EP2096186B1

EP2096186B1 - Free-cutting steel excellent in manufacturability

Info

Publication number: EP2096186B1
Application number: EP07849980A
Authority: EP
Inventors: Masayuki Hashimura; Atsushi Mizuno; Kenichiro Miyamoto; Jun Aoki; Seiji Ito
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2006-11-28
Filing date: 2007-11-27
Publication date: 2012-10-24
Anticipated expiration: 2027-11-27
Also published as: AU2007326255B2; US20100054984A1; WO2008066194A1; KR101118852B1; BRPI0719310B1; JP5212111B2; TWI363804B; KR20090055648A; BRPI0719310A2; CN101573463A; TW200840875A; AU2007326255A1; JPWO2008066194A1; EP2096186A1; EP2096186A4

Description

TECHNICAL FIELD

The present invention relates to low carbon machining steel used for automobiles, general machinery, etc. where machinability is required more than strength characteristics, more particularly relates to machining steel superior in tool life at the time of machining, finished surface roughness, chip evacuation, and other machinability, accompanied with little melt loss of plate refractories of the continuous casting sliding nozzles, and superior in manufacturability with good ductility in hot rolling.

BACKGROUND ART

General machinery and automobiles are manufactured by assembling a large number of types of parts. The parts are in most cases produced through machining processes from the viewpoint of the required precision and manufacturing efficiency. At that time, reduction of costs and improvement of production efficiency are demanded. Improvement of the machinability is being demanded from the steel as well. In particular, low carbon sulfur machining steel SUM23 and low carbon sulfur-lead composite machining steel SUM24L have been invented stressing the machinability. Up to now, to improve the machinability, it has been known that addition of S, Pb, and other machinability improving elements is effective. However, depending on the user, sometimes use of Pb is avoided due to the environmental load. The amount of use is being reduced as a general direction.
Up to now as well, when desiring not to add Pb, the technique has been used of forming inclusions such as sulfides mainly comprised of MnS which become soft under the machining environment so as to improve the machinability. However, the low carbon sulfur-lead composite machining steel SUM24L has the same amount of S added to it as low carbon sulfur machining steel SUM23. Therefore, it is necessary to add an amount of S greater than the past. However, with the addition of a large amount of S, just making the sulfides mainly comprised of MnS coarse was not effective for improving the machinability. Further, the problems arise that it is not possible to make the matrix sufficiently brittle and deterioration of the finished surface roughness along with the phenomena of pieces of the built-up cutting edge breaking off and chips not being removed and that chip evacuation becomes poor due to insufficient removal of chips. Furthermore in the rolling, forging, and other production processes, coarse sulfides mainly comprised of MnS become starting points of breakage and cause numerous problems in production such as rolling defects. There are limits with just the increase of the amount of S. Further, addition of machinability improving elements other than S such as Te, Bi, P, N, etc. can also improve the machinability to a certain extent, but at the time of rolling or hot forging, deterioration of the surface properties such as cracks and defects are caused, so it is considered desirable that these be as small as possible. It is not possible to achieve both machinability and manufacturability.
Japanese Patent Publication (A) No. 11-222646 proposes the method of introducing 30 or more independent sulfides of 20 µm or more or groups of sulfides of lengths of a plurality of sulfides connected in substantially straight lines of 20 µm or more in a 1 mm² field of the cross-section in the rolling direction so as to improve the chip evacuation. However, in actuality, no allusion is made, including of the method of production, of the dispersion of submicron level sulfides most effective for machinability. Further, this cannot be expected from the ingredients either.
There have been examples of attempts to use inclusions other than sulfides to improve the machinability up to now as well. For example, Japanese Patent Publication (A) No. 9-17840 , Japanese Patent Publication (A) No. 2001-329335 , Japanese Patent Publication (A) No. 2002-3991 , and Japanese Patent Publication (A) No. 2000-178683 are art using BN to improve the machinability. However these are not intended for improving the finished surface roughness. In Japanese Patent Publication (A) No. 9-17840 , Japanese Patent Publication (A) No. 2001-329335 , and Japanese Patent Publication (A) No. 2000-178683 , the object is the improvement of the tool life, while in Japanese Patent Publication (A) No. 2002-3991 , the object is the improvement of the chip evacuation. In applications in the chemical ingredients of the ranges of the examples disclosed in these, a sufficient effect cannot be obtained in improvement of the finished surface roughness. Specifically, unless the matrix is made uniform by the fine dispersion of BN in the steel, the effect of improvement of the finished surface roughness cannot be obtained, but these patent documents do not describe this art. US 2006/0013720 discloses a steel with good machinability.
The art disclosed in Japanese Patent Publication (A) No. 2004-176176 is also an example of attempted use of BN for improvement of the machinability. This considers the balance with the amount of addition of N. However, in this art, the balance of the chemical ingredients of the steel for completely suppressing the occurrence of rolling defects while securing the machinability - an opposite property - and the method of suppressing the amount of oxides of B with a high affinity with oxygen to make B precipitate as BN were not discovered.
Japanese Patent Publication (A) No. 5-345951 is art improving the machinability by increasing the concentration of oxygen in the steel so as to make the MnS larger in size. However, in this art, the reduction of MnS due to the increase in the oxygen and the accompanying reduction of the machinability are not alluded to at all. Furthermore, measures for preventing melt loss of refractories, increase of surface defects, and other remarkable deterioration of the manufacturability are not touched upon either.
Further, Japanese Patent Publication (A) No. 2001-329335 , to improve the hot ductility, discloses the art of suppressing grain boundary embrittlement due to precipitation of BN at the grain boundaries and furthermore limiting the amount of N added for making use of the action of solid-solute B in preventing grain boundary embrittlement. However, this only reduces the amount of N. Control of the amount of solid-solute N in the BT heating to work temperature range is not sufficiently considered. The amount of solid-solute N is not sufficiently reduced as required for preventing defects. Further, the amount of N is limited to one lower than the stoichiometric composition, so the amount of BN is insufficient for improving the finished surface roughness. Using other art for making up for this is not considered at all as well, so it is not possible to obtain a good finished surface roughness.
Further, Japanese Patent Publication (A) No. 2004-27297 proposes the art of reducing the surface defects by limiting the amount of oxygen in the steel. However, the method of control of the amount of oxygen in the steel is not alluded to at all. In unkilled low carbon machining steel, without special control, it is impossible to limit the amount of oxygen in the steel and prevent occurrence of defects.
There have been examples of adding Ca for improving the machinability in low carbon machining steel up to now as well. For example, in Japanese Patent Publication (A) No. 2000-160284 , the specific effect of improving the machinability is not described. Further, the range of the amount of addition of Ca is broad. The amount of addition effective for improving the machinability is also not described.
Further, when producing low carbon machining steel with the addition of B by continuous casting, there is the problem of easy melt loss of the plate refractories of the sliding nozzles. No prior art document solving this problem can be found.

DISCLOSURE OF THE INVENTION

The present invention provides low carbon machining steel used for automobiles, general machinery, etc. particularly machining steel superior in tool life at the time of machining, finished surface roughness, chip evacuation, and other machinability, accompanied with little melt loss of plate refractories of the continuous casting sliding nozzles, and superior in ductility in hot rolling, and able to prevent deterioration of the surface properties due to hot rolling.
Machining is a phenomenon of removal of chips. Promoting this is one of the key points. However, as already explained, there are limits with just increasing the S. Further, to achieve both machinability and manufacturability, it is also necessary to consider the amounts of the machinability improving elements.
Therefore, the inventors discovered that by controlling the amount of solid-solute N in the rolling temperature range and controlling the ratio of the amounts of B and N required for obtaining the BN required for machinability at room temperature where machining is performed, it is possible to achieve both hot ductility and machinability. Here, the "solid-solute N" is the total amount of N minus the amount of compound N. The "amount of compound N" substantially shows the amount of N forming BN. This solid-solute N is produced in large amounts since the BN becomes solid solute by heating in the rolling temperature range of 800 to 1100°C. For good rolling with little occurrence of surface defects, it is necessary to reduce the amount of solid-solute N in this temperature range.
Further, the inventors discovered that to improve the yield of Mn, which is easily consumed as an oxide in the molten steel, as MnS and the yield of B as BN so as to improve the machinability and hot ductility and to improve the machinability and suppress the melt loss of plate refractories of the continuous casting sliding nozzles, it is necessary to reduce the amount of production of MnO in the steel.
The present invention was made based on the above discovery and has as its gist the following:

(1) Machining steel superior in manufacturability containing, by mass%,
- C: 0.005 to 0.2%
- Si: 0.001 to 0.5%
- Mn: 0.3 to 3.0%
- P: 0.001 to 0.2%
- S: 0.30 to 0.60%
- B: 0.0003 to 0.015%
- O: 0.005 to 0.012%
- Ca: 0.0001 to 0.0010%, and
- Al≤0.01%, optionally one or more of
  - V: 0.05 to 1.0%
  - Nb: 0.005 to 0.2%
  - Cr: 0.01 to 2.0%
  - Mo: 0.05 to 1.0%
  - W: 0.05 to 1.0%
  - Ni: 0.05 to 2.0%
  - Cu: 0.01 to 2.0%
  - Sn: 0.005 to 2.0%
  - Zn: 0.0005 to 0.5%
  - Ti: 0.0005 to 0.1%
  - Zr: 0.0005 to 0.1%
  - Mg: 0.0003 to 0.005%
  - Te: 0.0003 to 0.2%
  - Bi: 0.005 to 0.5%
  - Pb: 0.005 to 0.5% , and
having an N content satisfying N≥0.0020% and 1.3×B-0.0100≤N≤1.3×B+0.0034, and having a balance of Fe and unavoidable impurities, wherein
further, regarding the MnO in the steel, in a cross-section of the steel material perpendicular to the rolling direction, the area of MnO of a circle equivalent diameter of 0.5 µm or more being 15% or less of the area of the total Mn-based inclusions.
(2) Machining steel superior in manufacturability as set forth in (1), wherein, regarding the sulfides mainly comprised of MnS, in a cross-section of the steel material perpendicular to the rolling direction, a density of sulfides of a circle equivalent diameter of 0.1 to 0.5 µm is 10000/mm² or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 gives conceptual views showing a plunge cutting test method, in which (a) is a bird's eye view and (b) is a plan view.
FIG. 2 gives conceptual views showing a longitudinal turning test method and the quality of the finished surface roughness, in which (a) is a plan view and (b) is an enlarged view of a finished surface (feed marks).
FIG. 3 is an optical micrograph showing an example of measurement of MnO by EPMA.
FIG. 4 gives an (a) TEM replica photograph and (b) optical micrograph of sulfides mainly comprised of MnS of an example of the present invention.
FIG. 5 gives an (a) TEM replica photograph and (b) optical micrograph of sulfides mainly comprised of MnS of a comparative example of the present invention.
FIG. 6 is a view showing changes in machinability due to MnO by the finished surface roughness by longitudinal turning after machining 800 pieces.
FIG. 7 is a view showing a balance of finished surface roughness by longitudinal turning and hot ductility in invention examples and comparative examples.
FIG. 8 is an explanatory view of a depth position of 1/4 of a cast slab thickness.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention provides low carbon machining steel in which machinability is required more than strength characteristics, which improves the machinability, without adding Pb, by adding B and making it precipitate as BN, wherein, regarding the composition of ingredients of the steel, in particular B and N are added so as to satisfy a suitable relationship to thereby improve the machinability and the ductility at the time of hot rolling and wherein MnO in the steel is reduced so as to improve the machinability and the lifetime of the refractories for control of the amount of injection in continuous casting, whereby the invention is completed. Furthermore, the present invention finely disperses MnS-based inclusions in the steel to improve the machinability. Below, the composition of ingredients prescribed in the present invention and the reasons for limitation will be explained.

[C] 0.005 to 0.2%

C is related to the basic strength of the steel material and the amount of oxygen in the steel, so has a large effect on the machinability. If adding a large amount of C to improve the strength, the machinability is reduced, so the upper limit was made 0.2%. On the other hand, if simply using blow refining and overly reducing the amount of C, not only will the costs swell, but also the oxygen will no longer be removed by the C, so a large amount of oxygen will remain in the steel and will cause pinholes and other problems. Therefore, an amount of C of 0.005% able to easily prevent pinholes and other problems was made the lower limit.

[Si] 0.001 to 0.5%

Excessive addition of Si forms hard oxides which lower the machinability, but suitable addition softens the oxides and does not cause a drop in the machinability. The upper limit is 0.5%. Above that, hard oxides form. If less than 0.001%, softening of the oxides becomes difficult and the cost swells industrially.

[Mn] 0.3 to 3.0%

Mn is required for fixing and dispersing the sulfur in the steel as MnS. Further, it is necessary for softening the oxides in the steel and rendering the oxides harmless. The effect depends on the amount of S added, but if less than 0.3%, the added S is sufficiently fixed as MnS leading to surface defects and S becomes FeS leading to embrittlement. If the amount of Mn becomes large, the hardness of the material also becomes greater and the machinability and cold workability fall, so 3.0% was made the upper limit.

[P] 0.001 to 0.2%

P causes a greater hardness of the material in the steel. Not only the cold workability, but also the hot workability and casting properties fall, so the upper limit has to be made 0.2%. On the other hand, this is an element effective for improvement of the machinability, so the lower limit was made 0.001%.

[S] 0.30 to 0.60%

S bonds with Mn and is present as sulfides mainly comprised of MnS. Sulfides mainly comprised of MnS improve the machinability, while sulfides mainly comprised of flattened MnS constitute one cause of anisotropy at the time of forging. Large sulfides mainly comprised of MnS should be avoided, but from the viewpoint of the improvement of the machinability, addition of a large amount is preferable. Therefore, causing sulfides mainly comprised of MnS to finely disperse is preferable. For improvement of the machinability when not adding Pb, addition of 0.30% or more is necessary. On the other hand, if the amount of addition of S is too great, not only is formation of coarse sulfides mainly comprised of MnS unavoidable, but also cracks occur during the manufacture due to the casting property due to FeS etc., the deterioration cf the deformation characteristics, etc. For this reason, the upper limit was made 0.60%.

[B] 0.0003 to 0.015%

If B precipitates as BN, there is an effect of improvement of the machinability. In particular, by coprecipitating with sulfides mainly comprised of MnS and finely dispersing in the matrix, the effect becomes more remarkable. These effects are not remarkable if less than 0.0003%, while if added over 0.015%, the reaction with the refractories in the molten steel becomes severer and the melt loss of the refractories at the time of casting becomes greater and the manufacturability is remarkably impaired. Therefore, the range was made 0.0003% to 0.015%.
B easily forms oxides, so if the dissolved O in the molten steel is high, it ends up being consumed as oxides and the amount of BN effective for improvement of the machinability is sometimes reduced. Adding Ca to lower the dissolved oxygen (free oxygen) to a certain extent, then adding B to improve the yield of the amount of B substantially becoming BN is effective for improving the machinability.

[O] 0.005 to 0.012%

When O does not form oxides, but remains alone, it forms bubbles at the time of cooling and causes pinholes. Sometimes it forms hard oxides causing deterioration of the machinability or defects, so control is necessary. Furthermore, it ends up consuming the Mn and B added for improving the machinability as oxides in the molten steel and thereby reduces the Mn becoming MnS and B becoming BN to have an effect on the machinability. If less than 0.005%, sulfides mainly comprised of MnS of a form called Type II of Sims are formed and thereby the machinability is degraded. Furthermore, a desulfurization reaction easily occurs in the molten steel and stable addition of S no longer becomes possible. Therefore, 0.005% was made the lower limit. If the amount of O exceeds 0.012%, oxides of Mn and B easily form in the molten steel and the Mn becoming MnS and B becoming BN are de facto reduced whereby the machinability is degraded. Furthermore, a large amount of hard oxides are formed and the amount of damage is increased. Furthermore, the melt loss of the refractories also becomes greater. Therefore, 0.012% was made the upper limit. For the control of O, addition of Ca is essential.

[Ca] 0.0001 to 0.0010%

Ca is a deoxidizing element. It can control the amount of dissolved oxygen (free oxygen) in the steel material, stabilizes the yields of the easily oxide forming Mn and B, and furthermore can suppress the formation of hard oxides. Further, if slight in amount, it forms soft oxides and acts to improve the machinability. If less than 0.0001%, this effect is nonexistent, while if over 0.0010%, a large amount of soft oxides are formed and deposit on the tool cutting edges as relief shapes, so the finished surface roughness becomes extremely bad. Not only this, but also a large amount of hard oxides are produced. Furthermore, the machinability and the hot ductility are lowered. Therefore, the range of the ingredient was defined as 0.0001 to 0.0010%.

[Al] Al≤0.01%

Al is a deoxidizing element and forms Al₂O₃ or AIN in the steel. However, Al₂O₃ is hard, so becomes a cause of tool damage and promotes wear at the time of machining. Further, by forming AIN, the amount of N for forming BN ends up being reduced and the machinability falls. Therefore, the amount was made 0.01% or less where Al₂O₃ and AIN are not produced in large amounts.

[N contained satisfying N≥0.0020% and 1.3×B-0.0100≤N≤1.3×B+ 0.0034]

N bonds with B to form BN which improves the machinability. BN forms inclusions improving the machinability. By finely dispersing them in a high density, the machinability is remarkably improved. B and N bond exactly by a stoichiometric ratio, by mass ratio, of B:N=10.8:14 (=1:1.3) whereby BN is formed. BN has solubility with respect to steel. Along with a rise in the steel temperature, its solubility becomes greater and the amount of solid-solute N increases. If the amount of N becoming solid solute in the rolling temperature range (800 to 1100°C) is great, this will become a cause of rolling defects, so it is necessary to limit the amount of solid-solute N to a certain amount or less. It is necessary to control the amount of N added to the steel material in accordance with the amount of addition of B. Therefore, if exceeding the amount of N bonding exactly with B (1.3×B) by +0.0034%, the occurrence of rolling defects becomes remarkable, so the upper limit of the amount of N was made 1.3×B+0.0034. On the other hand, if the amount of N added becomes too small, the amount of formation of BN is reduced. If less than the amount of N bonding exactly with B (1.3×B) by -0.0100%, the amount of BN necessary for improvement of the machinability cannot be obtained, so the lower limit of the amount of N with respect to the amount of B was made 1.3×B-0.0100 or more. Further, if the amount of N is less than 0.0020%, the absolute amount of N becomes insufficient and the distance of dispersion to places where B is present in the steel becomes greater, so even with an amount of addition of N of the stoichiometric ratio, sufficient BN cannot be produced. For this reason, it is necessary to secure 0.0020% or more. Due to the above, to achieve both manufacturability and machinability, it is necessary that the N content satisfy N≥0.0020% and 1.3xB-0.0100≤N≤1.3×B+0.0034.

[MnO] Area of MnO of Circle Equivalent Diameter of 0.5 µm or More Not More Than 15% of Area of Total Mn-Based Inclusions

Mn is an element strong in affinity with oxygen. Formation of MnO becomes unavoidable in the presence of a certain amount of dissolved oxygen (free oxygen) in the molten steel. MnO is an inclusion with relatively low melting point and softness. It itself does not cause remarkable deterioration of the tool life and other aspects of machinability like a hard inclusion such as Al₂O₃. However, if the MnO increases, the amount of Mn forming MnS is reduced and the fine dispersion of the MnS is obstructed, so the machinability deteriorates. Furthermore, in an environment where a large amount of MnO is produced, the dissolved oxygen (free oxygen) in the molten steel becomes a high concentration. Therefore, the amount of formation of B oxides also increases, the amount of B forming BN is reduced, and the machinability is further degraded. Further, if the Mn forming MnS is reduced, it is no longer possible to fix the S at a high temperature, so a large number of FeS particles are formed and therefore the hot ductility is degraded.
Furthermore, due to the MnO in the molten steel, the melt loss of the plate refractories of the continuous casting sliding nozzles becomes severer and the manufacturability is remarkably degraded. If the area of the MnO in the steel having a circle equivalent diameter of 0.5 µm or more in the cross-section of the steel material perpendicular to the rolling direction is over 15% of the area of the total Mn-based inclusions, the deterioration of the machinability and manufacturability becomes remarkable, so to obtain good machinability and manufacturability, it is necessary that the MnO in the steel be not more than 15% of the total Mn-based inclusions.
If MnO is, by circle equivalent diameter, 0.5 µm or less, its area rate is extremely small, therefore the amount of Mn consumed by the MnO is also slight, so the amount of production of MnS is not greatly affected. For this reason, it is defined as having a circle equivalent diameter of 0.5 µm or more.
Here, the identification of the MnO referred to in the present invention and the method of measurement of the area will be explained.
MnO is usually present as MnO alone and is also sometimes present bonded with other oxides, but in the present invention, what is measured by the following method is identified as the "MnO" and its area is found.
An example of measurement of the MnO by EPMA is shown in FIG. 3. A test piece cut out from a position of the steel material at a depth of 1/4 of the diameter of the cross-section perpendicular to the rolling direction, buried in resin, and polished was measured by an electron probe microanalyzer (EPMA) for at least 20 fields, each field being 200 µm×200 µm. The MnO's 13 in the steel of the steel material are present in a state contained in sulfides mainly comprised of MnS 14, so in elemental area analysis by EPMA, the parts where Mn and O overlap are deemed MnO and that area is found.
The "total Mn-based inclusions" is the general term for all of the inclusions combined with Mn in the steel. This covers the later explained sulfides mainly comprised of MnS, oxides of MnO alone, and oxides of MnO bonded with other oxides. The total Mn-based inclusions can also be identified by elemental area analysis by EPMA and their area measured, so the ratio of the area of the MnO measured with respect to the area of the total Mn-based inclusions measured is found.
To reduce the amount of formation of MnO, it is possible to reduce the concentration of dissolved oxygen (free oxygen) in the molten steel before LF. It is preferable to make the dissolved oxygen (free oxygen) concentration 200 ppm or less. However, if overly reducing it, a desulfurization reaction proceeds between the metal/slag and securing the S in the steel for maintaining the machinability becomes difficult, so sufficient care is required. Making this 150 ppm or more is preferable. As the method for control of the dissolved oxygen (free oxygen), advance desulfurization before LF treatment is effective. For control of the free oxygen, addition of Ca is essential, but in addition adding Si, Al, Ti, Zr, Mg, etc. alone or in combination is also effective.

[Dispersion of Sulfides Mainly Comprised of MnS] Density of sulfides of circle equivalent diameter of 0.1 to 0.5 µm of 10000/mm² or more

Sulfides mainly comprised of MnS are inclusions for improving the machinability. By finely dispersing them at a high density, the machinability is remarkably improved. In particular, in the case of a machining method like longitudinal turning which proceeds while forming peaks called "feed marks" at the finished surface, the presence of surface relief has a great effect on the height of the peaks, that is, the finished surface roughness, but sulfides mainly comprised of MnS dispersed finely at a high density make the steel material uniform and thereby can improve the breaking characteristics of the steel material, reduce the surface relief, and improve the finished surface roughness. This is more effective for improvement of the finished surface roughness of parts such as shafts of office automation equipment machined by longitudinal turning. To obtain this effect, a density of 10000/mm² or more is necessary. The dimensions have to be a circle equivalent diameter of 0.1 to 0.5 µm. Usually, the distribution of sulfides mainly comprised of MnS is observed under an optical microscope to measure the dimensions and density. Sulfides mainly comprised of MnS of these dimensions cannot be confirmed by observation by an optical microscope and can only first be observed by a transmission electron microscope (TEM). Sulfides mainly comprised of MnS are of dimensions where even if there is no difference in dimensions and density in observation under an optical microscope, clear differences are observed by observation under a TEM. In the present invention, this is controlled and the state of presence is converted into a numerical value so as to differentiate the invention from the prior art. To ensure the presence of sulfides mainly comprised of MnS over these dimensions by a density of 10000/mm² or more, addition of a large amount of S over the claims is considered necessary, but if adding this in a large amount, the probability rises of coarse sulfides mainly comprised of MnS also ending up present in large numbers and defects occuring more at the time of hot rolling. With the amount of addition of S of the claims, if sulfides mainly comprised of MnS exceed these dimensions, the amount of sulfides mainly comprised of MnS will become insufficient and the density required for improving the finished surface roughness will no longer be able to be maintained. Further, sulfides of less than the minimum diameter of 0.1 µm do not substantially affect the machinability. Therefore, the density of sulfides mainly comprised of MnS having a circle equivalent diameter of 0.1 to 0.5 µm was made 10000/mm². The sulfides mainly comprised of MnS form nuclei for precipitation of the Bn which is hard to make uniformly finely disperse in the matrix, whereby the BN can be made to uniformly finely disperse and the effect of improvement of the machinability, in particular the finished surface roughness, by BN can be made more remarkable.
Note that the "sulfides mainly comprised of MnS" include not only pure MnS, but also include inclusions of sulfides of Fe, Ca, Ti, Zr, Mg, REM, etc. solid solute with MnS or bonded together for copresence, inclusions such as MnTe where elements other than S form compounds with Mn to become solid solute or bond with MnS for copresence, the above inclusions precipitated with oxides as their nuclei, that is, inclusions able to be expressed by the chemical formula (Mn,X)(S,Y) (where X: sulfide forming elements other than Mn and Y: elements bonding with Mn other than S). This is the general term for Mn sulfide-based inclusions.
To obtain dimensions and a density of sulfides mainly comprised of MnS, it is more effective if the ratio Mn/S of the Mn and S contained is made 1.2 to 2.8.
Furthermore, to effectively produce fine sulfides mainly comprised of MnS, it is sufficient to control the range of the solidification and cooling rate. If the cooling rate is less than 10°C/min, the solidification becomes too slow and the precipitated sulfides mainly comprised of MnS end up becoming coarser and fine dispersion becomes difficult, while if the cooling rate is more than 100°C/min, the density of the produced fine sulfides mainly comprised of MnS becomes saturated, the hardness of the steel slab rises, and the danger of cracking increases. Therefore, the cooling rate at the time of casting should be 10 to 100°C/min. This cooling rate can be easily obtained by controlling the size of the casting mold cross-section, the casting speed, etc. to suitable values. This can be applied to both continuous casting and ingot making.
The "solidification and cooling rate" referred to here, as shown in FIG. 8, means the speed at the time of cooling from the liquidus temperature to the solidus temperature at the depth position 18 (see FIG. 8(b)) of 1/4 the thickness (L) of the cast slab in the horizontal cross-section 17 of the cast slab 16 produced by the casting direction 15 shown by the arrow. The cooling rate is found from the distance between the secondary dendrite arms of the solidified structure in the thickness direction of the cast slab after solidification by calculation by the following formula: $Rc = {(\frac{λ 2}{770})}^{- \frac{1}{0.41}}$

where, Rc: cooling rate (°C/min), λ2: distance between secondary dendrite arms 2 (µm)
That is, the distance between secondary dendrite arms changes according to the cooling conditions, so this was measured to confirm the controlled cooling rate.
Next, the reasons for defining the freely added optional elements will be explained.

[Steel Strengthening Elements]

[V] 0.05 to 1.0%

V forms carbonitrides which can strengthen the steel by secondary precipitation hardening. If less than 0.05%, there is no effect on strengthening, while if added over 1.0%, a large amount of carbonitrides precipitate and conversely the mechanical properties are impaired, so this was made the upper limit.

[Nb] 0.005 to 0.2%

Nb also forms carbonitrides which can strengthen the steel by secondary precipitation hardening. If less than 0.005%, there is no effect on strengthening, while if added over 0.2%, a large amount of carbonitrides precipitate and conversely the mechanical properties are impaired, so this was made the upper limit.

[Cr] 0.01 to 2.0%

Cr is an element improving the hardenability and imparting resistance to tempering softening. Therefore, it is added to steel requiring higher strength. In that case, addition of 0.01% or more is required. However, if adding a large amount, Cr carbides form and cause embrittlement, so 2.0% was made the upper limit.

[Mo] 0.05 to 1.0%

Mo is an element imparting resistance to tempering softening and improving the hardenability. If less than 0.05%, the effect is not recognized, while even if added over 1.0%, the effect becomes saturated, so 0.05% to 1.0% was made the range of addition.

[W] 0.05 to 1.0%

W forms carbonitrides which can strengthen the steel by secondary precipitation hardening. If less than 0.05%, there is no effect on strengthening, while if added over 1.0%, a large amount of carbonitrides precipitate and conversely the mechanical properties are impaired, so this was made the upper limit.

[Ni] 0.05 to 2.0%

Ni strengthens the ferrite, improves the ductility, and is also effective for improving the hardenability and improving the corrosion resistance. If less than 0.05%, that effect is not recognized, while even if added over 2.0%, the effect becomes saturated in terms of the mechanical properties, so this was made the upper limit.

[Cu] 0.01 to 2.0%

Cu strengthens the ferrite and is effective for improving the hardenability and improving the corrosion resistance. If less than 0.01%, the effect is not recognized, while even if added over 2.0%, the effect becomes saturated in respect to the mechanical properties, so this was made the upper limit. In particular, the hot ductility is reduced. This easily becomes a cause of defects at the time of rolling. Therefore, addition simultaneously with Ni is preferable.

[Machinability Improving Elements Using Embrittlement]

[Sn] 0.005 to 2.0%

Sn makes the ferrite brittle, extends tool life, and improves the surface roughness as an effect. If less than 0.005%, this effect is not recognized, while even if added over 2.0%, the effect becomes saturated, so this was made the upper limit.

[Zn] 0.0005 to 0.5%

Zn makes the ferrite brittle, extends tool life, and improves the surface roughness as an effect. If less than 0.0005%, this effect is not recognized, while even if added over 0.5%, the effect becomes saturated, so this was made the upper limit.

[Machinability Improving Elements Using Adjustment of Deoxidation]

[Ti] 0.0005 to 0.1%

Ti is a deoxidizing element which can control the amount of oxygen in the steel and can stabilize the yields of the easily oxide forming Mn and B. Further, if slight in amount, it forms soft oxides and acts to improve the machinability. If less than 0.0005%, this effect is nonexistent, while if over 0.1%, a large amount of hard oxides are formed and the Ti becoming solid solute without forming oxides bonds with N to form hard TiN which lowers the machinability. Therefore, the range of the ingredient was made 0.0005 to 0.1%. Ti forms TiN and thereby consumes the N required for forming BN. Therefore, the amount of addition of Ti is preferably 0.01% or less.

[Zr] 0.0005 to 0.1%

Zr is a deoxidizing element which can control the amount of oxygen in the steel and can stabilize the yields of the easily oxide forming Mn and B. Further, if slight in amount, it forms soft oxides and acts to improve the machinability. If less than 0.0005%, this effect is nonexistent, while if over 0.1%, a large amount of soft oxides are formed and deposit on the tool cutting edges as relief shapes, so the finished surface roughness becomes extremely bad. Not only this, but also a large amount of hard oxides are produced. Furthermore, the machinability is lowered. Therefore, the range of the ingredient was defined as 0.0005 to 0.1%.

[Mg] 0.0003 to 0.005%

Mg is a deoxidizing element which can control the amount of oxygen in the steel. It can stabilize the yields of easily oxide forming Mn and B. Further, if slight in amount, it forms soft oxides and acts to improve the machinability. If less than 0.0003%, this effect is nonexistent, while if over 0.005%, a large amount of soft oxides are formed and deposit on the tool cutting edges as relief shapes, so the finished surface roughness becomes extremely bad. Not only this, but also a large amount of hard oxides are produced. Furthermore, the machinability is lowered. Therefore, the range of the ingredient was defined as 0.0003 to 0.005%.

[Machinability Improving Elements Using Control of Sulfide Form and Lubrication Between Tool and Steel Material]

[Te] Te: 0.0003 to 0.2%

Te is a machinability improving element. Further, it forms MnTe and, by copresence with MnS, lowers the deformability of MnS to control the flattening of the MnS shapes. Therefore, this element is effective for reducing anisotropy. This effect is not recognized if less than 0.0003%, while even if added over 0.2%, not only does the effect become saturated, but also the hot ductility falls and defects are easily caused.

[Bi] 0.005 to 0.5%

Bi is a machinability improving element. Its effect is not recognized if less than 0.005%, while even if added over 0.5%, not only does the effect of improvement of the machinability become saturated, but also the hot ductility falls and defects are easily caused.

[Pb] 0.005 to 0.5%

Pb is a machinability improving element. Its effect is not recognized if less than 0.005%, while even if added over 0.5%, not only does the effect of improvement of the machinability become saturated, but also the hot ductility falls and defects are easily caused.

EXAMPLES

The effects of the present invention will be explained next using examples. Steels of the invention examples of Examples 1 to 72 shown in Tables 1 to 4 were produced in a 270t converter, then cast by a solidification and cooling rate of 4 to 18°C/min. The casting was classified so that, among these, the solidification and cooling rates of the steel types of claim 1 of Examples 1 to 8 and the steel types of claim 6 of Examples 62 to 72 were 1 to 7°C/min, while the solidification and cooling rates of the steel types of claims 2 to 6 of Examples 9 to 61 were 12 to 85°C/min. The steels of the comparative examples of Examples 73 to 102 shown in Tables 5 to 6 were produced in a 270t converter, then cast by a solidification and cooling rate of 4 to 7°C/min. In both the invention examples and the comparative examples, the 270t converter material was bloomed to a billet, then rolled to φ9.5. This φ9.5 mm rolled material was drawn to φ8 mm and used as the test material. For evaluation of the hot ductility, before the rolling, test pieces were taken from the billet and a 180 mm square cast material. Further, the solidification and cooling rate were adjusted by control of the size of the casting mold cross-section and casting speed.
The machinability of the material was evaluated by three typical types of machining methods of a drilling test showing the conditions in Table 7, a plunge cutting test showing the conditions in Table 8, and a longitudinal turning test showing the conditions in Table 9. The drilling test is the method of evaluating the machinability by the highest cutting speed enabling machining up to a cumulative hole depth of 1000 mm, (so-called VL1000, unit: m/min). The plunge cutting test is the method of evaluating the finished surface roughness by transferring the tool shape by a piercing tool of high speed steel (builtup cutting edge shape). A summary of this test method is shown in FIG. 1. In the test, the finished surface roughness when cutting 200 grooves was measured by a contact type roughness meter. This was used as an indicator showing the finished surface roughness of the 10 point surface roughness Rz (unit: µm). The longitudinal turning test is a machining method cutting into the outer circumference of the steel material of the test piece 2 in the machining direction 3 while feeding the carbide tool 1 in the longitudinal direction. In the same way as plunge cutting, this method repeatedly measures and evaluates the finished surface roughness of the measurement surface 4 of surface roughness in transfer of the tool shape. A summary of this test method is shown in FIG. 2. This method performs the test while rotating the test piece 2, feeding the carbide tool 1 along the test piece 2 (0.05 mm/rev), and machining by a predetermined depth of cut 6 (1 mm). It is advanced while forming peaks called "feed marks 5" on the finished surface 7 to form a surface roughness measurement plane 8. The presence of any deterioration 9 of the relief shapes forms peak heights which becomes the roughness of the surface relief surface (theoretical roughness+surface relief) 10. That is, this becomes the finished surface roughness and has a great effect on the good surface roughness (theoretical roughness) 11 (see FIG. 2(b)). If there is no surface relief, the value becomes close to the theoretical roughness, but if surface relief occurs, the roughness is degraded by that amount. Sulfides mainly comprised of MnS finely dispersed at a high density make the steel material uniform and thereby reduce the surface relief and enable a good finished surface roughness, so it is possible to express the effect of the sulfides mainly comprised of MnS dispersed at a high density remarkably well. Further, this method can express the quality of the finished surface roughness resulting from the transfer of tool surface relief due to tool wear after a large amount of machining remarkably well, so in this test, the evaluation was performed using the finished surface roughness after machining 800 pieces - which enables evaluation of the difference of machinability in the state where tool wear has progressed. The finished surface roughness was measured by a contact type roughness meter. The 10-point surface roughness Rz (unit: µm) was used as an indicator showing the finished surface roughness. For chip evacuation, examples where the radius at the time of chip curling is small or examples where the chips break off are preferable and were evaluated as "G (good)". Examples where the number of curls is large and the radius of curvature is small or examples where the chip lengths do not reach 100 mm are good and were evaluated as "G". Chips with a radius of curvature of over 20 mm, curling continuously by three curls or more, and extending long are poor and were evaluated as "P".
For the MnO in the steel material, the area rate of MnO of a circle equivalent diameter of 0.5 µm or more in the cross-section perpendicular to the rolling direction of the steel material was measured by an electron probe microanalyzer (EPMA) using a test piece cut out from a depth position of 1/4 of the diameter of the cross-section perpendicular to the rolling and drawing direction after φ8 mm drawing, buried in resin, and polished. The measurement was performed for 20 fields or more each of 200 µm×200 µm. The area rate was found using the area of MnO in the inclusions measured by the elemental area analysis as a ratio with respect to the area of the total Mn-based inclusions. The MnO in the steel material is present in a state contained in MnS, so in analysis by EPMA, the area where Mn and O overlap is deemed the area of MnO as differentiated from MnS. The Mn and O were overlaid by image processing. An example of measurement by EPMA is shown in FIG. 3.
The density of sulfides mainly comprised of MnS of dimensions of a circle equivalent diameter of a maximum diameter of 0.5 µm and a minimum diameter of 0.1 µm was measured by a transmission electron microscope using a test piece obtained by the extract replica method from a position of a depth of 1/4 the diameter of the cross-section perpendicular to the rolling and drawing direction after φ8 mm drawing. The measurement was performed at 10000 power for 40 fields or more, each field of 80 µm². The result was converted to the number of sulfides mainly comprised of MnS per mm².
The hot ductility was evaluated by the value of the reduction rate in a high temperature tensile test at 1000°C. If the reduction rate is 50% or more, good rolling is possible, but if less than 80%, numerous surface defects are formed, the area for removal of defects and touchup after rolling becomes greater, and use is not possible for high grade products with severe demands on surface properties. If a value of the reduction rate of 80% or more can be obtained, the formation of surface defects is remarkably reduced, use even without touchup becomes possible, and use for high grade products becomes possible. Furthermore, the touchup costs can also be slashed. Therefore, a reduction rate of 80% or more was evaluated as a "G (good)" hot ductility, while one of less than 80% was evaluated as "P (poor)".
The state of melt loss of the plate refractories of continuous casting sliding nozzles was evaluated using MgO-C (MgO=87%, Al₂O₃=10%, C=3%) as the material of the sliding nozzle plates. The melt loss rate is a value indexing the melt loss rates to the melt loss rate of refractories when the area of MnO of 0.5 µm or more size constitutes 15% of the total area of Mn-based inclusions as "1". If the melt loss rate exceeds 1, the melt loss of the refractories becomes worse, so a melt loss rate of 1 or less was evaluated as "G (good)" and one over 1 was evaluated as "P (poor)". The invention examples of Examples 1 to 72 were all better than the comparative examples of Examples 73 to 102 in drill tool life and finished surface roughness in plunge cutting and longitudinal turning, had a hot ductility of a value of 80% or more, and enabled good manufacturability with a low melt loss rate. For example, it was possible to control the amount of N by balanced amounts of addition of B and N like in the invention examples of Examples 1 to 8 and possible to obtain a high value of hot ductility and a low melt loss rate without deterioration of the machinability when the MnO area rate is low by control of the amount of O by addition of Ca. Further, it was possible to obtain an extremely good machinability by balanced amounts of addition of B and N and a low MnO area rate. When the density of fine sulfides mainly comprised of MnS satisfies claim 2 like in Examples 9 to 18 and 56 to 59, the value of the finished surface roughness, in particular the value at the time of longitudinal turning, becomes even better. Even in the examples of addition of the freely added optional elements of claims 3 to 6 of Examples 19 to 55 and 60 to 72, it is learned that a good finished surface roughness and manufacturability are obtained. Among these, in Examples 47, 52, 60, and 62 to 67 to which a slight amount of Pb, known as a free cutting element, is added, in Examples 45, 48, 50, 53, 61, 68, and 69 to which a slight amount of Te, also known as a free cutting element, is added, and furthermore in Examples 55 and 70 to 72 to which both Pb and Te are added, it is learned that good hot ductility and machinability are obtained.
As opposed to this, the comparative examples were all cast by a slow solidification cooling rate, so the density of fine sulfides mainly comprised of MnS becomes smaller and, overall, poor values of machinability, in particular the finished surface roughness by longitudinal turning, are shown. Compared with the invention examples of claim 1 of Examples 1 to 8 produced by the same level of small solidification and cooling rate, poor values are exhibited since the chemical ingredients are outside the ranges of the present invention. For example, when the area rate of MnO is high like in the comparative example of Example 76, the reduction in the amount of MnS and the amount of BN results in a poor value of finished surface roughness. The melt loss rate becomes a large value. In the comparative example of Example 80, the MnO area rate of 15% or less is satisfied, but the amounts of S and Ca are outside the invention ranges, so the hot ductility becomes a poor value. When Ca is not added like in the comparative example of Example 81, the O cannot be control and the large numbers of MnO and hard oxides formed result in poor manufacturability of a hot ductility of less than 80% and a large value of melt loss rate. Furthermore, Examples 90 and 91 are comparative examples with amounts of N below the lower limit. The increase of solid-solute B invites an increase in hardness and a low value of hot ductility is exhibited. Further, Example 93 is a comparative example with amounts of S and N above the upper limits. Due to the increase in solid-solute N, a poor value of hot ductility is exhibited. Example 102 is a comparative example with a high MnO. Poor values of both the finished surface roughness and melt loss index are exhibited..
FIG. 4 gives an (a) TEM replica photograph and (b) optical micrograph of sulfides mainly comprised of MnS of an example of the present invention. FIG. 5 gives an (a) TEM replica photograph and (b) optical micrograph of sulfides mainly comprised of MnS of a comparative example of the present invention. In this way, in the invention examples and the comparative examples, with (b) observation by an optical microscope, there is no large difference in the dimensions and density of the sulfides mainly comprised of MnS, but with (a) observation by a TEM replica, clear differences are seen in both the dimensions and density.
FIG. 6 shows changes in machinability due to the MnO area rate using as an example the finished surface roughness by longitudinal turning after machining 800 pieces. Tool wear remarkably progresses at the time of a large amount of machining when the MnO area rate is greater than 15%, so the difference in finished surface roughness, which is governed by the transfer of surface relief due to tool wear, appears remarkably at this as the borderline.
FIG. 7 is a view showing a balance of finished surface roughness by longitudinal turning and hot ductility in invention examples and comparative examples. The invention examples are good in finished surface roughness and have a hot ductility of a good region cf 80% or more. In the comparative examples, the finished surface roughness and the hot ductility are both in the poor range or even if the hot ductility is good, the finished surface roughness is poor.

Due to this, it is learned that the invention examples, which are balanced in amount of B and amount of N and where the amount of MnO can be controlled, the manufacturability and machinability are both good.

Table 7

Cutting conditions	Drill	Others
Cutting speed: 10 to 200	φ3 mm	Hole depth: 9 mm
m/min	NACHI general	Tool life: until
Feed: 0.25 mm/rev Non-water soluble cutting fluid	drill	breakage

Table 8

Cutting conditions	Tool	Others
Cutting speed: 80 m/min	Corresponding	Evaluation
Feed: 0.05 mm/rev	to SKH51	timing: 200th
Lubrication: Non-water	Rake angle	15°	groove
soluble cutting fluid	Relief angle	6°

Table 9

Cutting conditions	Tool	Others
Cutting speed: 80 m/min	Corresponding	Evaluation
Feed: 0.05 mm/rev	to carbide tool	timing: 800th
Depth of cut: 1 mm	type P10	piece
Lubrication: Water-	rake angle 10°
soluble cutting fluid	relief angle	7°

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide machining steel superior in tool life at the time of machining, finished surface roughness, chip evacuation, and other machinability, accompanied with little melt loss of plate refractories of the continuous casting sliding nozzles, and superior in manufacturability with good ductility in hot rolling.

Claims

Machining steel superior in manufacturability containing, by mass%,
C: 0.005 to 0.2%
Si: 0.001 to 0.5%
Mn: 0.3 to 3.0%
P: 0.001 to 0.2%
S: 0.30 to 0.60%
B: 0.0003 to 0.015%
O: 0.005 to 0.012%
Ca: 0.0001 to 0.0010%, and
Al≤0.01%, optionally one or more of
V: 0.05 to 1.0%
Nb: 0.005 to 0.2%
Cr: 0.01 to 2.0%
Mo: 0.05 to 1.0%
W: 0.05 to 1.0%
Ni: 0.05 to 2.0%
Cu: 0.01 to 2.0%
Sn: 0.005 to 2.0%
Zn: 0.0005 to 0.5%
Ti: 0.0005 to 0.1%
Zr: 0.0005 to 0.1%
Mg: 0.0003 to 0.005%
Te: 0.0003 to 0.2%
Bi: 0.005 to 0.5%
Pb: 0.005 to 0.5%, and
having an N content satisfying
N≥0.0020% and 1.3×B-0.0100≤N≤1.3×B+0.0039, and
having a balance of Fe and unavoidable impurities, wherein
further, regarding the MnO in the steel, in a cross-section of the steel material perpendicular to the rolling direction, the area of MnO of a circle equivalent diameter of 0.5 µm or more being 15% or less of the area of the total Mn-based inclusions.
Machining steel superior in manufacturability as set forth in claim 1, wherein, regarding the sulfides mainly comprised of MnS, in a cross-section of the steel material perpendicular to the rolling direction, a density of sulfides of a circle equivalent diameter of 0.1 to 0.5 µm is 10000/mm² or more.