase has the characteristic of accumulating
like a membrane once separated. Thus, it was possible to be determined whether to be
liquid phase separation or solid phase separation according to the separation form. In a
case where the separation form was granular and each granule was so minute as to be
less than 1 f.Ull, it was possible to be determined that a Cu solid phase was not separated,
15 that is, liquid membrane embrittlement did not occur. Thus, if the separation was
membranous, it was determined that the cracking susceptibility was large.
[0093] b. A state of progress of roughening the shapes of interfaces between scales
and the parent phase of the surface portion of a specimen. This was because: if the
shapes of interfaces were smooth, the separated Cu liquid phase was accumulated,
20 which made embrittlement easy to occur; in contrast, if roughening of the shapes of
interfaces progressed, the separated Cu liquid phase did not accumulate on the
interfaces but was taken into scales, and thus, removal of the liquid phase was
promoted; therefore, Cu embrittlement was inhibited. If the forms of boundaries of
scales and the parent phase were largely roughening forms, embrittlement was difficult
to occur. In contrast, if the boundaries were flat, the Cu liquid phase easily
accumulated like a membrane. When the boundaries between scales and the parent
phase were roughened by 50 Jlii1 or over in height, the separated Cu liquid phase did not
accumulate on the interfaces but was taken into scales. So no embrittlement occurred.
5 Thus, it was determined that if the boundaries between scales and a parent phase were
roughened by less than 50 J.Lm in height, the cracking susceptibility was large.
[0094] c. A state of uniformity of an internal oxidation layer in thickness. This
was because if an internal oxidation layer was not uniform in thickness, the separated
Cu liquid phase was concentrated on grain boundaries of a part of a specimen where the
10 internal oxidation layer was thin, and embrittlement was easy to occur. It was
determined that if an internal oxidation layer was not uniform in thickness (difference
between the maximum and the minimum of the thickness was 30 J.Lffi or more), the
cracking susceptibility was large.
[0095] d. Whether or not the separated Cu liquid phase penetrated grain boundaries
15 of a priory (austenite) phase (grain boundaries at llOO"C. Hereinafter referred to as
"prior y grain boundaries") of the surface portion of a specimen. This was because
penetration of the Cu liquid phase was evidence of embrittlement of a steel material. If
a Cu phase penetrated prior y grain boundaries, this was a Cu liquid phase, which was
evidence of liquid membrane embrittlement. Thus, it was determined that if a Cu
20 phase penetrated surface y grain boundaries, the cracking susceptibility was large.
[0096] 1-3. Evaluation Results
The evaluation of the above a to d was combined, and the cracking
susceptibility was evaluated. Table 2 represents the proportion of Ah03 in composite
oxides generated in an internal oxidation layer in addition to evaluation of items of a to
d and evaluation of the cracking susceptibility as the combined evaluation. In Table 2,
in a case where the boundaries between scales and the parent phase of the surface
portion of a specimen were roughened by 50 ~m or more in height, it was determined to
be "Roughened", and in a case where there was no such roughening, it was determined
5 to be "Smooth". "Thickness of Internal Oxidation Layer" was determined to be
"Uniform" in a case where difference between the maximum and minimum of the
thickness was less than 30 J..Lm, and it was determined to be "Not Uniform" in a case
where the difference was 30 ~m or more. "Proportion of Ah03 in Oxides Contained in
Internal Oxidation Layer" is represented by o if the proportion was no less than 15%
10 and no more than 40%, and represented by x if the proportion was less than 15% or
beyond 40%. It is noted that concerning the steel material of No. 8 represented in
Table 1, the AhOJ content in composite oxides contained in the internal oxidation layer
was 29.3% as the mean value obtained by composition analysis on randomly selected
10 composite oxides with an EDS.
15 (0097] [Table 2]
Separation of Penetration ofCu
State of
Thickness of Proportion of AhOJ in
Cracking
No. aass. Cu Liquid Liquid Phase into
Interfaces
Internal Oxides Contained in
Susceptibility
Phase Grain Boundaries Oxidation Layer Internal Oxidation Layer
I Ref. Ex. Partially No Roughened Unifonn 0 Low
2 Ref. Ex. Partially No Roughened Unifonn 0 Low
3 Ref.Fx · Partially No Roughened Unifonn 0 Low
4 Ref. Ex. Partially No Roughened Unifonn 0 Low
5 Ret: Ex. Partially No Roughened Uniform 0 Low
6 Ref. Ex. Partially No Roughened Uniform 0 Low
7 Ref. Ex. Partially No Roughened Unifonn 0 Low
8 Ref. Ex. Partially No Roughened Uniform 0 Low
9 Ref. Ex. Partially No Roughened Uniform 0 Low
10 Ref. Ex. Partially No Roughened Unifonn 0 Low
II Ret: Ex. None No Roughened Unifonn 0 Low
12 Ref. Ex. None No Roughened Uniform 0 Low
13 Comp. Fx Partially Yes Roughened Not Uniform X High
14 Comp. Ex. Partially Yes Smooth Not Uniform 0 High
15 Comp.Fx All Yes Smooth Not Uniform 0 High
16 Comp.Fx Partially Yes Roughened Not Uniform X High
17 Comp. Ex. Partially Yes Roughened Uniform X High
18 Comp. Ex. Partially Yes Roughened Not Uniform 0 High
19 Comp.Fx All Yes Smooth Not Uniform 0 High
20 Comp.Fx Partially Yes Roughened Not Uniform 0 High
21 Comp.Fx Partially Yes Roughened Not Uniform X High
22 Comp.Fx Partially Yes Roughened Not Uniform X High
[0098] As represented in Table 2, in each No. 1 to 10 among the reference
examples, while the separation of a Cu liquid phase on the interfaces between scales and
the parent phase of the surface portion of a specimen partially occurred, no separated Cu
5 liquid phase penetrated prior 'Y grain boundaries of the surface portion of the specimen,
and the cracking susceptibility was low. It was considered that this was because
roughening of the interfaces of scales and the parent phase of the surface portion of the
specimen progressed and removal of the separated Cu liquid phase effectively
progressed, and because an internal oxidation layer of uniform thickness was generated
10 inside scales and the separated Cu liquid phase was not concentrated on the prior 'Y grain
33
boundaries of the specimen.
[0099] In each No. 11 and 12 among the reference examples, the separation of a Cu
liquid phase on the interfaces between scales and the parent phase of the surface portion
of a specimen never occurred, and the cracking susceptibility was low. It was
5 considered that this was because a predetermined amount of AhOJ was contained in
composite oxides that were generated by internal oxidation since AI of 0.06% or more
was contained and the condition of K 1 was satisfied, and because generation of the Cu
liquid phase was inhibited and roughening of the interfaces between scales and the
parent phase progressed since Ni of 0.05% or more was contained and the condition of
10 K2 was satisfied, and in addition, because an internal oxidation layer of uniform
thickness was generated inside scales since the condition of K3 was satisfied.
[0 1 00] In all the reference examples, composite oxides that were contained in an
internal oxidation layer contained Ah03 of no less than 5% and less than 90%. The
total amount of Ah03 that composite oxides contained (the content of AhOJ in
15 composite oxides) was no less than 15% and no more than 40%.
[0101] On the contrary, in each No. 13 to 17, which were comparative examples,
the separation of a Cu liquid phase occurred on a part of or all the interfaces between
scales and the parent phase of the surface portion of a specimen, and the separated Cu
liquid phase penetrated the prior y grain boundaries of the surface portion of a specimen.
20 Thus, the cracking susceptibility was high. Open cracking also occurred to part of the
prior y grain boundaries. The proportion of Ah03 in oxides contained in an internal
oxidation layer was less than 15% or beyond 40% by mass.
[0102] In each No. 14 and 15 among the comparative examples, the interfaces
between scales and the parent phase of the surface portion of a specimen were smooth,
and an internal oxidation layer was not uniform in thickness. Thus, it was considered
that the Cu liquid phase accumulated on the interfaces was concentrated to penetrate the
prior y grain boundaries.
[0 1 03] In each No. 13 and 16 among the comparative examples, roughening of the
5 interfaces between scales and the parent phase of the surface portion of a specimen
progressed. An internal oxidation layer was not generated because the surface portion
of a specimen was partially high-Ni alloyed, and thus an internal oxidation layer was
not uniform in thickness. Oxidation of the prior y grain boundaries far progressed and
the Cu liquid phase accumulated on the interfaces penetrated around crystal grains of
10 the surface portion, which was high-Ni alloyed.
[0104] In No. 17 among the comparative examples, roughening of the interfaces
between scales and the ·parent phase of the surface portion of the specimen also
progressed. Although the internal oxidation layer of uniform thickness was generated,
excessive AhOJ was separated on grain boundaries a lot, which encouraged penetration
15 of the Cu liquid phase into the prior y grain boundaries.
[0105] In No. 18 that represents a comparative example, the separation of a Cu
liquid phase partially occurred to the interfaces between scales and the parent phase of
the surface portion of the specimen, and the separated Cu liquid phase penetrated the
prior y grain boundaries of the surface portion of the specimen. Thus, the cracking
20 susceptibility was high. In No. 18, roughening ofthe interfaces between scales and the
parent phase of the surface portion of the specimen progressed. The internal oxidation
layer was not generated because the surface portion of the specimen was partially
high-Ni alloyed, and thus the internal oxidation layer was not uniform in thickness.
Oxidation of the prior y grain boundaries far progressed and the Cu liquid phase
3S
accumulated on the interfaces penetrated around crystal grains of the surface portion,
which was high-Ni alloyed.
[0 1 06] In No. 19 that represents a comparative example, the separation of a Cu
liquid phase occurred on all the interfaces between scales and the parent phase of the
5 surface portion of the specimen, and the separated Cu liquid phase penetrated the prior y
grain boundaries of the surface portion of the specimen. Thus, the cracking
susceptibility was high. In No. 19, the interfaces between scales and the parent phase
of the surface portion of the specimen were smooth. Thus, it was considered that the
Cu liquid phase accumulated on the interfaces was concentrated to penetrate the prior y
10 grain boundaries.
[0107] In No. 20 that is a comparative example, roughening of the interfaces
between scales and the parent phase of the surface portion of the specimen progressed.
The surface portion of the steel material was FeNi alloyed, and the growth of the
internal oxidation layer is partially inhibited, which made ununiformity occur to the
15 thickness of the internal oxidation layer, and which encouraged oxidation of the grain
boundaries. The separation of a Cu liquid phase partially occurred to the interfaces
between scales and the parent phase of the surface portion of the specimen, and the
separated Cu liquid phase penetrated the prior y grain boundaries of the surface portion
of the specimen.
20 [0108] In each No. 21 and 22 that represents a comparative example, the separation
of a Cu liquid phase partially occurred to the interfaces between scales and the parent
phase of the surface portion of a specimen, and the separated Cu liquid phase penetrated
the prior y grain boundaries of the surface portion of a specimen. Thus, the cracking
susceptibility was high. The proportion of Ah03 in oxides contained in an internal
oxidation layer was less than 15% or beyond 40% by mass. In No. 21, the internal
oxidation layer was not uniform in thickness. Oxides in this internal oxidation layer
were mainly SiMn oxides. Oxidation of the gr~ boundaries remarkably progressed,
and the separated Cu liquid phase penetrated the grain boundaries deeply to bring about
5 embrittlement. In No. 22, the surface portion of the steel material was FeNi alloyed,
and oxidation of its inside was partially inhibited. As a result, the internal oxidation
layer was not uniform in thickness, and oxidation of the grain boundaries was
encouraged. The separation of the Cu liquid phase partially occurred to the interfaces
between scales and the parent phase of the surface portion of the specimen, and the
10 separated Cu liquid phase penetrated the prior y grain boundaries of the surface portion
of the specimen.
15
,[0109] 2. Final Test
Next, in view of the results of the preliminary test, the final test with a
continuous casting machine was done.
[0110] 2-1. Method of Test
Cu-Sn coexisting steel having the composition of each No. 23 and 24
represented in Table 3 was manufactured by melting in a melting furnace. No. 23
represents an example of the present invention where the composition and values of Kl
to K3 satisfied the specification of the present invention. No. 24 represents a
20 comparative example where the values of K 1 and K3 did not satisfy the specification of
the present invention.
[0111] [Table 3]
Composition(% by Mass)
No. Class. Cu/Sn K1 K2 K3
c Si Mn p s Cu Sn Ni AJ'
23 Ex. ofThis 0.14 0.23 0.90 Invention 0.011 0.002 0.45 0.18 0.30 0.18 2.50 0.11 0.22 0.60
24 Comp. Ex. 0.14 0.25 0.95 0.011 0.002 0.44 0.19 0.25 0.03 2.32 0.02 0.18 0.12
• AI represents the content of acid soluble AI.
[0112] Continuous casting was carried out with a vertical continuous casting
machine as such that: the manufactured molten steel by melting of 2.5 t was poured into
a tundish via a ladle, and was supplied into a vibrating internal water cooled mold of a
copperplate via a submerged nozzle with 50 to 1o·c of superheat at the casting speed of
5 0.8 m/min. The property values of used mold flux arranged on the molten steel in the
mold were; solidification temperature: 1235·c; viscosity at 13oo·c: 0.04 Pa·s; and
basicity (value obtained from division ofthe content ofCaO (%by mass) by the 90ntent
of Si02 (%by mass)): 1.8.
[0113] Spray cooling was carried out downward the mold with a specific water
10 flow of 1. 7 L per 1 kg of a slab, to manufacture a slab of 100 mm in thickness, 800 mm
in width and 3500 mm in length. The obtained slab was cooled to room temperature.
[0114] Part of the cooled slab was cut, to take a specimen for examining whether
surface cracking existed on the slab or not, and a steel material for a hot-rolling test.
The hot-rolling test was done as such that: the taken steel material was heated in the
15 atmosphere to 11 oo·c, and after that, was rolled with the reduction of 75%.
[0115] 2-2. Method of Evaluation
Evaluation items were whether surface cracking on a slab existed or not, and
whether surface cracking on a steel material after rolled (hereinafter referred to as
"rolled steel material") existed or not. Whether grain boundary cracking existed or not
20 was examined on both cases of the surface cracking by dye check (dye penetrant
inspection).
58
5
[0116] 3. Evaluation Results
There was no surface cracking on both of the slab and rolled steel material of
No. 23, which represented an example of the present invention. Cu embrittlement was
inhibited.
[0117] In contrast, surface cracking was confirmed on both of the slab and rolled
steel material of No. 24, which represented a comparative example. Occurrence of fin
cracking was also confirmed at an end part of the rolled steel material.
[0 118] The present invention is described concerning the embodiment that is, at the
present, the most practical and preferable. The present invention is not limited to the
10 embodiment disclosed in the description of the present application, but can be properly
modified within the scope of the summary and idea of the invention readable from the
claims and whole of the description. It must be understood that the Cu-Sn coexisting
steel and the method for manufacturing the same accompanied by such modification are
also encompassed in the technical scope of the present invention.
15
20
Industrial Applicability
[0119] According to the method for manufacturing Cu-Sn coexisting steel of the
present invention, slabs of a good quality where surface cracking and surface defects
accompanied by Cu embrittlement are inhibited from occurring can be manufactured.
[0120] In addition, the Cu-Sn coexisting steel of the present invention has no
surface cracking or surface defects, and surface cracking does not occur thereto even in
hot-rolling that is a post process. Thus, a steel material of a good surface quality can
be manufactured by means of the Cu-Sn coexisting steel of the present invention as a
material.
-------------------·------·----·-·-·
Reference Sings List
[0 I2I] S I, S2 ... method for manufacturing Cu-Sn coexisting steel
S II, S21 ... step of adjusting the composition of molten steel
5 S12, S22 ... step of forming an internal oxidation layer
1 0 ... Cu-Sn coexisting steel

Wec1aim:
Claim 1 A method for manufacturing Cu-Sn coexisting steel by continuous
casting of molten steel, the method comprising:
adjusting composition of molten steel so as to satisfy conditions represented by
the following formulas (1) to (3), the molten steel containing, as chemical composition,
C: 0.04 to 0.20%, Si: 0.05 to 1.00%, Mn: 0.20 to 2.50%, P: no more than 0.05%, S: no
more than 0.02%, Cu: 0.20 to 1.50% and Sn: 0.06 to 0.50% and further contains AI:
0.06 to 1.00% and Ni: 0.05 to 1.00% by mass, and Fe and impurities as the remainder;
forming an internal oxidation layer by oxidizing a surface of a slab in a process

of cooling the slab; and
making composite oxides that are generated in the internal oxidation layer,
contain A}z03:
[Al]/(3[Si] + [Mn]) ~ 0.050 ... (1)
[Ni]/([Cu] + 5[Sn]) ~ 0.10 ... (2)
[Al]/[Ni] ~ 0.20 ... (3)
wherein [Al], [Si], [~n], [Ni], [Cu] and [Sn] represent contents(% by mass) of
Al, Si, Mn, Ni, Cu and Sn in the molten steel respectively.
Claim 2 The method for manufacturing Cu-Sn coexisting steel according to
claim 1, wherein
a content of Al203 in the composite oxides that are generated in the internal
oxidation layer is 15 to 40% by mass.
Claim 3 The~ method for manufacturing Cu-Sn coexisting steel according to
claim 1 or 2, wherein
the composition of the molten steel is adjusted so as to further satisfy a
condition represented by the following formula (4):
1.0 ~ (Cu]/[Sn] ~ 8.0 ... (4).
Claim 4 Cu-Sn coexisting steel that is manufactured by the method for
manufacturing Cu-Sn coexisting steel according to any one of claims 1 to 3.