WO2020136990A1 - 高強度溶融亜鉛めっき鋼板およびその製造方法 - Google Patents
高強度溶融亜鉛めっき鋼板およびその製造方法 Download PDFInfo
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- WO2020136990A1 WO2020136990A1 PCT/JP2019/033082 JP2019033082W WO2020136990A1 WO 2020136990 A1 WO2020136990 A1 WO 2020136990A1 JP 2019033082 W JP2019033082 W JP 2019033082W WO 2020136990 A1 WO2020136990 A1 WO 2020136990A1
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- hot
- steel sheet
- less
- dip galvanized
- strength
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
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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
-
- 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
- C21D2211/004—Dispersions; Precipitations
-
- 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
Definitions
- the present invention relates to a high-strength hot-dip galvanized steel sheet suitable for automobile members and a method for producing the same.
- Patent Documents 1 and 2 disclose a steel plate having excellent delayed fracture resistance when it has a sheared end face by controlling the amount, shape, and distribution of inclusions.
- various technologies have been developed such as removing residual stress on the end face for delayed fracture from the end face, and it is considered that delayed fracture from the end face can be resolved by end face treatment in the future. ..
- Patent Document 3 discloses a steel sheet in which delayed fracture resistance is improved by controlling S and MnS of a base material.
- Patent Documents 1 and 2 are effective for delayed fracture from the sheared end face, it is not clear whether they are also effective for improving the delayed fracture characteristics of the base material.
- This technique is mainly for controlling inclusions and micron-level microstructure, and there is room for improvement because the control of P segregation of old austenite grain boundaries has not been studied.
- the technology related to S disclosed in Patent Document 3 only reduces P, and has not studied P segregation control of grain boundaries, and there is room for improvement.
- the present invention has been made in view of the above problems, and an object thereof is to provide a high-strength hot-dip galvanized steel sheet having high strength and excellent delayed fracture resistance, and a method for manufacturing the same.
- the present inventors have found the following as a result of intensive research. That is, in mass%, C: 0.08 to 0.35%, Si: 0.01 to 3.0%, Mn: 2.0 to 4.0%, P: 0.010% or less (including 0 No), S: 0.002% or less (not including 0), Al: 0.01 to 1.50%, B: 0.0005 to 0.010%, and Mo: 0.03 to 2 0.0%, Ti: 0.010 to 0.10%, at least one selected from the group consisting of Fe and unavoidable impurities, and the microstructure observation at 300 to 400 ⁇ m from the surface of the steel sheet.
- the total area ratio of bainite containing martensite and carbide is 60 to 100%, the average grain size of the former austenite is 15 ⁇ m or less, and the P in the former austenite grain boundary is 300 to 400 ⁇ m from the surface of the steel sheet.
- TS tensile strength
- high strength means a TS of 1100 MPa or more
- excellent delayed fracture resistance means a strain rate of 5 ⁇ m/min, a solution of 3 mass% NaCl+3 g/l NH 4 SCN, and an applied current density of 0.05 mA/cm 2. It means that the ratio of the maximum stress at the time of addition to the maximum stress at the time of not adding hydrogen is 0.70 or more in the SSRT test.
- the present invention has been made based on such findings, and the summary thereof is as follows. [1]% by mass, C: 0.08 to 0.35%, Si: 0.01 to 3.0%, Mn: 2.0 to 4.0%, P: 0.010% or less (0: (Not including), S: 0.002% or less (not including 0), Al: 0.01 to 1.50%, B: 0.0005 to 0.010%, and Mo: 0.03. To 2.0%, Ti: 0.010 to 0.10%, and the balance is Fe and inevitable impurities.
- the total area ratio of bainite containing martensite and carbide is 60 to 100%
- the average grain size of the former austenite is 15 ⁇ m or less in the area ratio
- the range of 300 to 400 ⁇ m in the plate thickness direction from the steel plate surface layer In, a steel structure in which the ratio of the peak height of the P auger electron spectrum at a position 5 nm or more away from the old austenite grain boundary to the peak height of the P auger electron spectrum at the old austenite grain boundary is 0.20 or more
- the component composition further contains, in mass%, one or more selected from Nb: 0.005 to 0.20% and V: 0.005 to 2.0%.
- the composition of the components is, in mass %, Cr: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Cu: 0.005 to 2.0%, Ca: 0.0. It contains at least one selected from 0002 to 0.0050%, REM: 0.0002 to 0.0050%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, [ The high-strength galvanized steel sheet according to [1] or [2].
- Annealing then hot dip galvanizing, or further alloying, and then cooling up to the Ms point at an average cooling rate of 1°C/s or more, and in the subsequent cooling step, the following (1)
- a method for producing a high-strength hot-dip galvanized steel sheet which satisfies the formula and gives a strain of more than 0% and 0.067% or less at 250°C to Ms point.
- a hot rolled sheet produced by hot rolling a slab having the component composition according to any one of [1] to [3], followed by cooling and hot rolling is pickled. Then, tempering is performed under the condition that the maximum reached temperature is less than 400° C., then cold rolling is performed at a reduction rate of 70% or more, and then heating is performed at 750 to 950° C. and after holding for 10 to 600 s, 3° C./ It is cooled to 550°C at an average cooling rate of s or more, annealed at Ms point to 550°C for 300s or less, then hot-dip galvanized, or further alloyed, and then heated to Ms point.
- High-strength melting that cools at an average cooling rate of 1°C/s or more, satisfies the following formula (1) in the subsequent steps, and imparts a strain of more than 0% and 0.067% or less at 250°C to Ms point.
- Manufacturing method of galvanized steel sheet Log e ([P] ⁇ [C] ⁇ (8.65-474 ⁇ D A ⁇ [B]) ⁇ t)+35.4 ⁇ 13320/(273+T)+4831/(273+T-100 ⁇ [Si])... ⁇ (1)
- D A ⁇ [B] ⁇ 0.00912 D A ⁇ [B] is set to 0.00912
- [P], [C], and [B] in the above formula (1) are used.
- [Si] is the P respectively in steel, C, B, the content of Si (mass%), D a is the prior austenite grain size ( ⁇ m), T is the retention temperature (° C.), t is the total residence time (s) at the residence temperature.
- C 0.08 to 0.35%
- C is an element effective in increasing bainite by forming bainite containing martensite and carbide. If the C content is less than 0.08%, such an effect cannot be obtained, and the strength and steel structure of the steel sheet according to this embodiment cannot be obtained. Therefore, the amount of C needs to be 0.08% or more.
- the C content is preferably 0.10% or more, more preferably 0.13% or more.
- the C content when the C content exceeds 0.35%, martensite is hardened and the delayed fracture resistance is deteriorated. Therefore, the C content needs to be 0.35% or less.
- the C content is preferably 0.33% or less.
- Si 0.01 to 3.0% Si is an element necessary to enhance TS by suppressing solid solution strengthening and tempering of martensite and to obtain excellent delayed fracture resistance.
- the Si amount needs to be 0.01% or more.
- the amount of Si is preferably 0.1% or more, more preferably 0.2% or more.
- the Si amount needs to be 3.0% or less.
- the amount of Si is preferably 2.5% or less, more preferably 2.0% or less.
- Mn 2.0 to 4.0%
- Mn is an element effective in increasing bainite by producing bainite containing martensite and carbide. If the Mn content is less than 2.0%, such an effect cannot be obtained. Therefore, the Mn amount needs to be 2.0% or more.
- the amount of Mn is preferably 2.1% or more, more preferably 2.2% or more.
- the Mn content exceeds 4.0%, the steel becomes brittle and the delayed fracture resistance of the steel sheet according to this embodiment cannot be obtained. Therefore, the Mn amount needs to be 4.0% or less.
- the Mn content is preferably 3.7% or less, and more preferably 3.4% or less.
- P 0.010% or less (not including 0) Since P makes the grain boundaries brittle and deteriorates the delayed fracture resistance, it is desirable to reduce the amount as much as possible, but in the steel sheet according to this embodiment, it can be allowed up to 0.010%.
- the lower limit of the amount of P does not have to be specified, but if the amount of P of the steel sheet is less than 0.0005%, a large load will occur in refining and the production efficiency will decrease. Therefore, the lower limit of the amount of P is preferably 0.0005%.
- S 0.002% or less (not including 0) S increases inclusions and deteriorates delayed fracture resistance. Therefore, it is preferable that the amount of S is as small as possible, but in the steel sheet according to the present embodiment, up to 0.002% can be allowed.
- the lower limit of the amount of S does not have to be specified, but if it is less than 0.0001%, a large load occurs in refining, and the production efficiency decreases. Therefore, the lower limit of the amount of S is preferably 0.0001%.
- Al 0.01 to 1.50% Since Al acts as a deoxidizing agent, it is preferably added in the deoxidizing step. In order to obtain such an effect, the amount of Al needs to be 0.01% or more.
- the Al amount is preferably 0.015% or more.
- the amount of Al needs to be 1.50% or less.
- the Al amount is preferably 1.00% or less, and more preferably 0.70% or less.
- B 0.0005 to 0.010%
- B is an important element in the steel sheet according to the present embodiment for improving delayed fracture resistance.
- the mechanism is not clear, but it is presumed that the grain boundaries are strengthened by segregating to the former austenite grain boundaries. It is an element that enhances the hardenability of steel sheets, forms martensite and bainite, and is effective for strengthening.
- the B amount needs to be 0.0005% or more.
- the amount of B is preferably 0.0007% or more.
- the amount of B needs to be 0.010% or less.
- the amount of B is preferably 0.0050% or less, more preferably 0.0040% or less.
- Mo and Ti suppress the carbonization and nitriding of B and exhibit the above effect as solid solution B. It is an element necessary for In order to obtain such an effect, it is necessary to set one or more kinds selected from Mo and Ti to the respective lower limit amounts or more. On the other hand, when Mo and Ti exceed the respective upper limit amounts, coarse carbides are generated, the amount of solute C decreases, and the steel structure of the steel sheet according to the present embodiment cannot be obtained. Therefore, it is necessary to contain at least one selected from Mo: 0.03% to 2.0% and Ti: 0.010% to 0.10%.
- the steel sheet according to the present embodiment may contain one or more selected from the following elements, if necessary.
- Nb 0.005 to 0.20%
- V 0.005 to 2.0%
- Nb and V are elements that form fine carbides and are effective in increasing the strength.
- the content of one or more selected from Nb and V be 0.005% or more.
- the content of one or more selected from Nb and V is preferably Nb: 0.005% or more and 0.20% or less, and V: 0.005% or more and 2.0% or less.
- Cr 0.005-2.0%
- Ni 0.005-2.0%
- Cu 0.005-2.0%
- Ca 0.0002-0.0050%
- REM 0.0002- 0.0050%
- Sn 0.001-0.05%
- Sb 0.001-0.05%
- Cr, Ni, and Cu are elements effective in increasing the strength by producing martensite and bainite.
- the amounts of Cr, Ni and Cu are preferably 0.005% or more.
- the amounts of Cr, Ni, and Cu are preferably 2.0% or less.
- Ca and REM are effective elements for improving delayed fracture resistance by controlling the morphology of inclusions.
- the Ca and REM contents are preferably 0.0002% or more.
- the Ca and REM contents are preferably 0.0050% or less.
- ⁇ Sn and Sb are elements that are effective in suppressing the strength reduction of steel by suppressing denitrification, deboron, etc.
- the Sn and Sb amounts are preferably 0.001% or more.
- the Sn and Sb amounts are preferably 0.05% or less.
- the contents of Cr, Ni, Cu, Ca, REM, Sn, and Sb are Cr: 0.005 to 2.0%, Ni: 0.005 to 2.0%, and Cu: 0.005 to 2. 0%, Ca: 0.0002 to 0.0050%, REM: 0.0002 to 0.0050%, Sb: 0.001 to 0.05%, Sn: 0.001 to 0.05% preferable.
- the steel sheet according to the present embodiment contains the above component composition, and the balance other than the above component composition contains Fe (iron) and inevitable impurities.
- the balance is preferably Fe and inevitable impurities.
- the above optional components are contained in less than the lower limit amounts, the components are included as unavoidable impurities.
- N may be contained in an amount of 0.01% or less.
- the other elements Zr, Mg, La, and Ce may be contained in a total amount of 0.002% or less.
- the steel sheet according to the present embodiment has a total area ratio of bainite containing martensite and carbide of 60 to 100% and an average grain size of old austenite in an area ratio of 300 to 400 ⁇ m in the thickness direction from the steel plate surface layer. 15 ⁇ m or less, and in the range of 300 to 400 ⁇ m in the plate thickness direction from the surface layer of the steel sheet, the Auger electron spectrum of P at a position 5 nm or more away from the former austenite grain boundary with respect to the peak height of the Auger electron spectrum of P at the former austenite grain boundary The ratio of the peak heights is 0.20 or more.
- the steel sheet surface layer means the interface between the hot-dip galvanized layer and the steel sheet.
- Total of martensite and bainite containing carbide 60-100% Bainite containing martensite and carbide is a structure required for TS increase and excellent delayed fracture resistance. In order to obtain such effects, the total area ratio of bainite containing martensite and carbide needs to be 60% or more.
- the total of martensite and bainite containing carbide is preferably 75% or more, more preferably 90% or more.
- the upper limit of the total of martensite and bainite containing carbide is 100%.
- Average grain size of prior austenite 15 ⁇ m or less
- Former austenite has a remarkable P segregation and greatly affects the occurrence of delayed fracture.
- concentration of grain boundary stress is relaxed, and delayed fracture resistance of the steel sheet according to this embodiment is obtained. Therefore, the average crystal grain size of prior austenite must be 15 ⁇ m or less.
- the average crystal grain size of prior austenite is preferably 12 ⁇ m or less, more preferably 10 ⁇ m or less, and further preferably 9 ⁇ m or less.
- the ratio of the peak height of the austenite electron spectrum of P at a position 5 nm or more away from the former austenite grain boundary to the peak height of the austenite electron spectrum of P in the former austenite grain boundary 0.20 or more Segregation remarkably deteriorates the delayed fracture resistance, so that suppression thereof is extremely important.
- the ratio of the peak height of the P auger electron spectrum at a position 5 nm or more away from the old austenite grain boundary to the peak height of the P auger electron spectrum at the old austenite grain boundary was used. If the ratio is less than 0.20, delayed fracture occurs at the former austenite grain boundary, and the steel sheet according to the present embodiment cannot have excellent delayed fracture resistance.
- the ratio of the peak height of the P auger electron spectrum at the position 5 nm or more away from the old austenite grain boundary to the peak height of the P auger electron spectrum at the old austenite grain boundary needs to be 0.20 or more.
- the ratio of the peak height of the P auger electron spectrum at a position 5 nm or more away from the old austenite grain boundary to the peak height of the P auger electron spectrum at the former austenite grain boundary is preferably 0.30 or more, It is more preferably 35 or more, still more preferably 0.40 or more.
- the ratio of the peak height of the P auger electron spectrum at a position 5 nm or more away from the old austenite grain boundary to the peak height of the P auger electron spectrum at the old austenite grain boundary can be calculated by the following formula (2).
- the area ratio of bainite containing martensite or carbide in the present embodiment is the ratio of the area of each structure to the observed area, and these area ratios cut out a sample from the steel sheet after annealing, in the rolling direction. After polishing the parallel plate thickness cross section, it is corroded with 1% by mass of Nital, and the range of 300 ⁇ m or more and 400 ⁇ m or less in the plate thickness direction near the steel plate surface and from the steel plate surface by SEM (scanning electron microscope) at a magnification of 1500 times, respectively, 3 times.
- the area ratio of each tissue is obtained from the obtained image data by using Image-Pro manufactured by Media Cybernetics, and the average area ratio of these fields of view is set as the area ratio of each tissue.
- bainite containing carbides are distinguished as gray or dark gray including orientation of uniform carbide 10 7 / mm 2 or more. Martensite is distinguished as dark gray, grey, light gray with non-orientated carbides or lumpy white without carbides. The retained austenite is also lumpy white, but is distinguished from martensite by the method described below. Carbides are distinguished as white dots or lines. Bainite containing no ferrite or carbide is distinguished as black or dark gray, and perlite is distinguished as black and white layered texture.
- the average crystal grain size of the former austenite was re-polished on the sample cross-section where the structure was observed, etched with a mixed solution of picric acid and ferric chloride, etc., to reveal the former austenite grain boundaries, and then to an optical microscope. Then, 3 to 10 fields of view are photographed at a magnification of 400 times, and a total of 20 straight lines of 10 ⁇ 10 are drawn at equal intervals and the obtained image data is obtained by the cutting method.
- the area ratio of retained austenite was determined by grinding the annealed steel plate to 1 ⁇ 4 of the plate thickness and then polishing the surface 0.1 mm by chemical polishing using a K ⁇ ray of Mo with an X-ray diffractometer and using fcc iron (austenite). ), (200) face, (220) face, (311) face and bcc iron (ferrite) (200) face, (211) face, (220) face integrated reflection intensity is measured, and each bcc iron The volume ratio is calculated from the intensity ratio of the integrated reflection intensity from each face of fcc iron to the integrated reflection intensity from the face, and this is taken as the area ratio of retained austenite. By calculating the area ratio of retained austenite in this way, retained austenite and martensite are distinguished.
- the steel sheet according to the present embodiment is, for example, subjected to hot rolling on a slab having the above components, then cooled, pickled to produce a hot rolled sheet produced by hot rolling and then cold rolling. And then heated to 750 to 950° C., held for 10 to 600 s, cooled to 550° C. or less at an average cooling rate of 3° C./s or more, and annealed to hold at Ms point to 550° C.
- the slab having the above components is subjected to hot rolling, followed by cooling, pickling the hot-rolled sheet produced by subjecting to hot rolling and picking, and then subjecting to tempering under conditions of a maximum reaching temperature of 400° C.,
- cold rolling is performed at a rolling reduction of 70% or more, then heating to 750 to 950° C., holding for 10 to 600 s, cooling to 550° C. or less at an average cooling rate of 3° C./s or more, and Ms point Annealing at 300 to 550° C.
- Annealing temperature 750-950°C If the annealing temperature is less than 750° C., the austenite formation is insufficient and the steel structure of the steel sheet according to this embodiment cannot be obtained. Therefore, the annealing temperature needs to be 750° C. or higher. On the other hand, when the annealing temperature exceeds 950° C., the grains are coarsened and the steel structure of the steel sheet according to this embodiment cannot be obtained. Therefore, the annealing temperature needs to be 950° C. or lower.
- Annealing holding time 10-600s
- the main annealing holding time is less than 10 s, austenite is insufficiently produced, and the steel structure of the steel sheet according to this embodiment cannot be obtained. Therefore, the annealing holding time needs to be 10 seconds or more.
- the annealing holding time is preferably 20 seconds or longer, more preferably 30 seconds or longer.
- the annealing holding time needs to be 600 s or less.
- the annealing holding time is preferably 500 s or less, more preferably 400 s or less.
- Average cooling rate from annealing temperature to 550° C. 3° C./s or more
- the average cooling rate from the annealing temperature to 550°C needs to be 3°C/s or more.
- the average cooling rate from the annealing temperature to 550°C is preferably 5°C/s or more.
- the upper limit of the average cooling rate from the annealing temperature to 550°C need not be specified, but from the viewpoint of shape stability, it is preferably less than 100°C/s.
- the average cooling rate is calculated by dividing the temperature difference between the annealing temperature and 550°C by the time required for cooling from the annealing temperature to 550°C.
- Holding temperature Ms point to 550°C If the holding temperature exceeds 550° C., ferrite is generated and the steel structure of the steel sheet according to this embodiment cannot be obtained. Therefore, the holding temperature needs to be 550° C. or lower. On the other hand, when the holding temperature is lower than the Ms point, martensite transformation and bainite transformation excessively proceed, and the P distribution of the steel sheet according to this embodiment cannot be obtained. Therefore, the holding temperature needs to be equal to or higher than the Ms point.
- the MS point is the temperature at which martensitic transformation starts and can be determined by Formaster.
- Holding time at Ms point to 550° C. 300 s or less If the holding time at Ms point or more and 550° C. or more exceeds 300 s, bainite containing no excess carbide is generated, and the steel structure of the steel sheet according to the present embodiment is obtained. Absent. Therefore, the holding time at the Ms point or higher and 550° C. or lower needs to be 300 s or less.
- the holding time at the Ms point or higher and 550° C. or lower is preferably 200 s or less, and more preferably 120 s or less.
- the temperature does not have to be constant as long as it is in the range of Ms point to 550° C., and cooling or heating may be performed.
- Average cooling rate up to Ms point 1° C./s or more
- bainite transformation excessively progresses during cooling and the P distribution of the steel sheet according to the present embodiment is obtained. I can't. Therefore, the average cooling rate up to the Ms point needs to be 1° C./s or more.
- the average cooling rate is calculated by dividing the temperature difference between the cooling start temperature and the Ms point by the time required for cooling from the cooling start temperature to the Ms point.
- Strain applied at 250° C. to Ms point Greater than 0% and less than or equal to 0.067% P can be suppressed from segregating to the prior austenite grain boundaries by applying strain during martensitic transformation.
- the applied strain exceeds 0.067%, the P segregation amount increases, and the P distribution of the steel sheet according to this embodiment cannot be obtained.
- the lower limit of the strain applied at 250° C. or higher and the Ms point or lower is preferably 0.010%.
- the method for imparting strain is not particularly limited, and tension imparting, rolling or the like may be used.
- the cooling process after cooling up to the Ms point at an average cooling rate of 1° C./s or more satisfies the formula (1) thereby suppressing P segregation to the austenite grain boundaries, and excellent resistance to the steel sheet according to the embodiment. Delayed fracture characteristics can be obtained.
- Isothermal holding and reheating may be performed as long as the cooling step after cooling to the Ms point at an average cooling rate of 1° C./s or more satisfies the formula (1).
- Whether or not the cooling step after cooling up to the Ms point at an average cooling rate of 1° C./s or more satisfies the formula (1) is, for example, 1/K at each temperature when continuously cooled at K° C./s. It may be considered that the equation has been held for a second, and the equation (1) may be calculated every 1° C. to determine whether or not the equation (1) is satisfied.
- the hot-rolled sheet before cold rolling may be subjected to heat treatment for the purpose of improving cold rollability.
- the highest temperature reached in the heat treatment of the hot-rolled sheet is 400° C. or higher
- segregation of P to the prior austenite grain boundaries of the hot-rolled sheet structure becomes remarkable.
- This segregation of P is not alleviated by cold rolling.
- the maximum temperature that it reaches must be lower than 400°C.
- the lower limit temperature may not be specified, but is preferably 100° C. or higher for softening the hot rolled sheet.
- Cold reduction 70% or more
- the reduction ratio of cold rolling is less than 70%, P segregation to the prior austenite grain size generated during the heat treatment of the hot rolled sheet is not sufficiently relaxed, and the delayed fracture resistance property described in Examples described later is deteriorated. Therefore, when the hot rolled sheet is heat-treated under the above conditions, the cold rolling reduction needs to be 70% or more.
- the slab is preferably manufactured by a continuous casting method in order to prevent macro segregation. It may be manufactured by the ingot making method or the thin slab casting method. In order to hot-roll the slab, the slab may be once cooled to room temperature and then reheated to be hot-rolled. Alternatively, the slab may be charged into a heating furnace without being cooled to room temperature and hot-rolled. Alternatively, an energy saving process in which hot rolling is performed immediately after slight heat retention may be applied. When the slab is heated, it is preferable to heat it so that the slab temperature is 1100° C. or higher and 1300° C. or lower. By heating the slab temperature to 1100° C. or higher, it is possible to dissolve the carbide and suppress an increase in rolling load. By increasing the slab temperature to 1300° C. or lower, increase in scale loss can be suppressed.
- the slab temperature is the temperature of the slab surface.
- the rough bar after rough rolling may be heated.
- a so-called continuous rolling process may be applied in which coarse bars are joined together and finish rolling is continuously performed.
- perform lubrication rolling so that the friction coefficient becomes 0.10 to 0.25 in all or some of the passes of finish rolling. Is preferred.
- the rolled steel sheet is subjected to cold rolling, annealing, and hot dip galvanizing after removing the scale by pickling. After plating, it is preferable to cool to room temperature and to temper-roll at an elongation of 1% or less in order to control the surface and yield strength (YS). If necessary, tempering treatment may be further performed within a range satisfying the formula (1), but this temperature is preferably 200° C. or lower. Leveler correction may be applied to adjust the shape and YS.
- hot-dip galvanized steel sheets (GI) and alloyed hot-dip galvanized steel sheets (GA) 1 to 31 were produced in the laboratory using a heat treatment and plating treatment apparatus.
- the hot-dip galvanized steel sheet was prepared by immersing the hot-dip galvanized steel sheet in a plating bath at 465° C. and forming a coating layer having an adhesion amount of 40 to 60 g/m 2 on each side on both sides of the steel sheet.
- the alloyed hot-dip galvanized steel sheet was produced by further performing an alloying treatment at 540° C. for 1 to 60 s after the above plating. After the plating treatment, it was cooled to room temperature and then temper-rolled with an elongation of 0.1%. From the plating to the Ms point, cooling was performed at an average cooling rate of 3° C./s. Part of them was further subjected to tempering heat treatment. The strain was applied by applying tension. Table 2 shows the production conditions of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet.
- the obtained hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet were evaluated for tensile properties and delayed fracture resistance according to the following test methods.
- ⁇ Tensile test> JIS (Japanese Industrial Standards) No. 5 tensile test pieces (JIS Z2201: 1998) are taken from the annealed plate in a direction perpendicular to the rolling direction, and the strain rate is set to 10 ⁇ 3 /s JIS (Japanese Industrial Standards) Z 2241. : TS was determined by performing a tensile test in accordance with the regulations of 2011. In the present embodiment, TS of 1100 MPa or more was passed.
- a test piece having a parallel part width of 6 mm and a parallel part length of 15 mm was sampled from the annealed plate with the width direction parallel to the rolling direction.
- the test piece was ground over its entire surface and evaluated with a plate thickness of 1.0 mm. This was soaked in a solution of 3 mass% NaCl and 3 g/l NH 4 SCN, and the applied current density was set to 0 and 0.05 mA/cm 2 for 24 hours and then the SSRT test was started at a pulling speed of 5 ⁇ m/min. Then, the test was terminated at the time of breaking.
- the applied current density for TS in conditions with 0 mA / cm 2 was stress ratio the ratio of the TS in conditions with 0.05 mA / cm 2, the stress ratio was 0.70 or more
- the delayed fracture resistance was rated as “ ⁇ ”, and when the stress ratio was less than 0.70, the delayed fracture resistance was rated as “x”.
- the steel structure, the average grain size of prior austenite and the ratio of the peak height of the austenite electron spectrum of P to the peak height of the austenite electron spectrum of P at the former austenite grain boundary are as described above. It was measured by the method described above. Table 3 shows these evaluation results and the steel structure of the steel sheet.
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Abstract
Description
[1]質量%で、C:0.08~0.35%、Si:0.01~3.0%、Mn:2.0~4.0%、P:0.010%以下(0を含まない)、S:0.002%以下(0を含まない)、Al:0.01~1.50%、B:0.0005~0.010%を含有し、かつ、Mo:0.03~2.0%、Ti:0.010~0.10%の中から選ばれる1種以上を含有し、残部がFeおよび不可避的不純物からなる成分組成と、鋼板表層から板厚方向に300~400μmの範囲において、面積率で、マルテンサイトと炭化物を含むベイナイトの合計が60~100%であり、旧オーステナイトの平均粒径が15μm以下であり、鋼板表層から板厚方向に300~400μmの範囲において、旧オーステナイト粒界におけるPのオージェ電子スペクトルのピーク高さに対する旧オーステナイト粒界から5nm以上離れた位置におけるPのオージェ電子スペクトルのピーク高さの比が0.20以上である鋼組織と、を有し、鋼板表面に溶融亜鉛めっき層を有する、高強度溶融亜鉛めっき鋼板。
[2]前記成分組成は、さらに質量%で、Nb:0.005~0.20%、V:0.005~2.0%の中から選ばれる1種以上を含有する、[1]に記載の高強度溶融亜鉛めっき鋼板。
[3]前記成分組成は、さらに質量%で、Cr:0.005~2.0%、Ni:0.005~2.0%、Cu:0.005~2.0%、Ca:0.0002~0.0050%、REM:0.0002~0.0050%、Sn:0.001~0.05%、Sb:0.001~0.05%から選ばれる1種以上を含有する、[1]または[2]に記載の高強度溶融亜鉛めっき鋼板。
[4]前記溶融亜鉛めっき層は、合金化溶融亜鉛めっき層である、[1]から[3]の何れか1つに記載の高強度溶融亜鉛めっき鋼板。
[5][1]から[3]の何れか1つに記載の成分組成を有するスラブに熱間圧延を施した後冷却し、巻き取る熱延を施して製造した熱延板を酸洗し、次いで、冷間圧延を施し、次いで、750~950℃に加熱し、10~600s保持後、3℃/s以上の平均冷却速度で550℃まで冷却し、Ms点~550℃で300s以下保持する焼鈍を施し、次いで、溶融亜鉛めっき処理を施し、あるいはさらに合金化処理を施し、その後、Ms点までを1℃/s以上の平均冷却速度で冷却し、その後の冷却工程において下記(1)式を満たし、かつ250℃~Ms点において0%より大きく0.067%以下のひずみを付与する、高強度溶融亜鉛めっき鋼板の製造方法。
Loge([P]×[C]×(8.65-474×DA×[B])×t)+35.4≧13320/(273+T)+4831/(273+T-100×[Si])・・・(1)
但し、上記(1)式において、DA×[B]≧0.00912の場合はDA×[B]を0.00912とし、上記(1)式の[P]、[C]、[B]、[Si]はそれぞれ鋼中のP、C、B、Siの含有量(質量%)であり、DAは旧オーステナイト粒径(μm)であり、Tは滞留温度(℃)であり、tは滞留温度における総滞留時間(s)である。
[6][1]から[3]の何れか一項に記載の成分組成を有するスラブに熱間圧延を施した後冷却し、巻き取る熱延を施して製造した熱延板を酸洗し、次いで、最高到達温度400℃未満の条件で焼戻しを施し、次いで、70%以上の圧下率で冷間圧延を施し、次いで、750~950℃に加熱し、10~600s保持後、3℃/s以上の平均冷却速度で550℃まで冷却し、Ms点~550℃で300s以下保持する焼鈍を施し、次いで、溶融亜鉛めっき処理を施し、あるいはさらに合金化処理を施し、その後、Ms点までを1℃/s以上の平均冷却速度で冷却し、以降の工程において下記(1)式を満たし、かつ250℃~Ms点において0%より大きく0.067%以下のひずみを付与する、高強度溶融亜鉛めっき鋼板の製造方法。
Loge([P]×[C]×(8.65-474×DA×[B])×t)+35.4≧13320/(273+T)+4831/(273+T-100×[Si])・・・(1)
但し、上記(1)式において、DA×[B]≧0.00912の場合はDA×[B]を0.00912とし、上記(1)式の[P]、[C]、[B]、[Si]はそれぞれ鋼中のP、C、B、Siの含有量(質量%)であり、DAは旧オーステナイト粒径(μm)であり、Tは滞留温度(℃)であり、tは滞留温度における総滞留時間(s)である。
Cは、マルテンサイトや炭化物を含むベイナイトを生成させてTSを上昇させるのに有効な元素である。C量が0.08%未満ではこのような効果が得られず、本実施形態に係る鋼板の強度や鋼組織が得られない。したがって、C量は0.08%以上である必要がある。C量は0.10%以上であることが好ましく、0.13%以上であることがより好ましい。一方、C量が0.35%を超えるとマルテンサイトが硬化して耐遅れ破壊特性が劣化する。したがって、C量は0.35%以下である必要がある。C量は0.33%以下であることが好ましい。
Siは、固溶強化やマルテンサイトの焼き戻しの抑制によりTSを高め、優れた耐遅れ破壊特性を得るのに必要な元素である。このような効果を得るために、Si量は0.01%以上である必要がある。Si量は0.1%以上であることが好ましく、0.2%以上であることがより好ましい。一方、Si量が3.0%を超えると過剰なフェライト生成を招き、本実施形態に係る鋼板の鋼組織が得られない。したがって、Si量は3.0%以下である必要がある。Si量は2.5%以下であることが好ましく、2.0%以下であることがより好ましい。
Mnは、マルテンサイトや炭化物を含むベイナイトを生成させてTSを上昇させるのに有効な元素である。Mn量が2.0%未満ではこのような効果が得られない。したがって、Mn量は2.0%以上である必要がある。Mn量は2.1%以上であることが好ましく、2.2%以上であることがより好ましい。一方、Mn量が4.0%を超えると鋼が脆化して本実施形態に係る鋼板の耐遅れ破壊特性が得られない。したがって、Mn量は4.0%以下である必要がある。Mn量は3.7%以下であることが好ましく、3.4%以下であることがより好ましい。
Pは、粒界を脆化させて耐遅れ破壊特性を劣化させるため、その量は極力低減することが望ましいが、本実施形態に係る鋼板では0.010%まで許容できる。P量の下限は規定しなくてよいが、鋼板のP量を0.0005%未満とするには精錬に多大な負荷が生じ、生産能率が低下する。このため、P量の下限は0.0005%であることが好ましい。
Sは、介在物を増加させて耐遅れ破壊特性を低下させる。このため、S量は極力少ないことが好ましいが、本実施形態に係る鋼板では0.002%まで許容できる。S量の下限は規定しなくてよいが、0.0001%未満にするには精錬に多大な負荷が生じ、生産能率が低下する。このため、S量の下限は0.0001%であることが好ましい。
Alは、脱酸剤として作用するので、脱酸工程で添加することが好ましい。このような効果を得るために、Al量は0.01%以上である必要がある。Al量は0.015%以上であることが好ましい。一方、Al量が1.50%を超えるとフェライトの生成が過剰になり、本実施形態に係る鋼板の鋼組織が得られない。したがって、Al量は1.50%以下である必要がある。Al量は1.00%以下であることが好ましく、0.70%以下であることがより好ましい。
Bは耐遅れ破壊特性を向上させるため本実施形態に係る鋼板に重要な元素である。そのメカニズムは明らかではないが旧オーステナイト粒界等に偏析することで粒界強化していると推測される。鋼板の焼入れ性を高め、マルテンサイトやベイナイトを生成させ、高強度化に有効な元素でもある。これらの効果を十分に得るため、B量は0.0005%以上である必要がある。B量は0.0007%以上であることが好ましい。一方、B量が0.010%を超えると介在物が増加して、耐遅れ破壊特性が低下する。したがって、B量は0.010%以下である必要がある。B量は0.0050%以下であることが好ましく、0.0040%以下であることがより好ましい。
MoおよびTiはBの炭化や窒化を抑制して、固溶Bとして上記効果を発現させるのに必要な元素である。このような効果を得るにはMoおよびTiから選ばれる1種以上をそれぞれの下限量以上とする必要がある。一方、MoおよびTiがそれぞれの上限量を超えると粗大炭化物が生成し、固溶C量が低下して本実施形態に係る鋼板の鋼組織が得られない。したがって、Mo:0.03%以上2.0%以下、Ti:0.010%以上0.10%以下から選ばれる1種以上を含有する必要がある。
Nb、Vは、微細炭化物を形成し、高強度化に有効な元素である。このような効果を得るため、Nb、Vから選ばれる1種以上は0.005%以上であることが好ましい。一方、含有量がそれぞれの上限量を超えると、鋼中の固溶炭素量が低下し、多量にフェライトが生成して本実施形態に係る鋼板の鋼組織が得られない場合がある。したがって、Nb、Vから選ばれる1種以上の含有量はNb:0.005%以上0.20%以下、V:0.005%以上2.0%以下であることが好ましい。
Cr、Ni、Cuは、マルテンサイトやベイナイトを生成させ、高強度化に有効な元素である。このような効果を得るため、Cr、Ni、Cu量は0.005%以上であることが好ましい。一方、Cr、Ni、Cu量が2.0%を超えると、粒界が脆化して本実施形態に係る鋼板の耐遅れ破壊特性が得られない場合がある。このため、Cr、Ni、Cu量は2.0%以下であることが好ましい。
マルテンサイトと炭化物を含むベイナイトは、TS上昇および優れた耐遅れ破壊特性に必要な組織である。このような効果を得るために、マルテンサイトと炭化物を含むベイナイトの面積率の合計は60%以上である必要がある。マルテンサイトと炭化物を含むベイナイトの合計は75%以上であることが好ましく、90%以上であることがより好ましい。マルテンサイトと炭化物を含むベイナイトの合計の上限は100%である。
旧オーステナイトはP偏析が顕著で遅れ破壊発生に大きく影響する。旧オーステナイトの平均結晶粒径を15μm以下とすることで粒界応力の集中が緩和され、本実施形態に係る鋼板の耐遅れ破壊特性が得られる。したがって、旧オーステナイトの平均結晶粒径は15μm以下である必要がある。旧オーステナイトの平均結晶粒径は12μm以下であることが好ましく、10μm以下であることがより好ましく、9μm以下であることが更に好ましい。
Pの旧オーステナイト粒界への偏析は、耐遅れ破壊特性を顕著に低下させるので、その抑制は極めて重要である。Pの粒界偏析の指標として旧オーステナイト粒界におけるPのオージェ電子スペクトルのピーク高さに対する旧オーステナイト粒界から5nm以上離れた位置におけるPのオージェ電子スペクトルのピーク高さの比を用いた。当該比が0.20未満となると、旧オーステナイト粒界で遅れ破壊が生じて本実施形態に係る鋼板の優れた耐遅れ破壊特性が得られない。したがって、旧オーステナイト粒界におけるPのオージェ電子スペクトルのピーク高さに対する旧オーステナイト粒界から5nm以上離れた位置におけるPのオージェ電子スペクトルのピーク高さの比は0.20以上である必要がある。旧オーステナイト粒界におけるPのオージェ電子スペクトルのピーク高さに対する旧オーステナイト粒界から5nm以上離れた位置におけるPのオージェ電子スペクトルのピーク高さの比は0.30以上であることが好ましく、0.35以上であることがより好ましく、0.40以上であることがさらに好ましい。
Pのオージェ電子スペクトルのピーク高さはオージェ電子分光装置を用いて、切欠を有する破断用サンプルを試料とし、オージェ電子分光装置内の真空チャンバー内で破断して破面を分析することで測定した。サンプルの破断は液体窒素を用いた冷却条件で実施した。分析はArイオンスパッタにより行った。
但し、上記(1)式において、DA×[B]≧0.00912の場合はDA×[B]を0.00912とし、上記(1)式の[P]、[C]、[B]、[Si]はそれぞれ鋼中のP、C、B、Siの含有量(質量%)であり、DAは旧オーステナイト粒径(μm)であり、Tは滞留温度(℃)であり、tは滞留温度における総滞留時間(s)である。
焼鈍温度が750℃未満ではオーステナイトの生成が不十分となり本実施形態に係る鋼板の鋼組織が得られない。したがって、焼鈍温度は750℃以上である必要がある。一方、焼鈍温度が950℃を超えると粗粒化して本実施形態に係る鋼板の鋼組織が得られない。したがって、焼鈍温度は950℃以下である必要がある。
本焼鈍保持時間が10s未満ではオーステナイトの生成が不十分となり、本実施形態に係る鋼板の鋼組織が得られない。したがって、焼鈍保持時間は10s以上である必要がある。焼鈍保持時間は20s以上であることが好ましく、30s以上であることがより好ましい。一方、焼鈍保持時間が600sを超えるとオーステナイト粒界へのP偏析が過剰になって本実施形態に係る鋼板の耐遅れ破壊特性が得られない。したがって、焼鈍保持時間は600s以下である必要がある。焼鈍保持時間は500s以下であることが好ましく、400s以下であることがより好ましい。
焼鈍温度から550℃までの平均冷却速度が3℃/s未満ではフェライトが過剰に生成して本実施形態に係る鋼板の鋼組織が得られない。したがって、焼鈍温度から550℃までの平均冷却速度は3℃/s以上である必要がある。焼鈍温度から550℃までの平均冷却速度は5℃/s以上であることが好ましい。焼鈍温度から550℃までの平均冷却速度の上限は規定しなくてよいが、形状安定性の観点からは100℃/s未満とすることが好ましい。平均冷却速度は、焼鈍温度と550℃との温度差を焼鈍温度から550℃までの冷却に要した時間で除することによって算出する。
保持温度が550℃を超えるとフェライトが生成して本実施形態に係る鋼板の鋼組織が得られない。したがって、保持温度は550℃以下である必要がある。一方、保持温度がMs点未満となるとマルテンサイト変態やベイナイト変態が過剰に進行して、本実施形態に係る鋼板のP分布が得られない。したがって、保持温度はMs点以上である必要がある。MS点とは、マルテンサイト変態が開始する温度でありフォーマスタにより求めることができる。
Ms点以上550℃以下での保持時間が300sを超えると過剰な炭化物を含まないベイナイトが生成し、本実施形態に係る鋼板の鋼組織が得られない。したがって、Ms点以上550℃以下での保持時間は300s以下である必要がある。Ms点以上550℃以下での保持時間は200s以下であることが好ましく、120s以下であることがより好ましい。保持中はMs点以上550℃以下の範囲であれば温度一定である必要はなく、冷却や加熱をしてもよい。
Ms点までの平均冷却速度が1℃/s未満では、冷却中にベイナイト変態が過剰に進行して本実施形態に係る鋼板のP分布が得られない。したがって、Ms点までの平均冷却速度は1℃/s以上である必要がある。平均冷却速度は、冷却開始温度とMs点との温度差を冷却開始温度からMs点までの冷却に要した時間で除することで算出する。
マルテンサイト変態させる際のひずみ付与により旧オーステナイト粒界へのP偏析を抑制できる。一方、付与するひずみが0.067%を超えると、逆にP偏析量が増大して本実施形態に係る鋼板のP分布が得られない。250℃未満の温度域で、ひずみを付与しても、このような効果が得られない。したがって、250℃以上Ms点以下において0.000%より大きく0.067%以下のひずみを付与する必要がある。250℃以上Ms点以下において付与するひずみの下限は0.010%であることが好ましい。ひずみ付与の方法は特に限定せず、張力付与やロール等でも曲げによるものでもよい。
但し、上記(1)式において、DA×[B]≧0.00912の場合はDA×[B]を0.00912とし、上記(1)式の[P]、[C]、[B]、[Si]はそれぞれ鋼中のP、C、B、Siの含有量(質量%)であり、DAは旧オーステナイト粒径(μm)であり、Tは滞留温度(℃)であり、tは滞留温度における総滞留時間(s)である。
本実施形態において冷間圧延前の熱延板に冷間圧延性改善などを目的として熱処理を施してもよい。この場合、熱延板の熱処理において最高到達温度が400℃以上となると熱延板組織の旧オーステナイト粒界へのPの偏析が顕著になる。このPの偏析は、冷間圧延によっても緩和されない。そして、その後の焼鈍においてもこれが引き継がれ、耐遅れ破壊特性が低下する。したがって、熱延板を熱処理する場合、その最高到達温度は400℃未満である必要がある。下限温度は規定しなくてよいが、熱延板軟質化のためには100℃以上とすることが好ましい。
最高到達温度を400℃未満とした条件で熱延板の熱処理を施す場合、70%以上の圧下率で冷間圧延を施す必要がある。冷間圧延の圧下率が70%未満では熱延板の熱処理時に生じた旧オーステナイト粒径へのP偏析が十分に緩和されず、後述する実施例に記載の耐遅れ破壊特性が低下する。したがって、上記条件で熱延板を熱処理した場合の冷間圧下率は70%以上である必要がある。
これらの鋼スラブを1250℃に加熱後粗圧延し、熱間圧延、熱延板の熱処理を施した。次いで、1.4mmまで冷間圧延して冷延板とした。得られた冷延板を焼鈍に供した。焼鈍は実験室にて熱処理およびめっき処理装置を用いて溶融亜鉛めっき鋼板(GI)および合金化溶融亜鉛めっき鋼板(GA)1~31を作製した。溶融亜鉛めっき鋼板は、465℃のめっき浴中に浸漬し、片面あたり付着量40~60g/m2のめっき層を鋼板両面に形成させることで作製した。合金化溶融亜鉛めっき鋼板は、上記めっき後さらに、540℃で1~60s保持する合金化処理を行うことで作製した。めっき処理後は室温まで冷却し、その後伸長率0.1%の調質圧延を施した。めっき後からMs点までは3℃/sの平均冷却速度で冷却した。一部はさらに焼戻し熱処理を施した。ひずみは張力付与により行った。溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板の製造条件を表2に示す。鋼板Fは、めっき前にMs点以下になったのでめっき後のひずみ量の規定の対象外とし、表2の「250℃~Msでの付与ひずみ量」の列に「-」を記載した。(1)式におけるDA(旧オーステナイト粒径)は、鋼板の板幅中央部の圧延方向断面の1/4t部の旧オーステナイト粒径をJIS(日本工業規格) G 0551:2013の規定に準拠して測定した。表2の「(1)式合否」の列の「〇」は(1)式を満たすことを意味し、「×」は(1)式を満たさないことを意味する。
得られた溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板について、以下の試験方法にしたがい、引張特性、耐遅れ破壊特性を評価した。
<引張試験>
焼鈍板より圧延方向に対して直角方向にJIS(日本工業規格)5号引張試験片(JIS Z2201:1998)を採取し、歪速度を10-3/sとするJIS(日本工業規格) Z 2241:2011の規定に準拠した引張試験を行い、TSを求めた。本実施例ではTSが1100MPa以上を合格とした。
<耐遅れ破壊特性>
圧延方向と平行となる方向を幅方向として、焼鈍板から平行部幅が6mm、平行部長さが15mmの試験片を採取した。試験片は全面研削し、板厚1.0mmで評価した。これを3質量%NaClと、3g/lのNH4SCNの溶液に浸漬させ、印加電流密度を0および0.05mA/cm2として24hr保持後、引張速度を5μm/minとしたSSRT試験を開始し、破断した時点で試験を終了した。印加電流密度を0mA/cm2とした条件でのTSに対する、印加電流密度を0.05mA/cm2とした条件でのTSの比を応力比とし、当該応力比が0.70以上であった場合に耐遅れ破壊特性を「○」とし、応力比が0.70未満であった場合に耐遅れ破壊特性を「×」とした。
Claims (6)
- 質量%で、
C:0.08~0.35%、
Si:0.01~3.0%、
Mn:2.0~4.0%、
P:0.010%以下(0を含まない)、
S:0.002%以下(0を含まない)、
Al:0.01~1.50%、
B:0.0005~0.010%を含有し、かつ、
Mo:0.03~2.0%、
Ti:0.010~0.10%の中から選ばれる1種以上を含有し、残部がFeおよび不可避的不純物からなる成分組成と、
鋼板表層から板厚方向に300~400μmの範囲において、面積率で、マルテンサイトと炭化物を含むベイナイトの合計が60~100%であり、旧オーステナイトの平均粒径が15μm以下であり、鋼板表層から板厚方向に300~400μmの範囲において、旧オーステナイト粒界におけるPのオージェ電子スペクトルのピーク高さに対する旧オーステナイト粒界から5nm以上離れた位置におけるPのオージェ電子スペクトルのピーク高さの比が0.20以上である鋼組織と、
を有し、鋼板表面に溶融亜鉛めっき層を有する、高強度溶融亜鉛めっき鋼板。 - 前記成分組成は、さらに質量%で、
Nb:0.005~0.20%、
V:0.005~2.0%の中から選ばれる1種以上を含有する、請求項1に記載の高強度溶融亜鉛めっき鋼板。 - 前記成分組成は、さらに質量%で、
Cr:0.005~2.0%、
Ni:0.005~2.0%、
Cu:0.005~2.0%、
Ca:0.0002~0.0050%、
REM:0.0002~0.0050%、
Sn:0.001~0.05%、
Sb:0.001~0.05%から選ばれる1種以上を含有する、請求項1または請求項2に記載の高強度溶融亜鉛めっき鋼板。 - 前記溶融亜鉛めっき層は、合金化溶融亜鉛めっき層である、請求項1から請求項3の何れか一項に記載の高強度溶融亜鉛めっき鋼板。
- 請求項1から請求項3の何れか一項に記載の成分組成を有するスラブに熱間圧延を施した後冷却し、巻き取る熱延を施して製造した熱延板を酸洗し、次いで、冷間圧延を施し、次いで、750~950℃に加熱し、10~600s保持後、3℃/s以上の平均冷却速度で550℃まで冷却し、Ms点~550℃で300s以下保持する焼鈍を施し、次いで、溶融亜鉛めっき処理を施し、あるいはさらに合金化処理を施し、その後、Ms点までを1℃/s以上の平均冷却速度で冷却し、その後の冷却工程において下記(1)式を満たし、かつ250℃~Ms点において0%より大きく0.067%以下のひずみを付与する、高強度溶融亜鉛めっき鋼板の製造方法。
Loge([P]×[C]×(8.65-474×DA×[B])×t)+35.4≧13320/(273+T)+4831/(273+T-100×[Si])・・・(1)
但し、上記(1)式において、DA×[B]≧0.00912の場合はDA×[B]を0.00912とし、上記(1)式の[P]、[C]、[B]、[Si]はそれぞれ鋼中のP、C、B、Siの含有量(質量%)であり、DAは旧オーステナイト粒径(μm)であり、Tは滞留温度(℃)であり、tは滞留温度における総滞留時間(s)である。 - 請求項1から請求項3の何れか一項に記載の成分組成を有するスラブに熱間圧延を施した後冷却し、巻き取る熱延を施して製造した熱延板を酸洗し、次いで、最高到達温度400℃未満の条件で焼戻しを施し、次いで、70%以上の圧下率で冷間圧延を施し、次いで、750~950℃に加熱し、10~600s保持後、3℃/s以上の平均冷却速度で550℃まで冷却し、Ms点~550℃で300s以下保持する焼鈍を施し、次いで、溶融亜鉛めっき処理を施し、あるいはさらに合金化処理を施し、その後、Ms点までを1℃/s以上の平均冷却速度で冷却し、以降の工程において下記(1)式を満たし、かつ250℃~Ms点において0%より大きく0.067%以下のひずみを付与する、高強度溶融亜鉛めっき鋼板の製造方法。
Loge([P]×[C]×(8.65-474×DA×[B])×t)+35.4≧13320/(273+T)+4831/(273+T-100×[Si])・・・(1)
但し、上記(1)式において、DA×[B]≧0.00912の場合はDA×[B]を0.00912とし、上記(1)式の[P]、[C]、[B]、[Si]はそれぞれ鋼中のP、C、B、Siの含有量(質量%)であり、DAは旧オーステナイト粒径(μm)であり、Tは滞留温度(℃)であり、tは滞留温度における総滞留時間(s)である。
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