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(11) | EP 1 029 938 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Rolled steel having few inclusion defects |
| (57) The present invention provides rolled steel having few inclusion defects suitable
for steel sheets used for automobiles, steel sheets used for deeply drawn cans and
steel pipes, the basic composition of which is C: 0.0002 to 0.7 mass %, Si: 0.001
to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05 mass %, S: 0.0005 to 0.15
mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001 to 0.1 mass %, the selective
composition of which is Ca: not more than 50 ppm and Mg: not more than 50 ppm, at
least one of Ca and Mg being contained, wherein created oxide inclusions are mainly
composed of one of the crystallized phase, the principal component of which is titanium
oxide, and the crystallized phase, the principal component of which is alumina, and
further composed of at least one of the crystallized phase, the principal component
of which is CaO, and the crystallized phase, the principal component of which is MgO,
and the crystallized phases of oxide inclusions exist in steel. |
[0001] The present invention relates to rolled steel having few inclusion defects suitable for producing steel sheets for automobile use, steel sheets for making deeply drawn cans, and steel pipes.
[0002] In general, pieces of rolled steel such as steel sheets and steel pipes are made of aluminum killed steel obtained when molten steel made by a converter, which has not been deoxidized yet, is deoxidized by aluminum. After the killed steel has been rolled, surface defects and internal defects such as sliver flaws (linear flaws) caused in the process of cold rolling, cracks and pin holes caused in the case of deep drawing and defects detected in weld zones of steel pipes by the ultrasonic test are caused by inclusions in some cases. It is known that those inclusion defects are caused by the inclusion of oxides, such as alumina, created in the process of deoxidation conducted in molten steel in refining.
[0003] In order to remove the oxide inclusions, the following methods have been conventionally adopted.
(1) A deoxidizing agent such as aluminum is thrown into molten steel in the process of tapping from a converter so that the period of time, in which the oxide inclusions are raised to the surface of molten steel by coagulation and coalescence, can be extended as much as possible.
(2) Rise and separation of oxide inclusions are facilitated when molten steel is forcibly agitated by the treatment of CAS (Composition Adjustment by Sealed Argon Gas Bubbling) or RH which is one of the secondary refining methods.
(3) Alumina is changed into CaO-Al2O3 by adding Ca into molten steel so that it can be easily crushed in the process of rolling, and the alumina becomes harmless.
[0004] However, the following problems may be encountered in the above methods (1) and (2). Effects of the above methods (1) and (2), by which oxide inclusions can be raised to the surface of molten steel so that the inclusions can be separated from molten steel, are limited. Therefore, it is impossible to perfectly prevent the occurrence of sliver flaws, cracks, pin holes and UST defects. Further, the following problems may be encountered in the above method (3) in which oxide inclusions are reformed by Ca. Material of Ca is expensive, and the yield is very low. Accordingly, the cost of alloy is raised. Further, particles of CaO-Al2O3, which are created when Ca is added into molten steel, are enlarged, and the thus created particles of CaO-Al2O3 can not be raised to the surface of molten steel, that is, the thus created particles of CaO-Al2O3 remain in molten steel. In this case, defects are caused by the particles of CaO-Al2O3.
SUMMARY OF THE INVENTION
[0005] The present invention has been accomplished to solve the above conventional problems. It is an object of the present invention to provide rolled steel having few inclusion defects in which particles of oxide inclusions are kept fine and capable of being dispersed in rolled steel.
[0006] In order to solve the above problems, the present invention provides rolled steel having few inclusion defects, the basic composition of which is C: 0.0002 to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05 mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001 to 0.1 mass %, wherein created oxide inclusions are mainly composed of a crystallized phase, the principal component of which is titanium-oxide, and a crystallized phase, the principal component of which is alumina, and the crystallized phases of oxide inclusions exist in steel.
[0007] In the same manner, in order to solve the above problems, the present invention provides rolled steel having few inclusion defects, the basic composition of which is C: 0.0002 to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05 mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001 to 0.1 mass %, the selective composition of which is Ca: not more than 50 ppm and Mg: not more than 50 ppm, at least one of Ca and Mg being contained, wherein created oxide inclusions are mainly composed of a crystallized phase, the principal component of which is titanium-oxide, and a crystallized phase, the principal component of which is alumina, and further composed of at least one of a crystallized phase, the principal component of which is CaO, and a crystallized phase, the principal component of which is MgO, and the crystallized phases of oxide inclusions exist in steel.
[0008] It is preferable that the crystallized phases of oxide inclusions are dispersed in rows in the direction of rolling near the center of a piece of rolled steel. It is preferable that Micro-Vickers hardness of the oxide inclusions at the room temperature is in a range from 600 to 1300 Hv. Further, it is preferable that the maximum diameter of the particles of oxide inclusions obtained by slime extraction is not more than 300 µm. Furthermore, it is preferable that the number of the particles of oxide inclusions obtained by slime extraction, the diameter of which is not less than 38 µm, is not more than 50 pieces/kg.
[0010] In the present invention, rolled steel includes steel sheets, steel pipes, shape steel, bar steel and wire rods. The basic composition of the rolled steel is C: 0.0002 to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05 mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001 to 0.1 mass %.
[0011] In the present invention, rolled steel includes steel sheets, steel pipes, shape steel, bar steel and wire rods. The basic composition of the rolled steel is C: 0.0002 to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05 mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001 to 0.1 mass %, and the selective composition of the rolled steel is Ca: not more than 50 ppm and Mg: not more than 50 ppm, wherein at least one of Ca and Mg is contained.
[0012] Carbon is an essential element to stably enhance the mechanical strength of steel. Therefore, the content of carbon is adjusted in a range from 0.0002 to 0.7% according to the desired mechanical strength of material. In order to ensure the mechanical strength or hardness, it is necessary that rolled steel contains carbon at not less than 0.0002%, however, when the content of carbon is higher than 0.7%, the workability is lowered. Therefore, the content of carbon is kept so that it cannot exceed 0.7%.
[0013] The reasons why the contents of other components are kept in the above ranges are described as follows.
[0014] The reason why the content of Si is kept in a range from 0.001 to 0.5% is described below. When the content of Si is in a range lower than 0.001%, it becomes necessary to conduct pretreatment of material, and the cost of refining is increased, that is, it is not economical to keep the content of Si in a range lower than 0.001%. When the content of Si is higher than 0.5%, defects are caused in the process of plating, and the surface property and the corrosion resistance are impaired.
[0015] The reason why the content of Mn is kept in a range from 0.005 to 2.0% is described below. When the content of Mn is lower than 0.005%, the refining time is extended, which is not economical. When the content of Mn exceeds 2.0%, the workability at steel is greatly impaired.
[0016] The reason why the content of P is kept in a range from 0.001 to 0.05% is described below. In order to keep the content of P lower than 0.001%, it takes time to treat molten pig iron and the cost is raised, which is not economical. When the content of P exceeds 0.05%, the workability of steel is greatly impaired.
[0017] The reason why the content of S is kept in a range from 0.0005 to 0.15% is described below. In order to keep the content of S lower than 0.0005%, it takes time to treat molten pig iron and the cost is raised, which is not economical. When the content of S exceeds 0.15%, the workability and the corrosion resistance of steel are greatly impaired.
[0018] The reason why the content of Ti is kept in a range from 0.001 to 0.25% is described below. When the content of Ti is lower than 0.001%, it becomes difficult to cast molten steel. When the content of Ti is higher than 0.25%, only titanium oxide, which tends to become clusters, is created, and the diameters of inclusion particles are enlarged. As a result, sliver flaws are caused in the same manner as that of alumina.
[0019] The reason why the content of dissolved Al (sol Al) is kept in a range from 0.001 to 0.1% is described below. When the content of dissolved Al is lower than 0.001%, it impossible to conduct a sufficient deoxidation treatment. When the content of dissolved Al exceeds 0.1%, only alumina is created, and surface defects and internal defects are caused.
[0020] Both Ca and Mg form "crystallized phases", the principal component of which is oxide, in the oxide inclusions.
(1) Therefore, they contribute to make the crystallized phase itself fine.
(2) Also, they contribute to crushing the inclusion so as to make the inclusion fine along an interface of the fine crystallized phase in the process of rolling. The reason why at least one of Ca, the content of which is kept lower than 50 ppm, and Mg, the content of which is kept lower than 50 ppm, is contained is described as follows. Since the vapor pressure of Ca and that of Mg are high and the yield of Ca and that of Mg are low, the cost is raised when the contents of Ca and Mg are increased to a value higher than 50 ppm. The reason why the lower limits of Ca and Mg are not stated plainly is described as follows. Even when the concentration of Ca and that of Mg are lower than the lower limit of analysis in the composition analysis of steel, it is possible to make the inclusions contain at least one of CaO and MgO sufficiently.
[0021] The present invention provides rolled steel, the basic composition of which is described above, and the oxide inclusions created in the processes of deoxidation and coagulation are mainly composed of a crystallized phase, the principal component of which is Ti oxide, and a crystallized phase, the principal component of which is alumina, and the crystallized phases of oxide inclusions are dispersed in steel.
[0022] The present invention provides rolled steel, the basic composition and the selective composition of which are described above, and the oxide inclusions created in the processes of deoxidation and coagulation are mainly composed of a crystallized phase, the principal component of which is Ti oxide, and a crystallized phase, the principal component of which is alumina, and also composed of at least one of a crystallized phase, the principal component of which is CaO, and a crystallized phase, the principal component of which is MgO, and the crystallized phases of oxide inclusions are dispersed in steel. In this case, the crystallized phase is a crystal phase in a solid state, that is, the crystallized phase does not include a glass phase in a solid state. That is, when there is provided a crystallized phase composed of at least two phases of the crystallized phase, the principal component of which is Ti oxide, and the crystallized phase, the principal component of which is alumina, or alternatively when there is provided a crystallized phase composed of at least three phases of the crystallized phase, the principal component of which is Ti oxide, the crystallized phase, the principal component of which is alumina, and at least one of the crystallized phase, the principal component of which is CaO, and the crystallized phase, the principal component of which is MgO, the crystallized phase itself is made to be fine, and further the crystallized phase is easily crushed to more fine particles. As a result, the occurrence of flaws such as sliver flaws can be prevented, and rolled steel having few inclusion defects can be obtained.
[0023] It is preferable that the crystallized phases of oxide inclusions are dispersed in rows in the direction of rolling near the center with respect to the thickness of a piece of rolled steel. Since the oxide inclusions seldom exist on the surface of the piece of rolled steel, it is possible to obtain rolled steel having few inclusion defects.
[0024] When consideration is given to the deformability of steel in the process of rolling conducted after the completion of hot rolling, it is preferable that Micro-Vickers hardness of oxide inclusions at the room temperature is in a range from 600 to 1300 Hv. The reason why the hardness is kept in the above range is described as follows. When the hardness is lower than 600 Hv, the inclusions are excessively elongated. When the hardness is higher than 1300 Hv, the inclusions are seldom elongated, and it becomes difficult to crush and disperse the inclusions by rolling.
[0025] When the maximum diameter of the particles of oxide inclusions obtained by slime extraction is not larger than 300 µm and further the number of the particles of oxide inclusions, the diameters of which are not less than 38 µm, is kept to be not more than 50 pieces/kg, there is little possibility that the particles of oxide inclusions on the surface of rolled steel are drawn out in rows, and it is possible to obtain rolled steel having few inclusion defects.
[0026] As described above, the present invention provides rolled steel, the characteristics of which are described as follows. Oxide inclusions created in the processes of deoxidation and coagulation are mainly composed of a crystallized phase, the principal component of which is Ti oxide, and a crystallized phase, the principal component of which is alumina. When the crystallized phases concerned are dispersed in rolled steel, oxide inclusions are made to be oxides composed of the two phases of the crystallized phase, the principal component of which is Ti oxide, and the crystallized phase, the principal component of which is alumina, and the crystallized phases of the oxides are made to be fine. Next, the oxide inclusions are further crushed and dispersed by rolling in rows on an interface of the crystallized phase, the particles of which are made to be fine. In this way, when the inclusion is made to be inclusion of the crystallized phase, the principal component of which is fine particles of Ti oxide, and/or the crystallized phase, the principal component of which is alumina, the product defects, which are caused by oxide inclusions, such as sliver flaws in the process of cold rolling, cracks, pin holes and defects detected in the process of UST, can be greatly reduced.
[0027] Further, the present invention provides rolled steel, the characteristics of which are described as follows. Oxide inclusions created in the processes of deoxidation and coagulation are mainly composed of a crystallized phase, the principal component of which is Ti oxide, and a crystallized phase, the principal component of which is alumina. Further, oxide inclusions created in the processes of deoxidation and coagulation are mainly composed of at least one of the crystallized phase, the principal component of which is CaO, and the crystallized phase, the principal component of which is MgO. When the crystallized phases concerned are dispersed in rolled steel, oxide inclusions are made to be oxides composed of at least three phases of the crystallized phase, the principal component of which is Ti oxide, the crystallized phase, the principal component of which is alumina, and at least one of the crystallized phase, the principal component of which is CaO, and the crystallized phase, the principal component of which is MgO. Next, the oxide inclusions are further crushed and dispersed by rolling in rows on the interface of the crystallized phase, the particles of which are made to be fine. In this way, when the inclusions are made to be inclusions of the crystallized phase, the principal component of which is fine particles of Ti oxide, and/or the crystallized phase, the principal component of which is alumina, and also when the inclusions are made to be inclusions of at least one of the crystallized phase, the principal component of which is CaO, and the crystallized phase, the principal component of which is MgO, the product defects, which are caused by oxide inclusions, such as sliver flaws in the process of cold rolling, cracks, pin holes and defects detected in the process of UST, can be greatly reduced.
Example 1
[0028] Pieces of rolled steel were produced by a vertical bend-type continuous casting machine under the condition that the slab size was 245 mm thickness × 1200 to 1600 mm width, the casting speed was 1.4 to 1.7 m/min, and the temperature of molten steel in the tundish was 1560°C. After that, the slabs were hot-rolled, and then the pieces of hot-rolled steel were subjected to acid pickling, cold rolling, annealing and secondary cold rolling when necessary. In this way, products shown on Table 1 were produced.
[0029] Deoxidizing alloy used in the production process and the principal components contained in the crystallized phase of oxide inclusions are shown in Table 2. The hardness of oxide inclusions, the existing formation and the ratio of occurrence of defects are shown in Table 3. It can be seen from these tables that the present invention can greatly reduce the defects of products caused by oxide inclusions so that the productivity can be enhanced.
[0030] The components of the crystallized phase of inclusions shown in Table 2 were identified in such a manner that the inclusions extracted from a piece of rolled steel of full thickness by means of slime electrolytic extraction (the minimum mesh was 38 µm) was subjected to component identification by SEM (Scanning Electron Microscope) having EDX (Energy Dispersive X-ray Spectrometer). Further, concerning the additional component detected in the above component identification, the content was found by the integral intensity of the peak of the characteristic X-rays.
[0031] The existing formation of inclusion, which is shown in Table 2, on the section in the rolling direction was determined by the profile of the product as follows.
[0032] In the case of a sheet, the full thickness of a section parallel to the rolling direction was observed by an optical microscope, and the existing inclusion formation was determined by an optical microscopic photograph (the magnification was 400 and the total number of field of view was 50) which was taken at a position where the inclusion exists.
[0033] In the case of a wire, the full thickness of a section parallel to the drawing direction (the rolling direction) was observed by an optical microscope, and the existing inclusion formation was determined by an optical microscopic photograph (the magnification was 400 and the total number of field of view was 50) which was taken at a position where the inclusion exists.
[0034] In the cases of a pipe and rod, local positions, which were located below the front or the rear surface by 0.1 mm, 1/8t, 1/4t, 3/8t, 1/2t, 5/8t, 3/4t and 7/8t wherein t is thickness, were observed by an optical microscope, and the existing inclusion formation was determined by an optical microscopic photograph (the magnification was 400 and the total number of field of view was 50 for each local position) which was taken at a position where the inclusion exists.
[0035] In this connection, meanings of *1 to *9 shown in Tables 2 and 3 are described as follows.
*1: Level of dissolved oxygen, A: Not less than 400 ppm, B: Not less than 200 and lower than 400 ppm, C: Not less than 100 and lower than 200 ppm, and D: Lower than 100 ppm
*2: Principal component in the crystallized phase is controlled by a quantity of alloy added in the process of deoxidation.
*3: MnO and SiO2 are contained by not more than 10 weight % as additional components in the crystallized phase.
*4: TiOx is contained by not more than 5 weight % as an additional component in the crystallized phase.
*5: Al2O3 is contained by not more than 5 weight % as an additional component in the crystallized phase.
*6: An average is calculated at the room temperature for 10 particles of inclusion when a load of 25 g is given to each of three positions with respect to one type of inclusion.
*7, *8: The maximum diameter of the inclusion particles and the number of the inclusion
particles are controlled by dissolved oxygen before deoxidation.
The method of measuring the maximum diameter of the inclusion particles is described
below. Inclusions, which were extracted by means of slime electrolytic extraction
(the minimum mesh was 38 µm) from a piece of rolled steel of full thickness of the
weight of 1 ± 0.1 kg, were photographed by a stereoscopic microscope, the magnification
of which was 40, and the averages of the major and the minor axis of the inclusion
particles on the photograph were found with respect to all the inclusion particles,
and the maximum value of the thus found averages was determined to be the maximum
diameter of the inclusion particles. The number of the inclusion particles was found
as follows. The number of all the inclusion particles, which was extracted by means
of slime electrolytic extraction (the minimum mesh was 38 µm) and observed by an optical
microscope (the magnification was 100) was converted into the number per the unit
of 1 kg.
*9: The ratio of occurrence of defects is determined by the following formulas.
Example 2
[0036] Pieces of rolled steel were produced by a vertical bend-type continuous casting machine under the condition that the slab size was 245 mm thickness × 1200 to 1600 mm width, the casting speed was 1.4 to 1.7 m/min, and the temperature of molten steel in the tundish was 1560°C. After that, the slabs were hot-rolled, and then the pieces of hot-rolled steel were subjected to acid pickling, cold rolling, annealing and secondary cold rolling when necessary. In this way, products shown in Tables 4, 7 and 10 were produced.
[0037] Deoxidizing alloy used in the production process and the principal components contained in the crystallized phase of oxide inclusions are shown in Tables 5, 8, 11 and 12. The hardness of oxide inclusions, the existing formation and the ratio of occurrence of defects are shown in Tables 6, 9 and 13. It can be seen from these tables that the present invention can greatly reduce the defects of products caused by oxide inclusions so that the productivity can be enhanced.
[0038] The components of the crystallized phases of inclusions shown in Tables 5, 8 and 12 were identified in such a manner that the inclusions extracted from pieces of rolled steel of full thickness, the weight of which was 1 ± 0.1 kg, by means of slime electrolytic extraction (the minimum mesh was 38 µm) were identified by SEM having EDX. Further, concerning the detected additional component, the content was found from the integral intensity of the peak of the characteristic X-rays.
[0039] The existing inclusion formations shown in Tables 5, 8 and 12 on the section of the rolling direction were determined by the profiles of products as follows.
[0040] In the case of a sheet, the full thickness of a section parallel to the rolling direction was observed by an optical microscope, and the existing inclusion formation was determined by an optical microscopic photograph (the magnification was 400 and the total number of field of view was 50) which was taken at a position where the inclusion exists.
[0041] In the case of a wire, the full thickness of a section parallel to the drawing direction (the rolling direction) was observed by an optical microscope, and the existing inclusion formation was determined by an optical microscopic photograph (the magnification was 400 and the total number of field of view was 50) which was taken at a position where the inclusion exists.
[0042] In the cases of a pipe and rod, local positions, which were located below the front or the rear surface by 0.1 mm, 1/8t, 1/4t, 3/8t, 1/2t, 5/8t, 3/4t and 7/8t wherein t is thickness, were observed by an optical microscope, and the existing formation of inclusion was determined by an optical microscopic photograph (the magnification was 400 and the total number of field of view was 50 for each local position) which was taken at a position where the inclusion exists.
[0043] In this connection, the meaning of *1 to *11 shown in Tables 4 to 13 are described as follows.
*1: Tr: Not more than the lower limit capable of being analyzed, -: Ca or Mg is not added.
*2: Level of dissolved oxygen
A: Not less than 400 ppm, B: Not less than 200 ppm and lower than 400 ppm, C: Not less than 100 ppm and lower than 200 ppm, D: Lower than 100 ppm
*3: Principal component in the crystallized phase is controlled by a quantity of alloy added in the process of deoxidation.
*4: Not more than 10 weight % of MnO and SiO2 are contained as additional components in the crystallized phase.
*5: Not more than 5 weight % of TiOx is contained as an additional component in the crystallized phase.
*6: Not more than 5 weight % of Al2O3 is contained as an additional component in the crystallized phase.
*7: Not more than 5 weight % of Al2O3 is contained as an additional component in the crystallized phase, and not more than 10 weight % of MnO and SiO2 are also contained as additional components in the crystallized phase.
*8: An average is calculated at the room temperature for 10 particles of inclusion when a load of 25 g is given to each of three positions with respect to one type of inclusion.
*9, *10: The maximum diameter of the inclusion particles and the number of the inclusion
particles are controlled by dissolved oxygen before deoxidation.
The method of measuring the maximum diameter of the inclusion particles is described
below. Inclusions, which were extracted by means of slime electrolytic extraction
(the minimum mesh was 38 µm) from a piece of rolled steel of full thickness of the
weight of 1 ± 0.1 kg, were photographed by a stereoscopic microscope, the magnification
of which was 40, and the averages of the major and the minor axis of the inclusion
particles on the photograph were found with respect to all the inclusion particles,
and the maximum value of the thus found averages was determined to be the maximum
diameter of the inclusion particles. The number of the inclusion particles was found
as follows. The number of all the inclusion particles, which was extracted by means
of the slime electrolytic extraction (the minimum mesh was 38 µm) and observed by
an optical microscope (the magnification was 100) was converted into the number per
the unit of 1 kg.
*11: The ratio of occurrence of defects is determined by the following formulas.
1. Rolled steel having few inclusion defects, the basic composition of which is C: 0.0002
to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05
mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001
to 0.1 mass %, wherein created oxide inclusions are mainly composed of a crystallized
phase, the principal component of which is titanium oxide, and a crystallized phase,
the principal component of which is alumina, and the crystallized phases of oxide
inclusions exist in steel.
2. Rolled steel having few inclusion defects, the basic composition of which is C: 0.0002
to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05
mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001
to 0.1 mass %, wherein created oxide inclusions are mainly composed of a crystallized
phase, the principal component of which is titanium oxide, and a crystallized phase,
the principal component of which is alumina, and the crystallized phases of oxide
inclusions exist in steel being dispersed in rows in the rolling direction near the
center of rolled steel.
3. Rolled steel having few inclusion defects, the basic composition of which is C: 0.0002
to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05
mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001
to 0.1 mass %, wherein created oxide inclusions are mainly composed of a crystallized
phase, the principal component of which is titanium oxide, and a crystallized phase,
the principal component of which is alumina, Micro-Vickers hardness at the room temperature
of the oxide inclusions is 600 to 1300 Hv, and the crystallized phases of oxide inclusions
exist in steel.
4. Rolled steel having few inclusion defects, the basic composition of which is C: 0.0002
to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05
mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001
to 0.1 mass %, wherein created oxide inclusions are mainly composed of a crystallized
phase, the principal component of which is titanium oxide, and a crystallized phase,
the principal component of which is alumina, Micro-Vickers hardness at the room temperature
of the oxide inclusions is 600 to 1300 Hv, and the crystallized phases of oxide inclusions
exist in steel being dispersed in rows in the rolling direction near the center of
rolled steel.
5. Rolled steel having few defects of inclusion, the basic composition of which is C:
0.0002 to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to
0.05 mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al:
0.001 to 0.1 mass %, the selective composition of which is Ca: not more than 50 ppm
and Mg: not more than 50 ppm, at least one of Ca and Mg being contained, wherein created
oxide inclusions are mainly composed of a crystallized phase, the principal component
of which is titanium oxide, and a crystallized phase, the principal component of which
is alumina, and further composed of at least one of a crystallized phase, the principal
component of which is CaO, and a crystallized phase, the principal component of which
is MgO, and the crystallized phases of oxide inclusions exist in steel.
6. Rolled steel having few inclusion defects, the basic composition of which is C: 0.0002
to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05
mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001
to 0.1 mass %, the selective composition of which is Ca: not more than 50 ppm and
Mg: not more than 50 ppm, at least one of Ca and Mg being contained, wherein created
oxide inclusions are mainly composed of a crystallized phase, the principal component
of which is titanium oxide, and a crystallized phase, the principal component of which
is alumina, and further composed of at least one of a crystallized phase, the principal
component of which is CaO, and a crystallized phase, the principal component of which
is MgO, and the crystallized phases of oxide inclusions exist in steel being dispersed
in rows in the rolling direction near the center of steel.
7. Rolled steel having few inclusion defects, the basic composition of which is C: 0.0002
to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05
mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001
to 0.1 mass %, the selective composition of which is Ca: not more than 50 ppm and
Mg: not more than 50 ppm, at least one of Ca and Mg being contained, wherein created
oxide inclusions are mainly composed of a crystallized phase, the principal component
of which is titanium oxide, and a crystallized phase, the principal component of which
is alumina, and further composed of at least one of a crystallized phase, the principal
component of which is CaO, and a crystallized phase, the principal component of which
is MgO, Micro-Vickers hardness of the oxide inclusions at the room temperature is
600 to 1300 Hv, and the crystallized phases of oxide inclusions exist in steel.
8. Rolled steel having few inclusion defects, the basic composition of which is C: 0.0002
to 0.7 mass %, Si: 0.001 to 0.5 mass %, Mn: 0.005 to 2.0 mass %, P: 0.001 to 0.05
mass %, S: 0.0005 to 0.15 mass %, Ti: 0.001 to 0.25 mass % and dissolved Al: 0.001
to 0.1 mass %, the selective composition of which is Ca: not more than 50 ppm and
Mg: not more than 50 ppm, at least one of Ca and Mg being contained, wherein created
oxide inclusions are mainly composed of a crystallized phase, the principal component
of which is titanium oxide, and a crystallized phase, the principal component of which
is alumina, and further composed of at least one of a crystallized phase, the principal
component of which is CaO, and a crystallized phase, the principal component of which
is MgO, Micro-Vickers hardness of the oxide inclusions at the room temperature is
600 to 1300 Hv, and the crystallized phases of oxide inclusions exist in steel being
dispersed in rows in the rolling direction near the center of steel.
9. Rolled steel according to any one of claims 1 to 8, wherein the maximum diameter of
particles of oxide inclusions obtained by slime extraction is not more than 300 µm.
10. Rolled steel having few inclusion defects according to claim 9 wherein the number
of particles of oxide inclusions obtained by slime extraction, the diameters of which
are not less than 38 µm, is not more than 50 pieces/kg.