WO2014175381A1 - 鋼板 - Google Patents
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- WO2014175381A1 WO2014175381A1 PCT/JP2014/061573 JP2014061573W WO2014175381A1 WO 2014175381 A1 WO2014175381 A1 WO 2014175381A1 JP 2014061573 W JP2014061573 W JP 2014061573W WO 2014175381 A1 WO2014175381 A1 WO 2014175381A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/06—Deoxidising, e.g. killing
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- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/068—Decarburising
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/0426—Hot rolling
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
Definitions
- the present invention relates to a carbon steel plate having a carbon content of more than 0.25% and less than 0.50% by mass%, and particularly to a carbon steel plate that is formed into a product shape by punching, hole expanding, forging, or the like.
- Patent Documents 1 to 3 propose the following techniques.
- Patent Document 1 by mass%, C: 0.15% to 0.50%, S: 0.01% or less, and the relationship [% P] ⁇ 6 ⁇ [% B] +0.005 is satisfied.
- a steel reclining seat gear made of a steel plate having an excellent notch tensile elongation rate has been proposed.
- Patent Document 1 paying attention to the fact that there is a strong correlation between punchability and notch tensile elongation, the notch tensile elongation and hence punchability can be increased by increasing the particle size of carbides dispersed in the steel sheet. It suggests that it can be improved.
- Patent Document 2 proposes a high-carbon steel containing C: 0.70% to 1.20% by mass and having a controlled particle size of carbides dispersed in a ferrite matrix. This steel is also excellent in punching workability because the notch tensile elongation which is closely related to the punching workability is improved. Further, this steel further contains Ca to control the form of MnS, and as a result, the punching workability is further improved.
- Patent Document 3 in steel containing C: 0.10% to 0.40% and S: 0.010% or less by mass%, the shapes of inclusions are classified based on the ASTM-D method, We propose to provide steel for gears with excellent cold forgeability by keeping the shape and number of objects within a certain range.
- Ca and / or REM Reare Earth Metal
- the present inventors also added oxides (inclusions) generated in steel by adding Ca and REM to a structural thick steel plate containing 0.08% to 0.22% C by mass%. Controlled as a mixed phase state of a high melting point phase and a low melting point phase, this oxide (inclusion) is prevented from stretching during rolling, and the continuous casting nozzle does not cause melting damage or internal inclusion defects
- oxides inclusions
- Japanese Unexamined Patent Publication No. 2000-265238 Japanese Unexamined Patent Publication No. 2000-265239 Japanese Unexamined Patent Publication No. 2001-329339 Japanese Unexamined Patent Publication No. 2011-68949
- Patent Document 1 aims to suppress the connection of microvoids by increasing the particle size of the carbide, assuming that the microvoids generated from the carbide are the starting points of cracking.
- Patent Document 2 proposes to increase the carbide particle size. Further, Patent Document 2 proposes that Ca is contained in order to suppress generation of MnS in the steel, focusing on the fact that MnS stretched (during rolling) in the steel sheet becomes a crack starting point.
- Patent Document 3 states that stretched oxide inclusions (B system of ASTM-D method) and non-stretched oxide inclusions (D system of ASTM-D method) cause the forgeability to deteriorate.
- the length and the total number are defined based on the classification of the ASTM-D method.
- Patent Document 3 defines the size, length, and total number of stretched oxide inclusions and non-stretched oxide inclusions, and shows specific measures for realizing this rule. Not.
- Patent Document 4 the inclusion number density is controlled by adding Ca and / or REM.
- the C content of the steel described in Patent Document 4 is 0.08% by mass to 0.22% by mass, and when used as a material for mechanical structural parts having a complicated shape, the strength (tensile strength, Abrasion, hardness, etc.) may be insufficient.
- Patent Document 4 does not disclose a method for controlling the inclusion number density to a desirable level in steel that is required to contain C in excess of 0.25% by mass.
- the present invention has been devised in view of the above-described problems, and contains C in an amount of more than 0.25% and less than 0.50%, and is used for manufacturing products with complicated shapes including gears.
- An object is to provide a carbon steel sheet having suitable workability.
- the chemical composition is mass%, C: more than 0.25% and less than 0.50%, Si: 0.10% to 0.60%, Mn: 0.00. 40% to 0.90%, Al: 0.003% to 0.070%, Ca: 0.0005% to 0.0040%, REM: 0.0003% to 0.0050%, Cu: 0% to 0 0.05%, Nb: 0% to 0.05%, V: 0% to 0.05%, Mo: 0% to 0.05%, Ni: 0% to 0.05%, Cr: 0% to 0% 50%, B: 0% to 0.0050%, P: 0.020% or less, S: 0.0070% or less, Ti: 0.050% or less, O: 0.0040% or less, N: limited to 0.0075% or less, the balance is made of iron and impurities, and the content expressed by mass% of each element in the chemical component is represented by the following formula 1 and the following formula 2 Simultaneously satisfy, present alone, the number density of Ti
- the chemical component is further in mass%, Cu: 0.01% to 0.05%, Nb: 0.01% to 0.05%, V: 0.01% to 0.05%, Mo: 0.01% to 0.05%, Ni: 0.01% to 0.05%, Cr: 0.01% to 0.50%, B: 0.0.
- One or more of 0010% to 0.0050% may be contained.
- the steel sheet according to (1) or (2) further includes a composite inclusion containing Al, Ca, O, S, and REM, and the Ti-containing carbonitride on the surface of the composite inclusion.
- An attached inclusion may be included.
- the content represented by mass% of each element in the chemical component may satisfy the following formula 3. 18 ⁇ (REM / 140) ⁇ O / 16 ⁇ 0 (Formula 3)
- the content expressed by mass% of each element in the chemical component may satisfy the following formula 4.
- the number density of the A-based inclusions, the number density of the B-based inclusions, the number density of the C-based inclusions in the steel is coarse and has an angular shape and is independent.
- Non-metallic inclusions and carbonitrides can be cited as causes for reducing the workability of steel sheets. These serve as starting points for cracking of the steel sheet when stress is applied to the steel sheet.
- Inclusions include oxides and sulfides that exist in the molten steel or that are generated during solidification of the molten steel.
- the size (long side) of inclusions ranges from several ⁇ m to several hundred ⁇ m when stretched by rolling. Therefore, in order to improve the workability of the steel sheet, it is important to reduce the content of inclusions.
- the state where the size of the inclusions in the steel sheet is small and the number is small that is, the state of “high cleanliness” of the steel sheet is preferable.
- Inclusions vary in shape, distribution, and the like. For example, in JIS G 0555, inclusions are distinguished from A-based inclusions, B-based inclusions, and C-based inclusions. In the present embodiment, the inclusions are classified into three types according to the following definition.
- A-based inclusions Non-metallic inclusions in steel that are viscously deformed by processing. A steel sheet having high stretchability and subjected to processing is often stretched along the processing direction.
- inclusions having an aspect ratio (major axis / minor axis) of 3.0 or more are defined as A-based inclusions.
- B-based inclusions Among non-metallic inclusions in steel, the inclusions form a group in the processing direction and are discontinuously granular inclusions, often have an angular shape, and have low stretchability. is there. In the present embodiment, an inclusion group in which three or more inclusions are aligned along the processing direction, an inclusion group in which the separation distance between the inclusions is 50 ⁇ m or less is formed, and an aspect ratio ( Inclusions having a major axis / minor axis of less than 3.0 are defined as B-based inclusions.
- C-based inclusion Dispersed irregularly without viscous deformation, often has an angular shape or a spherical shape, and has low stretchability.
- an inclusion having an aspect ratio (major axis / minor axis) of less than 3.0 and randomly distributed is defined as a C-based inclusion.
- Ti-containing carbonitrides that are very hard and square are generally classified as C-based inclusions, but in this embodiment, they may be distinguished from C-based inclusions.
- the influence on the properties of the steel sheet is greater than other C-based inclusions (C-based inclusions that are not Ti-containing carbonitrides).
- the “Ti-containing carbonitride present alone” refers to Ti-containing carbonitride that is not attached to inclusions that do not contain Ti.
- the Ti-containing carbonitride when the Ti-containing carbonitride is present in a state of being attached to other inclusions (for example, composite inclusions including Al, Ca, O, S, and REM), the Ti-containing carbonitride gives the properties of the steel sheet The impact is the same level as other C inclusions.
- the Ti-containing carbonitride adhering to other inclusions is regarded as a C-based inclusion that is not a Ti-containing carbonitride.
- “number density of C-based inclusions” means “number density of C-based inclusions that are not Ti-containing carbonitrides (including those in which Ti-containing carbonitrides adhere to C-based inclusions). ”And“ number density of Ti-containing carbonitrides present alone ”. Ti-containing carbonitrides can be distinguished from other C-based inclusions by their shape and color tone.
- inclusions having a particle size (in the case of a substantially spherical inclusion) or a long diameter (in the case of a deformed inclusion) of 1 ⁇ m or more are considered.
- Inclusions having a particle diameter or major axis of less than 1 ⁇ m have little influence on the workability of steel even if they are contained in steel, and therefore, such inclusions are not considered in this embodiment.
- the above-mentioned major axis is defined as a line segment having the maximum length among the line segments connecting the apexes that are not adjacent to each other in the cross-sectional contour of the inclusion on the observation surface.
- the above-mentioned minor axis is defined as a line segment that is the minimum length among the line segments that connect the apexes that are not adjacent to each other in the cross-sectional contour of the inclusion on the observation surface.
- the long side mentioned later is defined as the line segment which becomes the maximum length among the line segments which connect each adjacent vertex in the cross-sectional outline of the inclusion on an observation surface.
- Patent Document 4 proposes a technique that prevents the occurrence of defects.
- the present inventors also include the above-described A-based inclusions and B by adding Ca and REM with respect to a steel containing more than 0.25% and less than 0.50% by mass%. And the conditions for reducing C-based inclusions were examined. As a result, the present inventors have found a condition that can simultaneously reduce A-based inclusions and B and C-based inclusions. The specific contents are shown below.
- the inventors of the present invention have examined the addition of Ca and REM to a steel containing, by mass%, C more than 0.25% and less than 0.50%. As a result, when the content expressed by mass% of each element in the chemical component satisfies the following formula I, A-based inclusions in steel, in particular, MnS constituting the A-based inclusions can be greatly reduced. I found.
- the inclusions in the hot-rolled steel sheet are magnified 400 times with an optical microscope (however, when measuring the shape of inclusions in detail) (1000 times) and a total of 60 fields of view.
- the inclusions having a particle size (in the case of spherical inclusions) or a long diameter (in the case of deformed inclusions) of 1 ⁇ m or more are observed, and these inclusions are classified as A-based inclusions, They were classified into B-based inclusions and C-based inclusions, and their number density was measured.
- the number density of the square Ti containing carbonitride which exists independently among C type inclusions was also measured.
- SEM scanning electron microscope
- EPMA Electro Probe Micro Analysis
- EDX Electronic X-Ray Analysis
- the Charpy impact value at room temperature was measured as an index of workability of the hot-rolled steel sheet obtained above.
- the Charpy impact value is a numerical value indicating the toughness of the steel sheet.
- the Charpy impact value decreases as the number of inclusions serving as crack initiation points or the size of inclusions increases. That is, there is a strong correlation between the Charpy impact value and workability.
- the value of the limit strain at which cracking occurs varies depending on the processing method, but has a correlation with the Charpy impact value.
- the Charpy impact value and the number density of inclusions have a correlation. Specifically, it has been clarified that when the number density of A-based inclusions in steel exceeds 6 / mm 2 , the Charpy impact value deteriorates rapidly. Further, it has been clarified that even when the number density of B-based inclusions and C-based inclusions exceeds 6 / mm 2 in total, the impact value rapidly deteriorates. In addition, for Ti-containing carbonitrides that are C-based inclusions, if the number density of coarse Ti-containing carbonitrides having a long side of 5 ⁇ m or more that is present alone exceeds 5 / mm 2 , the impact value was found to worsen rapidly.
- FIG. 1 circles indicate the results of steel containing a chemical component containing Ca and not containing REM (hereinafter referred to as “Ca alone”), and square marks (“REM + Ca” in FIG. 1).
- Ca alone a chemical component containing Ca and not containing REM
- REM + Ca square marks
- FIG. 1 shows the results for a steel with a chemical component that contains Ca and also contains REM (hereinafter referred to as the combined inclusion of REM and Ca).
- the above R1 was calculated on the assumption that the REM content was 0. From FIG. 1, it was found that there is a correlation between the number density of A-based inclusions and the above R1 both in the case of containing Ca alone and in the case of containing REM and Ca in combination.
- the value of R1 is 0.3000 or more, the number density of A-based inclusions is reduced, and the number density is 6 pieces / mm 2 or less. As a result, the Charpy impact value is improved.
- the major axis of the A-based inclusions in the steel becomes longer than in the case of containing REM and Ca in combination.
- a CaO—Al 2 O 3 -based low melting point oxide is formed as an A-based inclusion, and this oxide is stretched during rolling. Therefore, in consideration of the major axis of inclusions that adversely affect the properties of the steel sheet, the combined inclusion of REM and Ca is preferable to the inclusion of Ca alone.
- the number density of A-based inclusions in the steel can be preferably reduced to 6 pieces / mm 2 or less under the conditions satisfying the above-mentioned formula I and in the case of composite inclusion of REM and Ca. I understood.
- R1 the value of R1 is 1.000, as an average composition, 1 equivalent of Ca and REM which couple
- MnS may be generated in the microsegregation part between dendrite branches.
- the value of R1 is 2.000 or more, it is possible to preferably prevent MnS formation at the microsegregation part between dendritic branches.
- the value of R1 is preferably 5.000 or less. That is, the upper limit value on the right side of Formula I is preferably 5.000.
- B-type inclusions and C-type inclusions As described above, by observing the observation surface of the hot-rolled steel sheet, B-type inclusions and C-type inclusions having an aspect ratio (major axis / minor axis) of less than 3 and a grain size or major axis of 1 ⁇ m or more. The number density was measured. As a result, the inventors have found that the number density of B-based inclusions and C-based inclusions increases as the Ca content increases in both cases of containing Ca alone or containing REM and Ca in combination. I found it. On the other hand, the inventors have found that the REM content does not greatly affect the number density of these inclusions.
- FIG. 2 shows the relationship between the Ca content in steel and the total number density of B-based inclusions and C-based inclusions when Ca is contained alone and when REM and Ca are combined.
- circles indicate the results of containing Ca alone
- square marks (explained as “REM + Ca” in FIG. 2) indicate the results of combined inclusion of REM and Ca.
- the total number density of B-based inclusions and C-based inclusions increases as the Ca content in steel increases in the case of containing Ca alone or in the case of containing REM and Ca in combination.
- the surface of the Ca-REM composite inclusion is in a liquid phase in the molten steel, and the aggregation and coalescence behavior is presumed to be similar to the CaO—Al 2 O 3 inclusion produced when Ca alone is contained. Therefore, it is considered that a large number of Ca-REM composite inclusions remain dispersed in the slab and the total number density of B-type inclusions and C-type inclusions increases.
- the total number density of B-based inclusions and C-based inclusions has a correlation with Ca content and C content.
- the present inventors have found that the total number density of B-based inclusions and C-based inclusions increases as the C content increases even with the same Ca content. More specifically, in order to make the total number density of B-based inclusions and C-based inclusions 6 pieces / mm 2 or less, the content expressed by mass% of each element in the chemical component is set as follows: It was found that it was necessary to control to the range represented by the formula II.
- This Formula II indicates that the upper limit value of the Ca content needs to be changed depending on the C content, that is, the upper limit value of the Ca content needs to be lowered as the C content increases.
- the lower limit value of the above formula II is not particularly limited, but 0.0005 which is the lower limit value of the Ca content in mass% is the substantial lower limit value of the right side of the above formula II. .
- the reason why the total number density of B inclusions and C inclusions increases as the C content increases is that the solidification temperature range from the liquidus temperature to the solidus temperature increases as the C concentration in the molten steel increases. Is considered to be caused by the increase in the length of the dendrite structure. That is, it is presumed that the dendrite structure develops for a long time, so that inclusions are easily trapped between the dendrite trees (it is difficult to be discharged into the bulk molten steel from between the dendrite trees). Therefore, it is necessary to lower the upper limit of the Ca content so as to satisfy the above-described formula II as the steel having a higher C content and thus a longer dendrite structure during solidification tends to develop.
- the phase at the time of solidification of the steel having the carbon concentration range (C: more than 0.25% and less than 0.50%) is a liquid phase + ⁇ phase above the peritectic temperature. Below the crystallization temperature, it is a liquid phase + ⁇ phase. That is, the degree of microsegregation of solute elements such as S is different at the peritectic temperature.
- the solid-liquid partition coefficient that affects inclusion trapping because it is a surface active element is greater when the phase is liquid phase + ⁇ phase than when the phase is liquid phase + ⁇ phase. Is small. When the solid-liquid distribution coefficient of S is small, the amount of S distributed to the solid phase decreases, and the amount of S distributed to the liquid phase increases.
- Ti-containing carbonitride When Ti is mixed from auxiliary materials such as alloys and scrap, Ti-containing carbonitrides such as TiN are produced in the steel. This Ti-containing carbonitride has a high hardness and has a square shape. For this reason, when coarse Ti-containing carbonitrides are produced alone in the steel, the carbonitrides are likely to be the starting point of fracture, so that the Charpy impact value of the steel and, consequently, the workability deteriorates.
- Ti-containing carbonitride As described above, as a result of examining the relationship between the content of Ti-containing carbonitride and the workability of the steel sheet, the number density of Ti-containing carbonitrides having a long side length of 5 ⁇ m or more, which is present alone, is It was found that if it is 5 pieces / mm 2 or less, breakage hardly occurs and deterioration of workability can be prevented.
- Ti carbide Ti nitride, Ti carbonitride, Ti-containing carbonitride includes TiNb carbide, TiNb nitride, TiNb carbonitride, etc. in the case of containing Nb as a selective element. .
- the number density of a coarse single Ti-containing carbonitride having a long side length of 5 ⁇ m or more can be preferably reduced to 5 pieces / mm 2 or less.
- the Ti-containing carbonitride that has been compositely deposited on the REM-containing composite inclusion is unlikely to become a starting point of fracture.
- the reason for this is considered that the Ti-shaped carbonitride is compound-deposited on the REM-containing composite inclusions, thereby reducing the angular portion of the Ti-containing carbonitride.
- the shape of Ti-containing carbonitride is a cube or a rectangular parallelepiped, when present alone in steel, all of the eight corners of Ti-containing carbonitride are in contact with the matrix. Since the corner is the starting point of fracture, the Ti-containing carbonitride having eight corners has eight fracture starting points.
- Ti-containing carbonitride when Ti-containing carbonitride is complex-deposited on the REM-containing composite inclusion, for example, when only half of Ti-containing carbonitride contacts the matrix, only four places of Ti-containing carbonitride are in the matrix. Touch. That is, the corners of the Ti-containing carbonitride in contact with the matrix are reduced from 8 places to 4 places. As a result, the starting point of fracture due to the Ti-containing carbonitride is reduced from 8 to 4 locations.
- Ti-containing carbonitride is likely to be preferentially precipitated on the REM-containing composite inclusion.
- Ti-containing carbonitride is precipitated on a specific crystal plane of the REM composite inclusion, This is presumably because the lattice matching between the specific crystal plane of the REM composite inclusion and the Ti-containing carbonitride is good.
- a composite of a Ti-containing carbonitride and a REM-containing inclusion ie, an inclusion in which a Ti-containing carbonitride is attached to the surface of a composite inclusion containing Al, Ca, O, S, and REM
- it Since it has less adverse effects on various properties of the steel sheet than Ti-containing carbonitrides present, it is considered to be a C-based inclusion that is not a Ti-containing carbonitride present alone.
- C more than 0.25% and less than 0.50%
- C (carbon) is an important element in securing the strength (hardness) of the steel sheet.
- C content is 0.25% or less, the hardenability of the steel sheet is lowered, so that the strength required for products manufactured using the steel sheet as a raw material, such as gears, cannot be obtained.
- the C content is 0.50% or more, a long time is required for the heat treatment to ensure workability. Therefore, the workability of the steel sheet may deteriorate unless the heat treatment is prolonged.
- the C content increases, the total number density of B-based inclusions and C-based inclusions increases.
- the C content is controlled to be more than 0.25% and less than 0.50%.
- the preferable lower limit of C content is 0.27%.
- the higher the C content the greater the hardness and tensile strength after heat treatment (quenching and tempering).
- a strength of 1300 MPa or more can be sufficiently secured after performing quenching and low-temperature tempering treatment.
- FIG. 3 is a graph showing the relationship between C content and tensile strength.
- the inventors measured the tensile strength of steel sheets in which conditions other than the C content were made to satisfy the conditions of the steel sheet according to the present embodiment and the C content was varied. As a result, it was revealed that when the C content is 0.27% or more, the steel sheet surely has a tensile strength of 1300 MPa. Furthermore, in the steel plate according to the present embodiment, the lower limit of the C content is preferably 0.30%, and the upper limit of the C content is preferably 0.48%.
- Si 0.10% to 0.60%
- Si silicon
- Si is an element that acts as a deoxidizer and is effective in improving the hardenability and improving the strength (hardness) of the steel sheet. If the Si content is less than 0.10%, the above-described content effect cannot be obtained. On the other hand, if the Si content exceeds 0.60%, the surface properties of the steel sheet may be deteriorated due to scale defects during hot rolling. Therefore, the Si content is controlled to 0.10% to 0.60%.
- the lower limit of the Si content is preferably 0.15%, and the upper limit of the Si content is preferably 0.55%.
- Mn manganese
- Mn manganese
- Mn is an element that acts as a deoxidizer and is an effective element for improving the hardenability and improving the strength (hardness) of the steel sheet. If the Mn content is less than 0.40%, the effect cannot be obtained sufficiently. On the other hand, if the Mn content exceeds 0.90%, the workability of the steel sheet may be deteriorated. Therefore, the Mn content is controlled to 0.40% to 0.90%.
- the lower limit of the Mn content is preferably 0.50%, and the upper limit of the Mn content is preferably 0.75%.
- Al 0.003% to 0.070%
- Al aluminum
- Al is an element that acts as a deoxidizer and is an element effective for improving the workability of the steel sheet by fixing N. If the Al content is less than 0.003%, the above-described effects cannot be obtained sufficiently, so 0.003% or more must be contained. On the other hand, when the Al content exceeds 0.070%, the content effect is saturated, and coarse inclusions increase. Due to the coarse inclusions, workability may be deteriorated or surface defects may be easily generated. Therefore, the Al content is controlled to 0.003% to 0.070%.
- the lower limit of the Al content is preferably 0.010%, and the upper limit of the Al content is preferably 0.040%.
- Ca 0.0005% to 0.0040%
- Ca is an effective element for controlling the form of inclusions and thereby improving the workability of the steel sheet. If the Ca content is less than 0.0005%, the above effect cannot be obtained sufficiently. Control of the form of inclusions is also possible by REM, but when the Ca content is less than 0.0005%, the nozzle clogging occurs during continuous casting as in the case where REM described later is contained alone. Stability is hindered, and further, high specific gravity inclusions are deposited on the lower surface side of the slab, which may deteriorate the workability of the steel sheet.
- the Ca content exceeds 0.0040%, for example, coarse low-melting point oxides such as CaO—Al 2 O 3 inclusions and / or inclusions such as CaS inclusions that are easily stretched during rolling. It becomes easy to produce
- the lower limit of the Ca content is preferably 0.0007%, more preferably 0.0010%.
- the upper limit of the Ca content is preferably 0.0030%, more preferably 0.0025%.
- the upper limit of the Ca content according to the C content. Specifically, it is necessary to control the content represented by mass% of C and Ca in the chemical component within a range represented by the following formula III.
- the Ca content does not satisfy the following formula III, the total number density of B-based inclusions and C-based inclusions exceeds 5 / mm 2 .
- REM 0.0003% to 0.0050%
- REM Rare Earth Metal
- the steel plate according to the present embodiment contains at least one element selected from these.
- REM is often selected from Ce (cerium), La (lanthanum), Nd (neodymium), Pr (praseodymium) and the like because of its availability.
- adding as a misch metal which is a mixture of these elements in steel is widely performed.
- the main components of misch metal are Ce, La, Nd, and Pr.
- the total amount of these rare earth elements contained in the steel plate is defined as the REM content.
- the average atomic weight of Misch metal is about 140, so the atomic weight of REM is 140.
- REM is an effective element for controlling the form of inclusions and improving the workability of the steel sheet. If the REM content is less than 0.0003%, the above effect cannot be obtained sufficiently, and the same problem as when Ca is contained alone occurs. That is, when the REM content is less than 0.0003%, the CaO—Al 2 O 3 inclusions and a part of CaS are stretched by rolling, thereby reducing the steel sheet properties (workability and toughness after processing). May occur. Furthermore, when the REM content is less than 0.0003%, the Ti-containing carbonitride is likely to be preferentially compounded, so there are few compound inclusions including Al, Ca, O, S, and REM, and thus it is generated alone in the steel sheet.
- the REM content is controlled to 0.0003% to 0.0050%.
- the lower limit of the REM content is preferably 0.0005%, more preferably 0.0010%.
- the upper limit of the REM content is preferably 0.0040%, more preferably 0.0030%.
- the content of Ca and REM according to the S content. Specifically, it is necessary to control the content expressed by mass% of each element in the chemical component within a range represented by the following formula IV.
- the number density of the A-based inclusions exceeds 6 / mm 2 .
- the value on the right side of the following formula IV is 2 or more, the form of inclusions can be more preferably controlled.
- the upper limit of the following formula IV is not specifically limited, when the value of the right side of the following formula IV exceeds 5, a coarse B-type or C-type inclusion having a maximum length exceeding 20 ⁇ m tends to be generated. Therefore, the upper limit value of the following formula IV is preferably 5.
- the steel sheet according to the present embodiment contains impurities in addition to the basic components described above.
- the impurities are auxiliary materials such as scrap, and P, S, Ti, O, N, Cd, Zn, Sb, W, Mg, Zr, As, Co, Sn, and the like mixed from the manufacturing process. It means an element such as Pb. Since the inclusion of these elements is not essential, the lower limit of the content of these elements is 0%.
- P, S, Ti, O, and N are limited as follows in order to preferably exhibit the above effects.
- the described% is mass%.
- P phosphorus
- the lower limit of the P content may be 0%. In consideration of current general refining (including secondary refining), the lower limit of the P content may be 0.005%.
- S sulfur
- S is an impurity element that inhibits the workability of the steel sheet by forming non-metallic inclusions. Therefore, the S content is limited to 0.0070% or less, preferably to 0.0050% or less.
- the lower limit of the S content may be 0%. In consideration of current general refining (including secondary refining), the lower limit of the S content may be 0.0003%.
- Ti titanium
- Ti titanium
- Ti is an element that deteriorates the workability of a steel sheet by forming a hard square carbonitride.
- the lower limit of the Ti content may be 0%. In consideration of current general refining (including secondary refining), the lower limit of the Ti content may be 0.0005%.
- O (oxygen) is an impurity element that forms an oxide (non-metallic inclusion), and this oxide aggregates and coarsens, thereby reducing the workability of the steel sheet. Therefore, the O content is limited to 0.0040% or less.
- the lower limit of the O content may be 0%. In consideration of current general refining (including secondary refining), the lower limit of the O content may be 0.0010%.
- the O content of the steel sheet according to the present embodiment is the total O content (TO content) obtained by adding up all O contents such as O dissolved in the steel and O present in the inclusions. Means.
- the O content and the REM content within the range represented by the following formula V using the content expressed by mass% of each element.
- the number density of A-based inclusions is further reduced, which is preferable.
- the upper limit of the following formula V is not specifically limited, 0.000643 becomes the upper limit of the left side of the following formula V from the upper limit and the lower limit of the O content and the REM content.
- REM 2 O 3 .11Al 2 O 3 (REM 2 O 3 and Al 2 O 3 molar ratio 1:11) and REM 2 O 3 .Al 2
- the A-based inclusions are further preferably reduced.
- REM / 140 indicates the number of moles of REM
- O / 16 indicates the number of moles of O.
- N nitrogen
- nitrogen is an impurity element that forms nitrides (non-metallic inclusions) and lowers the workability of the steel sheet. Therefore, the N content is limited to 0.0075% or less.
- the lower limit of the N content may be 0%. In consideration of current general refining (including secondary refining), the lower limit of the N content may be 0.0010%.
- the above basic components are controlled, and the balance is made of iron and the above impurities.
- the steel sheet according to the present embodiment may further contain the following selected components in the steel, if necessary, instead of a part of the remaining Fe.
- the hot-rolled steel sheet according to this embodiment may further contain one or more of Cu, Nb, V, Mo, Ni, and B as a selection component in addition to the basic components and impurities described above. Good.
- the numerical limitation range of the selected component and the reason for limitation will be described.
- the described% is mass%.
- Cu (copper) is a selective element having an effect of improving the strength (hardness) of the steel sheet. Therefore, if necessary, Cu may be contained within a range of 0.05% or less. Moreover, the said effect can be preferably acquired when the lower limit of Cu content shall be 0.01%. On the other hand, if the Cu content exceeds 0.05%, hot work cracking may occur during hot rolling due to molten metal embrittlement (Cu cracking). A preferable range of the Cu content is 0.02% to 0.04%.
- Nb 0.05% or less
- Nb niobium
- Nb is a selective element that forms carbonitrides and is effective in preventing coarsening of crystal grains and improving the workability of the steel sheet. Therefore, Nb may be contained within a range of 0.05% or less as necessary. Moreover, the said effect can be preferably acquired when the lower limit of Nb content shall be 0.01%. On the other hand, if the Nb content exceeds 0.05%, coarse Nb carbonitride may precipitate and cause deterioration in workability of the steel sheet. A preferable range of the Nb content is 0.02% to 0.04%.
- V vanadium
- V vanadium
- Nb vanadium
- V vanadium
- V vanadium
- the said effect can be preferably acquired when the lower limit of V content shall be 0.01%.
- the V content exceeds 0.05%, coarse inclusions may be generated and the workability of the steel sheet may be reduced.
- the preferred range is 0.02% to 0.04%.
- Mo mobdenum
- Mo mobdenum
- Mo is a selective element having an effect of improving the strength (hardness) of the steel sheet by improving hardenability and improving tempering softening resistance. Therefore, if necessary, Mo may be contained within a range of 0.05% or less. Moreover, the said effect can be preferably acquired when the lower limit of Mo content shall be 0.01%. On the other hand, when the Mo content exceeds 0.05%, the cost increases and the content effect is saturated. Furthermore, if the Mo content exceeds 0.05%, the workability of the steel sheet, particularly the cold workability, is deteriorated, which makes it difficult to process the steel sheet into a complicated shape (for example, a gear shape). . For the above reason, the upper limit of the Mo content is set to 0.05%. A preferable range of the Mo content is 0.01% to 0.05%.
- Ni (nickel) is a selective element effective for improving the strength (hardness) of a steel sheet by improving hardenability and improving workability. Further, it is a selective element that also has an effect of preventing molten metal embrittlement (Cu cracking) when Cu is contained. Therefore, if necessary, Ni may be contained within a range of 0.05% or less. Moreover, the said effect can be preferably acquired when the lower limit of Ni content shall be 0.01% or more. On the other hand, if the Ni content exceeds 0.05%, the cost increases, but the content effect is saturated, so the upper limit of the Ni content is set to 0.05%. A preferred range for the Ni content is 0.02% to 0.05%.
- Cr 0.50% or less
- Cr chromium
- Cr is an effective element for improving the hardenability and improving the strength (hardness) of the steel sheet. Therefore, if necessary, Cr may be contained within a range of 0.50% or less. Moreover, the said effect can be preferably acquired when the lower limit of Cr content shall be 0.01%. When the Cr content exceeds 0.50%, the cost increases while the content effect is saturated. Therefore, the Cr content is controlled to 0.50% or less.
- B boron
- B (boron) is a selective element that has the effect of increasing the hardenability and improving the strength (hardness) of the steel sheet. Therefore, you may contain B within the range of 0.0050% or less as needed. Moreover, the said effect can be preferably acquired when the lower limit of B content shall be 0.0010%. On the other hand, if the B content exceeds 0.0050%, a B-based compound is generated and the workability of the steel sheet is lowered, so the upper limit is made 0.0050%. A preferable range of the B content is 0.0020% to 0.0040%.
- the steel plate according to the present embodiment is made of, for example, a slab by continuous casting, using a blast furnace hot metal as a raw material, and by performing converter refining and secondary refining.
- the steel sheet is made by hot rolling, cold rolling and / or annealing as necessary.
- secondary refining in the ladle performs inclusion control by addition of Ca and REM along with adjustment of the steel components.
- molten steel melted in an electric furnace using iron scrap as a raw material may be used as a raw material.
- Ca and REM are added after adjusting the components of other contained elements and further floating Al 2 O 3 produced by Al deoxidation from the molten steel. If a large amount of Al 2 O 3 remains in the molten steel, Ca and REM are consumed by the reduction of Al 2 O 3 . Therefore, the contents of Ca and REM used for fixing S are reduced, and the generation of MnS cannot be sufficiently prevented.
- Ca has a high vapor pressure
- an alloy wire composed of these alloys may be used.
- REM may be added in the form of Fe-Si-REM alloy, misch metal, or the like.
- Misch metal is a mixture of rare earth elements, and specifically, it often contains about 40% to 50% Ce and about 20% to 40% La. For example, it is possible to obtain Misch metal made of Ce45%, La35%, Nd9%, Pr6%, and other impurities.
- the order of adding Ca and REM is not particularly limited. However, when Ca is added after REM addition, the size of inclusions tends to be slightly reduced. Therefore, it is preferable to add Ca after REM addition.
- Al 2 O 3 is generated after Al deoxidation, and a part of the Al 2 O 3 is clustered.
- REM addition is performed before Ca addition
- a part of the cluster is reduced and decomposed, and the size of the cluster can be reduced.
- Ca addition is performed before REM addition
- Al 2 O 3 changes to CaO—Al 2 O 3 -based inclusions having a low melting point, and the Al 2 O 3 cluster is converted into one coarse CaO—Al 2.
- Ca it is preferable to add Ca after REM addition.
- the ingredients were adjusted by ladle refining to produce 300 tons of molten steel having the ingredients shown in Table 2A.
- Al is first added to perform deoxidation, then components of other elements such as Ti are adjusted, and then Al 2 O 3 generated by Al deoxidation is floated for 5 minutes or more.
- REM was added and held for 3 minutes for uniform mixing before Ca was added.
- Misch metal was used for REM.
- the REM elements contained in this misch metal were Ce 50%, La 25%, and Nd 10%, and the balance was impurities. Therefore, the ratio of each REM element contained in the obtained steel plate is substantially the same as the ratio of each REM element described above. Since Ca has a high vapor pressure, a Ca—Si alloy was added to increase the yield.
- the molten steel after refining was cast into a slab of thickness 250 mm by continuous casting. Thereafter, the slab is heated to 1250 ° C. and held for 1 hour, and then hot-rolled so that the finishing temperature is 850 ° C. to a plate thickness of 5 mm, and then the coiling temperature is 580 ° C. Winded up.
- the pickled steel sheet was pickled and then annealed at 700 ° C. for 72 hours. This hot-rolled steel sheet was quenched at 900 ° C. for 30 minutes and further tempered at 100 ° C. for 30 minutes.
- the composition of inclusions and deformation behavior (ratio of major axis / minor axis after rolling; aspect ratio) were investigated.
- a field parallel to the rolling direction and the plate thickness direction was used as an observation surface, and 60 fields were observed with an optical microscope at a magnification of 400 times (however, when the inclusion shape was measured in detail, the magnification was 1000 times).
- the inclusions having a particle size (in the case of spherical inclusions) or a long diameter (in the case of deformed inclusions) of 1 ⁇ m or more are observed, and these inclusions are classified as A-based inclusions, They were classified into B-type inclusions and C-type inclusions, and their number density was measured. In addition, the number density of square Ti-containing carbonitrides precipitated alone in steel and having a long side exceeding 5 ⁇ m was also measured. Since the Ti-containing carbonitride is different in shape and color from other C-based inclusions, it can be determined by observation.
- an SEM Sccanning Electron Microscope, Scanning Electron Microscopy
- an EPMA Electro Probe Micro Analysis
- EDX Electro Dispersive X-Ray Analysis
- the inclusion evaluation criteria were as follows. Regarding the number density of A-type inclusions and the total number density of B-type inclusions and C-type inclusions, the case where the number density exceeds 6 / mm 2 is B (Bad), 4 / mm 2 and more than 6 / Mm 2 or less is G (Good), 2 / mm 2 is more than 4 pieces / mm 2 or less is VG (Very Good), and 2 / mm 2 or less is GG (Greyly Good).
- a B system and the C system relates the maximum length 20 ⁇ m or more coarse inclusions, the case where more than six / mm 2 B (Bad), 3 pieces / mm 2 Ultra-6 / mm 2 or less in the case G ( Good) 3 / mm 2 or less was designated as VG (Very Good).
- VG Very Good
- the number density exceeds 5 pieces / mm 2 and B (Bad), 3 pieces / mm 2 and more than 5 pieces / mm 2 In the case of G (Good), the case of 3 pieces / mm 2 or less was designated as VG (Very Good).
- the hot-rolled steel sheet obtained after quenching and tempering was evaluated for tensile strength (MPa), Charpy impact value (J / cm 2 ) at room temperature (about 25 ° C.), and hole expansibility (%).
- a steel plate having a tensile strength of 1200 MPa or more was regarded as a steel plate satisfying the acceptance criteria for tensile strength.
- the Charpy impact value at room temperature indicates toughness and is one of the indices for evaluating the workability of a steel sheet.
- the toughness of the product obtained by processing a steel plate can also be evaluated by the Charpy impact value.
- a steel plate having a Charpy impact value at room temperature of 6 J / cm 2 or more was regarded as a steel plate satisfying the acceptance criteria for toughness.
- Hole expandability is another index for evaluating workability.
- a punched hole having a diameter of 10 mm was formed in the center of a 150 mm ⁇ 150 mm steel plate, and then the punched hole was expanded with a 60 ° conical punch.
- the hole diameter D (mm) at the time when the plate thickness penetration crack occurred in the steel plate by this expansion treatment was measured.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry: ICP-MS (Inductively Coupled Plasma-Mass-Plasma-Mass-Plasma-Mass-Plasma-Mass-Plasma-Mass-Plasma-Mass) )
- ICP-MS Inductively Coupled Plasma-Mass-Plasma-Mass-Plasma-Mass-Plasma-Mass-Plasma-Mass-Plasma-Mass
- REM elements may be below the analysis limit, and in that case, the content is the largest as it is proportional to the content (Ce 50%, La 25%, Nd 10%) in the misch metal. It calculated using the ratio with respect to the analytical value of Ce.
- Comparative Example 2 since the Ca content exceeded the upper limit, a coarse CaO—Al 2 O 3 -based low melting point oxide was generated. Thereby, the evaluation of the number density of the A-type inclusions, the number density of the B-type + C-type inclusions, and the number density of the coarse inclusions in Comparative Example 2 was “B”.
- Comparative Example 3 since the REM content was less than the lower limit and did not satisfy Formula 3, a large number of coarse Ti-containing carbonitrides were produced alone in the matrix. Thereby, the evaluation of the number density of the Ti-containing carbonitride of Comparative Example 3 was “B”.
- Comparative Example 4 since the REM content exceeded the upper limit, the evaluation of the number density of B-type + C-type inclusions and the evaluation of the number density of coarse inclusions were “B”. Furthermore, nozzle clogging occurred during the casting of Comparative Example 4.
- Comparative Example 5 since the value on the right side of Formula 1 was less than 0.3, the evaluation of the number density of the A-based inclusions was “B”. Further, in Comparative Example 5, the C content was excessive, so the workability was low. Thereby, the impact value of Comparative Example 5 was insufficient. Since Comparative Example 6 did not satisfy Formula 2, the evaluation of the number density of B-based + C-based inclusions was “B”. In Comparative Example 7, since the C content was insufficient, the tensile strength was insufficient.
- Comparative Example 8 the number density of inclusions was at an appropriate level, but the C content was excessive, so that the workability was lowered. For this reason, the hole expansibility of the comparative example 8 was disqualified.
- Comparative Example 9 since the S content was excessive, coarse MnS inclusions were generated, and the evaluation of the number density of the A-based inclusions was “B”. Furthermore, the impact value and hole expansibility of Comparative Example 9 were insufficient.
- Comparative Example 10 since the Ti content was excessive, the evaluation of the number density of the Ti-containing carbonitride was “B”. Thereby, the impact value and hole expansibility of Comparative Example 10 were insufficient.
- Comparative Example 11 since the Ca content was excessive, a coarse oxide having a high CaO content was generated and stretched.
- Comparative Example 11 the evaluation of the number density of the A-based inclusions and B + C-based coarse inclusions in Comparative Example 11 was “B”. Furthermore, in Comparative Example 11, since the CaO content was high, the effect of the Ti-containing carbonitride adhering to the oxide surface was reduced. Thereby, the evaluation of the number density of the Ti-containing carbonitride of Comparative Example 11 was “B”. For the above reasons, the impact value and hole expansibility of Comparative Example 11 were insufficient. In Comparative Example 12, since the REM content was insufficient, the effect of attaching the Ti-containing carbonitride to the oxide surface was reduced. Thereby, the evaluation of the number density of the Ti-containing carbonitride of Comparative Example 12 was “B”.
- Comparative Example 12 since the REM content was excessive, the evaluation of the number density of coarse inclusions was “B”. Therefore, the impact value and hole expansibility of Comparative Example 13 were insufficient.
- Comparative Example 14 since the Mo content was excessive, the workability deteriorated despite the good evaluation of the number density of inclusions. Thereby, the impact value and hole expansibility of Comparative Example 14 were insufficient. Since Comparative Example 15 did not satisfy Formula 1, the evaluation of the number density of the A-based inclusions was “B”. Thereby, the impact value and hole expansibility of Comparative Example 15 were insufficient. Since Comparative Example 16 did not satisfy Formula 2, the evaluation of the number density of B + C inclusions was “B”. Therefore, the impact value and hole expansibility of Comparative Example 16 were insufficient.
- the C content, Ca content, and REM content of the steel sheet according to the present invention are expressed by the equation “0.3000 ⁇ ⁇ Ca / 40.88 + (REM / 140) / 2 ⁇ / (S / 32.07)”. And “Ca ⁇ 0.0058 ⁇ 0.0050 ⁇ C”.
- the number density of A-based inclusions having a long side of 1 ⁇ m or more of the steel plate according to the present invention is limited to 6 pieces / mm 2 or less
- the B-type having a long side of 1 ⁇ m or more of the steel plate according to the present invention is limited to 6 pieces / mm 2 or less.
- the number density of Ti carbonitrides having a long side of 5 ⁇ m or more and present alone in the steel sheet according to the present invention is limited to 5 pieces / mm 2 or less. According to the above aspect of the present invention, it is possible to reduce A-type inclusions, B-type inclusions, and C-type inclusions in steel, and to prevent generation of coarse Ti-containing carbonitrides that exist alone. Since it is possible to provide a steel sheet with excellent workability, the industrial applicability is high.
- the carbon steel sheet of the present invention can be used for manufacturing machine parts of various shapes, for example, vehicle gears, clutches, washers and the like.
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Abstract
Description
本願は、2013年4月25日に、日本に出願された特願2013-092408号に基づき優先権を主張し、その内容をここに援用する。
本発明の要旨は、以下の通りである。
0.3000≦{Ca/40.88+(REM/140)/2}/(S/32.07) ・・・(式1)
Ca≦0.0058-0.0050×C ・・・(式2)
(2)上記(1)に記載の鋼板は、前記化学成分が、さらに、質量%で、Cu:0.01%~0.05%、Nb:0.01%~0.05%、V:0.01%~0.05%、Mo:0.01%~0.05%、Ni:0.01%~0.05%、Cr:0.01%~0.50%、B :0.0010%~0.0050%、のうちの1種以上を含有してもよい。
(3)上記(1)または(2)に記載の鋼板は、さらに、Al、Ca、O、S、及びREMを含む複合介在物と、この複合介在物の表面に前記Ti含有炭窒化物が付着した介在物とを含んでもよい。
(4)上記(1)または(2)に記載の鋼板は、前記化学成分中の前記各元素の質量%で示した含有量が、下記の式3を満たしてもよい。
18×(REM/140)-O/16≧0 ・・・(式3)
(5)上記(3)に記載の鋼板は、前記化学成分中の前記各元素の質量%で示した含有量が、下記の式4を満たしてもよい。
18×(REM/140)-O/16≧0 ・・・(式4)
A系介在物:鋼中の非金属介在物のうち、加工によって粘性変形したものである。高延伸性を有し、加工を受けた鋼板においては加工方向に沿って延伸していることが多い。本実施形態では、アスペクト比(長径/短径)が3.0以上である介在物をA系介在物と定義する。
B系介在物:鋼中の非金属介在物のうち、加工方向に集団をなして不連続的に粒状の介在物がならんだものであり、角張った形状を有する場合が多く、低延伸性である。本実施形態では、加工方向に沿って3個以上の介在物が整列してなる介在物群であって、介在物同士の離間距離が50μm以下である介在物群を形成し、且つアスペクト比(長径/短径)が3.0未満である介在物をB系介在物と定義する。
C系介在物:粘性変形をしないで不規則に分散するものであり、角張った形状または球状形状を有する場合が多く、低延伸性である。本実施形態では、アスペクト比(長径/短径)が3.0未満であり、ランダムに分布する介在物をC系介在物と定義する。
非常に硬く、且つ角形状であるTi含有炭窒化物は、一般的にはこのC系介在物に分類されるが、本実施形態においてはC系介在物とは区別される場合がある。Ti含有炭窒化物は、単独で存在する場合、鋼板の特性に与える影響がその他のC系介在物(Ti含有炭窒化物ではないC系介在物)と比較して大きい。ここで「単独で存在するTi含有炭窒化物」とは、Tiを含有しない介在物に付着していない状態で存在するTi含有炭窒化物を示す。一方、Ti含有炭窒化物が他の介在物(例えばAl、Ca、O、S、及びREMを含む複合介在物)に付着した状態で存在する場合、Ti含有炭窒化物が鋼板の特性に与える影響は、その他のC系介在物と比較して同水準になる。本実施形態において、他の介在物に付着しているTi含有炭窒化物は、Ti含有炭窒化物ではないC系介在物であるとみなされる。
本実施形態において、「C系介在物の個数密度」とは、「Ti含有炭窒化物ではないC系介在物(C系介在物にTi含有炭窒化物が付着するものを含む)の個数密度」と、「単独で存在しているTi含有炭窒化物の個数密度」との合計である。Ti含有炭窒化物は、その形状およびその色調により他のC系介在物と区別することが可能である。
本発明者らは、質量%で、Cを0.25%超0.50%未満を含有する鋼に対して、さらにCaとREMとを含有させることについて検討した。その結果、化学成分中の各元素の質量%で示した含有量が、下記の式Iを満たすときに、鋼中のA系介在物、特に、A系介在物を構成するMnSを大きく低減できることを見出した。
真空溶解炉で、C含有量が、質量%で、0.45%であり、そして、トータルO(T.O.)、N、S、Ca、及びREMの含有量を表1に示す範囲で種々変更した化学成分を有する複数種類の鋼を、50kgインゴットとして作製した。これらのインゴットを、5mm厚となるように、仕上圧延温度が860℃の条件で熱間圧延し、そして、空冷して熱延鋼板を得た。
鋼中で、CaはSと結合してCaSを形成し、REMはS及びOと結合してREM2O2S(オキシサルファイド)を形成すると想定される。Sと結合するCaおよびREMの化学当量の合計R1は、Sの原子量を32.07、Caの原子量を40.88、REMの原子量を140とし、そして、化学成分中の各元素の質量%で示した含有量を用いて、
R1={Ca/40.88+(REM/140)/2}/(S/32.07)と表現することができる。
なお、Ca単独含有の場合の方が、REM及びCaの複合含有の場合よりも、鋼中のA系介在物の長径が長くなる。Ca単独含有の場合、CaO-Al2O3系の低融点酸化物がA系介在物として生成し、この酸化物が圧延時に延伸しているためと考えられる。したがって、鋼板の特性に悪影響を与える介在物の長径も考慮すると、Ca単独含有より、REM及びCaの複合含有が好ましい。
なお、R1の値が1.000であるとき、平均組成として、鋼中のSに結合する1当量のCaとREMとが鋼中に存在することになる。しかし実際には、R1の値が1.000であっても、デンドライト樹枝間のミクロ偏析部にMnSが生成するおそれがある。R1の値が2.000以上であるとき、デンドライト樹枝間のミクロ偏析部でのMnS生成を好ましく防止できる。一方、CaおよびREMを多量に含有させることにより、R1の値が5.000を超えると、最大長が20μmを超える粗大なB系またはC系介在物が生成する傾向がある。よって、R1の値は5.000以下であることが好ましい。すなわち、上記の式Iの右辺の上限値は、5.000であることが好ましい。
上記したように、熱延鋼板の上記観察面を観察して、アスペクト比(長径/短径)が3未満であり、粒径または長径が1μm以上であるB系介在物及びC系介在物の個数密度を計測した。その結果、Ca単独含有の場合、またはREM及びCaの複合含有の場合のいずれでも、Ca含有量が多いほど、B系介在物及びC系介在物の個数密度が増加することを発明者らは見出した。一方、REM含有量は、これらの介在物の個数密度に大きく影響しないことを発明者らは見出した。
なお、上述した炭素濃度範囲(C:0.25%超0.50%未満)を有する鋼の凝固時の相は、平衡状態図によると、包晶温度以上では液相+δ相であり、包晶温度以下では液相+γ相である。つまり、包晶温度を境に、S等の溶質元素のミクロ偏析度が異なる。ここで着目すべきは、界面活性元素であるので介在物捕捉に影響するSの固液分配係数は、相が液相+δ相である場合よりも、相が液相+γ相である場合の方が小さいということである。Sの固液分配係数が小さい場合、固相に分配されるSの量が少なくなり、液相に分配されるSの量が多くなる。液相に、界面活性元素であるSが多く分配された場合、液相と固相との間の界面エネルギーが低下するので、介在物が液相と固相との間の界面に捕捉されやすくなる。
鋼の温度が包晶温度以下(即ち、鋼の相が液相+γ相)である場合、Sが比較的多く液相に分配される。これにより、デンドライト樹枝(γ相)の間のSのミクロ偏析度が高くなる。したがって、包晶温度以下では特に介在物が捕捉されやすいと予想される。そして、C濃度が高いほどδ相が減少し、且つγ相が増加するので、介在物がデンドライト樹枝間に捕捉されやすくなる。式IIは、この効果も含めた評価、および観察結果に基づいて決定された。式IIは、鋼中のC濃度が包晶点よりも高い0.25%超0.50%未満である場合に成立する。
合金やスクラップ等の副原料からTiが混入すると、鋼中にTiNなどのTi含有炭窒化物が生成する。このTi含有炭窒化物は、その硬度が高く、さらにその形状が角形状である。そのため、鋼中に単独で粗大なTi含有炭窒化物が生成すると、この炭窒化物が破壊の起点となり易いので、鋼のシャルピー衝撃値、ひいては加工性が劣化する。
そこで、このような粗大なTi含有炭窒化物に起因する悪影響を減らすための別の手段について検討した結果、REM及びCaの複合含有が有効であることが本発明者らによって見出された。
REM及びCaの複合含有が行われた場合、まずAl、Ca、O、S、及びREMを含む複合介在物が鋼中に生成し、このREM含有複合介在物上に優先的に、Ti含有炭窒化物が複合析出する。REM含有複合介在物上にTi含有炭窒化物を優先的に複合析出させることにより、鋼中に単独で生成する角形状のTi含有炭窒化物を少なくすることができる。つまり、長辺の長さが5μm以上である粗大な単独のTi含有炭窒化物の個数密度を好ましく5個/mm2以下に減少させることができる。
Ti含有炭窒化物とREM含有介在物との複合物(すなわち、Al、Ca、O、S、及びREMを含む複合介在物の表面にTi含有炭窒化物が付着した介在物)は、単独で存在するTi含有炭窒化物よりも鋼板の諸特性に及ぼす悪影響が少ないので、単独で存在するTi含有炭窒化物ではないC系介在物であると見なされる。
C(炭素)は、鋼板の強度(硬度)を確保するうえで重要な元素である。C含有量を0.25%超とすることにより、鋼板の強度を確保する。C含有量が0.25%以下では、鋼板の焼入れ性が低下するので、この鋼板を素材として製造する製品、例えばギア類等に必要な強度が得られない。一方、C含有量が0.50%以上になると、加工性を確保する熱処理に長時間を要するので、熱処理を長時間化しなければ鋼板の加工性が悪化するおそれがある。さらに、C含有量が増大すると、B系介在物及びC系介在物の合計の個数密度が増加する。この原因は、C含有量が高い場合、溶鋼の凝固の際にデンドライト組織が長く発達し、デンドライト樹枝間に介在物が捕捉されやすくなるからであると推定される。よって、C含有量を0.25%超0.50%未満に制御する。
なお、C含有量の好ましい下限値は0.27%である。一般に、C含有量が高いほど、熱処理(焼入れ及び焼き戻し)を行った後の硬度および引張強さが増加する。特に、C含有量が0.27%以上であると、焼入れおよび低温焼き戻し処理を行った後に、1300MPa以上の強度を十分に確保できる。図3は、C含有量と引張強さとの関係を示すグラフである。本発明者らは、C含有量以外の条件を本実施形態に係る鋼板の条件を満たすようにし、且つC含有量を様々に異ならせた鋼板の引張強さを測定した。その結果、C含有量が0.27%以上である場合、鋼板が確実に1300MPaの引張強さを有することが明らかになった。さらに本実施形態に係る鋼板では、C含有量の下限を、好ましくは0.30%、C含有量の上限を、好ましくは0.48%とする。
Si(ケイ素)は、脱酸剤として作用し、また、焼入れ性を高めて鋼板の強度(硬度)を向上させるのに有効な元素である。Si含有量が0.10%未満では、上記含有効果が得られない。一方、Si含有量が0.60%を超えると、熱間圧延時のスケール疵に起因する鋼板の表面性状の劣化を招くおそれがある。よって、Si含有量を0.10%~0.60%に制御する。Si含有量の下限を、好ましくは0.15%、Si含有量の上限を、好ましくは0.55%とする。
Mn(マンガン)は、脱酸剤として作用する元素であるとともに、焼入れ性を高めて鋼板の強度(硬度)を向上させるのに有効な元素である。Mn含有量が0.40%未満では、その効果が十分得られない。一方、Mn含有量が0.90%を超えると、鋼板の加工性が劣化するおそれがある。よって、Mn含有量を0.40%~0.90%に制御する。Mn含有量の下限を、好ましくは0.50%、Mn含有量の上限を、好ましくは0.75%とする。
Al(アルミニウム)は、脱酸剤として作用する元素であるとともに、Nを固定することで鋼板の加工性を高めるのに有効な元素である。Al含有量が0.003%未満では、上記含有効果が十分に得られないので、0.003%以上を含有させる必要がある。一方、Al含有量が0.070%を超えると、上記含有効果は飽和し、さらに、粗大な介在物が増加する。この粗大な介在物によって、加工性が劣化する、または表面疵が発生し易くなるおそれがある。よって、Al含有量を0.003%~0.070%に制御する。Al含有量の下限を、好ましくは0.010%とし、Al含有量の上限を、好ましくは0.040%とする。
Ca(カルシウム)は、介在物の形態を制御し、これにより鋼板の加工性を向上させるために有効な元素である。Ca含有量が0.0005%未満では、上記効果が十分に得られない。介在物の形態の制御はREMによっても可能であるが、Ca含有量が0.0005%未満では、後述のREMを単独含有させた時と同様に、連続鋳造時にノズル詰まりが生じることにより操業の安定が妨げられ、さらに、高比重介在物が鋳片の下面側に堆積することにより鋼板の加工性が劣化するおそれがある。一方、Ca含有量が0.0040%を超えると、例えば、CaO-Al2O3系介在物などの粗大な低融点酸化物、および/またはCaS系介在物など圧延時に延伸し易い介在物が生成しやすくなり、これらによって鋼板の加工性が悪化するおそれがある。さらに、Ca含有量が0.0040%を超えると、ノズル耐火物が溶損しやすくなることにより連続鋳造の操業が安定しなくなるおそれがある。よって、Ca含有量を0.0005%~0.0040%に制御する。Ca含有量の下限を、好ましくは0.0007%、さらに好ましくは0.0010%とする。Ca含有量の上限を、好ましくは0.0030%、さらに好ましくは0.0025%とする。
REM(Rare Earth Metal)は希土類元素を意味し、スカンジウムSc(原子番号21)、イットリウムY(原子番号39)およびランタノイド(原子番号57のランタンから原子番号71のルテシウムまでの15元素)の17元素の総称である。本実施形態に係る鋼板では、これらのうちから選ばれる少なくとも1種以上の元素を含有する。一般的に、REMとして、入手のし易さから、Ce(セリウム)、La(ランタン)、Nd(ネオジム)、Pr(プラセオジム)などから選ばれることが多い。添加方法としては、例えば、鋼中にこれらの元素の混合物であるミッシュメタルとして添加することが広く行われている。ミッシュメタルの主成分はCe、La、Nd、およびPrである。本実施形態に係る鋼板では、鋼板に含有されるこれら希土類元素の合計量を、REM含有量とする。なお、上述したCaおよびREMの化学当量の合計R1の算出方法においては、ミッシュメタルの平均原子量が約140であるので、REMの原子量が140とされる。
P(リン)は、固溶強化の機能を有する。しかし、過剰な量のPの含有は、鋼板の加工性を阻害する。よって、P含有量を0.020%以下に制限する。P含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、P含有量の下限は0.005%であってもよい。
S(硫黄)は、非金属介在物を形成することにより、鋼板の加工性を阻害する不純物元素である。よって、S含有量を0.0070%以下に制限し、好ましくは、0.0050%以下に制限する。S含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、S含有量の下限は0.0003%であってもよい。
Ti(チタン)は、硬い角形状の炭窒化物を形成することにより、鋼板の加工性を劣化させる元素である。本実施形態においては、上述したようにREM含有介在物上に優先析出させることによって、加工性に及ぼす有害性を緩和する事が可能であるが、Ti含有量が0.050%を超えると加工性の劣化が顕在化する。よって、Ti含有量を0.050%以下に制限する。Ti含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、Ti含有量の下限は0.0005%であってもよい。
O(酸素)は、酸化物(非金属介在物)を形成し、この酸化物が凝集および粗大化することにより、鋼板の加工性を低下させる不純物元素である。よって、O含有量を0.0040%以下に制限する。O含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、O含有量の下限は0.0010%であってもよい。本実施形態に係る鋼板のO含有量は、鋼中に固溶するOや、介在物中に存在するOなどの、すべてのO含有量を合計したトータルO含有量(T.O含有量)を意味する。
N(窒素)は、窒化物(非金属介在物)を形成し、鋼板の加工性を低下させる不純物元素である。よって、N含有量を0.0075%以下に制限する。N含有量の下限は0%でもよい。また、現行の一般的な精錬(二次精錬を含む)を考慮すると、N含有量の下限は0.0010%であってもよい。
Cu(銅)は、鋼板の強度(硬度)を向上させる効果を有する選択元素である。そのため、必要に応じて、Cuを0.05%以下の範囲内で含有させても良い。また、Cu含有量の下限値を、0.01%とすると、好ましく上記効果を得ることができる。一方、Cu含有量が0.05%を超えると、溶融金属脆化(Cu割れ)によって熱間圧延時に熱間加工割れが生じる恐れがある。Cu含有量の好ましい範囲は0.02%~0.04%である。
Nb(ニオブ)は、炭窒化物を形成し、結晶粒の粗大化防止および鋼板の加工性の改善に有効な選択元素である。そのため、必要に応じて、Nbを0.05%以下の範囲内で含有させても良い。また、Nb含有量の下限値を、0.01%とすると、好ましく上記効果を得ることができる。一方、Nb含有量が0.05%を超えると、粗大なNb炭窒化物が析出して鋼板の加工性の低下を招く恐れがある。Nb含有量の好ましい範囲は0.02%~0.04%である。
V(バナジウム)は、Nbと同様に炭窒化物を形成し、結晶粒の粗大化防止や加工性の改善に有効な選択元素である。そのため、必要に応じて、Vを0.05%以下の範囲内で含有させても良い。また、V含有量の下限値を、0.01%とすると、好ましく上記効果を得ることができる。一方、V含有量が0.05%を超えると、粗大な介在物が生成して鋼板の加工性の低下を招く恐れがある。好ましい範囲は、0.02%~0.04%である。
Mo(モリブデン)は、焼入れ性の向上と焼戻し軟化抵抗性の向上とにより、鋼板の強度(硬度)を向上させる効果を有する選択元素である。そのため、必要に応じて、Moを0.05%以下の範囲内で含有させても良い。また、Mo含有量の下限値を、0.01%とすると、好ましく上記効果を得ることができる。一方、Mo含有量が0.05%を超えると、コストが増加し、且つ含有効果は飽和する。さらに、Mo含有量が0.05%を超えると、鋼板の加工性、特に冷間加工性が低下し、これにより、鋼板を複雑な形状(例えばギヤ形状など)に加工することが困難になる。以上の理由により、Mo含有量の上限を0.05%とする。Mo含有量の好ましい範囲は、0.01%~0.05%である。
Ni(ニッケル)は、焼入れ性の向上による鋼板の強度(硬度)の向上や、加工性の向上に有効な選択元素である。また、Cu含有時の溶融金属脆化(Cu割れ)を防止する効果も有する選択元素である。そのため、必要に応じて、Niを0.05%以下の範囲内で含有させても良い。また、Ni含有量の下限値を、0.01%以上とすると、好ましく上記効果を得ることができる。一方、Ni含有量が0.05%を超えると、コストが増加する一方で、含有効果は飽和するので、Ni含有量の上限を0.05%とする。Ni含有量の好ましい範囲は、0.02%~0.05%である。
Cr(クロム)は、焼入れ性を高めて鋼板の強度(硬度)を向上させるのに有効な元素である。そのため、必要に応じて、Crを0.50%以下の範囲内で含有させても良い。また、Cr含有量の下限値を0.01%とすると、好ましく上記効果を得ることができる。Cr含有量が0.50%を超えると、コストが増える一方で、含有効果は飽和する。よって、Cr含有量を0.50%以下に制御する。
B(ホウ素)は、焼入れ性を高めて鋼板の強度(硬度)を向上させる効果を有する選択元素である。そのため、必要に応じて、Bを0.0050%以下の範囲内で含有させても良い。また、B含有量の下限値を、0.0010%とすると、好ましく上記効果を得ることができる。一方、B含有量が0.0050%を超えると、B系化合物が生成して鋼板の加工性が低下するので上限を0.0050%とする。B含有量の好ましい範囲は、0.0020%~0.0040%である。
A系介在物の個数密度、ならびにB系介在物及びC系介在物の合計個数密度それぞれに関し、個数密度が6個/mm2を超える場合をB(Bad)、4個/mm2超6個/mm2以下の場合をG(Good)、2個/mm2超4個/mm2以下の場合をVG(Very Good)、2個/mm2以下の場合をGG(Greatly Good)とした。
B系及びC系であって最大長さ20μm以上の粗大介在物に関し、6個/mm2を超える場合をB(Bad)、3個/mm2超6個/mm2以下の場合をG(Good)、3個/mm2以下の場合をVG(Very Good)とした。
鋼中で単独に存在する長辺が5μm以上であるTi含有炭窒化物に関し、個数密度が5個/mm2を超える場合をB(Bad)、3個/mm2超5個/mm2以下の場合をG(Good)、3個/mm2以下の場合をVG(Very Good)とした。
比較例1は、Ca含有量が下限未満であったので、Caをほとんど含有しない介在物が生成した。これにより、比較例1にはB系介在物、C系介在物および粗大介在物が多数生成し、B系+C系介在物の個数密度の評価および20μm以上の粗大介在物の個数密度の評価が「B」であった。さらに、比較例1の鋳造中にノズル詰まりが生じた。
比較例2は、Ca含有量が上限を超えたので、粗大なCaO-Al2O3系低融点酸化物が生じた。これにより、比較例2のA系介在物の個数密度、B系+C系介在物の個数密度、および粗大介在物の個数密度の評価が「B」であった。
比較例3は、REM含有量が下限未満であり、且つ式3を満たさなかったので、マトリックス中に、粗大Ti含有炭窒化物が単独で多数生成した。これにより、比較例3のTi含有炭窒化物の個数密度の評価が「B」であった。
比較例4は、REM含有量が上限を超えたので、B系+C系介在物の個数密度の評価および粗大介在物の個数密度の評価が「B」であった。さらに、比較例4の鋳造中にノズル詰まりが生じた。
比較例5は、式1の右辺の値が0.3未満であったので、A系介在物の個数密度の評価が「B」であった。さらに、比較例5は、C含有量が過剰であったので、加工性が低かった。これにより、比較例5の衝撃値は不足した。
比較例6は、が式2を満たさなかったので、B系+C系介在物の個数密度の評価が「B」であった。
比較例7は、C含有量が不足していたので、引張強さが不足した。
比較例8は、介在物の個数密度が適切な水準であるが、C含有量が過剰であったので、加工性が低下した。このため、比較例8の穴拡げ性は不合格であった。
比較例9は、S含有量が過剰であったので、粗大なMnS介在物が生成し、A系介在物の個数密度の評価が「B」となった。さらに、比較例9の衝撃値および穴拡げ性は不十分であった。
比較例10は、Ti含有量が過剰であったので、Ti含有炭窒化物の個数密度の評価が「B」となった。これにより、比較例10の衝撃値および穴拡げ性は不十分であった。
比較例11は、Ca含有量が過剰であったので、CaO含有率が高い粗大酸化物が生成および延伸した。これにより、比較例11のA系介在物、およびB+C系粗大介在物の個数密度の評価が「B」となった。さらに比較例11では、CaO含有率が高いので、酸化物の表面にTi含有炭窒化物が付着する効果が低下した。これにより、比較例11のTi含有炭窒化物の個数密度の評価が「B」であった。以上の理由により、比較例11の衝撃値および穴拡げ性は不足した。
比較例12は、REM含有量が不足していたので、酸化物の表面にTi含有炭窒化物が付着する効果が低下した。これにより、比較例12のTi含有炭窒化物の個数密度の評価が「B」であった。そのため、比較例12の衝撃値および穴拡げ性は不足した。
比較例13は、REM含有量が過剰であったので、粗大介在物の個数密度の評価が「B」であった。そのため、比較例13の衝撃値および穴拡げ性は不足した。
比較例14は、Mo含有量が過剰であったので、介在物の個数密度評価が良好であるにもかかわらず、加工性が劣化した。これにより、比較例14の衝撃値および穴拡げ性は不足した。
比較例15は、式1を満たさなかったので、A系介在物の個数密度の評価が「B」であった。これにより、比較例15の衝撃値および穴拡げ性は不足した。
比較例16は、式2を満たさなかったので、B+C系介在物の個数密度の評価が「B」となった。そのため、比較例16の衝撃値および穴拡げ性は不足した。
Claims (5)
- 化学成分が、質量%で、
C :0.25%超0.50%未満、
Si:0.10%~0.60%、
Mn:0.40%~0.90%、
Al:0.003%~0.070%、
Ca:0.0005%~0.0040%、
REM:0.0003%~0.0050%、
Cu:0%~0.05%、
Nb:0%~0.05%、
V :0%~0.05%、
Mo:0%~0.05%、
Ni:0%~0.05%、
Cr:0%~0.50%、
B :0%~0.0050%、
を含有し、
P :0.020%以下、
S :0.0070%以下、
Ti:0.050%以下、
O :0.0040%以下、
N :0.0075%以下、
に制限し、
残部が鉄及び不純物からなり、
前記化学成分中の各元素の質量%で示した含有量が、下記の式1と下記の式2とを同時に満たし、
単独で存在する、長辺が5μm以上であるTi含有炭窒化物の個数密度が5個/mm2以下に制限されることを特徴とする鋼板。
0.3000≦{Ca/40.88+(REM/140)/2}/(S/32.07) ・・・(式1)
Ca≦0.0058-0.0050×C ・・・(式2) - 前記化学成分が、さらに、質量%で、
Cu:0.01%~0.05%、
Nb:0.01%~0.05%、
V :0.01%~0.05%、
Mo:0.01%~0.05%、
Ni:0.01%~0.05%、
Cr:0.01%~0.50%、
B :0.0010%~0.0050%、
のうちの1種以上を含有することを特徴とする請求項1に記載の鋼板。 - 前記鋼板が、さらに、Al、Ca、O、S、及びREMを含む複合介在物と、この複合介在物の表面に前記Ti含有炭窒化物が付着した介在物とを含むことを特徴とする請求項1または2に記載の鋼板。
- 前記化学成分中の前記各元素の質量%で示した含有量が、下記の式3を満たすことを特徴とする請求項1または2に記載の鋼板。
18×(REM/140)-O/16≧0 ・・・(式3) - 前記化学成分中の前記各元素の質量%で示した含有量が、下記の式4を満たすことを特徴とする請求項3に記載の鋼板。
18×(REM/140)-O/16≧0 ・・・(式4)
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JP2020084250A (ja) * | 2018-11-21 | 2020-06-04 | 日本製鉄株式会社 | 継目無鋼管用鋼材 |
JP2020084281A (ja) * | 2018-11-28 | 2020-06-04 | 日本製鉄株式会社 | 鋼板 |
WO2023276297A1 (ja) * | 2021-06-28 | 2023-01-05 | 日本製鉄株式会社 | 鋼材 |
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