WO2015004906A1 - 高炭素熱延鋼板およびその製造方法 - Google Patents
高炭素熱延鋼板およびその製造方法 Download PDFInfo
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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
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- C21D6/005—Heat treatment of ferrous alloys containing Mn
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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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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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
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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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- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- 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
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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
- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/32—Soft annealing, e.g. spheroidising
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/003—Cementite
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- C21—METALLURGY OF IRON
- 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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
Definitions
- the present invention relates to a high-carbon hot-rolled steel sheet excellent in hardenability and workability and a method for producing the same, and in particular, a high-carbon hot-rolled steel sheet to which B is added, It relates to the manufacturing method.
- Patent Document 1 contains, in mass%, C: 0.1 to 0.8%, Si: 0.15 to 0.40%, Mn: 0.3 to 1.0%, and P. 0.03% or less, S is 0.01% or less, T.I. Al is limited to a content of 0.1% or less, the hot-rolled steel sheet of hypoeutectoid steel, the balance of which is Fe and inevitable impurities, is subjected to cold rolling under light pressure of 20% to 30%, and then Ac1 After heating in the first stage that is held for 0.5 hours or more (excluding soaking 6 hours or more) in the temperature range of ⁇ 50 ° C. to less than Ac1, 0.5 to 20 in the temperature range of Ac1 to Ac1 + 100 ° C.
- the second stage heating for a time period and the third stage heating for 2 to 20 hours in the temperature range of Ar1-50 ° C. to Ar1 are continuously performed, and the third stage holding is performed from the second stage holding temperature.
- a method for producing a softened medium / high carbon steel sheet that is subjected to a three-stage annealing at a cooling rate of 5 to 30 ° C./h is disclosed.
- the invention described in Patent Document 1 aims to soften a hot-rolled steel sheet of medium / high carbon steel so that it can be sufficiently subjected to integral forming with a high workability while maintaining its hardenability. Is.
- Patent Document 2 discloses that when an annealing process using heating at an Ac1 point or higher is performed on a hot-rolled steel sheet containing C: 0.10 to 0.60% by mass, heating at an Ac1 point or higher is completed.
- metallographic alpha / gamma interface per gamma unit area is 0.5 [mu] m / [mu] m 2 or more in step, or one or more undissolved number carbides per 100 [mu] m 2 at the end of heating stage above Ac1 point, and, Excellent local ductility, characterized by having a metal structure with an ⁇ / ⁇ interface amount per unit area of ⁇ of 0.3 ⁇ m / ⁇ m 2 or more and then cooling to a temperature of Ar 1 point or less at a rate of 50 ° C./h or less.
- Patent Document 2 a method for producing a medium / high carbon steel sheet is disclosed.
- the invention described in Patent Document 2 can stably improve stretch flangeability in general medium and high carbon steel types without adding a special element, and can be hardened after parts are processed. It aims at providing the manufacturing method of the medium and high carbon steel plate raw material which can fully ensure.
- Patent Document 2 describes that an element that improves properties such as hardenability can be added.
- B describes that the hardenability of a steel material can be significantly improved by adding a very small amount. .
- the hot rolled steel sheet used as a material for press forming has an in-plane anisotropy ( ⁇ r) of r value (Lankford value) of 0 to ensure roundness and prevent uneven thickness. It may be required that it is close, ie, the absolute value of ⁇ r is small.
- Patent Document 1 it is necessary to perform cold rolling under light pressure before annealing.
- the technique described in Patent Document 1 intends to significantly reduce the hardness after annealing by performing three-stage annealing under predetermined conditions after performing such cold rolling under cold rolling.
- this technique requires a process of performing cold rolling under light rolling, which is a process that is not normally performed before annealing. For this reason, this technique has a problem that the production cost increases as compared with the case where such a process is not performed.
- B is described as an element that enhances hardenability by adding a trace amount.
- the inventors studied spheroidizing annealing in a nitrogen atmosphere generally used as spheroidizing annealing, and found that the hardenability could not be sufficiently secured even when B was added.
- the high carbon hot rolled steel sheet is required to have relatively low hardness and high elongation.
- a high carbon hot-rolled steel sheet that can be integrally formed with a cold press for automotive parts that have been manufactured in multiple processes such as hot forging, cutting, and welding has a Rockwell hardness HRB of 65 or less.
- a workability level such that the total elongation is 40% or more is required.
- excellent hardenability is desired for the high carbon hot-rolled steel sheet having good workability in this way, for example, to obtain a hardness of 440 or more with Vickers hardness (HV) after water quenching, Furthermore, it is desired to obtain a hardness of 500 or more with HV.
- HV Vickers hardness
- the present invention solves the above-mentioned problems, uses steel added with B as a raw material, and even if annealing is performed in a nitrogen atmosphere, stable and excellent hardenability is obtained, and before quenching treatment, HRB
- An object of the present invention is to provide a high carbon hot-rolled steel sheet having excellent workability such as 65 or less and a total elongation El of 40% or more and a method for producing the same.
- an object of the present invention is to provide a high carbon hot-rolled steel sheet having a small in-plane anisotropy of r value such that the absolute value of ⁇ r is 0.15 or less.
- the cementite density in the ferrite grains greatly affects the hardness and total elongation (hereinafter, also simply referred to as elongation) of the high carbon hot rolled steel sheet before quenching.
- elongation total elongation
- nitrogen in the atmosphere is nitrogend and concentrated in the steel sheet, and combined with B in the steel sheet to generate BN, so the amount of solute B in the steel sheet is greatly reduced.
- the nitrogen atmosphere is an atmosphere containing 90% by volume or more of nitrogen.
- the present invention has been made on the basis of such knowledge, and the gist thereof is as follows.
- the steel having the composition described in [1] or [2] is subjected to hot rolling at a finishing temperature: Ar3 transformation point or higher, and wound at a winding temperature: 500 to 750 ° C. After being taken, it is heated to the Ac1 transformation point or higher and held for 0.5 h or more, cooled to less than the Ar1 transformation point at 1 to 20 ° C./h, and held for 20 h or less below the Ar1 transformation point.
- high carbon hot-rolled steel sheet excellent in workability and workability.
- a high carbon hot-rolled steel sheet having excellent hardenability and cold workability (workability) can be produced.
- the high-carbon hot-rolled steel sheet of the present invention is suitable for automotive parts such as gears, transmissions, seat recliners, and hubs that require cold workability on the raw steel sheet.
- % which is a unit of component content, means “% by mass” unless otherwise specified.
- Composition C 0.20 to 0.48% C is an important element for obtaining strength after quenching.
- the amount of C is less than 0.20%, the desired hardness cannot be obtained by heat treatment after the steel sheet is formed into a part. For this reason, the amount of C needs to be 0.20% or more.
- the amount of C exceeds 0.48%, the steel plate becomes hard and the toughness and cold workability deteriorate. For this reason, the amount of C needs to be 0.48% or less.
- the C content is preferably 0.40% or less. Therefore, the C content is 0.20 to 0.48%.
- the C content is preferably 0.26% or more.
- the C content is preferably 0.32% or more.
- Si 0.10% or less
- Si is an element that increases the strength by solid solution strengthening. As the Si content increases, the steel sheet becomes hard and cold workability deteriorates, so the Si content is 0.10% or less. Preferably it is 0.05% or less. Since Si decreases the cold workability, the smaller the amount of Si, the better. However, if Si is excessively reduced, the refining cost increases, so the Si amount is preferably 0.005% or more.
- Mn 0.50% or less Mn is an element that improves hardenability and increases strength by solid solution strengthening.
- the amount of Mn exceeds 0.50%, a band structure due to segregation of Mn develops and the steel structure becomes non-uniform, so that cold workability is lowered. Therefore, the amount of Mn is 0.50% or less.
- the lower limit is not specified.
- the Mn content is preferably 0.20% or more in order to suppress the precipitation of graphite and dissolve the total amount of C in the steel sheet to obtain a predetermined quenching hardness.
- P 0.03% or less
- P is an element that increases the strength by solid solution strengthening. If the P content exceeds 0.03%, grain boundary embrittlement is caused and the toughness after quenching deteriorates. Therefore, the P content is 0.03% or less. In order to obtain excellent toughness after quenching, the P content is preferably 0.02% or less. P decreases the cold workability and toughness after quenching, so the smaller the amount of P, the better. On the other hand, if P is reduced excessively, the refining cost increases, so the amount of P is preferably 0.005% or more.
- S 0.010% or less
- S is an element that has to be reduced in order to form sulfides and to reduce the cold workability of the high carbon hot-rolled steel sheet and the toughness after quenching.
- the S amount is 0.010% or less.
- the S content is preferably 0.005% or less. Since S decreases cold workability and toughness after quenching, the smaller the amount of S, the better.
- the amount of S is preferably 0.0005% or more.
- sol. Al 0.10% or less sol.
- the amount of Al (acid-soluble aluminum) exceeds 0.10%, AlN is generated during heating in the quenching process and the austenite grains are excessively refined. As a result, generation of a ferrite phase is promoted during cooling of the quenching process. The steel structure becomes ferrite and martensite, the hardness after quenching decreases, and the toughness after quenching deteriorates. Therefore, the amount of sol.Al is 0.10% or less. Preferably, the amount of sol.Al is 0.06% or less. Note that sol. Al has a deoxidizing effect, and in order to sufficiently deoxidize, Al is preferably 0.005% or more.
- N 0.0050% or less
- the amount of N exceeds 0.0050%, the amount of dissolved B decreases due to the formation of BN.
- the formation of BN and AlN causes the austenite grains to become too fine during the heating of the quenching process, and as a result, the formation of the ferrite phase is promoted during the cooling of the quenching process.
- the toughness after quenching decreases. Therefore, the N content is 0.0050% or less. There is no specific lower limit.
- N is an element that forms BN and AlN, thereby appropriately suppressing the growth of austenite grains during heating in the quenching process and improving the toughness after quenching. 0005% or more is preferable.
- B 0.0005 to 0.0050%
- B is an important element that enhances hardenability.
- the amount of B is less than 0.0005%, a sufficient effect is not recognized, so the amount of B needs to be 0.0005% or more.
- the B amount is 0.0009% or more.
- the amount of B exceeds 0.0050%, the recrystallization of austenite after finish rolling is delayed. As a result, the texture of the hot-rolled steel sheet develops, and the anisotropy of the steel sheet after annealing is increased. growing. For this reason, the amount of B needs to be 0.0050% or less.
- the amount of B is 0.0035% or less. Therefore, the B amount is set to 0.0005 to 0.0050%.
- 0.002 to 0.030% in total of one or more of Sb, Sn, Bi, Ge, Te, Se Sb, Sn, Bi, Ge, Te, and Se are important elements for suppressing nitriding from the surface layer.
- the total amount of these elements is less than 0.002%, a sufficient effect is not recognized. For this reason, 1 or more types of Sb, Sn, Bi, Ge, Te, and Se are contained, and the minimum of the total amount of these elements shall be 0.002%.
- the lower limit of the total amount of these elements is 0.005%.
- even if these elements are added in a total content exceeding 0.030% the effect of preventing nitriding is saturated.
- the total content of Sb, Sn, Bi, Ge, Te, Se is 0.030% as an upper limit.
- the total content of Sb, Sn, Bi, Ge, Te, Se is 0.020% or less. Therefore, at least one of Sb, Sn, Bi, Ge, Te, and Se is contained, and the total content of these elements is set to 0.002 to 0.030%.
- the total content of Sb, Sn, Bi, Ge, Te and Se is 0.005 to 0.020%.
- At least one of Sb, Sn, Bi, Ge, Te, and Se is made 0.002 to 0.030% in total.
- solid solution B can be secured in the steel sheet after annealing.
- ⁇ (solid solution B amount) / (addition B amount) ⁇ ⁇ 100 (%) which is the ratio of the solid solution B amount in the steel sheet and the added B amount, can be 75 (%) or more.
- High hardenability can be obtained.
- the added B amount is the B content in the steel.
- the balance is Fe and inevitable impurities, but in order to further improve the hardenability, at least one of Ni, Cr, and Mo can be contained in a total amount of 0.50% or less. That is, at least one of Ni, Cr, and Mo can be contained, and the total content of Ni, Cr, and Mo can be 0.50% or less. Since Ni, Cr, and Mo are expensive, the total content is preferably 0.20% or less in order to suppress high costs. In order to obtain the effects described above, the total content of Ni, Cr, and Mo is preferably set to 0.01% or more.
- the microstructure of the steel sheet of the present invention is a microstructure composed of ferrite and cementite whose cementite density in the ferrite grains is 0.10 pieces / ⁇ m 2 or less.
- the cementite density in the ferrite grains is preferably 0.06 pieces / ⁇ m 2 or less, more preferably less than 0.04 pieces / ⁇ m 2 .
- the cementite density in the ferrite grains may be 0 / ⁇ m 2 .
- the cementite diameter present in the ferrite grains is about 0.15 to 1.8 ⁇ m as the major axis, which is an effective size for precipitation strengthening of the steel sheet. For this reason, in the steel plate of the present invention, the strength can be reduced by reducing the cementite density in the grains.
- the cementite density in the ferrite grain is defined as 0.10 pieces / ⁇ m 2 or less.
- the volume ratio of cementite is approximately 2.5% to 7.0%.
- the effect of the present invention is not impaired as long as the total volume ratio of the remaining structure is about 5% or less. Therefore, the remaining structure such as pearlite may be contained if the total volume ratio is 5% or less.
- the high carbon hot-rolled steel sheet of the present invention is required to reduce the hardness of the steel sheet to HRB 65 or less, increase the elongation to have El of 40% or more, and to have excellent workability and improve hardenability. , Has excellent hardenability.
- a water quenching process or an oil quenching process is performed.
- the water quenching process is, for example, a process of heating to approximately 850 to 1050 ° C., holding for approximately 0.1 to 600 seconds, and immediately cooling with water.
- the oil quenching process is, for example, a process of heating to approximately 800 to 1050 ° C., holding for approximately 60 to 3600 seconds, and immediately cooling with oil.
- the excellent hardenability is, for example, to obtain a hardness of 440 or more in terms of Vickers hardness (HV) by performing a water quenching process of holding at 870 ° C. for 30 s and immediately cooling with water, and more preferably at HV It is to obtain a hardness of 500 or more.
- the microstructure after the water quenching process or the oil quenching process is a martensite single phase structure or a mixed structure of martensite phase and bainite phase.
- the high-carbon hot-rolled steel sheet of the present invention is made of steel having the above composition, and after hot rough rolling, it is finish-rolled at a finishing temperature: Ar3 transformation point or higher, and a winding temperature: 500- After being wound at 750 ° C., heated to the Ac1 transformation point or higher and held for 0.5 h or more, cooled to less than the Ar1 transformation point at 1 to 20 ° C./h, and held for 20 h or more below the Ar1 transformation point Manufactured.
- a finishing temperature Ar3 transformation point or higher
- a winding temperature 500- After being wound at 750 ° C., heated to the Ac1 transformation point or higher and held for 0.5 h or more, cooled to less than the Ar1 transformation point at 1 to 20 ° C./h, and held for 20 h or more below the Ar1 transformation point Manufactured.
- Finishing temperature Ar3 transformation point or higher If the finishing temperature is less than the Ar3 transformation point, coarse ferrite grains are formed after hot rolling and after annealing, and the elongation is significantly reduced. For this reason, finishing temperature shall be more than Ar3 transformation point.
- the upper limit of the finishing temperature is not particularly required, but is preferably 1000 ° C. or lower in order to smoothly perform cooling after finishing rolling.
- Winding temperature 500-750 ° C
- the hot-rolled steel sheet after finish rolling is wound into a coil shape. If the coiling temperature is too high, the strength of the hot-rolled steel sheet becomes too low, and when it is wound into a coil shape, it may be deformed by its own weight. Therefore, the upper limit of the coiling temperature is 750 ° C. On the other hand, when the coiling temperature is too low, the hot-rolled steel sheet is hardened, which is not preferable. Therefore, the lower limit is set to 500 ° C.
- Heat to Ac1 transformation point or higher and hold for 0.5 h or longer (first annealing), cool to less than Ar1 transformation point at 1-20 ° C./h, and hold for 20 h or longer below Ar1 transformation point (second step)
- the hot-rolled steel sheet is heated to the Ac1 transformation point or higher and held for 0.5 h (0.5 hours) or more, and relatively fine carbides precipitated in the hot-rolled steel sheet. Is dissolved in the ⁇ phase. Thereafter, it is cooled to less than the Ar1 transformation point at 1 to 20 ° C./h, and maintained for 20 hours or more below the Ar1 transformation point, so that solid solution C is precipitated with relatively coarse undissolved carbides as nuclei.
- the cementite density in the ferrite grains is set to 0.10 pieces / ⁇ m 2 or less, and the dispersion of the carbide (cementite) is controlled. That is, in the present invention, by performing two-stage annealing under a predetermined condition, the dispersion form of carbide is controlled and the steel sheet is softened. In the high carbon steel sheet which is the subject of the present invention, it is important to control the dispersion form of carbides after annealing for softening. In the present invention, the high-carbon hot-rolled steel sheet is heated to the Ac1 transformation point or higher and held (first-stage annealing), thereby dissolving fine carbides and dissolving C in ⁇ (austenite).
- the ⁇ / ⁇ interface and undissolved carbide existing in the temperature range above the Ac1 point become nucleation sites, and relatively coarse carbide is formed. Precipitate.
- the atmospheric gas for annealing any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used.
- the atmosphere gas at the time of annealing may be any of the above gases, but a gas containing 90% by volume or more of nitrogen is preferable from the viewpoint of cost and safety.
- an annealing temperature shall be 800 degrees C or less, and it is preferable that holding time shall be 10 hours or less.
- Cooling to less than the Ar1 transformation point at 1 to 20 ° C./h After the first stage annealing described above, cooling is performed at 1 to 20 ° C./h to less than the Ar1 transformation point, which is the temperature range of the second stage annealing.
- C carbon
- the discharged C is precipitated as a relatively coarse spherical carbide using an ⁇ / ⁇ interface or undissolved carbide as a nucleation site. In this cooling, it is necessary to adjust the cooling rate so that pearlite is not generated.
- the cooling rate from the first stage annealing to the second stage annealing is less than 1 ° C./h, the production efficiency is poor, so the cooling rate is set to 1 ° C./h or more.
- the cooling rate is 5 ° C./h or more.
- the cooling rate is 20 ° C./h or less.
- the cooling rate is 15 ° C./h or less.
- cooling is performed at 1 to 20 ° C./h to less than the Ar1 transformation point that is the temperature range of the second stage annealing.
- it is cooled to a temperature range of 660 ° C. or higher which is lower than the Ar1 transformation point which is preferable as the temperature range of the second stage annealing.
- the second stage annealing After the first-stage annealing described above, by cooling at a predetermined cooling rate and maintaining it below the Ar1 transformation point, coarse spherical carbides are further grown by Ostwald growth, and fine carbides disappear. If the retention time below the Ar1 transformation point is less than 20 h, the carbide cannot be sufficiently grown, and the hardness after annealing becomes too large. For this reason, the second-stage annealing is held for 20 hours or longer below the Ar1 transformation point.
- the holding at 720 ° C. or lower is preferable. Preferably, the holding time is 22 hours or longer.
- the annealing temperature in the second stage is preferably set to 660 ° C. or higher in order to sufficiently grow carbide, and the holding time is preferably set to 30 h or less from the viewpoint of production efficiency. .
- both a converter and an electric furnace can be used.
- the high carbon steel thus melted is made into a slab by ingot-bundling rolling or continuous casting.
- the slab is usually heated and then hot rolled.
- slab heating temperature 1280 degrees C or less in order to avoid the deterioration of the surface state by a scale.
- the material to be rolled may be heated by a heating means such as a sheet bar heater during hot rolling.
- the above-mentioned finishing temperature in hot rolling is 900 ° C. or higher. If the finishing temperature is less than 900 ° C., the rolled structure (unmodified form) tends to remain, and the in-plane anisotropy of the r value after annealing may increase.
- the finishing temperature is set to 900 ° C. or more, the in-plane anisotropy of the r value of the hot-rolled steel sheet after annealing can be made 0.15 or less in absolute value, and ⁇ r can be brought close to 0. .
- finishing temperature when making the in-plane anisotropy of r value small, it is preferable that finishing temperature shall be 900 degreeC or more. Furthermore, in order to make the in-plane anisotropy of the r value 0.10 or less in absolute value, the finishing temperature is preferably set to 950 ° C. or more.
- the hardness of the steel plate after annealing A sample was taken from the center of the plate width of the steel plate (original plate) after annealing, and measured at five points using a Rockwell hardness meter (B scale) to obtain an average value.
- Microstructure of the steel sheet after annealing is obtained by cutting a sample taken from the center of the plate width, polishing the cut surface (thickness cross section in the rolling direction), applying nital corrosion, and using a scanning electron microscope Tissue photographs were taken at a magnification of 3000 times at five locations in the center of the thickness. Using the photographed structure photograph, the number of cementite having a major axis of 0.15 ⁇ m or more that was not on the grain boundary was measured, and this number was divided by the area of the field of view of the photograph to determine the cementite density in the grain.
- r ln (w / w0) / ln (t / t0) (1)
- w board width after 12% strain application
- w0 board width before test
- t board thickness after 12% strain application
- t0 board thickness before test.
- ⁇ r (r0 + r90-2r45) / 2 (2)
- r0, r45, and r90 are r values obtained by using tensile test pieces cut in directions of 0 °, 45 °, and 90 ° with respect to the rolling direction, respectively.
- the amount of nitrogen in the surface layer of 150 ⁇ m is the amount of nitrogen contained in the range from the steel plate surface to the depth of 150 ⁇ m in the plate thickness direction. Further, the amount of nitrogen in the surface layer of 150 ⁇ m was determined as follows.
- Cutting was started from the surface of the collected steel plate, the steel plate was cut to a depth of 150 ⁇ m from the surface, and chips generated at this time were collected as samples.
- the amount of N in this sample was measured to obtain a nitrogen amount of 150 ⁇ m on the surface layer.
- the amount of nitrogen at the surface layer of 150 ⁇ m and the average amount of N in the steel sheet were determined by measuring each amount of N by an inert gas transportation fusion-thermal conductivity method. If the difference between the nitrogen content of the surface layer 150 ⁇ m thus obtained (the nitrogen content in the range from the surface to a depth of 150 ⁇ m from the surface) and the average N content in the steel sheet (N content in the steel) is 30 mass ppm or less. It can be evaluated that the nitrification can be suppressed.
- the amount of solid solution B / the amount of added B is obtained by extracting BN in a steel plate with 10 (volume%) Br methanol by using a sample taken from the center of the plate width of the steel plate after annealing to form BN. The amount of B contained was measured, and the amount of B forming BN was subtracted from the total amount of B added. Further, the solid solution B amount / added B amount, which is the ratio between the solid solution B amount thus obtained and the added B amount (B content), was obtained. If ⁇ solid solution B amount (mass%) / added B amount (mass%) ⁇ ⁇ 100 (%) is 75 (%) or more, it can be evaluated that the decrease in the solid solution B amount can be suppressed.
- Steel plate hardness after quenching (quenching hardness) A flat plate test piece (width 15 mm ⁇ length 40 mm ⁇ plate thickness 4 mm) is collected from the center of the plate width of the steel plate after annealing, and subjected to quenching treatment by two methods of water cooling and 120 ° C. oil cooling as follows, The steel plate hardness (quenching hardness) after quenching was determined by each method. That is, the quenching treatment is a method in which the flat plate test piece is used and held at 870 ° C. for 30 s and immediately water-cooled (water cooling), and held at 870 ° C. for 30 s and immediately cooled with 120 ° C. oil (120 ° C. oil-cooled). It carried out in.
- the hardness of the cut surface of the test piece after the quenching treatment was measured with a Vickers hardness tester under the condition of a load of 1 kgf, and the average hardness was obtained.
- the quenching hardness was determined to be acceptable ( ⁇ ) when the conditions shown in Table 3 were satisfied after water cooling and after 120 ° C. oil cooling, and evaluated as excellent in quenchability.
- any of the hardness after water cooling and the hardness after 120 ° C. oil cooling did not satisfy the conditions shown in Table 3, it was judged as rejected ( ⁇ ) and evaluated as inferior in hardenability.
- Table 3 represents the quenching hardness according to the C content that can be evaluated as having sufficient quenchability from experience.
- hot-rolled steel sheet of the present invention embodiment it is found to have a microstructure comprising a cementite density in the ferrite grains from 0.10 pieces / [mu] m 2 or less and ferrite and cementite.
- the hot-rolled steel sheet of the example of the present invention has an HRB of 65 or less and a total elongation of 40% or more, and it is understood that it is excellent in cold workability and hardenability.
- the hot-rolled steel sheet of the example of the present invention manufactured with a finishing temperature of 900 ° C. or higher has ⁇ r of ⁇ 0.14 to ⁇ 0.07, and the absolute value of ⁇ r has reached 0.15 or less and is zero. It can be seen that a close ⁇ r is obtained and the anisotropy is small.
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Abstract
Description
i)焼入れ前の高炭素熱延鋼板の硬度、全伸び(以下、単に伸びともいう)には、フェライト粒内のセメンタイト密度が大きく影響する。フェライト粒内のセメンタイト密度を0.10個/μm2以下とすることで、硬さがHRBで65以下、全伸び(El)が40%以上といった優れた加工性が得られる。
ii)窒素雰囲気で焼鈍を施す場合、雰囲気中の窒素が浸窒して鋼板中に濃化し、鋼板中のBと結合してBNを生成するため、鋼板中の固溶B量が大幅に低下する。なお、窒素雰囲気とは、窒素を90体積%以上含む雰囲気である。一方、Sb、Sn、Bi、Ge、Te、Seの少なくとも1種を鋼中に所定量添加することで、このような浸窒を防止し、固溶B量の低下を抑制して優れた焼入れ性が得られる。
[5]前記仕上温度が900℃以上であることを特徴とする前記[4]に記載の焼入れ性および加工性に優れる高炭素熱延鋼板の製造方法。
C:0.20~0.48%
Cは、焼入れ後の強度を得るために重要な元素である。C量が0.20%未満の場合、鋼板を部品に成形した後の熱処理によって所望の硬さが得られない。このため、C量は0.20%以上にする必要がある。一方、C量が0.48%を超えると鋼板が硬質化し、靭性や冷間加工性が劣化する。このため、C量は0.48%以下にする必要がある。C量は0.40%以下とすることが好ましい。したがって、C量は0.20~0.48%とする。優れた焼入れ硬さを得るには、C量は0.26%以上とすることが好ましい。さらには安定して水焼入れ後のビッカース硬さ(HV)で500以上を得るためには、C量は0.32%以上とすることが好ましい。
Siは固溶強化により強度を上昇させる元素である。Si量の増加とともに鋼板が硬質化し、冷間加工性が劣化するため、Si量は0.10%以下とする。好ましくは0.05%以下である。Siは冷間加工性を低下させるため、Si量は少ないほど好ましいが、過度にSiを低減すると精錬コストが増大するため、Si量は0.005%以上が好ましい。
Mnは焼入れ性を向上させるとともに、固溶強化により強度を上昇させる元素である。Mn量が0.50%を超えると、Mnの偏析に起因したバンド組織が発達し、鋼組織が不均一になるため、冷間加工性が低下する。したがって、Mn量は0.50%以下とする。なお、下限はとくに指定しない。焼入れ時の溶体化処理において、グラファイト析出を抑制して鋼板中の全C量を固溶して所定の焼入れ硬さを得るためには、Mn量は0.20%以上が好ましい。
Pは固溶強化により強度を上昇させる元素である。P量が0.03%を超えて増加すると粒界脆化を招き、焼入れ後の靭性が劣化する。したがって、P量は0.03%以下とする。優れた焼入れ後の靭性を得るには、P量は0.02%以下が好ましい。Pは冷間加工性および焼入れ後の靭性を低下させるため、P量は少ないほど好ましい。一方、過度にPを低減すると精錬コストが増大するため、P量は0.005%以上が好ましい。
Sは硫化物を形成し、高炭素熱延鋼板の冷間加工性および焼入れ後の靭性を低下させるため、低減しなければならない元素である。S量が0.010%を超えると、高炭素熱延鋼板の冷間加工性および焼入れ後の靭性が著しく劣化する。したがって、S量は0.010%以下とする。優れた冷間加工性および焼入れ後の靭性を得るには、S量は0.005%以下が好ましい。Sは冷間加工性および焼入れ後の靭性を低下させるため、S量は少ないほど好ましい。一方、過度にSを低減すると精錬コストが増大するため、S量は0.0005%以上が好ましい。
sol.Al(酸可溶性アルミニウム)量が0.10%を超えると、焼入れ処理の加熱時にAlNが生成してオーステナイト粒が微細化し過ぎ、この結果、焼入れ処理の冷却時にフェライト相の生成が促進されて、鋼組織がフェライトとマルテンサイトとなり、焼入れ後の硬さが低下するとともに、焼入れ後の靭性が劣化する。したがって、sol.Al量は0.10%以下とする。好ましくは、sol.Al量は0.06%以下とする。なお、sol.Alは脱酸の効果を有しており、十分に脱酸するためには、0.005%以上とすることが好ましい。
N量が0.0050%を超えると、BNの形成により固溶B量が低下する。また、N量が0.0050%を超えると、BN、AlNの形成により焼入れ処理の加熱時にオーステナイト粒が微細化し過ぎ、この結果、焼入れ処理の冷却時にフェライト相の生成が促進されて、焼入れ後の硬さが低下するとともに、焼入れ後の靭性が低下する。したがって、N量は0.0050%以下とする。下限はとくに規定しない。なお、上記したように、NはBN、AlNを形成し、これにより焼入れ処理の加熱時にオーステナイト粒の成長を適度に抑制し、焼入れ後の靭性を向上させる元素であるため、N量は0.0005%以上が好ましい。
Bは焼入れ性を高める重要な元素である。B量が0.0005%未満の場合、十分な効果が認められないため、B量は0.0005%以上とする必要がある。好ましくは、B量は0.0009%以上とする。一方、B量が0.0050%を超えると、仕上圧延後のオーステナイトの再結晶が遅延し、その結果、熱延鋼板の集合組織(texture)が発達し、焼鈍後の鋼板の異方性が大きくなる。このため、B量は0.0050%以下とする必要がある。好ましくは、B量は0.0035%以下である。したがって、B量は0.0005~0.0050%とする。
Sb、Sn、Bi、Ge、Te、Seは表層からの浸窒抑制に重要な元素である。これら元素の合計の量が0.002%未満の場合、十分な効果が認められない。このため、Sb、Sn、Bi、Ge、Te、Seの1種以上を含有し、かつ、これら元素の合計量の下限を0.002%とする。好ましくは、これら元素の合計量の下限は0.005%である。一方、これらの元素を、その含有量の合計で0.030%超えとして添加しても、浸窒防止効果は飽和する。また、これらの元素は粒界に偏析する傾向があるため、これらの元素の含有量を合計で0.030%超えとすると、含有量が高くなりすぎ、粒界脆化を引き起こす可能性がある。したがって、Sb、Sn、Bi、Ge、Te、Seの含有量の合計は0.030%を上限とする。好ましくは、Sb、Sn、Bi、Ge、Te、Seの含有量の合計は0.020%以下である。よって、Sb、Sn、Bi、Ge、Te、Seのうち1種以上を含有し、これら元素の含有量の合計を0.002~0.030%とする。好ましくは、Sb、Sn、Bi、Ge、Te、Seの含有量の合計は、0.005~0.020%である。
フェライト粒内のセメンタイト密度が高いと、分散強化により硬質化し、伸びが低下する。本発明では、フェライト粒内のセメンタイト密度を0.10個/μm2以下とすることで、ロックウェル硬さがHRBで65以下、全伸び40%以上を達成することができる。このため、本発明の鋼板のミクロ組織は、フェライト粒内のセメンタイト密度が0.10個/μm2以下であるフェライトとセメンタイトからなるミクロ組織とする。フェライト粒内のセメンタイト密度は、好ましくは0.06個/μm2以下であり、さらに好ましくは0.04個/μm2未満である。フェライト粒内のセメンタイト密度は0個/μm2であってもよい。なお、フェライト粒内に存在するセメンタイト径は長径で0.15~1.8μm程度であり、鋼板の析出強化に有効なサイズである。このため、本発明の鋼板では、粒内のセメンタイト密度を低下することで強度低下を図ることができる。一方、フェライト粒界のセメンタイトは分散強化にほとんど寄与しないので、フェライト粒内のセメンタイト密度を0.10個/μm2以下と規定する。
本発明では、ギア、トランスミッション、シートリクライナーなどの自動車用部品を冷間プレスで成形するため優れた加工性が必要である。また、焼入れ処理により硬さを大きくして耐磨耗性を付与する必要がある。そのため、本発明の高炭素熱延鋼板は、鋼板の硬さを低減してHRB65以下とし、伸びを高めてElを40%以上として優れた加工性を有するとともに、焼入れ性を向上させる必要があり、優れた焼入れ性を有する。
本発明の高炭素熱延鋼板は、上記のような組成の鋼を素材とし、熱間粗圧延後、仕上温度:Ar3変態点以上で仕上圧延を行い、巻取温度:500~750℃で巻き取った後、Ac1変態点以上に加熱して0.5h以上保持し、1~20℃/hでAr1変態点未満に冷却して、Ar1変態点未満で20h以上保持することにより製造される。
以下、本発明の高炭素熱延鋼板の製造方法における限定理由について説明する。
仕上温度がAr3変態点未満では、熱間圧延後および焼鈍後に粗大なフェライト粒が形成され、伸びが著しく低下する。このため、仕上温度はAr3変態点以上とする。なお、仕上温度の上限は、特に規定する必要はないが、仕上圧延後の冷却を円滑に行うためには、1000℃以下とすることが好ましい。
仕上圧延後の熱延鋼板は、コイル形状に巻き取られる。巻取温度が高すぎると熱延鋼板の強度が低くなり過ぎて、コイル形状に巻き取られた際、コイルの自重で変形する場合があるため、操業上好ましくない。したがって巻取温度の上限を750℃とする。一方、巻取温度が低すぎると熱延鋼板が硬質化するため好ましくない。したがって下限を500℃とする。
本発明では、熱延鋼板をAc1変態点以上に加熱して0.5h(0.5時間)以上保持し、熱延鋼板中に析出していた比較的微細な炭化物を溶解してγ相中に固溶させる。その後1~20℃/hでAr1変態点未満に冷却し、Ar1変態点未満で20h以上保持することにより、比較的粗大な未溶解炭化物等を核として固溶Cを析出させる。これにより、フェライト粒内のセメンタイト密度を0.10個/μm2以下とし、炭化物(セメンタイト)の分散を制御された状態とする。すなわち、本発明では、所定条件で2段焼鈍を施すことで、炭化物の分散形態を制御し、鋼板を軟質化させる。本発明で対象とする高炭素鋼板では、軟質化する上で焼鈍後における炭化物の分散形態を制御することが重要となる。本発明では、高炭素熱延鋼板をAc1変態点以上に加熱して保持する(1段目の焼鈍)ことで、微細な炭化物を溶解するとともに、Cをγ(オーステナイト)中に固溶する。その後のAr1変態点未満の冷却段階や保持段階(2段目の焼鈍)において、Ac1点以上の温度域で存在するα/γ界面や未溶解炭化物が核生成サイトとなり、比較的粗大な炭化物が析出する。以下、このような2段焼鈍の条件について説明する。なお、焼鈍の際の雰囲気ガスは、窒素、水素、窒素と水素の混合ガスのいずれも使用できる。また、焼鈍の際の雰囲気ガスは、前記のガスのいずれであってもよいが、コストおよび安全性の観点から、窒素を90体積%以上含むガスが好ましい。
熱延鋼板をAc1点以上の焼鈍温度に加熱することにより、鋼板組織のフェライトの一部をオーステナイトに変態させ、フェライト中に析出していた微細な炭化物を溶解させ、Cをオーステナイト中に固溶させる。一方、オーステナイトに変態せずに残ったフェライトは高温で焼鈍されるため、転位密度が減少して軟化する。また、フェライト中には溶解しなかった比較的粗大な炭化物(未溶解炭化物)が残存するが、このような炭化物は、オストワルド成長(Ostwald growth)により、より粗大になる。焼鈍温度がAc1変態点未満では、オーステナイト変態が生じないため、炭化物をオーステナイト中に固溶させることができない。また、本発明では、Ac1変態点以上での保持時間が0.5h未満では微細な炭化物を十分に溶解することができない。このため、1段目の焼鈍として、Ac1変態点以上に加熱して0.5h以上保持することとする。好ましくは、(Ac1+10)℃以上に加熱する。また、好ましくは、1.0h以上の保持とする。なお、特に限定するものではないが、焼鈍温度は800℃以下とすることが好ましく、また、保持時間は10h以下とすることが好ましい。
上記した1段目の焼鈍の後、2段目の焼鈍の温度域であるAr1変態点未満に、1~20℃/hで冷却する。冷却途中において、オーステナイト→フェライト変態に伴いオーステナイトからC(炭素)が吐き出される。この吐き出されるCは、α/γ界面や未溶解炭化物を核生成サイトとして、比較的粗大な球状炭化物として析出する。この冷却においては、パーライトが生成しないように冷却速度を調整する必要がある。1段目の焼鈍後、2段目の焼鈍までの冷却速度が、1℃/h未満では生産効率が悪いため、該冷却速度は1℃/h以上とする。好ましくは、該冷却速度は5℃/h以上である。一方、該冷却速度が20℃/hを超えて大きくなると、パーライトが析出し、硬度が高くなるため、該冷却速度は20℃/h以下とする。好ましくは、該冷却速度は15℃/h以下である。このため、1段目の焼鈍後、2段目の焼鈍の温度域であるAr1変態点未満まで、1~20℃/hで冷却する。好ましくは、2段目の焼鈍の温度域として好ましいAr1変態点未満660℃以上の温度域まで冷却する。
上記した1段目の焼鈍後、所定の冷却速度で冷却してAr1変態点未満で保持することで、オストワルド成長により、粗大な球状炭化物をさらに成長させ、微細な炭化物を消失させる。Ar1変態点未満での保持時間は、20h未満では、炭化物を十分に成長させることができず、焼鈍後の硬度が大きくなりすぎる。このため、2段目の焼鈍はAr1変態点未満で20h以上保持とする。好ましくは、720℃以下での保持である。また、好ましくは、保持時間は22h以上である。なお、特に限定するものではないが、2段目の焼鈍温度は炭化物を十分成長させるため660℃以上とすることが好ましく、また、保持時間は生産効率の観点から、30h以下とすることが好ましい。
焼鈍後の鋼板(原板)の板幅中央部から試料を採取し、ロックウェル硬度計(Bスケール)を用いて5点測定し、平均値を求めた。
焼鈍後の鋼板(原板)から、圧延方向に対して0°の方向(L方向)に切り出したJIS5号引張試験片を用いて、島津製作所AG10TB AG/XRの引張試験機にて10mm/分で引張試験を行い、破断したサンプルを突き合わせて伸びを求めた。
焼鈍後の鋼板のミクロ組織は、板幅中央部から採取した試料を切断し、切断面(圧延方向板厚断面)を研磨後、ナイタール腐食を施し、走査型電子顕微鏡を用いて、板厚中央部の5箇所で3000倍の倍率で組織写真を撮影した。撮影した組織写真を用いて、粒界上になく、長径が0.15μm以上のセメンタイトの個数を測定し、この個数を写真の視野の面積で除して、粒内のセメンタイト密度を求めた。
焼鈍後の鋼板(原板)から、圧延方向に対して0°、45°、90°の方向に切り出したJIS5号引張試験片を用いて、島津製作所AG10TB AG/XRの引張試験機にて10mm/分で12%まで歪みを付与し、下記式(1)にて各方向のr値を求め、下記式(2)にてΔrを求めた。
r=ln(w/w0)/ln(t/t0)・・(1)
ただし、w:12%歪付与後の板幅、w0:試験前の板幅、t:12%歪付与後の板厚、t0:試験前の板厚。
Δr=(r0+r90-2r45)/2・・(2)
ただし、r0、r45、r90は、各々圧延方向に対して0°、45°、90°の方向に切り出した引張試験片を用いて求めたr値。
焼鈍後の鋼板の板幅中央部から採取した試料を用い、表層150μmの窒素量および鋼板中平均N量を測定して、表層150μmの窒素量と鋼板中の平均N量の差を求めた。ここで表層150μmの窒素量とは、鋼板表面から板厚方向に150μm深さまでの範囲に含有される窒素量である。また、表層150μmの窒素量は以下のように求めた。採取した鋼板の表面から切削を開始し、表面から150μmの深さまで鋼板を切削し、この際に発生した切りくず(chip)をサンプルとして採取した。このサンプル中のN量を測定し表層150μmの窒素量とした。表層150μmの窒素量と鋼板中平均N量は、不活性ガス融解-熱伝導度法(inert gas transportation fusion-thermal conductivity method)により各N量を測定し求めた。このようにして求めた表層150μmの窒素量(表面~表面から150μm深さの範囲の窒素量)と鋼板中の平均N量(鋼中のN含有量)の差が30質量ppm以下であれば、浸窒を抑制できていると評価できる。
固溶B量は、焼鈍後の鋼板の板幅中央から採取した試料を用い、鋼板中のBNを10(体積%)Brメタノールで抽出し、BNを形成しているB量を測定し、Bの全添加量からBNを形成しているB量を差し引き求めた。また、このようにして求めた固溶B量と、添加したB量(B含有量)の比である固溶B量/添加B量を求めた。{固溶B量(質量%)/添加B量(質量%)}×100(%)が75(%)以上であれば、固溶B量の低下を抑制できていると評価できる。
焼鈍後の鋼板の板幅中央から平板試験片(幅15mm×長さ40mm×板厚4mm)を採取し、以下のように水冷、120℃油冷の2通りの方法により焼入れ処理を施して、各々の方法で焼入れ後の鋼板硬さ(焼入れ硬さ)を求めた。すなわち、焼入れ処理は、上記平板試験片を用いて、870℃で30s保持して直ちに水冷する方法(水冷)、870℃で30s保持して直ちに120℃油で冷却する方法(120℃油冷)で実施した。焼入れ特性は焼入れ処理後の試験片の切断面について、ビッカース硬さ試験機で荷重1kgfの条件下で硬さを5点測定し平均硬さを求め、これを焼入れ硬さとした。焼入れ硬さは、表3の条件を水冷後硬さ、120℃油冷後硬さともに満足した場合、合格(○)と判定し、焼入れ性に優れると評価した。また、水冷後硬さ、120℃油冷後硬さのいずれかが表3に示す条件を満足しない場合、不合格(×)とし、焼入れ性に劣ると評価した。なお、表3は、経験上、焼入れ性が十分であると評価できる、C含有量に応じた焼入れ硬さを表したものである。
Claims (5)
- 質量%で、C:0.20~0.48%、Si:0.10%以下、Mn:0.50%以下、P:0.03%以下、S:0.010%以下、sol.Al:0.10%以下、N:0.0050%以下、B:0.0005~0.0050%を含有し、さらに、Sb、Sn、Bi、Ge、Te、Seのうち1種以上を合計で0.002~0.030%含有し、残部がFeおよび不可避的不純物からなる組成を有し、フェライト粒内のセメンタイト密度が0.10個/μm2以下であるフェライトとセメンタイトからなるミクロ組織を有し、硬さがHRBで65以下、全伸びが40%以上であることを特徴とする高炭素熱延鋼板。
- さらに、質量%で、Ni、Cr、Moのうちの少なくとも1種を合計で0.50%以下含有することを特徴とする請求項1に記載の高炭素熱延鋼板。
- r値の面内異方性(Δr)の絶対値が0.15以下であることを特徴とする請求項1または請求項2に記載の高炭素熱延鋼板。
- 請求項1または請求項2に記載の組成を有する鋼を、熱間粗圧延後、仕上温度:Ar3変態点以上で仕上圧延を行い、巻取温度:500~750℃で巻き取った後、Ac1変態点以上に加熱して0.5h以上保持し、1~20℃/hでAr1変態点未満に冷却して、Ar1変態点未満で20h以上保持することを特徴とする高炭素熱延鋼板の製造方法。
- 前記仕上温度が900℃以上であることを特徴とする請求項4に記載の高炭素熱延鋼板の製造方法。
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