HOT-DIP GALVANIZED HIGH-STRENGTH ALLOY STEEL SHEET AND METHOD OF MANUFACTURING THE SAME
Patent Information
- Authority / Receiving Office
- MX · MX
- Patent Type
- Applications
- Current Assignee / Owner
- JFE STEEL CORP
- Filing Date
- 2026-04-23
- Publication Date
- 2026-06-01
Abstract
Description
High-strength galvannealed steel sheet and method for manufacturing the same
[0001] The present invention relates to a high-strength galvannealed steel sheet and a method for producing the same.
[0002] From the viewpoint of protecting the global environment, in order to improve the fuel efficiency of automobiles, the weight of the automobile body may be reduced by increasing the strength and thinning of steel sheets used as automobile components. Furthermore, from the viewpoint of the corrosion prevention performance of the automobile body, the steel sheets used as automobile components may be subjected to a zinc-based plating treatment. For example, galvannealed steel sheets having a tensile strength (TS) of 590 MPa or more have been developed as steel sheets used for the framework of automobile cabins (Patent Documents 1 to 3).
[0003] JP 2012-219342 A JP 2013-117042 A JP 2014-37574 A
[0004] High-strength steel plates with a tensile strength of 590 MPa or more may have poor bendability due to a decrease in local ductility, and may develop cracks when subjected to bending deformation.
[0005] The present inventors formed a bent portion by applying bending deformation to a galvannealed steel sheet (hereinafter also simply referred to as "plated steel sheet") at a bending radius R larger than the bending radius R at which cracks occur in a bending test in accordance with JIS. Thereafter, the bent portion of the plated steel sheet was observed using a scanning electron microscope (SEM).
[0006] Fig. 1 is an SEM image showing a cross section of a galvannealed steel sheet. As shown in Fig. 1, it was found that in the bent portion of the galvannealed steel sheet, fine microcracks 3 originating from cracks in the galvannealed layer 2 were generated in the steel sheet 1, which could not be observed without an SEM.
[0007] When microcracks occur in a steel sheet (base steel sheet), the corrosion resistance of the bent portion of the plated steel sheet deteriorates, and furthermore, the impact absorption characteristics and fatigue characteristics may also deteriorate. In order to suppress the occurrence of microcracks, it is necessary to stop the cracks that occur in the galvannealed layer (hereinafter also simply referred to as the "plated layer") with the steel sheet, and the mechanism of occurrence of microcracks is significantly different from that of conventional cracks that occur on the steel sheet side (cracks large enough to be observed with a magnifying glass). The above-mentioned Patent Documents 1 to 3 disclose technologies for improving bendability (suppressing the occurrence of cracks), but all of them target conventional cracks, not microcracks.
[0008] Hereinafter, the bending property related to microcracks is also referred to as "microcrack resistance." The criteria for evaluating whether a material has excellent microcrack resistance will be described later (see the "Examples" section). Note that "microcrack resistance" may also be referred to as "bending property" for convenience.
[0009] The present invention has been made in view of the above points, and an object of the present invention is to provide a galvannealed steel sheet having a tensile strength of 590 MPa or more and excellent ductility and microcrack resistance. Another object of the present invention is to provide a method for producing the galvannealed steel sheet.
[0010] As a result of extensive investigations, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention. That is, the present invention provides the following [1] to [6]. [1] A steel sheet comprising: a steel sheet; and a galvannealed layer disposed on a surface of the steel sheet, wherein the steel sheet has a chemical composition in which a carbon equivalent Ceq represented by the following formula (1) is 0.370 or more and less than 0.520, a total content of Nb and Ti is 0.010 to 0.080 mass%, and the balance is Fe and unavoidable impurities, a martensite area ratio is 5 to 30% at a quarter-thickness position of the steel sheet; and a martensite area ratio is 5 to 30% in a range from the surface of the steel sheet to a depth of 20 μm. A high-strength galvannealed steel sheet having a martensite area ratio of 5 to 30%, a prior austenite grain size of 3.0 μm or less, a martensite area ratio of 50% or more, a total Nb and Ti content in precipitates of 100 nm or less of 50 ppm by mass or more, and a total Nb and Ti content in precipitates of more than 100 nm of 350 ppm by mass or less, and a long side length of Si and Mn oxides present on grain boundaries within a range of 1 μm deep from the surface of the steel sheet of 200 nm or less. Ceq = [C%] + ([Si%] / 24) + ([Mn%] / 6) + ([Ni%] / 40) + ([Cr%] / 5) + ([Mo%] / 4) + ([V%] / 14) (1) In the above formula (1), [M%] is the content of element M in the above chemical composition in unit mass%, and is 0 when element M is not contained. [2] The high-strength galvannealed steel sheet according to [1] above, wherein the chemical composition further contains, in mass%, C: 0.050 to 0.150%, Si: 0.30% or less, Mn: 1.70 to 3.50%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less.[3] The high-strength galvannealed steel sheet according to [2] above, wherein the chemical composition further contains, in mass%, at least one element selected from the group consisting of B: 0.0050% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, V: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less. [4] A method for producing a high-strength galvannealed steel sheet according to any one of [1] to [3] above, comprising: hot rolling a slab having the component composition according to any one of [1] to [3] above under the conditions of a slab heating temperature of 1200°C or higher, a final reduction rate of 5% or higher, a rolling completion temperature of 850 to 970°C, and a cooling time from the final reduction to 700°C or lower of 6.0 seconds or less to obtain a hot-rolled steel sheet; and then The plate is subjected to pickling, and the hot-rolled steel sheet after the pickling is subjected to cold rolling under conditions of a rolling reduction of 30% or more to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is subjected to pickling for 2.0 s or more, and the cold-rolled steel sheet after the pickling is subjected to a heating rate of 2.0 to 7.0 ° C. / s from 500 ° C. to 700 ° C., a dew point of an atmosphere at 700 ° C. or higher is −40 ° C. or less, a maximum temperature reached is 740 to 860 ° C., and a cooling rate v from 530 ° C. to 480 ° C. １ [5] A method for producing a high-strength galvannealed steel sheet, comprising: annealing the cold-rolled steel sheet under the condition that the cooling rate v is 2.0°C / s or less; and then subjecting the annealed cold-rolled steel sheet to a galvannealed hot-dip coating treatment including alloying at a temperature of 480°C or more. ２ [6] The method for producing a high-strength galvannealed steel sheet according to the above [4] or [5], wherein, during the annealing, a CO concentration in an atmosphere at 700°C or higher is 200 ppm by volume or less.
[0011] According to the present invention, it is possible to provide a galvannealed steel sheet having a tensile strength of 590 MPa or more and excellent ductility and microcrack resistance.
[0012] 1 is an SEM image showing a cross section of a galvannealed steel sheet.
[0013] [Galvannealed Steel Sheet] The high-strength galvannealed steel sheet of the present embodiment generally includes a steel sheet and a galvannealed layer disposed on the surface of the steel sheet. The term "high strength" means that the tensile strength (TS) is 590 MPa or more.
[0014] The high-strength galvannealed steel sheet of this embodiment has excellent ductility and microcrack resistance, and also excellent corrosion resistance, due to the steel sheet having the component composition and microstructure described below. Therefore, it is suitable for use in automobile components (for example, the framework of an automobile cabin). In this case, it can contribute to improving safety performance and reducing the weight of the automobile body, thereby improving the fuel efficiency of the automobile and reducing CO ２ This reduces emissions and contributes to the environment. It can also be actively applied to parts of automobiles where corrosion from rain and snow is a concern, such as undercarriage parts.
[0015] The high-strength galvannealed steel sheet of this embodiment can be applied not only to automobiles but also to fields such as civil engineering, construction, and home appliances.
[0016] <Steel Sheet> First, the steel sheet (base steel sheet) included in the high-strength galvannealed steel sheet of this embodiment will be described. The thickness of the steel sheet is not particularly limited and is, for example, 0.5 mm or more and 3.0 mm or less.
[0017] <<Composition>> First, the composition of the steel sheet (base steel sheet) will be described. The unit "%" in the composition means "mass %" unless otherwise specified.
[0018] (Carbon equivalent Ceq: 0.370 or more and less than 0.520) Because a tensile strength of 590 MPa or more can be obtained, the carbon equivalent Ceq is 0.370 or more, preferably 0.390 or more, and more preferably 0.410 or more. On the other hand, if the carbon equivalent Ceq is too high, ductility decreases. Because good ductility can be obtained, the carbon equivalent Ceq is less than 0.520, preferably 0.518 or less, more preferably 0.510 or less, and even more preferably 0.490 or less.
[0019] The carbon equivalent Ceq is expressed by the following formula (1): Ceq=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) (1) In the above formula (1), [M%] is the content (unit: mass%) of element M in the chemical composition of the steel sheet, and is 0 (zero) when element M is not contained.
[0020] (Nb + Ti: 0.010 to 0.080%) Nb and Ti are dispersed in the surface layer of the steel sheet as carbides and / or nitrides, thereby suppressing stress concentration and inhibiting the occurrence of microcracks. To achieve this effect, the total content of Nb and Ti is 0.010% or more, preferably 0.012% or more, more preferably 0.016% or more, and even more preferably 0.020% or more. On the other hand, excessive addition of Nb and Ti coarsens the precipitates in the surface layer of the steel sheet, promoting the occurrence of microcracks. For this reason, the total content of Nb and Ti is 0.080% or less, preferably 0.060% or less, and more preferably 0.040% or less.
[0021] (Other Elements 1) The chemical composition of the steel sheet may further contain the elements described below.
[0022] ((C: 0.050 to 0.150%)) C is an element effective in increasing the strength of steel, and particularly contributes to increasing strength by forming martensite, which is one of the hard phases in the steel structure. From the viewpoint of obtaining the desired high strength, specifically a tensile strength of 590 MPa or more, the C content is preferably 0.050% or more, more preferably 0.055% or more, and even more preferably 0.060% or more. On the other hand, if the C content is too high, ductility decreases. For this reason, the C content is preferably 0.150% or less, more preferably 0.140% or less, and even more preferably 0.130% or less.
[0023] ((Si: 0.30% or less)) When Si is added in excess, it generates Si oxides in the surface layer of the steel sheet, reducing microcrack resistance. Therefore, the Si content is preferably 0.30% or less, more preferably 0.25% or less, and even more preferably 0.20% or less. The lower limit of the Si content is not particularly limited and is, for example, 0.01%, but may also be 0 (zero).
[0024] ((Mn: 1.70 to 3.50%)) Mn is an element that contributes to increasing the strength of steel by solid solution strengthening and the formation of martensite. To achieve this effect, the Mn content is preferably 1.70% or more, more preferably 1.80% or more, and even more preferably 2.00% or more. On the other hand, if the Mn content is too high, Mn oxides are formed in the surface layer of the steel sheet, reducing microcrack resistance. For this reason, the Mn content is preferably 3.50% or less, more preferably 3.20% or less, and even more preferably 3.00% or less.
[0025] ((P: 0.100% or less)) P segregates at prior austenite grain boundaries and embrittles the grain boundaries, thereby reducing the ultimate deformability of the steel sheet. Therefore, if the P content is too high, the bendability will decrease. For this reason, the P content is preferably 0.100% or less, more preferably 0.070% or less, and even more preferably 0.040% or less. On the other hand, the lower limit of the P content is not particularly limited. However, P is a solid solution strengthening element and can increase the strength of the steel sheet. From this viewpoint, the P content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.
[0026] ((S: 0.0200% or less)) S exists as sulfides and reduces the ultimate deformability of the steel sheet. Therefore, if the S content is too high, the bendability decreases. Therefore, the S content is preferably 0.0200% or less, more preferably 0.0120% or less, and even more preferably 0.0050% or less. On the other hand, there is no particular restriction on the lower limit of the S content. However, due to constraints on production technology, the S content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.
[0027] ((Al: 0.100% or less)) Al is added as a deoxidizer. From the viewpoint of obtaining this effect, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more. However, if the Al content is too high, it may cause surface defects in the coating layer. Therefore, the Al content is preferably 0.100% or less, more preferably 0.090% or less, and even more preferably 0.080% or less.
[0028] ((N: 0.0100% or less)) N exists as nitride and reduces the ultimate deformability of the steel sheet. Therefore, if the N content is too high, the bendability decreases. Therefore, the N content is preferably 0.0100% or less, more preferably 0.0070% or less, and even more preferably 0.0050% or less. On the other hand, the lower limit of the N content is not particularly limited. However, due to constraints on production technology, the N content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.
[0029] ((O: 0.0100% or less)) O exists as an oxide and reduces the ultimate deformability of the steel sheet. Therefore, if the O content is too high, the bendability decreases. Therefore, the O content is preferably 0.0100% or less, more preferably 0.0070% or less, and even more preferably 0.0050% or less. On the other hand, there is no particular restriction on the lower limit of the O content. However, due to constraints on production technology, the O content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.
[0030] (Other Elements No. 2) The composition of the steel sheet may further contain at least one element selected from the group consisting of the elements described below.
[0031] ((B)) B is contained in Si and Mn oxides formed on the surface of the steel sheet and improves the wettability of these oxides with molten zinc, thereby improving the appearance of the coating layer. From the viewpoint of achieving this effect, the B content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more. On the other hand, if the B content is too high, coarse B compounds may be formed, which may result in a decrease in bendability. Therefore, the B content is preferably 0.0050% or less, more preferably 0.0045% or less, and even more preferably 0.0040% or less.
[0032] ((Ta and W)) When the contents of Ta and W are appropriate, large amounts of coarse precipitates or inclusions are not formed, and the ultimate deformability of the steel sheet is not reduced, so that bendability is not reduced. Therefore, the contents of Ta and W are each preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less. There are no particular restrictions on the lower limits of the contents of Ta and W. However, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the contents of Ta and W are each preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.
[0033] ((Cr, Ni, and Mo)) When the contents of Cr, Ni, and Mo are appropriate, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so that bendability is not reduced. Therefore, the contents of Cr, Ni, and Mo are each preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less. There are no particular restrictions on the lower limits of the contents of Cr, Ni, and Mo. However, Cr, Ni, and Mo are elements that improve hardenability. Therefore, the contents of Cr, Ni, and Mo are each preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.
[0034] ((V)) V is useful for precipitation strengthening of steel. However, if the V content is too high, workability may become insufficient. Therefore, the V content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less. On the other hand, when V is contained, in order to obtain the effect of adding V, the V content is preferably 0.01% or more, more preferably 0.04% or more, and even more preferably 0.07% or more.
[0035] ((Co)) If the Co content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that bendability will not be reduced. Therefore, the Co content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.006% or less. The lower limit of the Co content is not particularly limited. However, Co is an element that improves hardenability. Therefore, the Co content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
[0036] ((Cu)) If the Cu content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that bendability will not be reduced. Therefore, the Cu content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less. There is no particular limitation on the lower limit of the Cu content. However, Cu is an element that improves hardenability. Therefore, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.
[0037] ((Sn)) If the Sn content is appropriate, it does not cause cracks to form inside the steel sheet during casting or hot rolling, and does not reduce the ultimate deformability of the steel sheet, so that bendability does not decrease. Therefore, the Sn content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. There is no particular limitation on the lower limit of the Sn content. However, Sn is an element that improves hardenability. Therefore, the Sn content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.
[0038] ((Sb)) If the Sb content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that bendability will not be reduced. Therefore, the Sb content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. The lower limit of the Sb content is not particularly limited. However, Sb is an element that can adjust the strength by controlling the thickness of the softened layer present in the steel sheet. Therefore, the Sb content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.
[0039] ((Ca, Mg, and REM)) When the contents of Ca, Mg, and REM (rare earth metals) are appropriate, they do not increase coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, so that bendability does not decrease. Therefore, the contents of Ca, Mg, and REM are each preferably 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less. There are no particular restrictions on the lower limits of the contents of Ca, Mg, and REM. However, Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Ca, Mg, and REM are each preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.
[0040] ((Zr and Te)) When the contents of Zr and Te are appropriate, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet is not reduced, so that bendability is not reduced. Therefore, the contents of Zr and Te are each preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less. The lower limits of the contents of Zr and Te are not particularly limited. However, Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Zr and Te are each preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more.
[0041] ((Hf)) If the content of Hf is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that the bendability will not be reduced. Therefore, the Hf content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less. The lower limit of the Hf content is not particularly limited. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.
[0042] ((Bi)) If the Bi content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not be reduced, so that bendability will not be reduced. Therefore, the Bi content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less. There is no particular restriction on the lower limit of the Bi content. However, Bi is an element that reduces segregation. Therefore, the Bi content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.
[0043] Regarding the second other element described above, if the content thereof is less than the preferred lower limit value described above, it does not impair the effects of the present invention and is therefore considered to be an unavoidable impurity.
[0044] (Balance) The balance of the composition is composed of Fe and inevitable impurities. Examples of the inevitable impurities include Zn, Pb, and As. The total content of the inevitable impurities is preferably 0.100% or less. In this embodiment, it is preferable that the steel sheet contains only the above-mentioned elements and the balance as the composition, and the balance is Fe (iron) and inevitable impurities.
[0045] <Microstructure> Next, the microstructure (steel structure) of the steel sheet (base steel sheet) will be described.
[0046] (1 / 4 position of plate thickness) First, the microstructure at the 1 / 4 position of the plate thickness of the steel plate will be described.
[0047] ((martensite area ratio m １ The martensite area ratio is also referred to as "M area ratio." Martensite contributes to increasing the strength of steel sheets. From the viewpoint of obtaining a tensile strength of 590 MPa or more, the martensite area ratio at the 1 / 4 position of the sheet thickness of the steel sheet (M area ratio m １ ) is 5% or more, preferably 7% or more, and more preferably 11% or more. On the other hand, if the martensite content is too high, the desired ductility cannot be obtained. Therefore, the M area fraction m １ is 30% or less, preferably 28% or less, and more preferably 26% or less.
[0048] ((Remaining structure)) At the 1 / 4 position of the plate thickness of the steel plate, examples of the structure (remaining structure) other than martensite include ferrite, retained austenite, bainite, pearlite, and cementite. At the 1 / 4 position of the plate thickness of the steel plate, the area ratio of the remaining structure is, for example, 70 to 95%, or alternatively, 72 to 93%, or 74 to 89%.
[0049] (Range from the surface to a depth of 20 μm) Next, the microstructure in the range from the surface to a depth of 20 μm of the steel sheet will be described.
[0050] ((martensite area ratio m ２ From the viewpoint of obtaining a tensile strength of 590 MPa or more, the martensite area ratio (M area ratio m ２ ) is 5% or more, preferably 7% or more, and more preferably 11% or more. On the other hand, if the martensite content is too high, the desired ductility cannot be obtained. Therefore, the M area fraction m ２ is 30% or less, preferably 28% or less, and more preferably 26% or less.
[0051] ((Remaining structure)) In the range from the surface of the steel sheet to a depth of 20 μm, examples of structures other than martensite (remaining structure) include ferrite, retained austenite, bainite, pearlite, and cementite. In the range from the surface of the steel sheet to a depth of 20 μm, the area ratio of the remaining structure is, for example, 70 to 95%, alternatively, 72 to 93%, or alternatively, 74 to 89%.
[0052] ((Prior austenite grain size: 3.0 μm or less)) Austenite is expressed as "γ". Microcracks that occur in a steel sheet (base steel sheet) propagate from prior γ grain boundaries. By finely dispersing martensite present in the surface layer of the steel sheet and dispersing the stress applied to the prior γ grain boundaries, the occurrence of microcracks can be suppressed. For this reason, in order to achieve excellent microcrack resistance, the prior γ grain size (simply referred to as "prior γ grain size") having a martensite area ratio of 50% or more is 3.0 μm or less, preferably 2.7 μm or less, and more preferably 2.0 μm or less. There is no particular lower limit, and it is, for example, 0.5 μm.
[0053] The martensite area ratio is determined as follows. First, a test specimen is taken from a galvannealed steel sheet so that the cross section parallel to the rolling direction of the steel sheet (base steel sheet) and parallel to the sheet thickness direction serves as the observation surface. The coating layer of the test specimen is dissolved and removed using hydrochloric acid containing an inhibitor. Next, the observation surface of the test specimen is mirror-polished and then etched with 3% by volume of nital to reveal the microstructure on the observation surface. Then, a desired area on the observation surface of the test specimen is observed at 3,000x magnification using a scanning electron microscope (SEM). More specifically, five fields of view are observed at a position at 1 / 4 of the sheet thickness (including the position at 1 / 4 of the sheet thickness) and a range 20 μm deep from the surface of the steel sheet, and SEM images are obtained for each. The obtained SEM images are colored according to the structure, and the martensite area ratio (average value of five fields of view) is determined from the pixel count. In the SEM image, martensite appears as a white or light gray structure. Ferrite, for example, is a gray or dark gray structure with smooth grain boundaries and is distinguished from martensite, which includes autotempered martensite containing carbides.
[0054] When determining the prior γ grain size with a martensite area ratio of 50% or more, in addition to the above-mentioned method using SEM, the electron backscatter diffraction (EBSD) method is also used. That is, the prior austenite grain boundaries are identified by the EBSD method. Among the prior γ grains, prior γ grains with a martensite area ratio of 50% or more are selected, and the circle-equivalent diameter of each selected prior γ grain is determined from its area. The average value of the determined circle-equivalent diameters (average value of five fields of view) is taken as the prior γ grain size.
[0055] ((Nb + Ti in precipitates of 100 nm or less: 50 ppm by mass or more)) By dispersing fine precipitates (Nb, Ti precipitates) in the surface layer of the steel sheet, stress concentration at grain boundaries can be prevented and the occurrence of microcracks can be suppressed. For this reason, in order to achieve excellent microcrack resistance, the total content of Nb and Ti in fine precipitates (Nb, Ti precipitates) of 100 nm or less is 50 ppm by mass or more, preferably 100 ppm by mass or more, and more preferably 150 ppm by mass or more. The upper limit is not particularly limited and is, for example, 500 ppm by mass, preferably 400 ppm by mass, and more preferably 300 ppm by mass.
[0056] ((Nb + Ti in precipitates greater than 100 nm: 350 mass ppm or less)) On the other hand, if there are many coarse precipitates (Nb, Ti precipitates) in the surface layer of the steel sheet, stress concentrates on these precipitates, making them prone to become the starting points for microcracks. For this reason, in order to achieve excellent microcrack resistance, the total content of Nb and Ti in coarse precipitates greater than 100 nm (Nb, Ti precipitates) is 350 mass ppm or less, preferably 310 mass ppm or less, more preferably 250 mass ppm or less, even more preferably 200 mass ppm or less, and particularly preferably 150 mass ppm or less. The lower limit is not particularly limited, and is, for example, 10 mass ppm, preferably 30 mass ppm, and more preferably 70 mass ppm.
[0057] The Nb and Ti contents in the precipitates are determined as follows. First, a test piece measuring 20 mm x 50 mm is taken from the galvannealed steel sheet. The plating layer of the test piece is dissolved and removed using hydrochloric acid with an inhibitor added. Next, the test piece is electrolyzed using 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol as an electrolyte for deposit extraction. The amount of electrolysis is determined from the mass loss of the test piece. The electrolysis time is adjusted so that the amount of electrolysis is 20 μm in the depth direction from the surface of the steel sheet. The residue (precipitates) contained in the electrolyte after electrolysis are separated into those with a size of 100 nm or less and those with a size of more than 100 nm using a filter with a pore size of 100 nm. Each precipitate is subjected to acid decomposition, and then the Nb and Ti contents (unit: ppm by mass) are determined using ICP (inductively coupled plasma) atomic emission spectroscopy. Five test pieces are taken from one galvannealed steel sheet, and electrolysis is carried out on each of them, and the average value is adopted.
[0058] (Range from the surface to a depth of 1 μm) Next, the microstructure in the range from the surface to a depth of 1 μm of the steel sheet will be described.
[0059] ((Long Side of Si, Mn Oxide: 200 nm or Less)) Si, Mn oxides are mainly formed near grain boundaries and increase stress concentration at the grain boundaries, so if their size is too large, microcrack resistance deteriorates. Therefore, to achieve excellent microcrack resistance, the long side of Si, Mn oxides present on grain boundaries (simply referred to as "Si, Mn oxides") is 200 nm or less, preferably 180 nm or less, and more preferably 160 nm or less. There is no particular lower limit, and it is, for example, 10 nm, and 30 nm is preferable.
[0060] The long sides of the Si and Mn oxides are determined as follows. First, a sample is collected from a galvannealed steel sheet using a focused ion beam (FIB). The cross section of the sample (specifically, the area from the surface of the steel sheet to a depth of 1 μm) is observed at 10,000 to 30,000 magnifications using a transmission electron microscope (TEM), and the size of the long sides of the Si and Mn oxides present on the grain boundaries is measured. Si and Mn oxides present within 10 nm of the grain boundaries are also considered to be Si and Mn oxides present on the grain boundaries. Here, the grain boundaries are prior austenite grain boundaries, and are not block boundaries, packet boundaries, or lath boundaries. To measure the long sides of the Si and Mn oxides, a rectangle inscribed with the Si and Mn oxides is drawn on the TEM image. Multiple rectangles are drawn for each Si and Mn oxide, and the rectangle with the largest aspect ratio is selected. The long side of the rectangle is determined as the long side of the Si and Mn oxide. The average value of the measurement results from five fields of view is used. Whether or not an observed object on a TEM image is an Si, Mn oxide is determined using an EDX (energy dispersive X-ray analysis) device attached to the TEM. Observed objects with Si or Mn concentrations twice or more higher than that of the parent phase are treated as Si, Mn oxides.
[0061] The area ratio of each structure (martensite, etc.) in the "range from the surface of the steel plate to a depth of 1 μm" is the same as the area ratio of each structure in the "range from the surface of the steel plate to a depth of 20 μm" described above.
[0062] <Galvannealed layer> The high-strength galvannealed steel sheet of this embodiment has a galvannealed layer (galvannealed layer) on the surface of the steel sheet (base steel sheet). The galvannealed layer may be disposed on only one side of the steel sheet, but is preferably disposed on both sides of the steel sheet. The galvannealed layer is formed by a galvannealed coating treatment described later. The coating weight of the galvannealed layer is, for example, 20 to 80 g / m per side. ２ is.
[0063] [Method for manufacturing galvannealed steel sheet] Next, a method for manufacturing a high-strength galvannealed steel sheet of this embodiment will be described. Hereinafter, unless otherwise specified, each temperature refers to the surface temperature of a slab or steel sheet (hot-rolled steel sheet, cold-rolled steel sheet, etc.). In this embodiment, roughly speaking, a slab having the above-described component composition is used to perform hot rolling, cold rolling, annealing, and galvannealed coating treatments. The hot rolling, cold rolling, annealing, and galvannealed coating treatments are not particularly limited except for the conditions described below, and conditions in accordance with conventional methods can be appropriately adopted.
[0064] <Hot Rolling> First, a slab having the above-described composition is hot rolled to obtain a hot rolled steel sheet. The hot rolling conditions are described below.
[0065] <<Slab heating temperature: 1200°C or higher>> If the slab heating temperature is too low, the precipitates (Nb and Ti precipitates) formed during slab casting do not dissolve sufficiently, resulting in an increase in coarse precipitates. In this case, the Nb and Ti contents in the fine precipitates decrease. Therefore, the slab heating temperature is 1200°C or higher, preferably 1210°C or higher, and more preferably 1220°C or higher. The upper limit of the slab heating temperature is not particularly limited and is, for example, 1350°C, preferably 1330°C, and more preferably 1310°C.
[0066] <<Final Reduction: 5% or More>> A large amount of strain is introduced into the surface layer by rolling, which can refine the crystal grains (prior γ grains). Therefore, the final reduction is 5% or more, preferably 6% or more, and more preferably 7% or more. Note that if the final reduction is too high, the rolling load increases, so it is preferably 30% or less, more preferably 25% or less, and even more preferably 20% or less. The final reduction is calculated from the roll gap of the rolling stand used in hot rolling. Specifically, the final reduction is a value calculated using the roll gap (R1) of the final rolling stand and the roll gap (R2) of the rolling stand one step before the final one, using the following formula: (R2-R1) / R2×100
[0067] <<Rolling Completion Temperature: 850 to 970°C>> If the rolling completion temperature is too low, dynamic recrystallization does not occur, and the crystal grains (prior γ grains) cannot be refined. Therefore, the rolling completion temperature is 850°C or higher, preferably 860°C or higher, and more preferably 870°C or higher. On the other hand, if the rolling completion temperature is too high, the crystal grains (prior γ grains) cannot be refined due to grain growth after rolling. Therefore, the rolling completion temperature is 970°C or lower, preferably 950°C or lower, and more preferably 930°C or lower.
[0068] <<Cooling Time: 6.0 s or Less>> During cooling from the final reduction (reduction by the final rolling stand) to 700°C or less, fine precipitates (Nb and Ti precipitates) precipitate in the surface layer. If the cooling time from the final reduction to 700°C or less (simply referred to as "cooling time") is too long, the precipitates grow and become coarse, and the amount of fine precipitates decreases, resulting in coarsening of the prior γ grain size. Therefore, the cooling time is 6.0 s or less, preferably 5.5 s or less, and more preferably 5.0 s or less. The lower limit of the cooling time is not particularly limited and is, for example, 2.0 s, preferably 2.5 s.
[0069] <<Pickling>> The hot-rolled steel sheet obtained by hot rolling is subjected to pickling for the purpose of removing scale and Si and Mn oxides formed by hot rolling. The conditions for pickling are not particularly limited, and the pickling may be performed according to a conventional method.
[0070] <Cold Rolling> Next, the pickled hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The conditions for cold rolling are described below.
[0071] <<Rolling Reduction: 30%>> The strain introduced by rolling refines prior γ grains containing martensite. To achieve this effect, the cold rolling reduction is 30% or more, preferably 35% or more, and more preferably 40% or more. The upper limit of the cold rolling reduction is not particularly limited, and is, for example, 70%, preferably 65%, and more preferably 60%.
[0072] <<Pickling Time: 2.0 s or More>> The cold-rolled steel sheet obtained by cold rolling is also subjected to pickling again in order to remove Si and Mn oxides (to reduce the long sides of the Si and Mn oxides). To achieve this effect, the pickling time is 2.0 s or more, preferably 2.4 s or more, and more preferably 2.8 s or more. The upper limit of the pickling time is not particularly limited, and is, for example, 6.0 s, preferably 5.0 s. Conditions other than the pickling time are not particularly limited, and pickling may be performed according to a conventional method.
[0073] <Annealing> Next, the cold-rolled steel sheet after pickling is subjected to annealing (heat treatment). Annealing conditions will be described below.
[0074] <Heating Rate: 2.0 to 7.0°C / s> By increasing the heating rate from 500°C to 700°C (simply referred to as "heating rate"), prior γ grains containing martensite are refined. To achieve this effect, the heating rate is 2.0°C / s or more, preferably 2.5°C / s or more, and more preferably 3.0°C / s or more. On the other hand, if the heating rate is too fast, ferrite recrystallization becomes insufficient, resulting in too much martensite and reduced ductility. For this reason, the heating rate is 7.0°C / s or less, preferably 6.5°C / s or less, and more preferably 6.0°C / s or less. The heating rate is the average heating rate.
[0075] <Dew point: -40°C or lower> In order to suppress the generation of Si and Mn oxides and reduce their size (long side), the dew point of an atmosphere at 700°C or higher (simply referred to as "dew point") is -40°C or lower, preferably -41°C or lower, and more preferably -42°C or lower. On the other hand, lowering the dew point too much increases costs. For this reason, a dew point of -60°C or higher is preferred. The method for adjusting the dew point inside the furnace is not particularly limited, and examples include a method of removing moisture from the gas introduced into the furnace and the gas circulating inside the furnace using a filter or the like.
[0076] <<Maximum temperature: 740 to 860°C>> The martensite area ratio is controlled by the maximum temperature. To control the martensite area ratio within the above-mentioned range, the maximum temperature (soaking temperature) is 740°C or higher, preferably 760°C or higher, and more preferably 780°C or higher. For the same reason, the maximum temperature is 860°C or lower, preferably 840°C or lower, and more preferably 820°C or lower.
[0077] <Cooling rate v from 530°C to 480°C> １ : 2.0 ° C. / s or less >> At temperatures between 530 ° C. and 480 ° C., bainite transformation occurs and martensite decreases. In order to control the martensite area ratio within the above range, the cooling rate v １ (Simply "cooling rate v １ The cooling rate v is 2.0°C / s or less, preferably 1.8°C / s or less, and more preferably 1.5°C / s or less. １ The lower limit of the rate is not particularly limited, and is, for example, 0.5° C. / s, and preferably 0.8° C. / s.
[0078] <<Cooling rate v from 700 ° C to 600 ° C ２ : 5.0 ° C. / s or less Cooling rate v from 700 ° C. to 600 ° C. ２ (Simply "cooling rate v ２ ") is, for example, 10.0°C / s or less. At this time, by slowly cooling from 700°C to 600°C, ferrite grain growth occurs, and prior γ grains including martensite are further refined. In order to obtain this effect, the cooling rate v ２is preferably 5.0°C / s or less, more preferably 4.5°C / s or less, and even more preferably 4.0°C / s or less. <<CO concentration: 200 volume ppm or less>> The CO concentration in an atmosphere at 700°C or higher (simply referred to as "CO concentration") is, for example, 400 volume ppm or less. At this time, by reducing the CO concentration, coarsening of precipitates (Nb, Ti precipitates) in the surface layer can be further suppressed. To achieve this effect, the CO concentration is 200 volume ppm or less, preferably 180 volume ppm or less, and more preferably 150 volume ppm or less. On the other hand, if the CO concentration is lowered too much, costs will increase. For this reason, the CO concentration is preferably 10 volume ppm or more. The method for adjusting the CO concentration in the furnace is not particularly limited, and can be, for example, by adjusting the CO concentration in the gas introduced into the furnace and the CO (carbon monoxide) and CO circulating in the furnace. ２ One method is to remove carbon dioxide using a filter or the like.
[0079] <Galvannealed Hot-Dip Galvanizing Treatment> A galvannealed cold-rolled steel sheet is subjected to a galvannealed hot-dip galvanizing treatment. This results in a galvannealed hot-dip galvanized steel sheet. In the galvannealed hot-dip galvanizing treatment, a galvannealed hot-dip galvanizing treatment is first carried out. In the galvannealed hot-dip galvanizing treatment, for example, the annealed cold-rolled steel sheet is immersed in a zinc bath (Zn bath). Thereafter, the coating weight of the coating layer may be adjusted as appropriate by gas wiping or the like. For example, the zinc bath may have a component composition containing 0.10 to 0.23 mass% Al, with the balance consisting of Zn and unavoidable impurities. The bath temperature of the zinc bath is, for example, 440 to 500°C.
[0080] Alloying Temperature: 480°C or Higher After the hot-dip galvanizing treatment, alloying (specifically, for example, Zn—Fe alloying) is carried out. The alloying may be carried out according to a conventional method. In this case, the alloying temperature is 480°C or higher, preferably 485°C or higher, and more preferably 490°C or higher. On the other hand, the alloying temperature is preferably 600°C or lower, more preferably 550°C or lower, and even more preferably 530°C or lower.
[0081] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.
[0082] <Production of galvannealed steel sheet> Using a slab having the chemical composition shown in Table 1 below (the balance consisting of Fe and unavoidable impurities), hot rolling and cold rolling were carried out under the conditions shown in Table 2 below to produce a cold-rolled steel sheet having the thickness and width shown in Table 3 below. After hot rolling and cold rolling, the cold-rolled steel sheet was subjected to pickling. Thereafter, the cold-rolled steel sheet was subjected to annealing and galvannealed treatment using continuous hot-dip galvanizing equipment under the conditions shown in Table 2 below to produce a galvannealed steel sheet (plated steel sheet). In the galvannealed treatment, a zinc bath (bath temperature: 470°C) containing 0.14 mass% Al and the balance consisting of Zn and unavoidable impurities was used, and the coating weight of the coating layer was 45 g / m per side. ２ It was adjusted so that
[0083] <Observation of Microstructure> The following items were measured for the obtained plated steel sheets by the above-mentioned methods. The results are shown in Table 3 below. M area ratio m １ : Martensite area ratio at 1 / 4 position of steel plate thickness M area ratio m ２ : martensite area ratio in the range from the surface to a depth of 20 μm of the steel sheet Prior γ grain size: prior γ grain size with a martensite area ratio of 50% or more in the range from the surface to a depth of 20 μm of the steel sheet Nb + Ti in precipitates (100 nm or less): the total content of Nb and Ti in precipitates of 100 nm or less in the range from the surface to a depth of 20 μm of the steel sheet Nb + Ti in precipitates (more than 100 nm): the total content of Nb and Ti in precipitates of more than 100 nm in the range from the surface to a depth of 20 μm of the steel sheet Long side of Si, Mn oxides: the long side of Si, Mn oxides present on grain boundaries in the range from the surface to a depth of 1 μm of the steel sheet
[0084] <Evaluation> The obtained plated steel sheets were subjected to the tests described below to evaluate various properties. The results are shown in Table 3 below.
[0085] Tensile Test No. 5 test pieces according to JIS Z 2241 were taken from the obtained plated steel sheets, with the longitudinal direction (tensile direction) at an angle of 90° to the rolling direction (perpendicular to the rolling direction). Using the taken test pieces, a tensile test in accordance with JIS Z 2241 was carried out five times, and the tensile strength (TS) and total elongation at break (El) were calculated from the average values of the five tests. If TS is 590 MPa or more, the steel sheet can be evaluated as having high strength. If El is 14.0% or more, the steel sheet can be evaluated as having excellent ductility.
[0086] <<Bending Test>> A 30 mm × 100 mm test piece was taken from the width center of the plated steel sheet, with the end faces being ground. The 30 mm side of the test piece was parallel to the rolling direction (L direction) of the steel sheet, and the 100 mm side of the test piece was parallel to the width direction (C direction) of the steel sheet. A 90-degree V-bend test was performed using the taken test piece to form a bent portion in the test piece. As shown in Table 3 below, the bending test conditions included a ratio (R / t) of the bending radius R to the sheet thickness t in the range of 4.9 to 5.1. The test piece was then embedded in resin with the so-called C-section exposed and polished. The bent portion of the C-section of the test piece (including the V-bend apex) was observed using an SEM at 3,000x magnification, and the number of microcracks was counted. At this time, cracks that were connected to cracks in the plating layer and penetrated the steel sheet and had a depth d (see Figure 1) of more than 1 μm were defined as microcracks, and the number of such cracks was measured. The number of microcracks per unit length along the surface of the steel sheet was calculated as the microcrack amount (unit: cracks / mm). Five test pieces were taken from each plated steel sheet, and bending tests were performed on each, and the average value was used. The following criteria were recorded in the "Microcrack Resistance" column in Table 3 below: "A" if the microcrack amount was 25 cracks / mm or less; "B" if it was more than 25 cracks / mm and 50 cracks / mm or less; "C" if it was more than 50 cracks / mm and 75 cracks / mm or less; and "D" if it was more than 75 cracks / mm. A grade of "A" or "B" indicates that the steel sheet has excellent microcrack resistance.
[0087] <<Corrosion Resistance Test After Bending>> First, a 90-degree V-bend test was carried out in the same manner as above (except that the size of the test specimen was 70 mm (L direction) × 100 mm (C direction)). Using the test specimen after the bending test, a salt spray test was carried out for two days in accordance with JIS Z 2371 (2000). Then, the specimen was washed for one minute using chromic acid (concentration: 200 g / L, temperature: 80°C) to remove corrosion products formed by corrosion. The reduction in the plating layer per day (unit: g / (m ２ The reduction in the plating layer was determined by a weight method. ２ If it is less than 15g / (m ２ ・day) or more 25g / (m ２ If it is less than 25g / (m ２ If the corrosion resistance was greater than 100 days, the corrosion resistance was evaluated as "C" and entered in the "Corrosion Resistance" column in Table 3 below. If the corrosion resistance was evaluated as "A" or "B", it can be evaluated as being excellent in corrosion resistance.
[0088]
[0089]
[0090]
[0091] <Summary of Evaluation Results> As shown in Table 3 above, the plated steel sheets Nos. 1 to 3, 5, 8 to 10, 13, 18 to 21, 24, 26 to 27, 30 to 31, 33 to 36, 38 to 40, 43, and 45 to 62 (invention examples) all had a TS of 590 MPa or more, and were excellent in ductility and microcrack resistance as well as corrosion resistance. In contrast, the plated steel sheets Nos. 4, 6 to 7, 11 to 12, 14 to 17, 22 to 23, 25, 28 to 29, 32, 37, 41 to 42, and 44 (comparative examples) were insufficient in at least one of TS, ductility, and microcrack resistance.
[0092] 1: Steel plate 2: Galvannealed layer 3: Microcrack d: Depth of microcrack
Claims
1. A steel sheet comprising: a galvannealed layer disposed on a surface of the steel sheet; wherein the steel sheet has a carbon equivalent Ceq represented by the following formula (1) of 0.370 or more and less than 0.520; a total content of Nb and Ti of 0.010 to 0.080 mass%, with the balance being Fe and unavoidable impurities; a martensite area ratio of 5 to 30% at a 1 / 4 position of the sheet thickness of the steel sheet; and within a range from the surface of the steel sheet to a depth of 20 μm, a martensite area ratio of 5 to 30%, a prior austenite grain size having a martensite area ratio of 50% or more of 3.0 μm or less, a total content of Nb and Ti in precipitates of 100 nm or less of 50 ppm by mass or more, and a total content of Nb and Ti in precipitates of more than 100 nm of 350 ppm by mass or less; A high-strength galvannealed steel sheet, in which the long side of Si, Mn oxides present on grain boundaries in a range from the surface of the steel sheet to a depth of 1 μm is 200 nm or less. Ceq=[C%]+([Si%] / 24)+([Mn%] / 6)+([Ni%] / 40)+([Cr%] / 5)+([Mo%] / 4)+([V%] / 14) (1) In the formula (1), [M%] is the content of element M in the composition in unit mass%, and is 0 when element M is not contained.
2. The high-strength galvannealed steel sheet according to claim 1, wherein the chemical composition further contains, in mass%, C: 0.050 to 0.150%, Si: 0.30% or less, Mn: 1.70 to 3.50%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less.
3. The composition further includes, in mass%, B: 0.0050% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, V: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and The high-strength galvannealed steel sheet according to claim 2, further comprising at least one element selected from the group consisting of Bi: 0.200% or less.
4. A method for producing a high-strength galvannealed steel sheet according to any one of claims 1 to 3, comprising the steps of: hot rolling a slab having the component composition according to any one of claims 1 to 3 under conditions of a slab heating temperature of 1200°C or higher, a final reduction of 5% or higher, a rolling completion temperature of 850 to 970°C, and a cooling time from the final reduction to 700°C or lower of 6.0 seconds or less to obtain a hot-rolled steel sheet, and then pickling the hot-rolled steel sheet; cold rolling the hot-rolled steel sheet after the pickling under conditions of a reduction of 30% or higher to obtain a cold-rolled steel sheet, and then pickling the cold-rolled steel sheet for 2.0 seconds or more; The cold-rolled steel sheet after the pickling is heated at a heating rate of 2.0 to 7.0 ° C. / s from 500 ° C. to 700 ° C., the dew point of the atmosphere at 700 ° C. or higher is −40 ° C. or lower, the maximum temperature is 740 to 860 ° C., and the cooling rate v from 530 ° C. to 480 ° C. １ and subjecting the cold-rolled steel sheet after the annealing to a galvannealing treatment including alloying at a temperature of 480°C or higher.
5. In the annealing, the cooling rate v from 700 ° C to 600 ° C ２ The method for producing a high-strength galvannealed steel sheet according to claim 4, wherein the melting point is 5.0° C. / s or less.
6. The method for producing a high-strength galvannealed steel sheet according to claim 4 or 5, wherein the CO concentration in the atmosphere at 700° C. or higher during the annealing is 200 ppm by volume or less.