Technical Field
[0001] The present invention relates to low-carbon resulfurized free-machining steels which
are free of harmful lead (Pb) and exhibit good finished surface roughness.
Background Art
[0002] Low-carbon resulfurized free-machining steels are widely used as steels for hydraulic
parts of gear box units of automobiles, as well as for small parts, such as screws
and printer shafts, which do not require strength so high. When better finished surface
roughness and chip disposability are required, lead-sulfur free-machining steels comprising
the low-carbon resulfurized free-machining steels combined with lead (Pb) are used.
[0003] Lead (Pb) contained in free-machining steels is an element vary effective for improving
machinability, but is harmful to the human body. In addition, lead-containing free-machining
steels have some problems typically in fume of lead upon ingot making and chip disposability.
Accordingly, free-machining steels exhibiting good machinability without adding lead
(Pb) (lead-free) are demanded.
[0004] Various techniques have been proposed to improve the machinability of lead-free low-carbon
resulfurized free-machining steels without lead. Patent Document 1, for example, discloses
a technique for improving the machinability (finished surface roughness and chip disposability)
by controlling the size of sulfide inclusions. Patent Document 2 teaches that the
oxygen content in steel must be appropriately controlled in order to control the size
of sulfide inclusions. A technique for improving the machinability by specifying oxide
inclusions in steel has been proposed, for example, in Patent Document 3. Patent Document
4 proposes a technique for improving the machinability by specifying the ratio of
manganese (Mn) to sulfur (S) and by controlling the free oxygen content immediately
before casting.
[0005] Patent Documents 5 to 7, for example, each propose a technique for improving the
machinability by appropriately specifying the chemical composition of steel.
[0006] These conventional techniques are useful from the viewpoint of improving the machinability
of free-machining steels, but none of them can realize machinability in view of finished
surface roughness upon forming as good as with lead-containing steels.
[0007] It is important that such lead-free steels should have good productivity in addition
to satisfactory machinability. From this viewpoint, they must be produced by a continuous
casting process, be free typically from surface defects and be capable of easilybeing
rolled. The continuous casting process isbelieved to be disadvantageous for improving
the machinability of steels. It is, therefore, also important to produce free-machining
steels excellent in machinability with good productivity by a continuous casting process.
[0008] The continuous castingprocess realizes good surface quality, internal quality, and
good yield. Patent Document 8 discloses a technique for providing a free-machining
steel excellent in machinability (finished surface roughness) by the continuous casting
process. This technique indicates that a free-machining steel excellent in machinability
can be obtained in good yield according to a continuous casting process, by incorporating
a relatively large amount of oxygen of 100 to 300 ppm to a steel and incorporating
nitrogen (N) thereto in a larger amount than those of conventional equivalents. By
satisfying this, built-up edges can be suppressed, which built-up edges occur in a
tool surface upon machining.
[0009] However, if a steel is high both in oxygen content and nitrogen content, blow holes
caused by carbon monoxide gas (CO gas) and nitrogen gas (N
2 gas) are often formed, which may deteriorate the finished surface roughness of the
steel.
Patent Document 1: Japanese Patent Laid-Open No. 2003-253390
Patent Document 2: Japanese Patent Laid-Open No . H09-31522
Patent Document 3: Japanese Patent Laid-Open No. H10-158781
Patent Document 4: Japanese Patent Laid-Open No. 2005-23342
Patent Document 5: Japanese Patent Laid-Open No. 2001-152281
Patent Document 6: Japanese Patent Laid-Open No. 2001-152282
Patent Document 7: Japanese Patent Laid-Open No. 2001-152283
Patent Document 8: Japanese Patent Laid-Open No. H05-345951
Disclosure of Invention
Problems to be Solved by the Invention
[0010] The present invention has been achieved under these circumstances, and an object
of the present invention is to provide a low-carbon resulfurized free-machining steel
which exhibits good machinability typified by finished surface roughness, even being
free from lead, and can be produced with good productivity by a continuous casting
process while suppressing blow holes.
Means for Solving the Problems
[0011] The present invention has been accomplished to achieve the above object and provides
a low-carbon resulfurized free-machining steel excellent in machinability, containing:
0.02% to 0.15% (on the percent by mass basis; hereinafter the same) of carbon (C);
0.004% or less (exclusive of 0 percent) of silicon (Si);
0.6% to 3% of manganese (Mn);
0 .02%'tao 0.2% of phosphorus (P);
0.35% to 1% of sulfur (S);
0.005% or less (exclusive of 0%) of aluminum (A1);
0.008% to 0.03% of oxygen (0); and
0.007% to 0.03% of nitrogen (N), with the remainder being iron and inevitable impurities,
wherein the ratio [Mn]/[S] of the manganese content [Mn] to the sulfur content [S]
is within the range of 3 to 4, and wherein the carbon content [C], the manganese content
[Mn] and the nitrogen content [N] satisfy the following Expression (1):

wherein [C], [Mn] and [N] represent the contents on the percent by mass basis of carbon,
manganese, and nitrogen, respectively.
[0012] The low-carbon resulfurized free-machining steels according to the present invention
each preferably have a chemical composition in which (1) the content of soluble nitrogen
is 0.002% to 0.02% and/or (2) the total content of at least one selected from the
group consisting of Ti, Cr, Nb, V, Zr, and B is 0.02% or less (exclusive of 0%). By
satisfying these requirements, the low-carbon resulfurized free-machining steels according
to the present invention have further improved properties. The steels are preferably
produced by subjecting to electromagnetic stirring in which a magnetic field of 100
to 500 gausses is applied during casting. The resulting steels have better surface
quality.
Advantages
[0013] The present invention controls the contents of carbon, manganese, and nitrogen in
steel so as to satisfy a specific relational expression. By satisfying this, low-carbon
resulfurized free-machining steels good in finished surface roughness can be produced
with good productivity whilesuppressing blow holes even according to a continuous
casting process.
Brief Description of the Drawings
[0014]
FIG. 1 is a graph showing how the finished surface roughness (maximum height of irregularities
Rz) varies depending on the left-hand value of Expression (1) and on the presence
or absence of a magnetic field.
Best Mode for Carrying Out the Invention
[0015] The finished surface roughness of a free-machining steel varies significantly depending
on generation, size, shape and uniformity of built-up edges. Generation of built-up
edges is a phenomenon that part of a work attaches to a surface of a tool and actually
behaves as part (cutting edge) of the tool. It may adversely affect the finished surface
roughness of a work material. The built-up edges generate only under specific conditions,
but free-machining steels are generally cut in the art under such conditions as to
induce the built-up edges.
[0016] The built-up edges are believed to provide fatal defects due to variation in their
size. On the other hand, the built-up edges play a role to protect the edge of a tool
to thereby prolong the lifetime of the tool. All factors considered, therefore, it
is not advantageous to remove such a built-up edge fully, and the built-up edges must
be stably formed with uniformized sizes and shapes.
[0017] To stably form built-up edges with uniformized sizes and shapes, a large number of
fine cracks must be formed in primary and secondary shear zones in a region to be
cut. A large number of crack-forming sites must be introduced to form a large number
of fine cracks. MnS inclusions are known to be useful as sites for forming fine cracks.
Not all MnS inclusions but large-sized (wide) spherical MnS inclusions act as fine-crack-forming
sites. Such MnS inclusions elongate in the primary and secondary shear zones, but
if they become too thin as thin as matrix, they do not work as fine-crack-forming
sites. Accordingly, a work (steel to be cut) must comprise large-sized spherical MnS
inclusions before cutting.
[0018] Oxygen (total oxygen) in steel affects the size and sphericity of MnS inclusions
(for example, Patent Document 2), and it is believed that the size (diameter) of sulfides
increases with an increasing oxygen content of steel. Consequently, the oxygen content
of steel must be increased to some extent in order to make MnS inclusions larger and
more spherical. In addition, the manganese content and sulfur content must be higher
than those in conventional free-machining steels, such as Japanese Industrial Standards
(JIS) SUM 23 steel and SUM 24L steel, so as to increase MnS inclusions working as
fine-crack-forming sites.
[0019] The present inventors have found that soluble nitrogen in steel also significantly
affects the formation of fine cracks and that free-machining steels good in machinability
can be realized by appropriately adjusting the content of soluble nitrogen. The temperatures
in the primary and secondary shear zones significantly vary from a position to another.
The deformation resistance varies depending on temperatures at individual positions
when the soluble nitrogen is present in a certain amount. The difference (variation)
in deformation resistance causes fine-crack-forming sites. Accordingly, a certain
level or more of the soluble nitrogen can be effectively ensured by controlling the
total amount of Ti, Cr, Nb, V, Zr, B to a specific level or less. This is because
these components work to fix the soluble nitrogen, namely, theywork to form nitrides.
[0020] Specifically, the present inventors have found that built-up edges can be stably
formed with uniformized sizes and shapes, for example, by the two phenomena, namely,
(1) makingMnS inclusions become larger and spherical, and (2) increasing soluble nitrogen.
The resulting steels have dramatically improved finished surface roughness in forming
process and thereby exhibit properties as good as lead-free-machining steels.
[0021] The free-machining steels according to the present invention must have appropriately
specified chemical compositions. The reasons for specifying the contents of basic
components C, Si, Mn, P, S, A1, O, and N are as follows.
Carbon (C): 0.02% to 0.15%
[0022] Carbon (C) is an essential element to ensure the strength of steel, and, if added
to a specific amount or more, acts to improve the finished surface roughness. The
carbon content must be 0.02% or more to exhibit these activities. An excessively high
content thereof, however, may shorten the lifetime of a tool upon cutting to thereby
deteriorate the machinability, and may induce defects due to carbon monoxide (CO)
gas upon casting. From these viewpoints, the carbon content is preferably 0.15% or
less. Preferred lower and upper limits of the carbon content are 0.05% and 0.12%,
respectively.
Silicon (Si): 0.004% or less (exclusive of 0%)
[0023] Silicon (Si) is an element effective for ensuring the strength of steel as a result
of solid-solution strengthening, but it basically acts as a deoxidizing agent to form
silicon dioxide (SiO
2). The silicon dioxide SiO
2 serves to form MnO-SiO
2-Mn inclusions. If the silicon content exceeds 0.004%, the SiO
2 content in the inclusions becomes too high to ensure a necessary oxygen content in
MnS inclusions. Thus, the finished surface roughness deteriorates. From these viewpoints,
the silicon content must be 0.004% or less and is preferably 0.003% or less.
Manganese (Mn) : 0.6% to 3%
[0024] Manganese (Mn) acts to improve hardenability, to enhance the formation of the bainite,
and to improve the machinability. It is an element effective for ensuring the strength
of steel. Further, it combines with sulfur to form MnS and combines with oxygen to
form MnO to thereby form MnO-MnS composite inclusions. Thus, it acts to improve the
machinability. To exhibit these actions, the manganese content must be 0.6% or more,
but if it exceeds 3%, the strength increases excessively to deteriorate the machinability.
Preferred lower and upper limits of the manganese content are 1% and 2%, respectively.
Phosphorus (P) : 0. 02 o to 0.2%
[0025] Phosphorus (P) acts to improve the finished surface roughness. It also acts to significantly
improve the chip disposability by facilitating crack propagation in chip. To exhibit
these advantages, the phosphorus content must be 0.02% or more. An excessively high
phosphorus content, however, deteriorates the hot workability, and the phosphorus
content must be 0.2% or less Preferred lower and upper limits of the phosphorus content
are 0.05% and 0.15%, respectively.
Sulfur (S): 0.35% to 1%
[0026] Sulfur (S) is an element which combines with manganese in steel to form manganese
sulfide (MnS), thereby acts as a stress concentrator upon cutting. Thus, chips are
partitioned to thereby improve the machinability. To exhibit these actions, the sulfur
content must be 0. 350 or more. If the sulfur content is excessively high exceeding
1%, the hot workability may deteriorate. Accordingly, a preferred upper limit of the
sulfur content is 0.8%.
Total aluminum: 0.005% or less (exclusive of 0%)
[0027] Aluminum (Al) is an element useful for ensuring the strength of steel as a result
of solid-solution strengthening and for deoxidization. It also acts as a strong deoxidizing
agent to form an oxide (Al
2O
3). The oxide Al
2O
3 constitutes MnO-Al
2O
3-MnS inclusions. If the aluminum content exceeds 0.005%, the Al
2O
3 content of the inclusions becomes too high to ensure a necessary oxygen content in
MnS inclusions to thereby adversely affect the finished surface roughness. The aluminum
content is preferably 0.003% or less and more preferably 0.001% or less.
Oxygen (O): 0.008% to 0.03%
[0028] Oxygen (O) combines with manganese (Mn) to form manganese oxide (MnO) . The MnO contains
a large amount of sulfur to thereby constitute MnO-MnS composite inclusions. The MnO-MnS
composite inclusions are resistant to elongation upon rolling, are present as relatively
spherical inclusions and thereby act as stress concentrator zones upon cutting. Accordingly,
oxygen is positively added to the steel. If the oxygen content is less than 0.008%,
these actions are insufficient, but if it exceeds 0.03%, internal defects caused by
carbon monoxide gas may occur in steel ingots. Accordingly, the oxygen content (total
oxygen content) must be within the range of 0.008% to 0.03%.
[0029] Oxygen (total oxygen) forms manganese oxide (MnO) in molten steel, and the MnO contains
a large amount of sulfur to thereby form MnO-MnS composite inclusions. These MnO-MnS
composite inclusions act as nuclei so as to precipitate MnS inclusions during solidification.
Thus, the resulting billet (ingot prepared as a result of continuous casting) contains
MnO-MnS composite inclusions mainly comprising MnS. The billet then undergoes heating,
blooming, and wire rod rolling or bar mill rolling. With an increasing oxygen content,
the MnO-MnS composite inclusions mainly comprising MnS are more resistant to elongation
in the blooming, wire rod rolling or bar mill rolling, and they constitute large-sized
spherical MnS inclusions in final products such as wire steels and bar steels.
[0030] The lower limit of oxygen (total oxygen) is set in view of these mechanisms in which
the oxygen content is preferably high. However, the upper limit of oxygen content
is also set in actuality. The reasons of this will be explained below. Oxygen (total
oxygen) comprises oxygen in the form of oxides, and soluble oxygen (free oxygen) dissolved
inmolten steel. The oxygen in the form of oxides, namely, oxygen inMnO is very useful.
In contrast, the free oxygen (0) reacts with carbon (C) in molten steel to form CO
gas [C+O = CO (gas)], and the CO gas, if not released sufficiently, causes blow holes.
In addition, the nitrogen content of steel is increased according to the present invention,
and soluble nitrogen in molten steel forms N
2 (gas) [N+N = N
2 (gas)] during a solidification process, because the nitrogen solubility in molten
steel decreases with a descending temperature. The N
2 gas also causes blow holes. Specifically, blow holes mainly comprise CO (gas) and
N
2 (gas).
[0031] A feature (concept) of the present invention is that the free oxygen (O) and nitrogen
(N) contents are set highest within such ranges that the CO (gas) and N
2 (gas) do not form blow holes. The formation of blow holes in steel can also be improved
by carrying out electromagnetic stirring, in addition to setting the chemical composition
of steel. This is because blow holes, if formed, can be eliminated from the steel
by electromagnetic stirring carried out in a mold in continuous casting.
[0032] Under these ideas, the present inventors made investigations to determine which affects
the free oxygen (O) content and have found that the manganese content [Mn] and the
sulfur content [S] mainly affect the free oxygen (O) content. Accordingly, the amount
of CO (gas) can be controlled by [C], [Mn], and [S], and the amount of CO (gas)+N
2 (gas) can be determined according to Expression (1), wherein the nitrogen content
[N] is added to these parameters. Thus, blow holes can be controlled. The detail of
this will be described later.
[0033] The free oxygen (O) content in molten steel is preferably controlled to about 0.
0050% or less from the viewpoint of preventing internal defects caused by CO gas,
while it varies depending on the carbon and nitrogen contents [C] and [N] or electromagnetic
stirring conditions. Preferred lower and upper limits of the oxygen content (total
oxygen content) of steel are 0.01% and 0.03%, respectively.
Nitrogen (N): 0.007% to 0.03%
[0034] Nitrogen (N) is an element affecting the amount of built-up edges, and the content
thereof affects the finished surface roughness. If the nitrogen content is less than
0.007%, excessively large amounts of built-up edges occur to thereby adversely affect
the finished surface roughness. Nitrogen is liable to segregate in dislocations in
the matrix. It segregates in dislocations during cutting to thereby make the matrix
brittle and facilitate crack propagation. Thus, nitrogen serves to improve chip breakability
(chip disposability). However, an excessively high nitrogen content exceeding 0. 03%
causes bubbles (blow holes) upon casting, which may often become internal and surface
defects of the resulting ingot. The nitrogen content must therefore be controlled
to 0.03% or less. Preferred lower and upper limits of the nitrogen content are 0.005%
and 0.025%, respectively.
[0035] Specifying the chemical composition of the low-carbon resulfurized free-machining
steels according to the present invention as above alone is not enough to achieve
the objects of the present invention. In addition to this, the ratio [Mn]/[S] of the
manganese content [Mn] to the sulfur content must be controlled within a specific
appropriate range and these parameters must satisfy the condition represented by Expression
(1) . The reasons why these requirements are set are as follows.
Ratio [Mn]/[S]: 3 to 4
[0036] The ratio [Mn]/[S] is an important factor affecting, for example, cracking during
hot working. If the manganese content is insufficient relative to the sulfur content,
namely, [Mn] / [S] is less than 3, FeS often forms, and this causes hot crack. When
the ratio [Mn]/[S] is within the range of 3 to 4, the manganese content is sufficient
relative to the sulfur content, which prevents the formation of FeS to thereby prevent
hot crack. If the ratio [Mn]/[S] exceeds 4, this effect is saturated and the free
oxygen (O) content decreases to thereby adversely affect the finished surface roughness.
The free oxygen content varies depending on [Mn] and [S].
10[C]×[Mn]-0.94+1226 [N]2 ≤ 1.2
[0037] The above condition must be satisfied for preventing blow holes and ensuring satisfactory
machinability. If the left-hand value (10[C]×[Mn]
-0.94+1226[N]
2) exceeds 1.2, blow holes may form. The left-hand value is preferably 1.1 or less
and more preferably 0.9 or less.
[0038] The condition represented by Expression (1) has been determined after various experiments,
and the reason for which will be described below. Carbon (C), oxygen, and nitrogen
(N) dissolved in molten steel undergo microsegregation due to solid-liquid separation
and are enriched in a liquid. The soluble oxygen nearly equals the free oxygen (O)
, whereas the free oxygen means the oxygen activity. The solubilities of carbon, oxygen,
and nitrogen in the liquid decreases with a descending temperature. Specifically,
the enriched carbon, oxygen, and nitrogen due to microsegregation react as C+O = CO
(gas) and N = 1/2N
2 (gas) with decreasing solubilities with a descending temperature. The resulting gases,
if overcome the local pressure, form bubbles (blow holes) in the liquid part of the
molten steel. The local pressure mainly comprises the total of the atmospheric pressure,
molten steel static pressure, and (interfacial energy between liquid and gas) / (diameter
of bubble) . The bubbles often form in the vicinity of menisci in which the molten
steel static pressure is low. The gas (bubbles) comprises CO (gas) and N
2 (gas). If the gas (bubbles) floats due to difference in density and escapes from
the molten steel to the atmosphere, it does not remain as blow holes in the billet.
However, if it is engulfed, for example, by solidified crystals, it remains as blow
holes and as defects in the billet.
[0040] Initially, Expression (2) will be examined provided that a reaction proceeds from
the right to the left. The equilibrium constant K
CO in Expression (2) is given by the activity coefficient of carbon (f
C), the carbon content [C], the activity factor of oxygen (fo), the oxygen content
[O], and the CO partial pressure (P
CO). The equilibrium constant is determined according to Expression (4), in which T
represents the absolute temperature. The carbon content [C] and the oxygen content
[O] refer to contents after microsegregation and are determined according to the Sheil
Equation as in Expressions (5) and (6). In Expressions (5) and (6), C
C0 and C
O0 represent the initial carbon content [C] and oxygen content [O] of molten steel before
casting, respectively; and C
CL and C
OL represent the carbon content [C] and oxygen content [O] of the liquid phase during
solidification where a solid phase and a liquid phase are coexistent. The C
CL and C
OL represent the contents after enrichment due to microsegregation. By substituting
these parameters into Expression (3), the CO partial pressure (P
CO) can be represented by Expression (7). In these expressions, "f" represents the fraction
of solid phase; and K
C and k
O represent the equilibrium distribution coefficients of carbon and oxygen, respectively.
[0041] The phenomenon relating to nitrogen can be represented by following Expressions (8)
to (12).

Specifically, the equilibrium constant K
N2 in Expression (8) can be represented by Expression (9), and the equilibrium constant
can be represented by Expression (10) . The nitrogen content [N] of the molten steel
after microsegregation can be represented by Expression (11), and by substituting
this into Expression (9), the N
2 partial pressure (P
N2) can be represented by Expression (12).
[0042] Blow holes are formed when the total sum (P
CO + P
N2) of the partial pressures represented by Expressions (7) and (12) thus estimated
exceeds the total of the external pressure (atmospheric pressure), molten steel static
pressure, and (interfacial energy between liquid and gas) / (diameter of bubble),
as represented by following Expression (13):

wherein Pg represents the total sum of partial pressures of gases in molten steel;
Pa represents the external pressure;
ρLgh represents the liquid static pressure;
σ represents the interfacial energy between liquid and gas; and
"r" represents the diameter of bubble.
[0043] The present inventors examined how the occurrence frequency of blow holes varies
depending on the total of partial pressures (P
CO + P
N2) calculated according to the above-mentioned method of calculation having these physical
meanings. As a result, they have found that blow holes occur when the total of partial
pressures (P
CO + P
N2) exceeds 1.2 atm.
[0044] The present inventors made an attempt to convert the total of partial pressures (P
CO + P
N2) into an index. The carbon and nitrogen contents [C] and [N] can be easily determined
by on-line analyses, but the free oxygen content must be determined using a free-oxygen
analyzer. It may be accompanied by a large error in some determination procedures.
The present inventors examined what affects the free oxygen content [O] and have found
that the manganese content [Mn] and the sulfur content [S] affect the free oxygen
content [O]. This is also apparent from the fact that oxygen forms MnO-MnS oxide-sulfide
inclusions in molten steel. This shows that the formation of blow holes can be indicated
by a relational expression among [C], [Mn], [S], and [N]. In addition, the manganese
and sulfur contents [Mn] and [S] have a relation in which the ratio [Mn] / [S] is
3 to 4. In view of this relation, the formation of blow holes can be schematically
expressed by the relational expression among [C], [Mn], and [N].
[0045] Under these ideas, the right-hand value of Expression (7) and data of the contents
such as [Mn] experimentally show that P
CO equals 10[C]×[Mn]
-0.94, because Expression (7) shows that P
CO is proportional to [C] and [O], and [O] varies depending on [Mn] . The square of
the right-hand value of Expression (12) and data of the contents such as [N] show
that the nitrogen partial pressure P
N2 equals 1226[N]
2, because Expression (12) shows that P
N2 is proportional to [N]
2.
[0046] Blow holes occur to thereby cause surface defects with an increased total of the
partial pressures of carbon monoxide and nitrogen P
CO+P
N2 (= 10[C]×[Mn]
-0.94+1226[N]
2). The formation of blow holes naturally affects the finished surface roughness. The
total of the partial pressures of carbon monoxide and nitrogen P
CO+P
N2 has such a relation with the finished surface roughness as shown in after-mentioned
FIG. 1 . FIG. 1 shows that the threshold of the total of partial pressures is about
1.2 in view of the formation of surface defects and the finished surface roughness.
[0047] The low-carbon resulfurized free-machining steels according to the present invention
comprise the above-mentioned components with the remainder basicallybeing iron. However,
they can further comprise trace components in addition to these components, and those
further comprising such trace components are also included within the scope of the
present invention. The low-carbon resulfurized free-machining steels according to
the present invention comprise inevitable impurities such as Cu, Sn, and Ni, and these
inevitable impurities are accepted within ranges not adversely affecting the advantages
of the present invention.
[0048] The low-carbon resulfurized free-machining steels according to the present invention
preferably have (1) a content of soluble nitrogen of 0.002% to 0.02% and/or (2) a
total content of at least one selected from the group consisting of Ti, Cr, Nb, V,
Zr, and B of 0.02% or less (exclusive of 0 percent), according to necessity. The reasons
for setting these specific ranges are as follows.
Content of soluble nitrogen: 0.002% to 0.02%
[0049] As is described above, soluble nitrogen in steel affects the formation of fine cracks,
and free-machining steels with good machinability can be realized by appropriately
controlling the content of the soluble nitrogen. To exhibit these advantages, the
content of soluble nitrogen in steel is preferably controlled to 0.002% or more. If
it exceeds 0. 02%, however, surface defects may increase.
Total content of at least one element selected from the group consisting of Ti, Cr,
Nb, V, Zr, and B: 0.02% or less (exclusive of 0%)
[0050] These elements combine with nitrogen to form nitrides, and if the contents thereof
are excessively high, the content of soluble nitrogen becomes too small below the
necessary content of soluble nitrogen. From this viewpoint, the total content of these
elements is preferably controlled to 0.02% or less.
[0051] The low-carbon resulfurized free-machining steels according to the present invention
are basically produced by a continuous casting process. They can be specifically produced,
for example, according to the following procedure. Initially, carbon is blown down
to a carbon content of 0.04% or less in a converter so as to make molten steel have
a high free oxygen content (soluble oxygen content). The free oxygen content herein
is preferably 500 ppm or more. Next, alloys such as Fe-Mn alloy and Fe-S alloy are
added upon tapping. These alloys contain silicon and aluminum as impurities . By adding
these alloys to high-oxygen molten steel upon tapping from the converter, silicon
and aluminum are oxidized to form SiO
2 and Al
2O
3. These float and separate into slag upon subsequent ladle refining process of the
molten steel. Thus, silicon and aluminum remained in steel decrease to be target contents.
It is important in this processing that 70% or more of additional components such
as Fe-Mn alloy and Fe-S alloy added for adjusting the chemical composition is added
upon tapping from the converter so as to reduce aluminum and silicon, and the remainder
(30% or less) of them is added in the ladle refining process of the molten steel.
By carrying out these procedures, the steel can have a silicon content of 0.004% or
less.
[0052] In the production of steels, electromagnetic stirring is preferably carried out,
in which a predetermined magnetic field is applied to the steels upon casting. The
electromagnetic stirring is carried out from the viewpoint of reducing blow holes
to thereby prevent defects and to provide good surface quality. The production of
steels in combination with the electromagnetic stirring is very useful for making
MnS inclusions large-sized and spherical and for preventing the formation of blow
holes. The magnetic field to be applied in the electromagnetic stirring preferably
has an intensity of about 100 to about 500 gausses. If the intensity of the magnetic
field is less than 100 gausses, the effect of electromagnetic stirring may not be
exhibited. In contrast, if it exceeds 500 gausses, the molten steel in a continuous
casting mold may flow at an excessively high rate, and the steel may involve a mold
powder and make casting difficult.
[0053] The present invention will be illustrated in further detail with reference to several
examples below. It should be noted, however, that the following examples are never
intended to limit the scope of the present invention, various modifications and variations
are possible unless departing from the spirit and scope of the present invention,
and those modifications and variations are included within the technical scope of
the present invention.
[Examples]
[0054] A series of molten steels having varying contents of, for example, Si, Mn, S, Al,
and N were made using a 3-ton induction furnace, a 100-ton converter, and molten steel
refining facilities including a pouring ladle. In this procedure, the silicon and
aluminum contents were adjusted by varying the silicon and aluminum contents in Fe-Mn
alloys and Fe-S alloys to be added, respectively. The free oxygen contents in the
resulting molten steels were determined immediately before casting into a predetermined
mold using a free oxygen probe (the product of Heraeus Electro-Nite under the trade
name of "HYOP 10A-C150"), and they were defined as the free oxygen contents.
[0055] The molten steels were subjected to continuous casting using a (bloom-type) mold
having a sectional size of 300 mm wide and 430 mm long. Alternatively, they were cast
in the 3-ton induction furnace using a cast-iron mold having a sectional size of 300
mm wide and 430 mm long which had been designed to achieve a cooling rate as in bloomed
billets. Where necessary, a magnetic field was applied to the mold during casting
so as to carry out electromagnetic stirring.
[0056] Samples were taken from quenched portion in the surface layer of the resulting billets
or ingots and were chemically analyzed to determine their chemical compositions. The
results are shown in Table 1 below.
[0057]
Table 1
Sample No. |
Chemical composition (percent by mass) |
[Mn] / [S] |
C |
Si |
Mn |
P |
S |
Al |
Total oxygen |
Free oxygen |
N |
Pb |
Other components |
1 |
0.10 |
0.002 |
0.9 |
0.080 |
0.30 |
0.003 |
0.0284 |
0.0066 |
0.0124 |
- |
Ti: 0.06, Cr: 0.005, Zr: 0.005, V: 0.005 |
3.0 |
2 |
0.11 |
0.002 |
1.1 |
0.087 |
0.32 |
0.001 |
0.0223 |
0.0054 |
0.0160 |
- |
- |
3.4 |
3 |
0.11 |
0.001 |
1.3 |
0.081 |
0.36 |
0.002 |
0.0185 |
0.0046 |
0.0189 |
- |
- |
3.6 |
4 |
0.11 |
0.003 |
1.5 |
0.086 |
0.42 |
0.002 |
0.0160 |
0.0040 |
0.0210 |
- |
- |
3.6 |
5 |
0.11 |
0.007 |
2.1 |
0.079 |
0.59 |
0.003 |
0.0114 |
0.0029 |
0.0241 |
- |
Ti: 0.02, Cr: 0.003 |
3.6 |
6 |
0.08 |
0.003 |
1.2 |
0.082 |
0.31 |
0.001 |
0.0208 |
0.0049 |
0.0070 |
0.300 |
- |
3.8 |
7 |
0.11 |
0.003 |
1.7 |
0.080 |
0.49 |
0.001 |
0.0168 |
0.0036 |
0.0186 |
- |
Ti: 0.003, Cr: 0.004 |
3.5 |
8 |
0.12 |
0.004 |
1.9 |
0.081 |
0.53 |
0.003 |
0.0155 |
0.0032 |
0.0173 |
- |
Ti: 0.002, Cr: 0.006 |
3.6 |
9 |
0.11 |
0.007 |
2.3 |
0.082 |
0.65 |
0.009 |
0.0111 |
0.0027 |
0.0234 |
- |
Ti: 0.003, Cr: 0.003, Zr: 0.002 |
3.5 |
10 |
0.10 |
0.003 |
1.3 |
0.079 |
0.37 |
0.001 |
0.0198 |
0.0046 |
0.0150 |
- |
- |
3.5 |
11 |
0.10 |
0.003 |
1.5 |
0.081 |
0.45 |
0.002 |
0.0181 |
0.0040 |
0.0125 |
- |
Ti: 0.002, Cr: 0.003 |
3.3 |
12 |
0.08 |
0.004 |
1.7 |
0.084 |
0.50 |
0.004 |
0.0146 |
0.0036 |
0.0180 |
- |
- |
3.4 |
13 |
0.08 |
0.003 |
1.9 |
0.077 |
0.55 |
0.002 |
0.0125 |
0.0032 |
0.0143 |
- |
- |
3.5 |
14 |
0.08 |
0.003 |
2.1 |
0.079 |
0.59 |
0.005 |
0.0117 |
0.0029 |
0.0156 |
- |
Ti: 0.003, Cr: 0.004 |
3.6 |
15 |
0.08 |
0.004 |
2.3 |
0.075 |
0.65 |
0.003 |
0.0104 |
0.0027 |
0.0135 |
- |
Ti: 0.003, Cr: 0.005, Nb: 0.001 |
3.5 |
16 |
0.10 |
0.002 |
1.3 |
0.083 |
0.35 |
0.001 |
0.0222 |
0.0046 |
0.0155 |
- |
- |
3.7 |
17 |
0.08 |
0.001 |
1.5 |
0.077 |
0.43 |
0.001 |
0.0192 |
0.0040 |
0.0135 |
- |
Ti: 0.001, Cr: 0.003 |
3.5 |
18 |
0.09 |
0.003 |
1.7 |
0.083 |
0.48 |
0.002 |
0.0160 |
0.0036 |
0.0167 |
- |
Ti: 0.003, Cr: 0.005, Nb: 0.001 |
3.5 |
19 |
0.10 |
0.001 |
1.9 |
0.079 |
0.55 |
0.003 |
0.0145 |
0.0032 |
0.0165 |
- |
Ti: 0.003, Cr: 0.004 |
3.5 |
20 |
0.10 |
0.004 |
2.1 |
0.078 |
0.61 |
0.002 |
0.0135 |
0.0029 |
0.0205 |
- |
- |
3.4 |
21 |
0.15 |
0.003 |
2.3 |
0.077 |
0.70 |
0.001 |
0.0111 |
0.0027 |
0.0155 |
- |
Ti: 0.002, Cr: 0.004 |
3.3 |
[0058] The resulting billets and ingots were heated at 1250°C for one hour, subjected to
blooming to a sectional size of 155 mm wide and 155 mm long, rolled to a diameter
of 25 mm, subjected to acidpickling to yield cold finished steel bars having a diameter
of 22 mm, and subjected to cutting tests. The rolling herein was conducted at 1000°C
and the rolled steels were cooled forcedly at an average cooling rate from 800°C to
500°C of about 1.5°C per second. The temperatures of steels were determined using
a radiation pyrometer.
[0059] The steels were measured on content of soluble nitrogen and subjected to cutting
tests under the following conditions. The finished surface and surface defects of
the steels after cutting tests were evaluated according to the following criteria.
[Determination of content of soluble nitrogen]
[0060] The content of soluble nitrogen was determined as the difference between the total
nitrogen and the nitrogen in compound. The total nitrogen was determined according
to a method using a conductivity of an inert gas heat of fusion, and the nitrogen
in compound was determined by dissolving and extracting a sample with a methanol solution
containing 10% of acetylacetone and 1% of tetramethylammonium chloride, collecting
nitrogen through a 1-µm filter and determining nitrogen using an indophenol-absorptiometer.
[Cutting test conditions]
[0061]
Tool: high-speed tool steel SKH4A
Cutting rate: 100 m/min.
Feed rate: 0.01 mm per revolution
Depth of cut: 0.5 mm
Cutting oil: chlorine-containing water-insoluble cutting oil Length of cut: 500 m
[Criteria]
[0062] Finished surface evaluation: The surface roughness was evaluated in terms of the
maximum height of irregularities Rz according to JIS B 0601 (2001).
Surface defect evaluation: Surface defects on bloomed sample billets having a sectional
size of 155 mm wide and 155 mm long were detected using an automatic defect detector.
A sample in which no defect was detected by the automatic defect detector was evaluated
as "Good"; a sample having some defects that can be removed by a processing was evaluated
as "Fair"; and a sample having defects that are not removable was evaluated as "Failure"
.
[0063] The results in the cutting tests with other data, such as the left-hand value of
Expression (1) and the intensity of magnetic field, are shown in Table 2 below.
Table 2
Sample No. |
Left-hand value of Expression (1) |
Magnetic field (gauss) |
Dissolved nitrogen (percent by mass) |
Finished surface roughness Rz(µm) |
Surface defects |
Remarks |
1 |
1.293 |
- |
0.0052 |
45 |
Failure |
Comparative Example |
2 |
1.320 |
- |
0.0138 |
46 |
Failure |
Comparative Example |
3 |
1.298 |
- |
0.0162 |
43 |
Failure |
Comparative Example |
4 |
1.292 |
- |
0.0183 |
40 |
Failure |
Comparative Example |
5 |
1.260 |
- |
0.0198 |
43 |
Failure |
Comparative Example |
6 |
0.734 |
100 |
0.0036 |
17 |
Good |
Referential Example |
7 |
1.092 |
- |
0.0140 |
28 |
Fair |
Example |
8 |
1.023 |
- |
0.0130 |
27 |
Fair |
Example |
9 |
1.174 |
- |
0.0190 |
29 |
Fair |
Example |
10 |
1.057 |
100 |
0.0120 |
18 |
Good |
Example |
11 |
0.875 |
100 |
0.0090 |
17 |
Good |
Example |
12 |
0.833 |
100 |
0.0155 |
16 |
Good |
Example |
13 |
0.688 |
100 |
0.0113 |
17 |
Good |
Example |
14 |
0.697 |
100 |
0.0120 |
21 |
Good |
Example |
15 |
0.589 |
100 |
0.0080 |
25 |
Good |
Example |
16 |
1.076 |
500 |
0.0128 |
20 |
Good |
Example |
17 |
0.770 |
500 |
0.0101 |
16 |
Good |
Example |
18 |
0.888 |
500 |
0.0120 |
17 |
Good |
Example |
19 |
0.881 |
500 |
0.0125 |
19 |
Good |
Example |
20 |
1.013 |
500 |
0.0181 |
23 |
Good |
Example |
21 |
0.980 |
500 |
0.0113 |
26 |
Good |
Example |
[0064] These results show that samples satisfying the requirements specified in the present
invention (Sample Nos. 7 to 21) have small finished surface roughness (maximum height
of irregularities Rz) and exhibit good machinability; and that, among them, the samples
subjected to electromagnetic stirring (Sample Nos. 10 to 21) are reduced in surface
defect caused by blow holes.
[0065] In contrast, samples not satisfying at least one of the requirements specified in
the present invention are poor in at least one of the properties.
[0066] FIG. 1 shows how the finished surface roughness (maximumheight of irregularities
Rz) varies depending on the left-hand value of Expression (1) and on the presence
or absence of a magnetic field.