WO2020148948A1 - 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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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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Definitions
- the present invention relates to a high-strength hot-dip galvanized steel sheet having excellent workability, which is suitable for automobile members, and a method for producing the same.
- Patent Document 1 discloses a technique relating to a steel sheet having a tensile strength of over 1180 MPa and excellent in uniform ductility and local ductility by controlling the amount of retained austenite and a plurality of tempered martensite fractions.
- Patent Document 2 discloses a technology relating to a high-strength steel sheet excellent in elongation and stretch-flangeability by controlling tempered martensite and bainite as main phases and controlling the nanohardness and texture of the structure.
- Patent Document 3 discloses a technique relating to a method for manufacturing a steel sheet having a tensile strength of more than 540 MPa which is excellent in elongation and stretch flangeability by controlling the crystal orientation difference between ferrite and a hard phase.
- Patent Document 1 focuses only on the elongation due to tensile deformation, and has not yet been made to improve the workability of the flange portion, which is inevitable when actually processing parts.
- Patent Document 2 is attempting to improve the elongation and stretch flangeability, compatibility with YS, which is important as a skeletal component, has not been studied, and there is room for improvement.
- Patent Document 3 has excellent elongation and stretch flangeability, it does not achieve YS, elongation and stretch flangeability all at a high level at the same time.
- the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having excellent workability and a method for manufacturing the same.
- the present inventors have found that in a hot-dip galvanized steel sheet, the steel sheet has a predetermined composition and a steel sheet structure within a range of 300 to 400 ⁇ m from the steel sheet surface.
- the area ratio of martensite is 30% or less
- the area ratio of pearlite is 1% or less
- the total area ratio of tempered martensite and bainite containing carbide is 30% or more and 99% or less
- the area ratio of retained austenite is 1 to 20.
- the total area ratio of bainite containing no ferrite and carbide is 45% or less, and further has two or more crystal orientations among all retained austenite grains within the range of 300 to 400 ⁇ m from the steel plate surface. It has been found that when the area ratio of the retained austenite grains is 40% or less, the strength is high and the workability is excellent.
- the present invention has been made on the basis of such findings, and the summary thereof is as follows.
- a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet The steel sheet, in mass%, C: 0.12 to 0.35%, Si: 0.5-3.0%, Mn: 1.5 to 4.0%, P: 0.100% or less (0% is not included), S: 0.02% or less (0% is not included), and Al: 0.01 to 1.50%, and the balance is Fe and inevitable impurities.
- the area ratio of martensite is 30% or less
- the area ratio of pearlite is 1% or less
- the total area ratio of tempered martensite and bainite containing carbides is 30% with respect to the steel plate structure within a range of 300 to 400 ⁇ m from the steel plate surface.
- the area ratio of retained austenite is 1 to 20%
- the total area ratio of bainite containing no ferrite and carbide is 45% or less
- a high-strength hot-dip galvanized steel sheet in which the proportion of retained austenite grains having two or more crystal orientations in the total retained austenite grains within a range of 300 to 400 ⁇ m from the surface of the steel sheet is 40% or less in area ratio.
- the steel sheet further contains, by mass%, Cr: 0.005 to 2.0%, Ni: 0.005-2.0%, Cu: 0.005 to 2.0%, V: 0.1-1.5%, Mo: 0.1-1.5%, Ti: 0.005 to 0.10%, Nb: 0.005 to 0.10%, B: 0.0001 to 0.0050%, Ca: 0.0003 to 0.0050%, REM: 0.0003 to 0.0050%,
- the high-strength galvanized steel sheet according to [1] which contains at least one element selected from Sn: 0.005 to 0.50% and Sb: 0.005 to 0.50%.
- a slab having the component composition according to [1] or [2] is subjected to hot rolling, followed by cooling and winding in a hot rolling step, The hot-rolled sheet obtained in the hot-rolling step, or the cold-rolled sheet obtained by cold-rolling the hot-rolled sheet at a rolling reduction of 30% or more, in a temperature range of (Ac1-5°C) to (Ac1+10°C) for 15 seconds.
- the above is retained, and a tension of more than 0 to 10 MPa is applied during the retention, Then, it is heated to an annealing temperature of 750 to 940° C. and retained for 10 to 600 s, Then, cooling from the annealing temperature to 550° C. at a first average cooling rate of 3° C./s or more and at a first cooling stop temperature (Ms to 550° C.), Next, the plating treatment temperature (Ms to 580° C.) is retained for 10 to 300 s, and during the retention, a galvanization treatment or a plating alloying treatment is performed after the zinc plating treatment. Then, cooling from the plating treatment temperature to 350° C.
- a method for producing a high-strength hot-dip galvanized steel sheet which comprises heating to a reheating temperature of 300 to 500° C. above the second cooling stop temperature, allowing it to stay for 1 to 600 seconds, and then cooling to room temperature.
- high strength means that YS is 850 MPa or more.
- excellent in workability means that uniform elongation (UEL) is 9.0% or more, and YS ⁇ uniform elongation (UEL) ⁇ hole expansion ratio ⁇ is 270 GPa ⁇ % ⁇ % or more. ..
- the hot-dip galvanized steel sheet includes not only hot-dip galvanized steel sheet but also alloyed hot-dip galvanized steel sheet. Further, when it is necessary to distinguish between the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, these steel sheets are described separately.
- the present invention it is possible to provide a high-strength hot-dip galvanized steel sheet having excellent workability and a method for producing the same.
- the high-strength hot-dip galvanized steel sheet is suitably used as a material for automobile parts.
- Component composition The component composition of the steel sheet in the high-strength galvanized steel sheet of the present invention will be described.
- C 0.12-0.35%
- C is an element effective for strengthening bainite containing tempered martensite and carbide to increase strength and for obtaining retained austenite. If the C content is less than 0.12%, such effects cannot be sufficiently obtained, and the strength and steel structure of the present invention cannot be obtained. Therefore, the C content is 0.12% or more, preferably 0.14% or more, and more preferably 0.15% or more. On the other hand, when the C content exceeds 0.35%, the retained austenite is excessively increased and the steel structure of the present invention cannot be obtained. Therefore, the C content is 0.35% or less, preferably 0.32% or less.
- Si 0.5-3.0% Si is an element necessary for solid solution strengthening steel to increase strength and to obtain retained austenite. In order to sufficiently obtain such effects, the Si content needs to be 0.5% or more. The Si content is preferably 0.8% or more. On the other hand, if the Si content exceeds 3.0%, excessive ferrite is generated, and the steel structure of the present invention cannot be obtained. Therefore, the Si content is 3.0% or less, preferably 2.5% or less, and more preferably 2.0% or less.
- Mn 1.5-4.0% Mn is an element effective in generating martensite and bainite and increasing the strength. If the Mn content is less than 1.5%, such effects cannot be sufficiently obtained. Therefore, the Mn content is 1.5% or more, preferably 1.8% or more, and more preferably 2.0% or more. On the other hand, if the Mn content exceeds 4.0%, the steel becomes brittle and the excellent workability of the present invention cannot be obtained. Therefore, the Mn content is 4.0% or less, preferably 3.7% or less, and more preferably 3.4% or less.
- P 0.100% or less (0% is not included) P makes the steel brittle and reduces the workability, so it is desirable to reduce the amount as much as possible, but in the present invention, the P content can be allowed up to 0.100%. Note that it is difficult to set the P content to 0% in operation, so 0% is not included. Further, if the P content is less than 0.001%, the production efficiency is lowered, so 0.001% or more is preferable.
- S 0.02% or less (0% is not included) Since S increases the inclusions and deteriorates the workability, it is preferable to reduce the amount as much as possible, but in the present invention, the S content can be allowed up to 0.02%. Note that it is difficult to set the S content to 0% in operation, so 0% is not included. Further, when the S content is less than 0.0001%, the production efficiency is lowered, so 0.0001% or more is preferable.
- Al acts as a deoxidizing agent and is preferably added in the deoxidizing step. It is also an effective element for producing retained austenite. In order to exert such an effect, the Al content needs to be 0.01% or more. The Al content is preferably 0.02% or more. On the other hand, when the Al content exceeds 1.50%, ferrite is excessively formed and the steel structure of the present invention cannot be obtained. Therefore, the Al content is 1.50% or less, preferably 1.0% or less, and more preferably 0.70% or less.
- the above are the basic ingredients.
- the steel sheet of the present invention contains the above basic components, and the balance other than the above basic components has a component composition containing Fe (iron) and inevitable impurities.
- the steel sheet of the present invention contains the above-mentioned basic components, and the balance has a composition of Fe and inevitable impurities.
- the steel sheet of the present invention may further contain the following optional components in addition to the basic components, if necessary.
- the balance other than the basic and optional components is iron and inevitable impurities.
- Cr 0.005-2.0%
- Ni 0.005-2.0%
- Cu 0.005-2.0%
- V 0.1-1.5%
- Mo 0.1- 1.5%
- Ti 0.005 to 0.10%
- Nb 0.005 to 0.10%
- B 0.0001 to 0.0050%
- Ca 0.0003 to 0.0050%
- REM At least one selected from 0.0003 to 0.0050%, Sn: 0.005 to 0.50%, and Sb: 0.005 to 0.50% Cr, Ni, and Cu are martensite and bainite. It is an element that is generated and effective in increasing the strength. In order to obtain such effects, it is preferable that the above lower limit values are satisfied.
- the content of each of Cr, Ni, and Cu is preferably not more than the above upper limit value from the viewpoint of improving workability.
- V, Mo, Ti and Nb are effective elements for strengthening by strengthening precipitation. In order to obtain such effects, it is preferable that the above lower limit values are satisfied. On the other hand, when the contents of these elements exceed the above-mentioned upper limits, the carbides coarsen, the amount of solid solution carbon in the steel decreases, and a large amount of ferrite is generated, so that the steel structure of the present invention cannot be obtained.
- the B is an element that enhances the hardenability of steel sheets, forms martensite and bainite, and is effective for increasing the strength.
- the B content is preferably 0.0001% or more.
- the B content is preferably 0.0050% or less.
- Ca and REM are effective elements for improving workability by controlling the morphology of inclusions.
- the contents of Ca and REM are each set to the above lower limit value or more.
- the contents of Ca and REM exceed the above upper limits, the amount of inclusions increases and workability tends to decrease. Therefore, it is preferable to set the contents of Ca and REM to the respective upper limit values or less.
- ⁇ Sn and Sb are effective elements for suppressing denitrification, deboronization, etc., and for suppressing the strength reduction of steel.
- the contents of Sn and Sb are each set to the above lower limit value or more. If the contents of Sn and Sb exceed the respective upper limits, the steel tends to become brittle and the workability tends to deteriorate. Therefore, it is preferable to set the contents of Sn and Sb to not more than the above upper limits.
- the steel sheet of the present invention may further contain Zr, Mg, La, and Ce in a total amount of 0.002%, if necessary.
- the area ratio of the steel plate structure means the area ratio of the steel plate structure within the range of 300 to 400 ⁇ m from the surface of the steel plate.
- UEL uniform elongation
- ⁇ hole expansion ratio
- the area ratio of martensite 30% or less Martensite increases strength but reduces workability, so the area ratio needs to be 30% or less. If the area ratio exceeds 30%, the workability of the present invention cannot be obtained. Therefore, the area ratio of martensite is 30% or less, preferably 25% or less, more preferably 15% or less, and further preferably 10% or less. The lower limit is not specified, but is often 1% or more.
- Perlite area ratio 1% or less (including 0%) Since pearlite lowers the balance of uniform strength and elongation, it needs to be reduced as much as possible. If the area ratio exceeds 1%, the strength uniform elongation balance of the present invention cannot be obtained. Therefore, the area ratio of pearlite is 1% or less.
- Total area ratio of tempered martensite and bainite containing carbide 30% or more and 99% or less Bainite containing tempered martensite and carbide is a structure necessary for obtaining high strength and high workability. If the total area ratio of these is less than 30%, neither strength nor workability of the present invention can be obtained. Therefore, the total area ratio of tempered martensite and bainite containing carbide is 30% or more, preferably 40% or more, and more preferably 50% or more. From the viewpoint of obtaining high strength and high workability of the present invention, the upper limit of the total area ratio of tempered martensite and bainite containing carbide is not limited, but in relation to the area ratio of other structures, the total area ratio is It is 99% or less.
- Area ratio of retained austenite 1 to 20% Retained austenite is a structure necessary for increasing uniform elongation. If the area ratio of retained austenite is less than 1%, such an effect cannot be sufficiently obtained. Therefore, the area ratio of retained austenite is 1% or more, preferably 3% or more, and more preferably 5% or more. On the other hand, if the area ratio exceeds 20%, the hole expansion ratio ( ⁇ ) is lowered. Therefore, the area ratio of retained austenite is 20% or less, preferably 18% or less, and more preferably 17% or less.
- Proportion of retained austenite grains having two or more crystal orientations among all retained austenite grains 40% or less in area ratio
- "Retained austenite grains having two or more crystal orientations" in the present invention means 15° or more. It means a grain in which a plurality of austenites having different orientations are connected. In the present invention, the connection of the crystals of the retained austenite grains is extremely important. Among all the retained austenite grains, the austenite grains having two or more crystal orientations undergo martensite transformation at an early stage during processing, resulting in deterioration of workability.
- the proportion of the retained austenite grains having two or more crystal orientations in the total retained austenite grains needs to be 40% or less in area ratio, and preferably in area ratio. It is 35% or less, more preferably 30% or less in area ratio, and further preferably 20% or less in area ratio.
- the area ratio of martensite, pearlite, tempered martensite, and bainite containing carbide, and each structure occupied in the observed area Is the ratio of the area.
- a sample of the annealed steel plate is cut out, a plate thickness cross section parallel to the rolling direction is polished, and then corroded by 3% by volume of Nital, and a position within a range of 300 to 400 ⁇ m from the surface of the steel plate in the plate thickness direction is SEM (scanning electron 3 fields of view are taken with a microscope) at a magnification of 1500 times.
- the surface of the steel sheet refers to the interface between the galvanized layer and the steel sheet.
- the area ratio of each tissue is determined by Image-Pro manufactured by Media Cybernetics, and the average area ratio of each tissue in the visual field is taken as the area ratio of each tissue.
- martensite is white or light gray or gray containing unoriented carbides
- bainite containing carbide is gray or dark gray containing oriented carbides
- tempered martensite is unoriented carbides.
- Gray or dark gray containing, and perlite are distinguished as black and white layered tissue.
- Residual austenite is observed in white or light gray, similar to martensite.
- the area ratio of retained austenite is separately determined by EBSD (electron beam backscattering diffraction), and the area ratio of retained austenite obtained based on EBSD is subtracted from the total area ratio of martensite and retained austenite obtained by the above visual field observation. Then, the area ratio of martensite is calculated.
- EBSD electron beam backscattering diffraction
- bainite that does not contain ferrite and carbide.
- Ferrites are distinguished as black and bainite without carbides as dark gray. These residual structures are not preferable, but the total area ratio of bainite that does not contain ferrite and carbide is acceptable if it is 45% or less in the present invention.
- the total area ratio is preferably 40% or less, more preferably 35% or less, further preferably 20% or less, and particularly preferably 10% or less.
- the grain dilation treatment and the Neighbor CI correlation treatment are performed. From the data, the area ratio of grains in which a plurality of austenite having a misorientation of 15° or more are connected is obtained. Then, the ratio (%) of the area ratio of the grains in which a plurality of the austenites having the orientation difference of 15° or more are connected to the area ratio of all the austenites is obtained. This ratio (%) is defined as the ratio of the retained austenite grains having two or more crystal orientations among all the retained austenite grains.
- Hot-dip galvanized layer The high-strength hot-dip galvanized steel sheet of the present invention has a hot-dip galvanized layer on the surface of the steel sheet.
- the hot dip galvanized layer may be an alloyed hot dip galvanized layer.
- the plate thickness (not including the plating layer) of the high-strength galvanized steel plate of the present invention is not particularly limited, but is preferably 0.4 mm or more and 3.0 mm or less.
- the high-strength hot-dip galvanized steel sheet of the present invention is, for example, subjected to a hot rolling step of performing hot rolling, then cooling and winding the slab having the above component composition,
- the hot-rolled sheet obtained in the hot-rolling step or the cold-rolled sheet obtained by cold-rolling the hot-rolled sheet at a rolling reduction of 30% or more is used in a temperature range of (Ac1-5°C) to (Ac1+10°C) for 15 s or more. It is allowed to stay, and a tension of more than 0 to 10 MPa is applied during the stay, and then it is heated to an annealing temperature of 750 to 940° C.
- the first average cooling rate is 3° C./s or more and the cooling is performed at the first cooling stop temperature (Ms to 550° C.), and then the plating treatment temperature (Ms to 580° C.) is retained for 10 to 300 s, and during the retention.
- each temperature shown in the manufacturing conditions is the surface temperature of the steel sheet.
- the manufacturing method will be described in order.
- Reduction ratio in cold rolling 30% or more
- the hot rolled sheet obtained in the hot rolling step is cold rolled if necessary.
- the rolling reduction in cold rolling is 30% or more, preferably 35% or more.
- the upper limit is not particularly specified, but 90% or less is preferable from the viewpoint of shape stability and the like.
- the residence time is made longer, the retained austenite grains having two or more crystal orientations become smaller, and the effect of the present invention can be effectively obtained. Therefore, the upper limit of the residence time is not specified.
- Ac1 shows an Ac1 transformation point.
- Residual austenite grains having two or more crystal orientations by applying tension when austenite is generated by allowing the hot-rolled or cold-rolled sheet to stay in the temperature range of (Ac1-5°C) to (Ac1+10°C) Can be reduced.
- this mechanism is not clear, it can be considered to reduce generation of retained austenite grains having two or more crystal orientations and to promote subsequent bainite transformation.
- the tension in (Ac1-5° C.) to (Ac1+10° C.) is more than 0 to 10 MPa, preferably 0.5 to 10 MPa, more preferably 0.5 to 5 MPa.
- Tensile force is applied along the rolling direction (longitudinal direction) in the production line, for example.
- the method of applying tension is not particularly limited, but a method of controlling the peripheral speed of the pinch rolls to apply tension between the pinch rolls, or a method of applying tension by bending by the rolls is preferable.
- the direction in which the tension is applied is not limited to the rolling direction, and may be, for example, the direction intersecting the rolling direction on the steel plate surface.
- Annealing temperature 750-940°C
- the steel plate after the above steps is heated to an annealing temperature of 750 to 940°C. If the annealing temperature is lower than 750°C, the austenite formation is insufficient and the steel structure of the present invention cannot be obtained. On the other hand, when the temperature exceeds 940°C, the retained austenite grains having two crystal orientations increase and the steel structure of the present invention cannot be obtained. Therefore, the annealing temperature is 750 to 940°C, preferably 770 to 920°C.
- Residence time at annealing temperature 10-600s
- the steel sheet is allowed to stay at the annealing temperature for 10 to 600 seconds. If the residence time is less than 10 s, the austenite formation is insufficient and the steel structure of the present invention cannot be obtained. On the other hand, when the residence time at the annealing temperature exceeds 600 s, the retained austenite grains having two or more crystal orientations increase and the steel structure of the present invention cannot be obtained. Therefore, the residence time is 10 to 600 s, preferably 30 to 300 s.
- First average cooling rate from annealing temperature to 550° C. 3° C./s or more First average cooling rate of 3° C./s or more from annealing temperature to 550° C. , Cooling from the annealing temperature to 550°C or lower.
- the first 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. If the first average cooling rate is less than 3° C./s, ferrite is excessively generated and the steel structure of the present invention cannot be obtained. Therefore, the first average cooling rate is 3° C./s or more, preferably 5° C./s or more.
- the upper limit of the first average cooling rate is not particularly specified, it is preferably less than 100° C./s from the viewpoint of shape stability.
- First cooling stop temperature Ms-550°C
- the first cooling stop temperature when cooling the annealed steel sheet at the first average cooling rate is (Ms to 550° C.).
- Ms shows the Ms point (martensitic transformation start temperature) of a steel plate.
- the first cooling stop temperature is (Ms to 550°C), preferably 450 to 550°C. ..
- Residence time at plating temperature 10 to 300s
- the steel sheet cooled to the first cooling stop temperature is retained at the plating treatment temperature (Ms to 580° C.). If the residence time at (Ms to 580° C.) is less than 10 s or more than 300 s, the bainite transformation proceeds insufficiently and the retained austenite grains having two or more crystal orientations increase to obtain the steel structure of the present invention. I can't. Therefore, the residence time at (Ms to 580° C.) is set to 10 to 300 s.
- the plating temperature may be (Ms to 580° C.), and the temperature may be higher than the cooling stop temperature by heating if necessary after the first average cooling.
- the steel sheet is subjected to a galvanizing treatment, or a galvanizing treatment is further performed after the zinc plating treatment.
- the plating treatment is preferably performed by immersing the steel plate obtained above in a zinc plating bath at 440° C. or higher and 500° C. or lower, and then adjusting the amount of plating adhered by gas wiping or the like. Further, when galvanizing is alloyed, it is preferable to stay in the temperature range of 460° C. or more and 580° C. or less for 1 second or more and 120 seconds or less for alloying.
- Zinc plating is preferably formed using a zinc plating bath having an Al content of 0.08 mass% or more and 0.25 mass% or less. Further, the galvanized steel sheet may be subjected to various coating treatments such as resin and oil coating.
- Second average cooling rate from the plating temperature to 350° C. 50° C./s or more
- the steel sheet after the plating treatment has a second average cooling rate of 50° C./s or more from the plating temperature to 350° C. Cool from the plating temperature to 350° C. or lower.
- the second average cooling rate is calculated by dividing the temperature difference between the plating treatment temperature and 350° C. by the time required for cooling from the plating treatment to 350° C. If the second average cooling rate is less than 50° C./s, the promotion of bainite transformation is insufficient and the steel structure of the present invention cannot be obtained. Therefore, the second average cooling rate is 50° C./s or more.
- the upper limit of the second average cooling rate is not limited as long as it is 50° C./s or more, so the upper limit is not specified, but the second average cooling rate obtained industrially. Is about 1500° C./s.
- Second cooling stop temperature 50-350°C
- the second cooling stop temperature when cooling the plated steel sheet at the second average cooling rate is 50 to 350°C. If the second cooling stop temperature is lower than 50°C, the retained austenite of the present invention cannot be obtained. On the other hand, when the second cooling stop temperature exceeds 350° C., the promotion of bainite transformation is insufficient and the steel structure of the present invention cannot be obtained. Therefore, the second cooling stop temperature is 50 to 350°C, preferably 80 to 320°C.
- Reheating temperature above the second cooling stop temperature and 300 to 500°C
- the steel sheet cooled to the second cooling stop temperature is heated above the second cooling stop temperature and at a reheating temperature of 300 to 500°C.
- the reheating temperature is 300 to 500°C, preferably 325 to 475°C.
- Residence time at reheating temperature 1 to 600s
- the steel sheet cooled to the second cooling stop temperature is retained at the above reheating temperature for 1 to 600 s. If the residence time is less than 1 s, the bainite transformation is insufficient, the martensite and the retained austenite grains having two or more crystal orientations increase, and the steel structure of the present invention cannot be obtained. On the other hand, if it exceeds 600 s, pearlite is generated and the steel structure of the present invention cannot be obtained. Therefore, the residence time at the reheating temperature is 1 to 600 s, preferably 1 to 300 s, more preferably 1 to 120 s, and further preferably 1 to 60 s.
- the conditions for the slab production method and the hot rolling process are not particularly limited, but it is preferable to perform the following conditions, for example.
- the slab is preferably manufactured by a continuous casting method in order to prevent macrosegregation, but can also be manufactured by an ingot making method or a thin slab casting method.
- To hot-roll the slab the slab may be cooled to room temperature and then reheated to be hot-rolled, or the slab may be placed in a heating furnace without being cooled to room temperature and hot-rolled. You may go. Further, an energy-saving process in which hot rolling is performed immediately after a slight heat retention is also applicable.
- heating the slab it is preferable to heat it to 1100° C. or higher in order to dissolve the carbide and prevent the rolling load from increasing. Further, in order to prevent an increase in scale loss, it is preferable that the heating temperature of the slab is 1300° C. or lower.
- the temperature of the slab is the temperature of the slab surface.
- the condition of the finish rolling temperature is not particularly limited, but it is preferably 800 to 950° C. from the viewpoint of easily obtaining the steel sheet structure of the present invention and homogenizing the steel sheet structure.
- the rough bar after rough rolling can be heated. Further, it is possible to apply a so-called continuous rolling process in which coarse bars are joined together and finish rolling is continuously performed.
- hot rolling in order to reduce the rolling load and make the shape and material uniform, 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 coiling temperature of the steel sheet after rolling is not particularly limited, but it is preferably 400 to 550° C. from the viewpoint of easily obtaining the steel sheet structure of the present invention and stabilizing the plate shape.
- the rolled steel plate can be used for the subsequent process after removing the scale by pickling.
- temper rolling may be further performed.
- the elongation is preferably 0.05% or more and 1.00% or less from the viewpoint of further improving YS.
- the temper rolling is preferably performed at room temperature after the heat treatment.
- Annealing was performed in a laboratory using a heat treatment and plating apparatus under the main annealing conditions shown in Table 2-1 or Table 2-2 to produce alloyed galvanized steel sheets (GA) 1 to 39.
- the alloyed hot dip galvanized steel sheet is immersed in a plating bath at 465° C. to form a plating layer with an adhesion amount of 40 to 60 g/m 2 on one side on both sides of the steel sheet, and then retained at 540° C. for 1 to 60 s. It processed and produced.
- the steel sheet was temper-rolled with an elongation of 0.1%.
- the tensile property values and workability of the obtained alloyed galvanized steel sheet were evaluated according to the following methods. The results are shown in Table 3. In addition, the area ratio of each structure within the range of 300 to 400 ⁇ m from the steel plate surface and the ratio of the retained austenite grains having two or more crystal orientations among all the retained austenite grains within the range of 300 to 400 ⁇ m from the steel plate surface are also shown. 3 shows.
- martensite is white or light gray or gray containing unoriented carbides
- bainite containing carbide is gray or dark gray containing oriented carbides
- tempered martensite is unoriented carbides.
- pearlite is black and white layered structure
- ferrite is black
- bainite without carbide is dark gray.
- the area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained based on EBSD (electron backscattering diffraction) from the total area ratio of martensite and retained austenite obtained by the above visual field observation. It was
- ⁇ Tensile test> A JIS No. 5 tensile test piece (JIS Z2201) was taken from the produced hot-dip galvanized steel sheet in a direction perpendicular to the rolling direction, and the tensile rate was set according to JIS Z 2241 (2011) with a strain rate of 10 ⁇ 3 /s. Tests were performed to determine YS and uniform elongation (UEL). In the present invention, YS of 850 MPa or more and uniform elongation of 9.0% or more were accepted.
- ⁇ Hole expansion test> A 100 mm ⁇ 100 mm test piece was sampled from the hot-dip galvanized steel sheet produced, and a hole expansion test was performed three times using a 60° conical punch in accordance with JFST 1001 (Japan Iron and Steel Federation Standard, 2008), and the average hole was tested. The expansion ratio ⁇ (%) was determined and the stretch flangeability was evaluated. In the present invention, YS ⁇ uniform elongation (UEL) ⁇ hole expansion ratio ⁇ of 270 GPa ⁇ % ⁇ % or more was accepted as excellent workability.
- UEL uniform elongation
- the hot-dip galvanized steel sheet of the present invention example has YS of 850 MPa or more, uniform elongation (UEL) of 9.0% or more, and YS ⁇ uniform elongation (UEL) ⁇ hole expansion ratio ⁇ of 270 GPa ⁇ % ⁇ % or more, That is, it has high strength and excellent workability.
- the hot-dip galvanized steel sheet of the comparative example outside the scope of the present invention does not satisfy at least one of these items.
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Abstract
Description
前記鋼板が、質量%で、
C:0.12~0.35%、
Si:0.5~3.0%、
Mn:1.5~4.0%、
P:0.100%以下(0%は含まない)、
S:0.02%以下(0%は含まない)、および
Al:0.01~1.50%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を有し、
鋼板表面から300~400μmの範囲内の鋼板組織に対して、マルテンサイトの面積率が30%以下、パーライトの面積率が1%以下、焼戻しマルテンサイトと炭化物を含むベイナイトの合計面積率が30%以上99%以下、残留オーステナイトの面積率が1~20%、かつフェライトと炭化物を含まないベイナイトの合計面積率が45%以下であり、
前記鋼板表面から300~400μmの範囲内の全残留オーステナイト粒のうち、2つ以上の結晶方位を有する残留オーステナイト粒の割合が、面積率で40%以下である、高強度溶融亜鉛めっき鋼板。
[2]前記鋼板が、前記成分組成に加えてさらに、質量%で、
Cr:0.005~2.0%、
Ni:0.005~2.0%、
Cu:0.005~2.0%、
V:0.1~1.5%、
Mo:0.1~1.5%、
Ti:0.005~0.10%、
Nb:0.005~0.10%、
B:0.0001~0.0050%、
Ca:0.0003~0.0050%、
REM:0.0003~0.0050%、
Sn:0.005~0.50%、および
Sb:0.005~0.50%のうちから選ばれる少なくとも一種の元素を含有する、[1]に記載の高強度溶融亜鉛めっき鋼板。
[3]前記溶融亜鉛めっき層が、合金化溶融亜鉛めっき層である、[1]または[2]に記載の高強度溶融亜鉛めっき鋼板。
[4][1]または[2]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、巻き取る熱延工程を施し、
前記熱延工程で得られた熱延板、または前記熱延板にさらに圧下率30%以上で冷間圧延した冷延板を、(Ac1-5℃)~(Ac1+10℃)の温度域で15s以上滞留させ、かつ当該滞留の間に0超~10MPaの張力を付加し、
次いで、750~940℃の焼鈍温度まで加熱して10~600s滞留させ、
次いで、前記焼鈍温度から550℃までの間を第1平均冷却速度3℃/s以上、かつ第1冷却停止温度(Ms~550℃)で冷却し、
次いで、(Ms~580℃)のめっき処理温度で10~300s滞留させ、当該滞留の間に、亜鉛めっき処理、または前記亜鉛めっき処理後にめっき合金化処理を行い、
次いで、前記めっき処理温度から350℃までの間を第2平均冷却速度50℃/s以上、かつ第2冷却停止温度50~350℃で冷却し、
次いで、前記第2冷却停止温度超、かつ300~500℃の再加熱温度まで加熱して1~600s滞留させた後、室温まで冷却する、高強度溶融亜鉛めっき鋼板の製造方法。
本発明の高強度溶融亜鉛めっき鋼板における鋼板の成分組成について説明する。下記の成分組成の説明において、成分の含有量の単位である「%」は「質量%」を意味する。
Cは、焼戻しマルテンサイトや炭化物を含むベイナイトを強化して強度を上昇させ、また残留オーステナイトを得るのに有効な元素である。C含有量が0.12%未満ではこのような効果が十分得られず、本発明の強度や鋼組織が得られない。したがって、C含有量は0.12%以上であり、好ましくは0.14%以上であり、より好ましくは0.15%以上である。一方、C含有量が0.35%を超えると残留オーステナイトが過剰に多くなって本発明の鋼組織が得られなくなる。したがって、C含有量は0.35%以下であり、好ましくは0.32%以下である。
Siは、鋼を固溶強化して強度を上昇させたり、残留オーステナイトを得るのに必要な元素である。このような効果を十分に得るには、Si含有量を0.5%以上とする必要がある。Si含有量は、好ましくは0.8%以上である。一方、Si含有量が3.0%を超えると過剰なフェライト生成を招き、本発明の鋼組織が得られなくなる。したがって、Si含有量は3.0%以下であり、好ましくは2.5%以下であり、より好ましくは2.0%以下である。
Mnは、マルテンサイトやベイナイトを生成させて強度を上昇させるのに有効な元素である。Mn含有量が1.5%未満ではこうした効果が十分得られない。したがって、Mn含有量は1.5%以上であり、好ましくは1.8%以上であり、より好ましくは2.0%以上である。一方、Mn含有量が4.0%を超えると鋼が脆化して本発明の優れた加工性を得られない。したがって、Mn含有量は4.0%以下であり、好ましくは3.7%以下であり、より好ましくは3.4%以下である。
Pは、鋼を脆化させて加工性を低下させるため、その量は極力低減することが望ましいが、本発明ではP含有量を0.100%まで許容できる。なお、P含有量を0%にするのは操業上困難なため、0%は含まない。また、P含有量が0.001%未満では生産能率の低下を招くため、0.001%以上が好ましい。
Sは、介在物を増加させて加工性を低下させるため、その量は極力低減することが好ましいが、本発明ではS含有量を0.02%まで許容できる。なお、S含有量を0%にするのは操業上困難なため、0%は含まない。また、S含有量が、0.0001%未満では生産能率の低下を招くため、0.0001%以上が好ましい。
Alは、脱酸剤として作用し、脱酸工程で添加することが好ましい。また、残留オーステナイトを生成させるのに有効な元素である。このような効果を発現させるには、Al含有量を0.01%以上とする必要がある。Al含有量は、好ましくは0.02%以上である。一方、Al含有量が1.50%を超えるとフェライトが過剰に生成して本発明の鋼組織が得られない。したがって、Al含有量は1.50%以下であり、好ましくは1.0%以下であり、より好ましくは0.70%以下である。
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にはそれぞれ上記下限値以上とすることが好ましい。一方、Cr、Ni、Cuのそれぞれの含有量は、加工性を高める観点からは、上記上限値以下とすることが好ましい。
高強度溶融亜鉛めっき鋼板における鋼板組織について説明する。以下の説明において、鋼板組織の面積率とは、鋼板表面から300~400μmの範囲内における鋼板組織に対する面積率を意味する。なお、本発明にて、鋼板表面から300~400μmの範囲内の鋼板組織の制御が重要なのは、本発明で意図する均一伸び(UEL)や穴拡げ率(λ)の向上が、前記した鋼板の板厚の範囲で発生するネッキングやボイド生成に影響しているためと推測される。
マルテンサイトは強度を上昇させるが加工性を低下させるため、面積率を30%以下とする必要がある。面積率が30%を超えると本発明の加工性が得られなくなる。したがって、マルテンサイトの面積率は30%以下、好ましくは25%以下、より好ましくは15%以下、さらに好ましくは10%以下である。なお、下限については特に定めないが、1%以上であることが多い。
パーライトは強度均一伸びバランスを低下させるため、極力低減する必要がある。面積率が1%を超えると本発明の強度均一伸びバランスが得られなくなる。したがって、パーライトの面積率は1%以下である。
焼戻しマルテンサイトと炭化物を含むベイナイトは高強度と高加工性を得るのに必要な組織である。これらの合計面積率が30%未満では本発明の強度と加工性のいずれかが得られない。したがって、焼戻しマルテンサイトと炭化物を含むベイナイトの合計面積率は30%以上、好ましくは40%以上、より好ましくは50%以上である。本発明の高強度と高加工性を得る観点からは、焼戻しマルテンサイトと炭化物を含むベイナイトの合計面積率の上限は限られないが、他の組織の面積率との関係で、合計面積率は99%以下である。
残留オーステナイトは均一伸びを上昇させるのに必要な組織である。残留オーステナイトの面積率が1%未満ではこのような効果が十分に得られない。したがって、残留オーステナイトの面積率は1%以上であり、好ましくは3%以上であり、さらに好ましくは5%以上である。一方、面積率が20%を超えると穴拡げ率(λ)の低下を招く。したがって、残留オーステナイトの面積率は20%以下であり、好ましくは18%以下であり、より好ましくは17%以下である。
本発明でいう「2つ以上の結晶方位を有する残留オーステナイト粒」とは、15゜以上の方位差を持つ複数のオーステナイトが連結した粒であることを意味する。
本発明において、残留オーステナイト粒の結晶の連結は極めて重要である。全残留オーステナイト粒の内、2つ以上の結晶方位を有するオーステナイト粒は、加工の際に早期にマルテンサイト変態するため、加工性の低下を招く。本発明の優れた加工性を得るには、全残留オーステナイト粒のうち、2つ以上の結晶方位を有する残留オーステナイト粒の割合を面積率で40%以下とする必要があり、好ましくは面積率で35%以下であり、より好ましくは面積率で30%以下であり、さらに好ましくは面積率で20%以下である。
本発明において、マルテンサイト、パーライト、焼戻しマルテンサイト、および炭化物を含むベイナイトの面積率とは、観察面積に占める各組織の面積の割合のことである。
焼鈍後の鋼板のサンプルを切り出し、圧延方向に平行な板厚断面を研磨後、3体積%ナイタールで腐食し、鋼板表面から板厚方向に300~400μmの範囲内の位置をSEM(走査型電子顕微鏡)で1500倍の倍率でそれぞれ3視野撮影する。なお、鋼板表面とは、溶融亜鉛めっき層と鋼板の界面を示す。得られた画像データを用いて、Media Cybernetics社製のImage-Proにより各組織の面積率を求め、視野の各組織の平均面積率を、各組織の面積率とする。前記画像データにおいて、マルテンサイトは白色若しくは明灰色または方位の揃っていない炭化物を含む灰色、炭化物を含むベイナイトは方位の揃った炭化物を含む灰色または暗灰色、焼戻しマルテンサイトは方位の揃っていない炭化物を含む灰色または暗灰色、パーライトは黒色と白色の層状組織として区別される。
上記EBSDに基づいて残留オーステナイトの面積率を求める方法において、Grain Dilation処理と、Neighbor CI Correlation処理を行った後のデータから、15゜以上の方位差を持つ複数のオーステナイトが連結した粒の面積率を求める。そして、全オーステナイトの面積率に対する、当該15゜以上の方位差を持つ複数のオーステナイトが連結した粒の面積率の割合(%)を求める。この割合(%)を、全残留オーステナイト粒のうち、2つ以上の結晶方位を有する残留オーステナイト粒の割合とする。
本発明の高強度溶融亜鉛めっき鋼板は、鋼板の表面上に溶融亜鉛めっき層を有する。溶融亜鉛めっき層は、合金化溶融亜鉛めっき層でもよい。
本発明の高強度溶融亜鉛めっき鋼板の板厚(めっき層を含まない)は特に限定されないが、0.4mm以上3.0mm以下が好ましい。
本発明の高強度溶融亜鉛めっき鋼板は、例えば、上記成分組成を有するスラブに、熱間圧延を施した後、冷却し、巻き取る熱延工程を施し、前記熱延工程で得られた熱延板、または前記熱延板にさらに圧下率30%以上で冷間圧延した冷延板を、(Ac1-5℃)~(Ac1+10℃)の温度域で15s以上滞留させ、かつ当該滞留の間に0超~10MPaの張力を付加し、次いで、750~940℃の焼鈍温度まで加熱して10~600s滞留させ、次いで、前記焼鈍温度から550℃までの間を第1平均冷却速度3℃/s以上、かつ第1冷却停止温度(Ms~550℃)で冷却し、次いで、(Ms~580℃)のめっき処理温度で10~300s滞留させ、当該滞留の間に、亜鉛めっき処理、または前記亜鉛めっき処理後にめっき合金化処理を行い、次いで、前記めっき処理温度から350℃までの間を第2平均冷却速度50℃/s以上、かつ第2冷却停止温度50~350℃で冷却し、次いで、前記第2冷却停止温度超、かつ300~500℃の再加熱温度まで加熱して1~600s滞留させた後、室温まで冷却して製造される。なお、製造条件に示した各温度は、いずれも鋼板の表面温度である。
以下、上記製造方法を、順に説明する。
熱延工程で得られた熱延板を必要に応じて冷間圧延する。冷間圧延する場合、圧下率が30%未満となると2つの結晶方位を有する残留オーステナイト粒が多くなって、本発明の鋼組織が得られない。したがって、冷間圧延での圧下率は30%以上、好ましくは35%以上である。上限は特に規定しないが、形状安定性等の観点からは90%以下が好ましい。冷間圧延を施さない場合は、ベイナイト変態が促進されて2つの結晶方位を有する残留オーステナイトが減少する。なお、熱延板を焼鈍する場合は、形状矯正のために5%以下の調質圧延を施してもよい。
前記熱延工程で得られた熱延板、または熱延板に上記冷間圧延して得られた冷延板を、(Ac1-5℃)~(Ac1+10℃)の温度域で15s以上滞留させる。当該温度域での滞留時間が15s未満では、2つ以上の結晶方位を有する残留オーステナイト粒が多くなって、本発明の鋼組織が得られない。したがって、(Ac1-5℃)~(Ac1+10℃)の温度域での滞留時間は15s以上、好ましくは20s以上である。また、当該滞留時間をより長くすれば、2つ以上の結晶方位を有する残留オーステナイト粒がより少なくなり、本発明の効果が有効に得られるので、滞留時間の上限は規定していない。なお、Ac1はAc1変態点を示す。
上記熱延板または冷延板を(Ac1-5℃)~(Ac1+10℃)の温度域で滞留させオーステナイトを生成させる際に、張力を付加することで2つ以上の結晶方位を有する残留オーステナイト粒を低減することができる。このメカニズムは明らかではないが、2つ以上の結晶方位を有する残留オーステナイト粒の生成そのものを低減することやその後のベイナイト変態を促進することなどが考えられる。しかしながら、張力が10MPaを超えるとこのような効果はみられなくなる。したがって、(Ac1-5℃)~(Ac1+10℃)での張力は0超~10MPa、好ましくは0.5~10MPa、より好ましくは0.5~5MPaである。
上記工程後の鋼板を750~940℃の焼鈍温度まで加熱する。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発明の鋼組織が得られない。一方、940℃を超えると、2つの結晶方位を有する残留オーステナイト粒が増加して本発明の鋼組織が得られない。したがって、焼鈍温度は750~940℃、好ましくは770~920℃である。
鋼板を上記焼鈍温度で10~600s滞留させる。滞留時間が10s未満ではオーステナイトの生成が不十分となり、本発明の鋼組織が得られない。一方、上記焼鈍温度での滞留時間が600sを超えると、2つ以上の結晶方位を有する残留オーステナイト粒が増加して本発明の鋼組織が得られない。したがって、滞留時間は10~600s、好ましくは30~300sである。
上記焼鈍温度で焼鈍後の鋼板を、焼鈍温度から550℃までの間を第1平均冷却速度3℃/s以上で、焼鈍温度から550℃以下まで冷却する。第1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して算出する。第1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発明の鋼組織が得られない。したがって、第1平均冷却速度は3℃/s以上、好ましくは5℃/s以上である。第1平均冷却速度の上限は特に規定しないが、形状安定性の観点からは100℃/s未満が好ましい。
上記焼鈍後の鋼板を上記第1平均冷却速度で冷却するときの第1冷却停止温度は、(Ms~550℃)である。なお、Msは鋼板のMs点(マルテンサイト変態開始温度)を示す。冷却停止温度がMs未満になるとめっき熱処理前にオーステナイトへのC濃化が過剰となってパーライトが生成し、本発明の鋼組織が得られない。一方、550℃を超えると過剰なフェライトやパーライトの生成を招いて本発明の鋼組織が得られない、したがって、第1冷却停止温度は(Ms~550℃)、好ましくは450~550℃である。
第1冷却停止温度まで冷却した鋼板を、(Ms~580℃)のめっき処理温度で滞留させる。(Ms~580℃)での滞留時間が10s未満または300sを超えるとベイナイト変態の進行が不十分となって2つ以上の結晶方位を有する残留オーステナイト粒が増加して本発明の鋼組織が得られない。したがって、(Ms~580℃)での滞留時間は10~300sとする。
なお、めっき処理温度は(Ms~580℃)であればよく、上記第1平均冷却の後に必要に応じて加熱し、冷却停止温度以上の温度としてもよい。
上記めっき処理温度で滞留させる間に、鋼板に、亜鉛めっき処理、または前記亜鉛めっき処理後にさらにめっき合金化処理を行う。めっき処理は、上記により得られた鋼板を440℃以上500℃以下の亜鉛めっき浴中に浸漬し、その後、ガスワイピングなどによってめっき付着量を調整して行うことが好ましい。さらに亜鉛めっきを合金化する際は460℃以上580℃以下の温度域に1秒以上120秒以下滞留させて合金化することが好ましい。亜鉛めっきは、Al量が0.08質量%以上0.25質量%以下である亜鉛めっき浴を用いて形成することが好ましい。
また、亜鉛めっき後の鋼板には樹脂や油脂コーティングなどの各種塗装処理を施すこともできる。
めっき処理後の鋼板を、めっき処理温度から350℃までの間を第2平均冷却速度50℃/s以上で、めっき処理温度から350℃以下まで冷却する。第2平均冷却速度は、めっき処理温度と350℃との温度差をめっき処理から350℃までの冷却に要した時間で除して算出する。第2平均冷却速度が50℃/s未満ではベイナイト変態の促進が不十分となり、本発明の鋼組織が得られない。したがって、第2平均冷却速度は50℃/s以上とする。本発明の鋼組織を得る観点からは、第2平均冷却速度は50℃/s以上であれば上限は限られないため、上限を規定していないが、工業的に得られる第2平均冷却速度の上限は1500℃/s程度である。
めっき処理後の鋼板を、上記第2平均冷却速度で冷却するときの第2冷却停止温度は、50~350℃である。第2冷却停止温度が50℃未満では本発明の残留オーステナイトが得られない。一方、第2冷却停止温度が350℃を超えるとベイナイト変態の促進が不十分となって本発明の鋼組織が得られない。したがって、第2冷却停止温度は50~350℃、好ましくは80~320℃である。
第2冷却停止温度まで冷却した鋼板を、第2冷却停止温度超、かつ300~500℃の再加熱温度で加熱する。再加熱温度が300℃未満ではベイナイト変態が抑制されて2つ以上の結晶方位を有する残留オーステナイト粒が増加して本発明の鋼組織が得られない。一方、500℃を超えるとパーライトが生成して本発明の鋼組織が得られない。したがって、再加熱温度は300~500℃、好ましくは325~475℃である。
第2冷却停止温度まで冷却した鋼板を、上記再加熱温度で1~600s滞留させる。滞留時間が1s未満ではベイナイト変態が不足して、マルテンサイトや2つ以上の結晶方位を有する残留オーステナイト粒が増加して本発明の鋼組織が得られない。一方、600sを超えると、パーライトが生成して、本発明の鋼組織が得られなくなる。したがって、再加熱温度での滞留時間は1~600s、好ましくは1~300s、より好ましくは1~120s、さらに好ましくは1~60sである。
スラブは、マクロ偏析を防止するため、連続鋳造法で製造するのが好ましいが、造塊法、薄スラブ鋳造法により製造することもできる。スラブを熱間圧延するには、スラブを一旦室温まで冷却し、その後再加熱して熱間圧延を行ってもよいし、スラブを室温まで冷却せずに加熱炉に装入して熱間圧延を行ってもよい。また、わずかな保熱を行った後に直ちに熱間圧延する省エネルギープロセスも適用できる。
各鋼板のサンプルを切り出し、圧延方向に平行な板厚断面を研磨後、3体積%ナイタールで腐食し、鋼板表面から板厚方向に300~400μmの範囲内の位置をSEM(走査型電子顕微鏡)で1500倍の倍率でそれぞれ3視野撮影した。なお、鋼板表面とは、溶融亜鉛めっき層と鋼板の界面を示す。得られた画像データを用いて、Media Cybernetics社製のImage-Proにより各組織の面積率を求め、視野の各組織の平均面積率を、各組織の面積率とした。前記画像データにおいて、マルテンサイトは白色若しくは明灰色または方位の揃っていない炭化物を含む灰色、炭化物を含むベイナイトは方位の揃った炭化物を含む灰色または暗灰色、焼戻しマルテンサイトは方位の揃っていない炭化物を含む灰色または暗灰色、パーライトは黒色と白色の層状組織、フェライトは黒色、炭化物を含まないベイナイトは暗灰色として区別される。
上記EBSDに基づいて残留オーステナイトの面積率を求める方法において、Grain Dilation処理と、Neighbor CI Correlation処理を行った後のデータから、15゜以上の方位差を持つ複数のオーステナイトが連結した粒の面積率を求めた。そして、全オーステナイトの面積率に対する、当該15゜以上の方位差を持つ複数のオーステナイトが連結した粒の面積率の割合(%)を求めた。この割合(%)を、全残留オーステナイト粒のうち、2つ以上の結晶方位を有する残留オーステナイト粒の割合とした。
作製した溶融亜鉛めっき鋼板から圧延方向に対して直角方向にJIS5号引張試験片(JIS Z2201)を採取し、歪速度が10-3/sとするJIS Z 2241(2011)の規定に準拠した引張試験を行い、YSおよび均一伸び(UEL)を求めた。本発明では、YSが850MPa以上かつ均一伸びが9.0%以上を合格とした。
作製した溶融亜鉛めっき鋼板から100mm×100mmの試験片を採取し、JFST 1001(日本鉄鋼連盟規格、2008年)に準拠して60゜円錐ポンチを用いて穴拡げ試験を3回行って平均の穴拡げ率λ(%)を求め、伸びフランジ性を評価した。本発明では、YS×均一伸び(UEL)×穴拡げ率λが270GPa・%・%以上を加工性が優れるとして合格とした。
Claims (4)
- 鋼板の表面上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であって、
前記鋼板が、質量%で、
C:0.12~0.35%、
Si:0.5~3.0%、
Mn:1.5~4.0%、
P:0.100%以下(0%は含まない)、
S:0.02%以下(0%は含まない)、および
Al:0.01~1.50%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を有し、
鋼板表面から300~400μmの範囲内の鋼板組織に対して、マルテンサイトの面積率が30%以下、パーライトの面積率が1%以下、焼戻しマルテンサイトと炭化物を含むベイナイトの合計面積率が30%以上99%以下、残留オーステナイトの面積率が1~20%、かつフェライトと炭化物を含まないベイナイトの合計面積率が45%以下であり、
前記鋼板表面から300~400μmの範囲内の全残留オーステナイト粒のうち、2つ以上の結晶方位を有する残留オーステナイト粒の割合が、面積率で40%以下である、高強度溶融亜鉛めっき鋼板。 - 前記鋼板が、前記成分組成に加えてさらに、質量%で、
Cr:0.005~2.0%、
Ni:0.005~2.0%、
Cu:0.005~2.0%、
V:0.1~1.5%、
Mo:0.1~1.5%、
Ti:0.005~0.10%、
Nb:0.005~0.10%、
B:0.0001~0.0050%、
Ca:0.0003~0.0050%、
REM:0.0003~0.0050%、
Sn:0.005~0.50%、および
Sb:0.005~0.50%のうちから選ばれる少なくとも一種の元素を含有する、請求項1に記載の高強度溶融亜鉛めっき鋼板。 - 前記溶融亜鉛めっき層が、合金化溶融亜鉛めっき層である、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。
- 請求項1または請求項2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、巻き取る熱延工程を施し、
前記熱延工程で得られた熱延板、または前記熱延板にさらに圧下率30%以上で冷間圧延した冷延板を、(Ac1-5℃)~(Ac1+10℃)の温度域で15s以上滞留させ、かつ当該滞留の間に0超~10MPaの張力を付加し、
次いで、750~940℃の焼鈍温度まで加熱して10~600s滞留させ、
次いで、前記焼鈍温度から550℃までの間を第1平均冷却速度3℃/s以上、かつ第1冷却停止温度(Ms~550℃)で冷却し、
次いで、(Ms~580℃)のめっき処理温度で10~300s滞留させ、当該滞留の間に、亜鉛めっき処理、または前記亜鉛めっき処理後にめっき合金化処理を行い、
次いで、前記めっき処理温度から350℃までの間を第2平均冷却速度50℃/s以上、かつ第2冷却停止温度50~350℃で冷却し、
次いで、前記第2冷却停止温度超、かつ300~500℃の再加熱温度まで加熱して1~600s滞留させた後、室温まで冷却する、高強度溶融亜鉛めっき鋼板の製造方法。
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