WO2021006296A1 - 高強度鋼板 - Google Patents
高強度鋼板 Download PDFInfo
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- WO2021006296A1 WO2021006296A1 PCT/JP2020/026704 JP2020026704W WO2021006296A1 WO 2021006296 A1 WO2021006296 A1 WO 2021006296A1 JP 2020026704 W JP2020026704 W JP 2020026704W WO 2021006296 A1 WO2021006296 A1 WO 2021006296A1
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- steel sheet
- ferrite
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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
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- 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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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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- 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
- C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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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
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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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
- C21D1/78—Combined heat-treatments not provided for above
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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
- C21D2211/001—Austenite
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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
- C21D2211/002—Bainite
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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
- C21D2211/005—Ferrite
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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
- C21D2211/008—Martensite
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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
- 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
- C21D8/0242—Flattening; Dressing; Flexing
Definitions
- the present invention relates to a high-strength steel sheet having excellent tensile strength, elongation, stretch flangeability and bendability, and excellent material stability.
- the present application claims priority based on Japanese Patent Application No. 2019-128612 filed in Japan on July 10, 2019, the contents of which are incorporated herein by reference.
- Steel sheets used for automobile parts are required to have various workability required at the time of part molding, such as press workability and weldability, as well as strength. Specifically, from the viewpoint of press workability, the steel sheet is often required to have excellent elongation (total elongation in a tensile test; EL) and elongation flangeability (hole expansion ratio; ⁇ ).
- the temperature history may vary in the width direction and the longitudinal direction, such as unevenness of how the cooling water is applied in the width direction and unevenness of the cooling rate depending on the position in the coil after winding. is there. Therefore, in the production of high-strength steel sheets, a technique for stabilizing the material is required, such as using a manufacturing method that reduces these temperature histories as much as possible, or designing a material that minimizes the influence of the temperature history.
- Non-Patent Document 1 discloses that the elongation and hole expandability of a steel sheet are improved by using the double annealing method in which the steel sheet is annealed twice.
- the present inventors conducted a search to obtain a steel sheet having both elongation and hole expandability. Since the method described in Non-Patent Document 1 performs annealing twice, there is a problem that fuel cost and the like increase as compared with the manufacturing method in which single annealing is performed. Therefore, the present inventors annealed the hot-rolled steel sheet in order to create a similar plate-like structure (that is, a structure having a large aspect ratio of austenite) without performing the annealing twice. I tried a manufacturing method to make it. Specifically, the present inventors have investigated a manufacturing method in which a hot-rolled steel sheet is wound at a low temperature of 450 ° C. or lower and then annealed.
- the structure of the hot-rolled steel sheet can be made mainly of a low-temperature transformation structure.
- the present inventors have considered that by annealing a hot-rolled steel sheet having a structure mainly composed of a low-temperature transformation structure, a plate-like structure can be obtained by one annealing.
- material instability occurred in the steel sheet obtained by this method. Specifically, the variation in the amount of ferrite measured along the plate width direction increased, and as a result, the variation in mechanical properties increased.
- An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent tensile strength, elongation, stretch flangeability and bendability, and excellent material stability.
- the material stability means that there is little variation in tensile strength and total elongation for each part in the steel sheet.
- the high-strength steel plate according to one aspect of the present invention has C: 0.030 to 0.280%, Si: 0.50 to 2.50%, Mn: 1.00 in mass% as a chemical component. ⁇ 4.00%, sol. Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0200% or less, N: 0.01000% or less, O: 0.0100% or less, B: 0 to 0.010 %, Ti: 0 to 0.20%, Nb: 0 to 0.20%, V: 0 to 1.000%, Cr: 0 to 1.000%, Mo: 0 to 1.000%, Cu: 0 ⁇ 1.000%, Co: 0 to 1.000%, W: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100% , REM: 0 to 0.0100%, Zr: 0 to 0.0100%, and the balance: Fe and impurities, and the balance
- the high-strength steel sheet according to (1) may have a standard deviation of surface roughness Ra of 0.5 ⁇ m or less at 10 positions at intervals of 50 mm in the plate width direction.
- the high-strength steel sheet according to (1) or (2) has B: 0.001% to 0.010%, Ti: 0.01 to 0.20%, in mass%, as the chemical component.
- Nb 0.01 to 0.20%
- V 0.005% to 1.000%
- Cr 0.005% to 1.000%
- Mo 0.005% to 1.000%
- Cu 0 .005% to 1.000%
- Co 0.005% to 1.000%
- W 0.005% to 1.000%
- Ni 0.005% to 1.000%
- Ca 0.0003 It is composed of a group consisting of% to 0.0100%, Mg: 0.0003% to 0.0100%, REM: 0.0003% to 0.0100%, and Zr: 0.0003% to 0.0100%. At least one may be contained.
- the present inventors have made extensive studies on the cause of impaired material stability in a steel sheet that has been annealed once. Then, the present inventors have found that the variation in the surface texture of the hot-rolled steel sheet before annealing affects the material stability of the steel sheet after annealing.
- the variation in the surface texture (surface roughness) of the hot-rolled steel sheet tends to be larger than that of the cold-rolled steel sheet. If there is unevenness in the surface roughness, the unevenness in the surface roughness causes unevenness in the emissivity in the process of raising the temperature for annealing, and the resulting temperature variation is brought to the steel sheet. As a result, the variation in the amount of ferrite increases in the annealed steel sheet.
- the present inventors have also found an effective hot rolling method for suppressing variations in the surface texture of a steel sheet (hot-rolled steel sheet) before annealing.
- the present inventors have discovered that the phenomenon in which the surface scale is pressed against the steel sheet by the hot-rolled roll during hot rolling greatly characterizes the surface texture of the steel sheet after hot rolling.
- it is important to control the scale growth during hot rolling and this is achieved by spraying a water film on the surface of the steel sheet under specific conditions during rolling. I found that I could do it.
- the rolling direction RD means the direction in which the steel sheet is moved by the rolling roll during rolling.
- the plate thickness direction TD is a direction perpendicular to the rolled surface 11 of the steel sheet.
- the plate width direction WD is a direction perpendicular to the rolling direction RD and the plate thickness direction TD.
- the rolling direction RD can be easily specified based on the stretching direction of the crystal grains of the steel sheet. Therefore, the rolling direction RD can be specified even in the steel sheet cut out from the material steel sheet after rolling.
- the amount of ferrite in the metal structure and the like are specified.
- the metallographic structure is evaluated in a cross section 12 parallel to the rolling direction RD and perpendicular to the rolling surface 11 (see FIG. 1).
- the cross section 12 parallel to the rolling direction RD and perpendicular to the rolling surface 11 may be simply referred to as a cross section parallel to the rolling direction RD.
- the detailed evaluation method of the metallographic structure will be described later.
- the ratio of the number of retained austenites having an aspect ratio of 2.0 or more to the total number of retained austenites is defined.
- Residual austenite is evaluated in a cross section parallel to the rolling direction RD and the plate thickness direction TD (see FIG. 2). The detailed evaluation method of retained austenite will be described later.
- the standard deviation of the area ratio of ferrite is specified.
- the area ratio of ferrite is measured at the plate thickness 1/4 position 121 of the cross section 12 parallel to the rolling direction RD and perpendicular to the rolling surface 11 (see FIG. 3).
- Ten cross sections 12 parallel to the rolling direction RD and perpendicular to the rolling surface 11 were created at intervals of 50 mm along the plate width direction WD, and the standard deviation of the area ratios of 10 ferrites measured on these surfaces is the present. It is considered to be the standard deviation of the area ratio of ferrite according to the embodiment.
- the plate thickness 1/4 position is a position at a depth of 1/4 of the thickness of the steel plate 1 from the rolled surface 11 of the steel plate 1.
- FIGS. 1 and 2 only the position at a depth of 1/4 of the thickness of the steel plate 1 from the rolled surface 11 on the upper side of the steel plate 1 is shown as the plate thickness 1/4 position.
- a position having a depth of 1/4 of the thickness of the steel plate 1 from the rolled surface 11 on the lower side of the steel plate 1 can also be treated as a 1/4 position of the plate thickness.
- FIG. 3 only a part of the 10 measurement surfaces is shown. Further, FIG.
- FIG. 3 merely conceptually shows the measurement points of the area ratio of ferrite, and it is not necessary to form the measurement surface of the number density as shown in FIG. 3 as long as a predetermined requirement is satisfied. A detailed evaluation method of the standard deviation of the area ratio of ferrite will be described later.
- the high-strength steel sheet according to this embodiment has a chemical composition of% by mass.
- C 0.030 to 0.280%, Si: 0.50 to 2.50%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0200% or less, N: 0.01000% or less, O: 0.0100% or less, B: 0 to 0.010%, Ti: 0 to 0.20%, Nb: 0 to 0.20%, V: 0 to 1.000%, Cr: 0 to 1.000%, Mo: 0 to 1.000%, Cu: 0 to 1.000%, Co: 0 to 1.000%, W: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, REM: 0-0.0100%, Zr: 0 to 0.0100% or less, and the balance: Fe and impurities included
- the metal structure is the balance: 0 to
- the standard deviation of the area ratio of ferrite measured at 10 points every 50 mm along the plate width direction at the plate thickness 1/4 position of the cross section parallel to the rolling direction and perpendicular to the surface is less than 10%.
- the tensile strength is 780 MPa or more.
- the composition of the high-strength steel sheet according to this embodiment will be described in detail below.
- the high-strength steel sheet according to the present embodiment contains a basic element as a chemical component, contains a selective element if necessary, and the balance is composed of Fe and impurities.
- C (C: 0.030% or more and 0.280% or less) C is an important element for ensuring the strength of the steel sheet. If the C content is less than 0.030%, the tensile strength of 780 MPa or more cannot be secured. Therefore, the C content is 0.030% or more, preferably 0.050% or more, 0.100% or more, 0.120% or more, or 0.140% or more.
- the C content is preferably 0.260% or less or 0.250% or less, more preferably 0.200% or less, 0.180% or less, or 0.160% or less.
- Si 0.50% or more and 2.50% or less
- Si is an important element for suppressing the precipitation of iron-based carbides and stabilizing the residual ⁇ . If the Si content is less than 0.50%, it is difficult to obtain residual ⁇ of 5% or more and the elongation deteriorates. Therefore, the Si content is set to 0.50% or more.
- the Si content is preferably 0.80% or more, 1.00% or more, or 1.20% or more.
- the Si content if the Si content exceeds 2.50%, the surface texture deteriorates, so the Si content should be 2.50% or less.
- the Si content is preferably 2.00% or less, more preferably 1.80% or less, 1.50% or less, or 1.30% or less.
- Mn 1.00% or more and 4.00% or less
- Mn is an element effective in increasing the mechanical strength of the steel sheet. If the Mn content is less than 1.00%, it is not possible to secure a tensile strength of 780 MPa or more. Therefore, the Mn content is set to 1.00% or more.
- the Mn content is preferably 1.50% or more, more preferably 1.80% or more, 2.00% or more, or 2.20% or more.
- the Mn content is set to 4.00% or less, preferably 3.00% or less, more preferably 2.80% or less, 2.60% or less, or 2.50% or less.
- Al is an element having an action of deoxidizing steel to make a steel sheet sound. sol. If the Al content is less than 0.001%, it cannot be sufficiently deoxidized. The Al content is 0.001% or more. However, when sufficient deoxidation is required, it is more desirable to add 0.010% or more. More preferably, sol. The Al content is 0.020% or more, 0.030% or more, or 0.050% or more.
- sol. When the Al content exceeds 2.000%, the weldability is remarkably lowered, and the oxide-based inclusions are increased, so that the surface texture is remarkably deteriorated. Therefore, sol.
- the Al content is 2.000% or less, preferably 1.500% or less, more preferably 1.000% or less, or 0.700% or less, and most preferably 0.090% or less, 0. It shall be 080% or less, or 0.070% or less.
- sol. Al means an acid-soluble Al that is not an oxide such as Al 2 O 3 and is soluble in an acid.
- the high-strength steel sheet according to this embodiment contains impurities as a chemical component.
- impurity refers to, for example, those mixed from ore or scrap as a raw material, or from the manufacturing environment, etc., when steel is industrially manufactured. Impurities mean, for example, elements such as P, S, and N. These impurities are preferably limited as follows in order to fully exert the effects of the present embodiment. Further, since the content of impurities is preferably small, it is not necessary to limit the lower limit value, and the lower limit value of impurities may be 0%.
- P 0.100% or less
- P is generally an impurity contained in steel, but since it has an effect of increasing tensile strength, P may be positively contained. However, when the P content exceeds 0.100%, the deterioration of weldability becomes remarkable. Therefore, the P content is limited to 0.100% or less.
- the P content is preferably limited to 0.080% or less, 0.070% or less, or 0.050% or less. In order to obtain the effect of the above action more reliably, the P content may be 0.001% or more, 0.002% or more, or 0.005% or more.
- S is an impurity contained in steel, and the smaller the amount, the more preferable it is from the viewpoint of weldability.
- the S content exceeds 0.0200%, the weldability is significantly lowered, the MnS precipitation amount is increased, and the low temperature toughness is lowered. Therefore, the S content is limited to 0.0200% or less.
- the S content is preferably limited to 0.0100% or less, more preferably 0.0080% or less, 0.0070% or less, or 0.0050% or less. From the viewpoint of desulfurization cost, the S content may be 0.0010% or more, 0.0015% or more, or 0.0020% or more.
- N is an impurity contained in steel, and the smaller the amount, the more preferable it is from the viewpoint of weldability.
- the N content may be limited to 0.01000% or less, preferably 0.00900% or less, 0.00700% or less, or 0.00500% or less.
- the lower limit of the N content is not particularly limited, but for example, the N content may be 0.00005% or more, 0.00010% or more, or 0.00020% or more.
- O is an impurity contained in steel, and the smaller the amount, the more preferable it is from the viewpoint of weldability.
- the O content exceeds 0.0100%, the weldability is significantly reduced. Therefore, the O content is limited to 0.0100% or less, preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less.
- the lower limit of the O content is not particularly limited, but for example, the O content may be 0.0005% or more, 0.0008% or more, or 0.0010% or more.
- the high-strength steel sheet according to the present embodiment may contain a selective element in addition to the basic elements and impurities described above.
- a selective element for example, B, Ti, Nb, V, Cr, Mo, Cu, Co, W, Ni, Ca, Mg, REM, and Zr are contained as selective elements in place of a part of Fe, which is the balance described above. May be good.
- These selective elements may be contained according to the purpose. Therefore, it is not necessary to limit the lower limit of these selective elements, and the lower limit may be 0%. Further, even if these selective elements are contained as impurities, the above effects are not impaired.
- the content of B is 0.010% or less
- the content of Ti and Nb is 0.20% or less, respectively
- the contents of V, Cr, Mo, Cu, Co, W and Ni are 1.0%, respectively.
- the content of B may be 0.008% or less, 0.007% or less, or 0.005% or less.
- the upper limit of the contents of Ti and Nb may be 0.18%, 0.15%, or 0.10%, respectively.
- the upper limit of the content of each of V, Cr, Mo, Cu, Co, W, and Ni may be 0.500% or less, 0.300% or less, or 0.100% or less.
- B 0.001% or more, 0.002% or more, or 0.004% or more, Ti: 0.01% or more, 0.02% or more, or 0.05% or more, Nb: 0.01% or more, 0.02% or more, or 0.05% or more, V: 0.005% or more, 0.008% or more, or 0.010% or more, Cr: 0.005% or more, 0.008% or more, or 0.010% or more, Mo: 0.005% or more, 0.008% or more, or 0.010% or more, Cu: 0.005% or more, 0.008% or more, or 0.010% or more, Co: 0.005% or more, 0.008% or more, or 0.010% or more, W: 0.005% or more, 0.008% or more, or 0.010% or more, and Ni: 0.005% or more, 0.008% or more, or 0.010% or more, at least one of them is contained. It is preferable to do.
- Ca, Mg, REM, and Zr are all elements that contribute to inclusion control, particularly fine dispersion of inclusions, and have an effect of enhancing toughness. Therefore, one or more of these elements may be contained. However, if each of the elements is contained in an amount of more than 0.0100%, deterioration of the surface texture may become apparent. Therefore, the contents of Ca, Mg, REM, and Zr are preferably 0.01% or less, or 0.0100% or less, respectively.
- the upper limit of the content of each of Ca, Mg, REM, and Zr may be 0.0080%, 0.0050%, or 0.0030%.
- the content of at least one of these elements is preferably 0.0003% or more, 0.0005% or more, or 0.0010% or more.
- REM refers to a total of 17 elements of Sc, Y and lanthanoid, and is at least one of them.
- the content of REM means the total content of at least one of these elements.
- lanthanoids they are industrially added in the form of misch metal.
- Ca 0.0003% or more and 0.0100% or less
- Mg 0.0003% or more and 0.0100% or less
- REM 0. It is preferable to contain at least one of 0003% or more and 0.0100% or less and Zr: 0.0003% or more and 0.0100% or less.
- the above steel composition may be measured by a general analysis method for steel.
- the steel component may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrum).
- C and S may be measured by using the combustion-infrared absorption method
- N may be measured by using the inert gas melting-thermal conductivity method
- O may be measured by using the inert gas melting-non-dispersion infrared absorption method.
- the metal structure is ferrite: 20% to 70%, retained austenite: 5% to 40%, fresh martensite: 0% to 30%, tempered martensite in terms of area ratio.
- Ferrite is a relatively soft structure that contributes to molding. Having ferrite improves elongation, hole expandability, and bendability. In order to obtain this effect, it is necessary to have 20% or more of ferrite. Therefore, the area ratio of ferrite in the metal structure is set to 20% or more. The area ratio of ferrite may be 25% or more, 30% or more, or 35% or more. If the ferrite content exceeds 70%, it becomes difficult to increase the tensile strength to 780 MPa or more. Therefore, the area ratio of ferrite in the metal structure is set to 70% or less. The area ratio of ferrite may be 65% or less, 60% or less, or 50% or less.
- Retained austenite is a tissue that contributes to elongation. To obtain this effect, 5% or more of retained austenite is required. Therefore, the area ratio of retained austenite in the metal structure is preferably 5% or more, preferably 8% or more, 10% or more, or 15% or more. In the production method according to the present embodiment, it is practically impossible to leave 40% or more of retained austenite. Therefore, the upper limit of the area ratio of retained austenite in the metal structure is 40%. The area ratio of retained austenite may be 35% or less, 30% or less, or 25% or less.
- Fresh martensite is a structure that inhibits moldability at the cost of contributing to strength. Therefore, fresh martensite does not have to be included, and its lower limit is set to 0%. On the other hand, in order to obtain the effect of improving the strength of fresh martensite, it is preferable to have fresh martensite at 2% or more, 5% or more, or 8% or more. On the other hand, if the amount of fresh martensite exceeds 30%, the elongation and the hole-spreading property are deteriorated. Therefore, the area ratio of fresh martensite in the metal structure is set to 30% or less. The area ratio of fresh martensite is preferably 20% or less, more preferably 15% or less, or 10% or less.
- Tempered martensite and bainite are structures that contribute to strength. In order to obtain a tensile strength of 780 MPa or more, a total of 20% or more of tempered martensite and bainite is required. Therefore, in the metal structure of the high-strength steel sheet according to the present embodiment, the total area ratio of tempered martensite and bainite is 20% or more, preferably 30% or more, 40% or more, or 50% or more. On the other hand, it is not necessary to set an upper limit on the total amount of tempered martensite and bainite.
- the metallographic structure of the steel sheet according to the present embodiment contains 20% or more of ferrite and 5% or more of retained austenite, all of which may be tempered martensite and bainite.
- the total area ratio of tempered martensite and bainite can be up to 75%.
- the total area ratio of tempered martensite and bainite may be 70% or less, 60% or less, or 55% or less.
- Total of pearlite and cementite 0% to 10%
- Pearlite and cementite are tissues that inhibit moldability. When the total area ratio of pearlite and cementite exceeds 10%, the moldability is significantly deteriorated, which is not preferable. Therefore, the total area ratio of pearlite and cementite is set to 10% or less in total.
- the total area ratio of pearlite and cementite may be 8% or less, 5% or less, or 3% or less. Since pearlite and cementite are not required to solve the problems of the present invention, the lower limit of the total area ratio is 0%. However, the total area ratio of pearlite and cementite may be 0.5% or more, 1% or more, or 2% or more.
- Metal structure measurement method Identification of bainite, tempered martensite, ferrite, pearlite, retained austenite and martensite constituting the metal structure of the high-strength steel plate according to the present embodiment as described above, confirmation of existence position and area fraction Is measured by the following method.
- a nital reagent and a cross section disclosed in Japanese Patent Application Laid-Open No. 59-219473 are used to corrode a cross section parallel to the rolling direction (that is, a cross section parallel to the rolling direction and perpendicular to the surface).
- a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol was used as solution A, and 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid were dissolved in 100 ml of water.
- the solution is solution B, and solution A and solution B are mixed at a ratio of 1: 1 to form a mixed solution, and nitric acid at a ratio of 1.5 to 4% is further added and mixed with respect to the total amount of this mixed solution.
- the solution is used as a pretreatment solution.
- a liquid obtained by adding and mixing the above-mentioned pretreatment liquid in a ratio of 10% with respect to the total amount of the 2% nital liquid to the 2% nital liquid is used as a post-treatment liquid.
- a cross section parallel to the rolling direction that is, a cross section parallel to the rolling direction and perpendicular to the surface
- a magnification of 1000 to 1000 is used using a scanning electron microscope.
- the metallographic structure is identified, the existence position is confirmed, and the area fraction is measured. Even if the measurement target is a steel plate that has not undergone any special machining after manufacturing (in other words, a steel plate that has not been cut out from the coil) or a steel plate that has been cut out from the coil, the center position in the plate width direction.
- the total area fraction of "bainite and tempered martensite” is obtained by measuring the area fraction of "upper bainite” and "lower bainite or tempered martensite".
- Upper bainite is an aggregate of laths, a structure containing carbides between laths.
- the lower bainite is a structure containing iron-based carbides having a major axis of 5 nm or more and extending in the same direction inside.
- Tempering martensite is a collection of lath-shaped crystal grains, and is a structure containing iron-based carbides having a major axis of 5 nm or more and extending in different directions.
- Ferrite is a region where the brightness is low and the substructure is not recognized.
- the region where the brightness is high and the substructure is not exposed by etching is judged as fresh martensite or retained austenite. Therefore, the area fraction of fresh martensite can be obtained as the difference between the area fraction of the uncorroded region observed by FE-SEM and the area fraction of retained austenite measured by X-ray described later. it can.
- Pearlite means a region where plate-shaped cementite and plate-shaped ferrite are arranged alternately.
- pearlite and the above-mentioned structure ferrite, bainitic ferrite, bainite, tempered martensite
- Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (electron backscatter diffraction image, Electron Backscattering Diffraction Pattern) analysis, and magnetic measurement methods, and the measured values may differ depending on the measurement method. ..
- the surface integral of retained austenite is measured by X-ray diffraction.
- the cross section parallel to the rolling direction that is, parallel to the rolling direction and perpendicular to the surface
- the integrated intensities of a total of 6 peaks of ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220) were obtained using Co-K ⁇ rays. Then, the area fraction of retained austenite is obtained by calculation using the intensity averaging method.
- Creating a plate-like structure of retained austenite contributes to the improvement of elongation, hole-spreading property, and bendability, and is one of the important structure-building points in the present invention.
- Making the retained austenite plate-shaped has the effect of suppressing strain distribution to the austenite during molding and appropriately stabilizing the retained austenite against plastic deformation, thereby improving elongation and hole expandability.
- the form of retained austenite having this effect has an aspect ratio of 2.0 or more.
- the number ratio of retained austenite having an aspect ratio of 2.0 or more in the range of 1/8 to 3/8 thickness needs to be 50% or more with respect to the total retained austenite. Therefore, the number ratio is 50% or more, preferably 70% or more. If the number ratio is less than 50%, it is difficult to achieve both excellent elongation, hole expandability, and bendability, which is not preferable.
- the aspect ratio and major axis of the retained austenite grains contained in the steel structure inside the steel sheet are evaluated by observing the crystal grains using FE-SEM and performing high-resolution crystal orientation analysis by the EBSD method (electron backscatter diffraction method).
- EBSD method electron backscatter diffraction method
- Crystal structure analysis is performed by the EBSD method for an area of ⁇ 10-9 m 2 or more (either a plurality of fields of view or the same field of view is acceptable).
- a crystal orientation map is drawn.
- the boundary that causes a crystal orientation difference of 10 ° or more is regarded as the grain boundary of the retained austenite grains.
- the aspect ratio is a value obtained by dividing the major axis length of the retained austenite grains by the minor axis length.
- the major axis is the major axis length of the retained austenite grains.
- the standard deviation of the area ratio of ferrite is 10%. Less than
- ferrite is important for ensuring elongation and hole expandability.
- the strength, elongation, and hole-spreading property change depending on the tissue fraction. Therefore, it is important for the material stability to be obtained that the structure fraction of ferrite is uniformly distributed in the thermal spread width direction. As shown in FIG.
- the area ratio of ferrite at the plate thickness 1/4 position 121 of the cross section parallel to the rolling direction (that is, the cross section 12 parallel to the rolling direction and perpendicular to the surface) is set in the plate width direction (that is, rolling).
- the standard deviation of the area ratio of ferrite is 10% or more, it causes variations in mechanical properties and material stability can be obtained. Absent. Therefore, the standard deviation of the area ratio of ferrite described above is set to less than 10%, preferably 8% or less, less than 5%, or 4% or less.
- the measurement points of the standard deviation of the area ratio of ferrite may be arranged on a straight line along the plate width direction.
- the measurement points of the standard deviation of the area ratio of ferrite are arranged on two or more straight lines along the plate width direction. It is good to do.
- the measurement points can be arranged as described above.
- Standard deviation of surface roughness Ra (Standard deviation of surface roughness Ra measured at 10 points every 50 mm along the plate width direction is preferably 0.5 ⁇ m or less)
- the steel sheet according to this embodiment is not particularly limited as long as the chemical composition, the metallographic structure, and the tensile strength described later are within a predetermined range.
- the standard deviation of the surface roughness Ra is 0.5 ⁇ m or less. May be.
- the standard deviation is 0.5 ⁇ m or less.
- the surface roughness of the steel sheet can be freely changed by additional machining.
- the high-strength steel sheet may be subjected to processing such as hairline processing to change the surface roughness. From this point of view, it is not essential that the standard deviation of the surface roughness Ra is within the above range.
- a contact type roughness meter (Mitutoyo surf test SJ-500) was used to acquire a roughness curve over a length of 5 mm in the plate width direction at each measurement position, and JIS B0601: 2001.
- the arithmetic mean roughness Ra is obtained by the method described in 1.
- the standard deviation of the surface roughness Ra is obtained by using the value of the arithmetic mean roughness Ra at each measurement position thus obtained.
- surface roughness Ra of the steel sheet means the surface roughness measured after removing the surface treatment film from the steel sheet.
- the surface roughness Ra of the steel sheet is the surface roughness of the base iron.
- the method for removing the surface treatment film can be appropriately selected depending on the type of the surface treatment film within a range that does not affect the surface roughness of the base iron.
- the galvanized layer may be dissolved with dilute hydrochloric acid to which an inhibitor is added. As a result, only the galvanized layer can be peeled off from the steel sheet.
- the inhibitor is an additive used to suppress the change in roughness due to the prevention of overdissolution of the base iron.
- a mixture of hydrochloric acid diluted 10 to 100 times with the hydrochloric acid pickling corrosion inhibitor "Ibit No. 700BK" manufactured by Asahi Chemical Industry Co., Ltd. added to a concentration of 0.6 g / L is galvanized. It can be used as a layer peeling means.
- the high-strength steel sheet according to the present embodiment has a tensile strength (TS) of 780 MPa or more as a sufficient strength that contributes to weight reduction of automobiles.
- the tensile strength of the steel sheet may be 800 MPa or more, 900 MPa or more, or 1000 MPa or more.
- the upper limit of the tensile strength in particular, but in the present embodiment, the upper limit of the substantial tensile strength can be set to 1470 MPa.
- the tensile strength of the steel sheet may be 1400 MPa or less, 1300 MPa or less, or 1200 MPa or less.
- the tensile test may be performed in the following procedure in accordance with JIS Z2241 (2011).
- JIS No. 5 test pieces are collected from 10 positions on the high-strength steel sheet at intervals of 50 mm in the plate width direction.
- the plate width direction of the steel plate and the longitudinal direction of the test piece are made to coincide with each other.
- each test piece is sampled at a position shifted in the rolling direction of the steel sheet so that the sampling positions of the test pieces do not interfere with each other.
- Tensile tests are carried out on these test pieces in accordance with the provisions of JIS Z 2241 (2011), the tensile strength TS (MPa) is obtained, and the average value of these is calculated. This average value is regarded as the tensile strength of the high-strength steel sheet.
- the high-strength steel sheet according to the present embodiment may have the following characteristics such as elongation and hole expandability as indicators of formability. These mechanical properties are obtained from the various properties of the high-strength steel sheet according to the present embodiment described above.
- the high-strength steel sheet according to the present embodiment may have a total elongation of 14% or more in a tensile test as an index of elongation. On the other hand, it is difficult to make the total elongation exceed 35% in the configuration of the present embodiment. Therefore, the upper limit of the substantial total growth may be 35%.
- the high-strength steel sheet according to the present embodiment may have a hole expansion rate of 25% or more as an index of hole expansion property.
- the upper limit of the substantial hole expansion rate may be 80%.
- the hole expansion rate can be evaluated by a hole expansion test based on the test method described in the Japan Iron and Steel Federation standard JFS T 1001-1996.
- the high-strength steel sheet according to the present embodiment has an R / t of 2.0 or less when the value R / t obtained by dividing the limit bending R (mm) by the plate thickness t (mm) is used as an index of bendability. You may. On the other hand, it is difficult to set the bendability index R / t to 0.1 or less in the configuration of the present embodiment. Therefore, the lower limit of the substantial bendability index R / t may be set to 0.1.
- the limit bending R is obtained by repeatedly performing bending tests applying various bending radii. In the bending test, bending is performed in accordance with JIS Z 2248 (V block 90 ° bending test).
- the bending radius (to be exact, the inner radius of the bending) is changed at a pitch of 0.5 mm.
- the minimum bending obtained in this test that does not cause tears and other defects in the steel sheet is regarded as the limit bending R.
- the value obtained by dividing this limit bending R by the thickness t of the steel sheet is used as an index R / t for evaluating the bendability.
- the high-strength steel plate according to the present embodiment has tensile test results measured at 10 points every 50 mm along the plate width direction (that is, the direction perpendicular to the rolling direction) as an index of the stability of the material.
- the standard deviation of TS may be 50 MPa or less, and the standard deviation of EL may be 1% or less.
- the method for obtaining the TS standard deviation and the EL standard deviation is the same as the above-mentioned tensile test method for obtaining the average value of the tensile strength.
- the TS standard deviation and the EL standard deviation can be obtained by obtaining the standard deviation of the results of 10 tensile tests by the above method.
- the standard deviation of R / t (limit bending R (mm), plate thickness t (mm)) measured at 10 points every 50 mm along the plate width direction is calculated. It may be 0.2 or less.
- the manufacturing process prior to hot rolling is not particularly limited. That is, following the melting in a blast furnace or an electric furnace, various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting, casting by the ingot method, or thin slab casting.
- a method such as ordinary continuous casting, casting by the ingot method, or thin slab casting.
- the cast slab may be cooled to a low temperature and then heated again and then hot-rolled, or the cast slab may be hot-rolled as it is after casting without being cooled to a low temperature.
- Good. Scrap may be used as the raw material.
- the cast slab is heated. In this heating step, it is preferable to heat the slab to a temperature of 1100 ° C. or higher and 1300 ° C. or lower. Coarse precipitates deposited in the slab (iron-based carbides, carbonitrides of alloying elements, etc.) may impair material stability, so the slab may be heated to 1100 ° C or higher to dissolve it. preferable.
- the slab heating temperature is preferably 1300 ° C. or lower.
- a rough rolling step is performed in which the heated slab is roughly rolled to obtain a rough rolled plate.
- the slab may have a desired size and shape, and the conditions thereof are not particularly limited.
- the thickness of the rough-rolled sheet affects the amount of temperature decrease from the tip to the tail of the hot-rolled steel sheet from the start of rolling to the end of rolling in the finish rolling process, so it should be determined in consideration of this. Is preferable.
- the rough-rolled plate is subjected to finish rolling.
- finish rolling is performed in a temperature range of 850 ° C to 1200 ° C under the condition of satisfying the following formula (1).
- Si * 140 ⁇ Si
- Si * 80.
- Si represents the Si content (mass%) of the steel sheet.
- K'in the above formula (1) is represented by the following formula (2).
- K ' D ⁇ (DT- 930) ⁇ 1.5 + ⁇ ((FT n -930) ⁇ S n) ⁇ (2)
- D is the spray amount per hour of hydraulic descaling before the start of finish rolling (m 3 / min)
- DT is the steel plate temperature (° C.) when hydraulic descaling is performed before the start of finish rolling
- FT n is finish. temperature of the steel strip in the n-th rolling (° C.), blowing amount per unit time when S n is blown onto the steel sheet of water on spray during n-1 stage and the n-th stage of the finish rolling (m 3 / min ).
- Si * is a parameter related to the steel sheet component that indicates the susceptibility to unevenness due to scale.
- the scale generated on the surface layer during hot rolling grows from wustite (FeO), which is relatively easy to descale and does not easily form irregularities on the steel sheet, and grows to form irregularities on the steel sheet.
- the ease of forming the unevenness of the surface layer by adding Si becomes particularly remarkable when 0.35% by mass or more of Si is added. Therefore, Si * is a function of Si when 0.35% by mass or more is added, but becomes a constant when 0.35% by mass or less is added.
- K' is a parameter of manufacturing conditions that indicates the difficulty of forming irregularities.
- the first item of the above formula (2) is that when hydraulic descaling is performed before the start of finish rolling in order to suppress the formation of unevenness, the larger the amount of hydraulic descaling sprayed per hour and the higher the steel sheet temperature. It is shown to be effective from the viewpoint of descaling. When performing multiple descaling before the start of finish rolling, the descaling value closest to the finish rolling is used.
- the second item of the above formula (2) is a section showing the effect of descaling the scale that could not be completely peeled off by descaling before finishing and the scale that was reformed during finish rolling during finish rolling. It is shown that at high temperatures, spraying a large amount of water onto the steel sheet on the spray makes it easier to descale.
- K'/ Si * is set to 2.5 or more, preferably 3.0 or more, and more preferably 3.5 or more.
- the standard deviation of the surface roughness Ra measured at 10 positions at 50 mm intervals in the plate width direction (that is, the direction perpendicular to the rolling direction), which is a preferable form of the steel sheet according to the present invention, is set to 0.5 ⁇ m or less. In order to do so, it is preferable that K'/ Si * is 3.0 or more (K'/ Si * ⁇ 3.0).
- cooling is performed at an average cooling rate of 50 ° C./s or higher, and winding is performed at a winding temperature of 450 ° C. or lower.
- the average cooling rate is a value obtained by dividing the difference in temperature between the start of cooling and the temperature before winding by that time.
- the average cooling rate is less than 50 ° C./s, ferrite transformation occurs, which hinders the control of the structure morphology in the subsequent annealing step, and the ratio of the number of retained austenites having an aspect ratio of 2.0 or more to the total number of retained austenites is 50%. It cannot be controlled more than that.
- the take-up temperature exceeds 450 ° C
- ferrite transformation occurs, and it becomes difficult to make the total of bainite and tempered martensite 20% or more of the total.
- the winding temperature exceeds 450 ° C.
- the winding temperature is 450 ° C. or lower, preferably 400 ° C. or lower, and more preferably 200 ° C. or lower.
- setting the winding temperature to 450 ° C. or lower also has an effect of suppressing the formation of internal oxides on the surface of the steel sheet after winding and increasing the roughness of the surface layer.
- the high-strength steel sheet produced in this way is pickled for the purpose of removing oxides on the surface of the steel sheet.
- the pickling treatment may be carried out, for example, in hydrochloric acid having a concentration of 3 to 10% at a temperature of 85 ° C. to 98 ° C. for 20 seconds to 100 seconds.
- the manufactured hot-rolled steel sheet may be lightly reduced with a reduction ratio of 20% or less for the purpose of shape correction.
- the reduction rate shall be 20% or less.
- the light reduction may be carried out before or after the pickling step. Light reduction after the pickling step has the effect of further reducing the roughness of the surface layer.
- the standard deviation of the surface roughness Ra is 0.5 ⁇ m. In order to satisfy the following, it is necessary to perform light rolling after the pickling step.
- the obtained steel sheet is annealed.
- the heating temperature is set to Acc1 point to Ac3 point-10 ° C. calculated by the following formula.
- a c1 723-10.7 ⁇ Mn-16.9 ⁇ Ni + 29.1 ⁇ Si + 16.9 ⁇ Cr
- a c3 879-346 x C + 65 x Si-18 x Mn + 54 x Al ... (9)
- ferrite-austenite transformation occurs from the carbides formed between the laths of the low-temperature transformation structure, and plate-shaped austenite is generated.
- the region that did not undergo austenite transformation can be thought of as a low-temperature transformation structure (tempered martensite or tempered bainite) that was tempered at high temperature, but the dislocation density was greatly reduced by tempering, and the substructure was also unclear. Therefore, it is a region to be evaluated as ferrite in the microstructure observation after annealing. Therefore, it is also referred to as ferrite here.
- the regions evaluated as tempered martensite or bainite in the microstructure observation after annealing were generated by bainite transformation or martensitic transformation while the austenite generated by heating was held at 150 ° C to 550 ° C, which will be described later. Mainly refers to the organization.
- the reason why the heating temperature is set to Ac1 point to Ac3 point to ⁇ 10 ° C. is to set an appropriate ferrite-austenite transformation fraction in order to set the area ratio of ferrite to 20% to 70%.
- the heating time is 10 seconds to 1000 seconds. If the holding time is less than 1 second, cementite in the steel may remain undissolved and the characteristics of the steel sheet may deteriorate. Since this effect saturates in more than 1000 seconds and leads to a decrease in productivity, the holding time is limited to 1000 seconds.
- heating or cooling may be performed in the temperature range. For example, once the temperature is lowered to 250 ° C. or lower to transform a part of retained austenite into martensite, and then reheated to a temperature range of about 400 ° C., the martensite becomes a nucleation site of bainite transformation and bainite transformation. The effect of accelerating is obtained.
- hot-dip galvanizing or alloying hot-dip galvanizing may be performed in this temperature range.
- General conditions can be used as the plating conditions such as the galvanizing bath temperature and the galvanizing bath composition in the hot-dip galvanizing step, and there is no particular limitation.
- the plating bath temperature may be 420 to 500 ° C.
- the penetration plate temperature of the steel sheet may be 420 to 500 ° C.
- the immersion time may be 5 seconds or less.
- the plating bath is preferably a plating bath containing 0.08 to 0.2% of Al, but may also contain impurities such as Fe, Si, Mg, Mn, Cr, Ti, and Pb.
- the basis weight of hot-dip galvanizing is usually 5 g / m 2 or more per side, but is preferably 25 to 75 g / m 2 , and more preferably 20 to 120 g / m 2 .
- the alloying treatment temperature is preferably 460 to 550 ° C. If the alloying treatment is less than 460 ° C., not only the alloying rate becomes slow and the productivity is impaired, but also the alloying treatment unevenness occurs. Therefore, the alloying treatment temperature is preferably 460 ° C. or higher.
- the alloying treatment temperature exceeds 550 ° C., pearlite transformation occurs and retained austenite cannot be sufficiently stabilized. Further, the alloying treatment is preferably performed under the condition that the iron concentration in the hot-dip galvanized layer is 6.0% by mass or more.
- an electrogalvanized layer may be formed on the steel sheet manufactured as described above. The electrogalvanized layer can be formed by a conventionally known method.
- the high-strength steel sheet according to the present embodiment can be manufactured by the above-mentioned manufacturing method.
- the high-strength steel sheet according to the present invention will be described in more detail below with reference to an example.
- the following examples are examples of the high-strength steel sheet of the present invention, and the high-strength steel sheet of the present invention is not limited to the following aspects.
- the conditions in the examples described below are one-condition examples adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to these one-condition examples.
- various conditions can be adopted as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
- the steels with the chemical components shown in Table 1 are cast, and after casting, they are cooled to room temperature and then reheated, heated to a temperature range of 1200 ° C to 1350 ° C, and then the slab is roughened at a temperature of 1100 ° C or higher.
- a rough-rolled plate was produced by rolling.
- values outside the scope of the invention are underlined.
- the rough-rolled plate was subjected to multi-step finish rolling consisting of 7 steps in all stages under the conditions shown in Table 2. After that, cooling and winding after finish rolling were performed under each condition shown in Table 3. After that, pickling was performed under all conditions, but under some conditions, light reduction was performed before or after the pickling. Then, the temperature was raised to the heating temperature shown in Table 3 at a heating rate of 30 ° C./s to 150 ° C./s. After heating, the temperature was maintained at the heating temperature for the time shown in Table 3. Then, under condition A, the mixture was cooled to 250 ° C. at 50 to 100 ° C./s, reheated at 400 ° C., and then held for 300 seconds.
- condition B the mixture was cooled to 360 ° C. at 50 to 100 ° C./s and held for 50 seconds.
- condition C which is a comparative example, the mixture was cooled to 100 ° C. at 100 ° C./s and held for 300 seconds. After that, some conditions were alloyed hot-dip galvanized or hot-dip galvanized. In the plating process, the steel sheet was in the temperature range of 400 ° C. to 520 ° C.
- the metallographic structure of the obtained high-strength steel sheet was observed by the following method.
- a Nital reagent and a reagent disclosed in JP-A-59-219473 were used to corrode a cross section parallel to the rolling direction and perpendicular to the surface.
- a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol was used as solution A, and 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid were dissolved in 100 ml of water.
- the solution is solution B, and solution A and solution B are mixed at a ratio of 1: 1 to form a mixed solution, and nitric acid at a ratio of 1.5 to 4% is further added and mixed with respect to the total amount of this mixed solution.
- the solution was used as a pretreatment solution. Further, a liquid obtained by adding and mixing the above-mentioned pretreatment liquid in a ratio of 10% with respect to the total amount of the 2% nital liquid to the 2% nital liquid was used as a post-treatment liquid.
- the cross section parallel to the rolling direction and perpendicular to the surface is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, then immersed in the posttreatment liquid for 3 to 20 seconds, washed with water and dried. As a result, the cross section was corroded.
- the region where the brightness was low and the substructure was not recognized was judged to be ferrite.
- the region where the brightness was high and the substructure was not exposed by etching was judged to be fresh martensite or retained austenite.
- the area fraction of fresh martensite was determined as the difference between the area fraction of the uncorroded region observed by FE-SEM and the area fraction of retained austenite measured by X-ray.
- the surface integral of retained austenite was measured by X-ray diffraction. First, in a cross section parallel to the rolling direction and perpendicular to the surface at a depth position of 1/4 of the plate thickness of the steel sheet, ⁇ (110), ⁇ (200), ⁇ (211), The integrated intensity of a total of 6 peaks of ⁇ (111), ⁇ (200), and ⁇ (220) was determined and calculated using the intensity averaging method to obtain the area fraction of retained austenite.
- the aspect ratio and major axis of the retained austenite grains contained in the steel structure inside the steel sheet are evaluated by observing the crystal grains using FE-SEM and performing high-resolution crystal orientation analysis by the EBSD method (electron backscatter diffraction method). did.
- a sample was taken with a cross section parallel to the rolling direction and the thickness direction of the steel sheet as an observation surface, and the observation surface was polished to a mirror surface.
- Crystal structure analysis was performed by the EBSD method for the area (either a plurality of fields of view or the same field of view is possible).
- a crystal orientation map was drawn.
- the boundary that causes a crystal orientation difference of 10 ° or more was regarded as the grain boundary of the retained austenite grains.
- the aspect ratio was the value obtained by dividing the major axis length of the retained austenite grains by the minor axis length.
- the major axis was the major axis length of the retained austenite grains.
- the area ratio of ferrite at the position of 1/4 of the plate thickness of the cross section parallel to the rolling direction and perpendicular to the surface was determined according to the above method.
- the area ratio of ferrite was obtained at 10 locations at intervals of 50 mm in the plate width direction, and the standard deviation of the area ratio was calculated.
- the standard deviation of the surface roughness Ra measured at 10 positions at 50 mm intervals in the plate width direction was determined by the following procedure. Using a contact type roughness meter (Mitutoyo surf test SJ-500), a roughness curve was obtained over a length of 5 mm in the plate width direction at each measurement position, and the arithmetic mean was obtained by the method described in JIS B0601: 2001. Roughness Ra was determined. The standard deviation of the surface roughness Ra was obtained by using the value of the arithmetic mean roughness Ra at each measurement position thus obtained.
- Tensile strength was determined by conducting a tensile test in accordance with the provisions of JIS Z 2241 (2011) using JIS No. 5 test pieces collected from a high-strength steel plate so that the plate width direction is the longitudinal direction.
- TS (MPa) and butt elongation (total elongation) EL (%) were determined. The sampling was performed from 10 positions of the steel plate at intervals of 50 mm in the plate width direction.
- the average value of the tensile strengths of the 10 test pieces was regarded as the tensile strength TS of the steel sheet, and when TS ⁇ 780 MPa was satisfied, it was regarded as a high-strength hot-rolled steel sheet and passed.
- the standard deviations of TS and EL were obtained at 10 positions at intervals of 50 mm in the plate width direction.
- a steel sheet having a TS standard deviation of 50 MPa or less and an EL standard deviation of 1% or less was determined to be a steel sheet having excellent material stability.
- the hole expansion rate was evaluated by a hole expansion test based on the test method described in the Japan Iron and Steel Federation standard JFS T 1001-1996.
- the bending test was performed in accordance with JIS Z2248 (V block 90 ° bending test), and the bending radius (mm) was tested at a pitch of 0.5 mm. Further, R / t was measured at 10 positions at intervals of 50 mm in the plate width direction, and the standard deviation was obtained.
- Comparative Example 11 the proportion of retained austenite having an aspect ratio of 2.0 or more was insufficient, and the hole-spreading property was impaired. It is presumed that this is because the average cooling rate after finish rolling was insufficient.
- Comparative Example 14 the proportion of retained austenite having an aspect ratio of 2.0 or more was insufficient, and the hole-spreading property was impaired. It is presumed that this is because the reduction rate under light reduction applied to the steel sheet before annealing of the steel sheet was excessive.
- Comparative Example 31 and Comparative Example 32 the amount of Si was insufficient. Therefore, in Comparative Example 31 and Comparative Example 32, the amount of retained austenite was insufficient, and the total elongation and hole expandability were impaired.
- High-strength steel plate (steel plate) 11 Surface (rolled surface) 12 Cross section parallel to the rolling direction and perpendicular to the surface 121 Plate thickness 1/4 position of the cross section parallel to the rolling direction and perpendicular to the surface 13 Measuring surface of retained austenite 131 1 from the surface (rolling surface) of the measuring surface of retained austenite Range from 8/8 thickness to 3/8 thickness RD Rolling Direction TD plate thickness direction (Thickness Direction) WD plate width direction (Wids Direction)
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Abstract
Description
本願は、2019年7月10日に、日本に出願された特願2019-128612号に基づき優先権を主張し、その内容をここに援用する。
また、非特許文献1には、鋼板を2回焼鈍する2回焼鈍法を用いることで、鋼板の伸びおよび穴広げ性が向上することが開示されている。
しかしながら、この方法によって得られた鋼板では、材質不安定化が生じた。具体的には、板幅方向に沿って測定されたフェライト量のばらつきが増大し、その結果、機械特性のばらつきが増大した。
(2)(1)に記載の高強度鋼板は、前記板幅方向に50mm間隔で10か所の位置において、表面粗さRaの標準偏差が0.5μm以下であってもよい。
(3)(1)又は(2)に記載の高強度鋼板は、前記化学成分として、質量%で、B:0.001%~0.010%、Ti:0.01~0.20%、Nb:0.01~0.20%、V:0.005%~1.000%、Cr:0.005%~1.000%、Mo:0.005%~1.000%、Cu:0.005%~1.000%、Co:0.005%~1.000%、W:0.005%~1.000%、Ni:0.005%~1.000%、Ca:0.0003%~0.0100%、Mg:0.0003%~0.0100%、REM:0.0003%~0.0100%、及びZr:0.0003%~0.0100%からなる群から構成される少なくとも1種を含有してもよい。
また、本発明者らは、焼鈍前の鋼板(熱延鋼板)の表面性状のばらつきを抑制するために効果的な熱間圧延方法も見出した。熱間圧延時に、表層スケールが熱延ロールによって鋼板に押しつけられる現象が、熱間圧延後の鋼板の表面性状を大きく特徴づけることを、本発明者らは発見した。そして、熱延鋼板の表面性状を制御するためには、熱間圧延中のスケール成長を制御することが重要であり、圧延中に鋼板表面に水膜を特定の条件で吹き付けることでこれを達成できることが見いだされた。
また、本実施形態に係る高強度鋼板においては、全残留オーステナイトの個数に対する、アスペクト比2.0以上の残留オーステナイトの個数割合が既定される。残留オーステナイトは、圧延方向RD及び板厚方向TDに平行な断面において評価される(図2参照)。詳細な残留オーステナイトの評価方法は後述される。
本実施形態に係る高強度鋼板は、化学成分として、質量%で、
C:0.030~0.280%、
Si:0.50~2.50%、
Mn:1.00~4.00%、
sol.Al:0.001~2.000%、
P:0.100%以下、
S:0.0200%以下、
N:0.01000%以下、
O:0.0100%以下、
B:0~0.010%、
Ti:0~0.20%、
Nb:0~0.20%、
V:0~1.000%、
Cr:0~1.000%、
Mo:0~1.000%、
Cu:0~1.000%、
Co:0~1.000%、
W:0~1.000%、
Ni:0~1.000%、
Ca:0~0.0100%、
Mg:0~0.0100%、
REM:0~0.0100%、
Zr:0~0.0100%以下、及び
残部:Fe及び不純物
を含み、
金属組織が、面積率で、
フェライト:20%~70%、
残留オーステナイト:5%~40%、
フレッシュマルテンサイト:0%~30%、
焼き戻しマルテンサイト及びベイナイトの合計:20%~75%、及び
パーライト及びセメンタイトの合計:0%~10%
からなり、
表面から1/8厚~3/8厚の範囲において、全残留オーステナイトの個数に対する、アスペクト比2.0以上の残留オーステナイトの個数割合が50%以上であり、
圧延方向に平行且つ前記表面に垂直な断面の板厚1/4位置において、板幅方向に沿って50mmおきに10か所で測定されたフェライトの面積率の標準偏差が10%未満であり、
引張強度が780MPa以上である。
以下、本実施形態に係る高強度鋼板の成分組成について詳細に説明する。本実施形態に係る高強度鋼板は、化学成分として、基本元素を含み、必要に応じて選択元素を含み、残部がFe及び不純物からなる。
Cは鋼板強度を確保する上で重要な元素である。C含有量が0.030%未満では、引張強度780MPa以上を確保することができない。したがって、C含有量は0.030%以上とし、好ましくは0.050%以上、0.100%以上、0.120%以上又は0.140%以上である。
Siは、鉄系炭化物の析出を抑制し、残留γを安定化させるのに重要な元素である。Si含有量が0.50%未満では、残留γを5%以上得ることが難しく、伸びが劣化するため、Si含有量は0.50%以上とする。Si含有量は好ましくは、0.80%以上、1.00%以上、又は1.20%以上である。
Mnは、鋼板の機械的強度を高める上で有効な元素である。Mn含有量が1.00%未満では、780MPa以上の引張強度を確保することができない。したがって、Mn含有量は、1.00%以上とする。Mn含有量は好ましくは1.50%以上であり、より好ましくは1.80%以上、2.00%以上、又は2.20%以上である。
Alは、鋼を脱酸して鋼板を健全化する作用を有する元素である。sol.Al含有量が、0.001%未満では、十分に脱酸できないため、sol.Al含有量は、0.001%以上とする。但し、脱酸が十分に必要な場合、0.010%以上の添加がより望ましい。さらに望ましくは、sol.Al含有量は0.020%以上、0.030%以上、又は0.050%以上である。
Pは、一般には鋼に含有される不純物であるが、引張強度を高める作用を有するのでPを積極的に含有させてもよい。しかし、P含有量が0.100%超では、溶接性の劣化が著しくなる。したがって、P含有量は0.100%以下に制限する。P含有量は好ましくは0.080%以下、0.070%以下、又は0.050%以下に制限する。上記作用による効果をより確実に得るためには、P含有量を0.001%以上、0.002%以上、又は0.005%以上にしてもよい。
Sは、鋼に含有される不純物であり、溶接性の観点からは少ないほど好ましい。S含有量が0.0200%超では、溶接性の低下が著しくなると共に、MnSの析出量が増加し、低温靭性が低下する。したがって、S含有量は0.0200%以下に制限する。S含有量は好ましくは0.0100%以下、さらに好ましくは0.0080%以下、0.0070%以下、又は0.0050%以下に制限する。なお、脱硫コストの観点から、S含有量は、0.0010%以上、0.0015%以上、又は0.0020%以上としてもよい。
Nは、鋼に含有される不純物であり、溶接性の観点からは少ないほど好ましい。N含有量が0.01000%超では、溶接性の低下が著しくなる。したがって、N含有量は0.01000%以下に制限し、好ましくは0.00900%以下、0.00700%以下、又は0.00500%以下としてもよい。N含有量の下限値は特に限定されないが、例えばN含有量を0.00005%以上、0.00010%以上、又は0.00020%以上としてもよい。
Oは、鋼に含有される不純物であり、溶接性の観点からは少ないほど好ましい。O含有量が0.0100%超では、溶接性の低下が著しくなる。したがって、O含有量は0.0100%以下に制限し、好ましくは0.0090%以下、0.0070%以下、又は0.0050%以下である。O含有量の下限値は特に限定されないが、例えばO含有量を0.0005%以上、0.0008%以上、又は0.0010%以上としてもよい。
(Ti:0%以上0.20%以下)
(Nb:0%以上0.20%以下)
(V:0%以上1.000%以下)
(Cr:0%以上1.000%以下)
(Mo:0%以上1.000%以下)
(Cu:0%以上1.000%以下)
(Co:0%以上1.000%以下)
(W:0%以上1.000%以下)
(Ni:0%以上1.000%以下)
B、Ti、Nb、V、Cr、Mo、Cu、Co、W、Niは、いずれも強度を安定して確保するために効果のある元素である。したがって、これらの元素を含有させてもよい。しかし、Bを0.010%超、Ti及びNbをそれぞれ0.20%超、V、Cr、Mo、Cu、Co、W、Niをそれぞれ1.000%超含有させても、上記作用による効果は飽和し易く経済的に不利となる場合がある。
B:0.001%以上、0.002%以上、又は0.004%以上、
Ti:0.01%以上、0.02%以上、又は0.05%以上、
Nb:0.01%以上、0.02%以上、又は0.05%以上、
V:0.005%以上、0.008%以上、又は0.010%以上、
Cr:0.005%以上、0.008%以上、又は0.010%以上、
Mo:0.005%以上、0.008%以上、又は0.010%以上、
Cu:0.005%以上、0.008%以上、又は0.010%以上、
Co:0.005%以上、0.008%以上、又は0.010%以上、
W:0.005%以上、0.008%以上、又は0.010%以上、及び
Ni:0.005%以上、0.008%以上、又は0.010%以上
のうち、少なくとも1種を含有していることが好ましい。
(Mg:0%以上0.0100%以下)
(REM:0%以上0.0100%以下)
(Zr:0%以上0.0100%以下)
Ca、Mg、REM、Zrは、いずれも介在物制御、特に介在物の微細分散化に寄与し、靭性を高める作用を有する元素である。したがって、これらの元素の1種または2種以上を含有させてもよい。しかし、いずれの元素についてもそれぞれ0.0100%を超えて含有させると、表面性状の劣化が顕在化する場合がある。したがって、Ca、Mg、REM、Zrの含有量はそれぞれ0.01%以下、又は0.0100%以下とすることが好ましい。Ca、Mg、REM、Zrそれぞれの含有量の上限を、0.0080%、0.0050%、又は0.0030%としてもよい。なお、上記作用による効果をより確実に得るには、これらの元素の少なくとも一つの含有量を0.0003%以上、0.0005%以上、又は0.0010%以上とすることが好ましい。
本実施形態に係る高強度鋼板では、金属組織が、面積率で、フェライト:20%~70%、残留オーステナイト:5%~40%、フレッシュマルテンサイト:0%~30%、焼き戻しマルテンサイト及びベイナイトの合計:20%~75%、及びパーライト及びセメンタイトの合計:0%~10%からなる。
フェライトは比較的軟質で成形に寄与する組織である。フェライトを有することで、伸び、穴広げ性、曲げ性が向上する。この効果を得るためには、フェライトを20%以上有する必要がある。そのため、金属組織におけるフェライトの面積率を20%以上とする。フェライトの面積率を25%以上、30%以上、又は35%以上としてもよい。
フェライトを70%超有すると、引張強度を780MPa以上とすることが困難になる。そのため、金属組織におけるフェライトの面積率を70%以下とする。フェライトの面積率を65%以下、60%以下、又は50%以下としてもよい。
残留オーステナイトは伸びに寄与する組織である。この効果を得るためには残留オーステナイトが5%以上必要である。そのため、金属組織における残留オーステナイトの面積率を5%以上とし、8%以上、10%以上、又は15%以上が好ましい。
本実施形態に係る製法では、残留オーステナイトを40%以上残存させることは実質的に不可能である。そのため、金属組織における残留オーステナイトの面積率の上限は40%である。残留オーステナイトの面積率を35%以下、30%以下、又は25%以下としてもよい。
フレッシュマルテンサイトは強度に寄与する代わりに成形性を阻害する組織である。そのため、フレッシュマルテンサイトは含まれなくてもよく、その下限を0%とする。
一方、フレッシュマルテンサイトによる強度を向上させる効果を得るためには、フレッシュマルテンサイトを2%以上、5%以上、又は8%以上有することが好ましい。
一方、フレッシュマルテンサイトを30%超有すると伸びや穴広げ性を劣化させるため、金属組織におけるフレッシュマルテンサイトの面積率を30%以下とする。フレッシュマルテンサイトの面積率は20%以下が好ましく、15%以下、又は10%以下が更に好ましい。
焼き戻しマルテンサイト及びベイナイトは、強度に寄与する組織である。引張強さ780MPa以上を得るためには、焼き戻しマルテンサイト及びベイナイトが合計で20%以上必要である。そのため、本実施形態に係る高強度鋼板の金属組織では、焼き戻しマルテンサイトとベイナイトとの合計面積率を20%以上とし、好ましくは30%以上、40%以上、又は50%以上である。
一方、焼き戻しマルテンサイトおよびベイナイトの合計の上限を規定する必要はない。上述のように、本実施形態に係る鋼板の金属組織は20%以上のフェライト及び5%以上の残留オーステナイトを含むが、その残部全てが焼き戻しマルテンサイトおよびベイナイトであってもよい。換言すると、焼き戻しマルテンサイトおよびベイナイトの合計面積率は、最大で75%とすることができる。焼き戻しマルテンサイトおよびベイナイトの合計面積率は、70%以下、60%以下、又は55%以下であってもよい。
パーライトとセメンタイトは成形性を阻害する組織である。パーライトとセメンタイトとの合計面積率が10%超の場合には、成形性の劣化が大きくなるため好ましくない。そのため、パーライトとセメンタイトとの合計面積率を合計で10%以下とする。パーライトとセメンタイトとの合計面積率を8%以下、5%以下、又は3%以下としてもよい。パーライト及びセメンタイトは、本発明の課題を解決するために必要とされないので、その合計面積率の下限値は0%である。しかしながら、パーライトとセメンタイトとの合計面積率が0.5%以上、1%以上、又は2%以上であってもよい。
以上のような本実施形態に係る高強度鋼板の金属組織を構成するベイナイト、焼き戻しマルテンサイト、フェライト、パーライト、残留オーステナイトおよびマルテンサイトの同定、存在位置の確認及び面積分率の測定は、以下の方法によって行う。
まず、ナイタール試薬及び特開昭59-219473号公報に開示の試薬を用いて、圧延方向に平行な断面(即ち、圧延方向に平行且つ表面に垂直な断面)を腐食する。断面の腐食について、具体的には、100mlのエタノールに1~5gのピクリン酸を溶解した溶液をA液とし、100mlの水に1~25gのチオ硫酸ナトリウムおよび1~5gのクエン酸を溶解した溶液をB液とし、A液とB液とを1:1の割合で混合して混合液とし、この混合液の全量に対して1.5~4%の割合の硝酸を更に添加して混合した液を前処理液とする。また、2%ナイタール液に、2%ナイタール液の全量に対して10%の割合の上記前処理液を添加して混合した液を後処理液とする。圧延方向に平行な断面(即ち、圧延方向に平行且つ表面に垂直な断面)を上記前処理液に3~15秒浸漬し、アルコールで洗浄して乾燥した後、上記後処理液に3~20秒浸漬した後、水洗し、乾燥することで、上記断面を腐食する。
次に、図1に示されるように、鋼板1の表面(圧延面11)から板厚の1/4深さ且つ板幅方向WDの中央の位置において、走査型電子顕微鏡を用いて倍率1000~100000倍で、40μm×30μmの領域を少なくとも3領域観察することによって、上記金属組織の同定、存在位置の確認、及び、面積分率の測定を行う。なお、測定対象が、製造後に特段の機械加工を受けていない鋼板(換言すると、コイルから切り出されていない鋼板)である場合でも、コイルから切り出された鋼板であっても、板幅方向中央位置とは、板幅方向WDで見た鋼板1両端から実質的に等距離にある位置のことである。
また、上述の測定方法により下部ベイナイトと焼き戻しマルテンサイトとを区別することは困難である。そのため、本実施形態では両者を区別する必要はない。すなわち、「ベイナイトおよび焼き戻しマルテンサイト」の合計の面積分率は、「上部ベイナイト」および「下部ベイナイトまたは焼き戻しマルテンサイト」の面積分率を測定することで得る。上部ベイナイトは、ラスの集合体であり、ラス間に炭化物を含む組織である。下部ベイナイトは、内部に長径5nm以上かつ同一方向に伸長した鉄系炭化物を含む組織である。焼き戻しマルテンサイトは、ラス状の結晶粒の集合であり、内部に長径5nm以上かつ異なる方向に伸長した鉄系炭化物を含む組織である。
残留オーステナイトの組織形態を板状に作り込むことは、伸び、穴広げ性、曲げ性の向上に寄与し、本発明における重要な組織作り込みポイントの一つである。残留オーステナイトを板状にすることは、成形時のオーステナイトへのひずみ分配を抑制し、残留オーステナイトを塑性変形に対して適度に安定化させることで、伸び、穴広げ性を向上させる効果がある。この効果を有する残留オーステナイトの形態はアスペクト比で2.0以上である。
この効果を得るには、1/8厚~3/8厚の範囲において、アスペクト比で2.0以上の残留オーステナイトの個数割合が全残留オーステナイトに対して50%以上である必要がある。そのため、当該個数割合を50%以上とし、70%以上が好ましい。当該個数割合が50%未満では、優れた伸びと穴広げ性、曲げ性の両立が困難になるため好ましくない。
まず、図2に示されるように、鋼板の圧延方向及び板厚方向に平行な断面を観察面13として試料を採取し、観察面を研磨して鏡面に仕上げる。次いで、観察面13における表面(圧延面)11から1/4厚の位置を中心とした1/8厚~3/8厚の範囲131の一つないし複数の観察視野において、合計で2.0×10-9m2以上(複数視野及び同一視野のいずれでも可)の面積についてEBSD法による結晶構造解析を行う。次に、上記の方法により測定した残留オーステナイト粒の結晶方位から、測定エラーを避けるため、長軸長さが0.1μm以上のオーステナイトのみを抜き出して、結晶方位マップを描く。10°以上の結晶方位差を生じる境界を残留オーステナイト粒の結晶粒界とみなす。アスペクト比は、残留オーステナイト粒の長軸長さを短軸長さで除した値とする。長径は、残留オーステナイト粒の長軸長さとする。測定に当たってEBSD法により得られたデータの解析には、TSL社製の「OIM Analysys 6.0」を用いる。また、評点間距離(step)は0.01~0.20μmとする。観察結果から、FCC鉄と判断される領域を残留オーステナイトとする。この結果から、1/8厚~3/8厚の範囲において全残留オーステナイトに占める、アスペクト比2.0以上の残留オーステナイトの個数割合を求める。
本発明において、フェライトは伸びや穴広げ性を担保するために重要である。一方で、その組織分率によって強度や伸び、穴広げ性が変化する。そのため、フェライトの組織分率が熱延幅方向に均一に分布していることは、材質安定性を得る上で重要である。
図3に示すように、圧延方向に平行な断面(即ち、圧延方向に平行且つ表面に垂直な断面12)の板厚1/4位置121におけるフェライトの面積率を、板幅方向(即ち、圧延方向RDに直角な方向)WDに沿って50mmおきに10か所で測定したとき、フェライトの面積率の標準偏差が10%以上であると、機械特性がばらつく原因となり、材質安定性が得られない。そのため、上述のフェライトの面積率の標準偏差を10%未満とし、好ましくは8%以下、5%未満、又は4%以下である。なお、測定対象となる鋼板の板幅方向に沿った大きさが十分に大きいときは、フェライトの面積率の標準偏差の測定箇所は、板幅方向に沿った1直線上に配置するとよい。一方、測定対象となる鋼板の板幅方向に沿った大きさが450mmに満たないときは、フェライトの面積率の標準偏差の測定箇所は、板幅方向に沿った2本以上の直線上に配置するとよい。フェライト以外の特性(例えば表面粗さ等)の板幅方向の標準偏差の測定の際にも、上述のように測定箇所を配置することができる。
(板幅方向に沿って50mmおきに10か所で測定した表面粗さRaの標準偏差が、好ましくは0.5μm以下)
化学成分、金属組織、及び後述する引張強度が所定の範囲内である限り、本実施形態に係る鋼板は特に限定されない。一方、板幅方向(即ち、圧延方向に直角な方向)に沿って50mmおきに10か所で圧延面11の表面粗さRaを測定したとき、表面粗さRaの標準偏差を0.5μm以下としてもよい。表面粗さRaのばらつきを抑制することにより、曲げ加工性のばらつきを抑制し、材質安定性を一層高めることができる。そのため、当該標準偏差を0.5μm以下とすることが好ましい。ただし、鋼板の表面粗さは追加工によって自在に変更することができる。例えば、後述する好ましい製造方法によって材質安定性に優れた高強度鋼板を製造した後に、この高強度鋼板にヘアライン加工などの表面粗さを変更する加工をしてもよい。この観点からも、表面粗さRaの標準偏差を上述の範囲内とすることは必須ではない。
(引張強度TS:780MPa以上)
本実施形態に係る高強度鋼板は、自動車の軽量化に寄与する十分な強度として、780MPa以上の引張強度(TS)を有する。鋼板の引張強度が800MPa以上、900MPa以上、又は1000MPa以上であってもよい。一方、本実施形態の構成で1470MPa超とすることは困難であると推定される。そのため、引張強度の上限は特に定める必要はないが、本実施形態において実質的な引張強度の上限を1470MPaとすることができる。また、鋼板の引張強度を1400MPa以下、1300MPa以下、又は1200MPa以下としてもよい。
本実施形態に係る高強度鋼板は、伸びの指標として引張試験における全伸びで14%以上を有してもよい。一方、本実施形態の構成で全伸びを35%超とすることは困難である。そのため、実質的な全伸びの上限は35%としてもよい。
本実施形態に係る高強度鋼板は、穴広げ性の指標として穴広げ率25%以上を有してもよい。一方、本実施形態の構成で穴広げ率を80%超とすることは困難である。そのため、実質的な穴広げ率の上限を80%としてもよい。
穴広げ率は、日本鉄鋼連盟規格JFS T 1001-1996記載の試験方法に準拠した穴広げ試験により評価できる。
本実施形態に係る高強度鋼板は、曲げ性の指標として限界曲げR(mm)を板厚t(mm)で除した値R/tを用いた場合、2.0以下のR/tを有してもよい。一方、本実施形態の構成で曲げ性の指標R/tを0.1以下とすることは困難である。そのため、実質的な曲げ性の指標R/tの下限値を0.1としてもよい。
限界曲げRは、種々の曲げ半径を適用した曲げ試験を繰り返し実施することによって求められる。曲げ試験では、JIS Z 2248(Vブロック90°曲げ試験)に準拠して曲げ加工を行う。曲げ半径(正確には、曲げの内側半径)は0.5mmピッチで変更する。曲げ試験における曲げ半径が小さいほど、鋼板に裂けきず及びその他の欠点が生じやすくなる。この試験において求められた、鋼板に裂けきず及びその他の欠点を生じさせない最小の曲げを限界曲げRとみなす。そして、この限界曲げRを鋼板の厚さtで割った値を、曲げ性を評価する指標R/tとして用いる。
次に、本実施形態に係る高強度鋼板の好ましい製造方法の一例について説明する。ただし、本実施形態に係る高強度鋼板の製造方法は特に限定されないことに留意されたい。上述の要件を満たす鋼板は、その製造方法に関わらず、全て本実施形態に係る鋼板であるとみなされる。
粗圧延は、スラブを所望の寸法形状にすればよく、その条件は特に限定しない。なお、粗圧延板の厚さは、仕上げ圧延工程における、圧延開始時から圧延完了時までの熱延鋼板先端から尾端までの温度低下量に影響を及ぼすため、これを考慮して決定することが好ましい。
K’/Si*≧2.5・・・(1)
ここで、Si≧0.35のときはSi*=140√Siとし、Si<0.35のときはSi*=80とする。なお、Siは鋼板のSi含有量(質量%)を表す。
K’=D×(DT-930)×1.5+Σ((FTn-930)×Sn)・・・(2)
ここで、Dは仕上げ圧延開始前の水圧デスケーリングの時間当たりの吹き付け量(m3/min)、DTは仕上げ圧延開始前の水圧デスケーリングを行う際の鋼板温度(℃)、FTnは仕上げ圧延のn段目における鋼板温度(℃)、Snは仕上げ圧延のn-1段目とn段目の間に水をスプレー上に鋼板に吹き付けるときの時間当たりの吹き付け量(m3/min)である。
焼鈍工程では、加熱温度を以下の式で計算されるAc1点~Ac3点-10℃とする。
Ac1=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr
Ac3=879-346×C+65×Si-18×Mn+54×Al・・(9)
加熱時には低温変態組織のラス間などに生成した炭化物からフェライト-オーステナイト変態が生じ、板状のオーステナイトが生成する。オーステナイト変態しなかった領域は高温で焼き戻された低温変態組織(焼き戻しマルテンサイトや焼き戻しベイナイト)と考えることもできるが、転位密度は焼き戻しにより大きく減少しており、下部組織も不明瞭となっていることから、焼鈍後の組織観察においてフェライトとして評価させる領域である。そのため、ここでもフェライトと呼称する。なお、焼鈍後の組織観察において焼き戻しマルテンサイトやベイナイトと評価される領域は、加熱で生成したオーステナイトが後述する150℃~550℃での保持中にベイナイト変態やマルテンサイト変態することで生成した組織を主に指す。
加熱温度をAc1点~Ac3点-10℃とする理由は、フェライトの面積率を20%~70%とするために、適切なフェライト-オーステナイト変態分率とするためである。加熱時間は10秒~1000秒とする。保持時間が1秒未満であると、鋼中のセメンタイトが溶け残り、鋼板の特性が劣化する懸念がある。この効果は1000秒超で飽和し、生産性の低下につながることから、保持時間は1000秒を上限とする。
この温度域では、オーステナイトの一部をベイナイト変態やマルテンサイト変態させ、ベイナイト変態に伴い固溶炭素をオーステナイトに吐き出させることや、マルテンサイトの焼き戻しに伴い固溶炭素をオーステナイトに吐き出させることで、オーステナイトを安定化させる効果がある。150℃以下ではオーステナイトの大部分がマルテンサイト変態し、十分な残留オーステナイト量を得ることができない。一方、550℃以上では、パーライト変態が生じ、残留オーステナイトを十分に安定化できない。保持時間が10秒未満では、炭素の拡散が十分に起きず、残留オーステナイトを十分に安定化できない。1000秒超では、残留オーステナイトを安定化させる効果が飽和し、生産性が低下する。
合金化処理を行う場合は、常法にしたがって行えばよいが、合金化処理温度は460~550℃とすることが好ましい。合金化処理が460℃未満であると、合金化速度が遅くなり生産性を損なうばかりでなく、合金化処理むらが発生するので、合金化処理温度は460℃以上とすることが好ましい。一方、合金化処理温度が550℃を超えると、パーライト変態が生じ、残留オーステナイトを十分に安定化できない。
また合金化処理は、溶融亜鉛めっき層中の鉄濃度が6.0質量%以上となるような条件で行うことが好ましい。
溶融亜鉛めっきや合金化溶融亜鉛めっきを施さなかった場合、上記のように製造した鋼板に、電気亜鉛めっき層を形成してもよい。電気亜鉛めっき層は、従来公知の方法により形成できる。
その後、表3に記載の各条件で仕上げ圧延後の冷却及び巻き取りを施した。
その後、全条件に対して酸洗を行ったが、一部の条件については酸洗の前または後工程で軽圧下を実施した。その後、加熱速度30℃/s~150℃/sの速度で表3に記載の加熱温度まで昇温した。加熱後、表3に記載の時間、加熱温度で保持した。その後、条件Aでは、50~100℃/sで250℃まで冷却し、400℃再加熱した後、300秒保持した。条件Bでは50~100℃/sで360℃まで冷却し、50秒保持した。比較例である条件Cでは、100℃/sで100℃まで冷却し、300秒保持した。
その後、一部の条件は合金化溶融亜鉛めっきや溶融亜鉛めっきを施した。めっき工程においては、鋼板は400℃~520℃の温度域にあった。
まず、ナイタール試薬及び特開昭59-219473号公報に開示の試薬を用いて、圧延方向に平行且つ表面に垂直な断面を腐食した。断面の腐食について、具体的には、100mlのエタノールに1~5gのピクリン酸を溶解した溶液をA液とし、100mlの水に1~25gのチオ硫酸ナトリウムおよび1~5gのクエン酸を溶解した溶液をB液とし、A液とB液とを1:1の割合で混合して混合液とし、この混合液の全量に対して1.5~4%の割合の硝酸を更に添加して混合した液を前処理液とした。また、2%ナイタール液に、2%ナイタール液の全量に対して10%の割合の上記前処理液を添加して混合した液を後処理液とした。圧延方向に平行且つ表面に垂直な断面を上記前処理液に3~15秒浸漬し、アルコールで洗浄して乾燥した後、上記後処理液に3~20秒浸漬した後、水洗し、乾燥することで、上記断面を腐食した。
なお、「ベイナイトおよび焼き戻しマルテンサイト」の合計の面積分率は、「上部ベイナイト」および「下部ベイナイトまたは焼き戻しマルテンサイト」の面積分率を測定することで得た。
まず、鋼板の圧延方向及び板厚方向に平行な断面を観察面として試料を採取し、観察面を研磨して鏡面に仕上げた。次いで、観察面における表面から1/4厚の位置を中心とした1/8厚~3/8厚の範囲の一つないし複数の観察視野において、合計で2.0×10-9m2以上(複数視野及び同一視野のいずれでも可)の面積についてEBSD法による結晶構造解析を行った。次に、上記の方法により測定した残留オーステナイト粒の結晶方位から、測定エラーを避けるため、長軸長さが0.1μm以上のオーステナイトのみを抜き出して、結晶方位マップを描いた。10°以上の結晶方位差を生じる境界を残留オーステナイト粒の結晶粒界とみなした。アスペクト比は、残留オーステナイト粒の長軸長さを短軸長さで除した値とした。長径は、残留オーステナイト粒の長軸長さとした。測定に当たってEBSD法により得られたデータの解析には、TSL社製の「OIM Analysys 6.0」を用いた。また、評点間距離(step)は0.01~0.20μmとした。観察結果から、FCC鉄と判断される領域を残留オーステナイトとした。この結果から、1/8厚~3/8厚の範囲において全残留オーステナイトに占める、アスペクト比2.0以上の残留オーステナイトの個数割合を求めた。
また、板幅方向に50mm間隔で10か所の位置におけるTS及びELの標準偏差を求めた。TSの標準偏差が50MPa以下であり、且つELの標準偏差が1%以下である鋼板を、材質安定性に優れた鋼板と判定した。
また、板幅方向に50mm間隔で10か所の位置でR/tを測定し、その標準偏差を求めた。
11 表面(圧延面)
12 圧延方向に平行且つ表面に垂直な断面
121 圧延方向に平行且つ表面に垂直な断面の板厚1/4位置
13 残留オーステナイトの測定面
131 残留オーステナイトの測定面における、表面(圧延面)から1/8厚~3/8厚の範囲
RD 圧延方向(Rolling Direction)
TD 板厚方向(Thickness Direction)
WD 板幅方向(Width Direction)
Claims (3)
- 化学成分として、質量%で、
C:0.030~0.280%、
Si:0.50~2.50%、
Mn:1.00~4.00%、
sol.Al:0.001~2.000%、
P:0.100%以下、
S:0.0200%以下、
N:0.01000%以下、
O:0.0100%以下、
B:0~0.010%、
Ti:0~0.20%、
Nb:0~0.20%、
V:0~1.000%、
Cr:0~1.000%、
Mo:0~1.000%、
Cu:0~1.000%、
Co:0~1.000%、
W:0~1.000%、
Ni:0~1.000%、
Ca:0~0.0100%、
Mg:0~0.0100%、
REM:0~0.0100%、
Zr:0~0.0100%、及び
残部:Fe及び不純物
を含み、
金属組織が、面積率で、
フェライト:20%~70%、
残留オーステナイト:5%~40%、
フレッシュマルテンサイト:0%~30%、
焼き戻しマルテンサイト及びベイナイトの合計:20%~75%、及び
パーライト及びセメンタイトの合計:0%~10%
からなり、
表面から1/8厚~3/8厚の範囲において、全残留オーステナイトの個数に対する、アスペクト比2.0以上の残留オーステナイトの個数割合が50%以上であり、
圧延方向に平行且つ前記表面に垂直な断面の板厚1/4位置において、板幅方向に沿って50mmおきに10か所で測定されたフェライトの面積率の標準偏差が10%未満であり、
引張強度が780MPa以上である
ことを特徴とする高強度鋼板。 - 前記板幅方向に50mm間隔で10か所の位置において、表面粗さRaの標準偏差が0.5μm以下であることを特徴とする請求項1に記載の高強度鋼板。
- 前記化学成分として、質量%で、
B:0.001%~0.010%、
Ti:0.01~0.20%、
Nb:0.01~0.20%、
V:0.005%~1.000%、
Cr:0.005%~1.000%、
Mo:0.005%~1.000%、
Cu:0.005%~1.000%、
Co:0.005%~1.000%、
W:0.005%~1.000%、
Ni:0.005%~1.000%、
Ca:0.0003%~0.0100%、
Mg:0.0003%~0.0100%、
REM:0.0003%~0.0100%、及び
Zr:0.0003%~0.0100%
からなる群から構成される少なくとも1種を含有する
ことを特徴とする請求項1又は2に記載の高強度鋼板。
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CN113933136A (zh) * | 2021-10-15 | 2022-01-14 | 华北理工大学 | 一种无Al医用锌基合金的枝晶腐蚀试剂、制备和使用方法 |
WO2024135365A1 (ja) * | 2022-12-23 | 2024-06-27 | 日本製鉄株式会社 | 熱間圧延鋼板 |
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WO2021172298A1 (ja) * | 2020-02-28 | 2021-09-02 | Jfeスチール株式会社 | 鋼板、部材及びそれらの製造方法 |
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- 2020-07-08 US US17/601,799 patent/US20220177995A1/en active Pending
- 2020-07-08 JP JP2021530716A patent/JP7168087B2/ja active Active
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Cited By (2)
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CN113933136A (zh) * | 2021-10-15 | 2022-01-14 | 华北理工大学 | 一种无Al医用锌基合金的枝晶腐蚀试剂、制备和使用方法 |
WO2024135365A1 (ja) * | 2022-12-23 | 2024-06-27 | 日本製鉄株式会社 | 熱間圧延鋼板 |
Also Published As
Publication number | Publication date |
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MX2021012787A (es) | 2021-12-10 |
KR20210134967A (ko) | 2021-11-11 |
KR102649505B1 (ko) | 2024-03-21 |
CN113748223B (zh) | 2022-12-16 |
US20220177995A1 (en) | 2022-06-09 |
JP7168087B2 (ja) | 2022-11-09 |
EP3998366A1 (en) | 2022-05-18 |
CN113748223A (zh) | 2021-12-03 |
EP3998366A4 (en) | 2023-06-07 |
JPWO2021006296A1 (ja) | 2021-12-16 |
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