High-strength plated steel sheet and method for producing high-strength plated steel sheet

Abstract

Provided is a high-strength plated steel sheet having excellent yield ratio, excellent fatigue resistance, and excellent hole expandability.　The high-strength plated steel sheet has a plating layer on at least one surface of a steel sheet. The steel sheet has a prescribed component composition. The steel structure of the steel sheet is such that the total area ratio of martensite and bainite is 80-100%, the area ratio of pearlite is 0-2%, the area ratio of retained austenite is 0-6%, the number density of precipitates A that are present in the martensite or bainite and that have a particle diameter of 3-20 nm and an aspect ratio of 1.0-2.0 is 1.0 × 10３ to 10.0 × 10３ per μm２; and the number density of a prescribed precipitate group is 1.0 per μm２ or below.

Description

High-strength plated steel sheet and method for manufacturing high-strength plated steel sheet

[0001] The present invention relates to high-strength plated steel sheets and methods for manufacturing high-strength plated steel sheets, and more particularly to high-strength plated steel sheets and methods for manufacturing high-strength plated steel sheets that are suitable as materials for automobile parts.

[0002] From the perspective of improving collision safety and fuel efficiency in automobiles, high-strength steel sheets are required for automotive parts. Among automotive parts, high-strength hot-rolled steel sheets have been widely used for suspension components, but from the perspective of corrosion prevention, the demand for plated materials is also increasing. Suspension components require various properties depending on their shape, but one property that is commonly required for all suspension components is hole-expandability. Furthermore, even if the tensile strength level is increased, a corresponding increase in fatigue strength is not obtained, making it one of the major challenges in increasing the strength of components that require fatigue resistance.

[0003] In response to these demands, various hot-rolled steel sheets and plated steel sheets have been developed, for example, as disclosed in Patent Documents 1 to 4.

[0004] Patent Document 1 discloses a technology relating to a plated steel sheet having a tensile strength of over 590 MPa and excellent yield ratio, elongation, and hole-expanding properties, wherein the material contains, by mass%, C: 0.04% to 0.13%, Si: 0.9% to 2.3%, Mn: 0.8% to 2.4%, P: 0.1% or less, S: 0.01% or less, Al: 0.01% to 0.1%, and N: 0.008% or less, with the remainder being iron and unavoidable impurities, and the structure has an area percentage of 94% or more of ferrite and 2% or less of martensite, the average grain size of the ferrite is 10 μm or less, the Vickers hardness of the ferrite is 140 or more, the average grain size of the carbides present on the grain boundaries of the ferrite is 0.5 μm or less, and the aspect ratio of the carbides present on the grain boundaries of the ferrite is 2.0 or less.

[0005] Patent Document 2 describes a material containing, by mass%, C: 0.03-0.10%, Si: 0.01-1.5%, Mn: 1.0-2.5%, P: 0.1% or less, S: 0.02% or less, Al: 0.01-1.2%, Ti: 0.06-0.15%, and N: 0.01% or less, with the remainder being iron and unavoidable impurities, and having a microstructure consisting of bainite with an area ratio of 40% or more, and the remainder being either or both of ferrite and martensite, with a Ti(C,N) precipitate density of 10 nm or less. １０ pieces / mm ３ In summary, a technology is disclosed relating to a steel sheet with a tensile strength exceeding 590 MPa, which has excellent yield ratio, elongation, and fatigue strength, by setting the ratio (Hvs / Hvc) of the hardness at the center of the sheet thickness (Hvs / Hvc) to 0.85 or higher.

[0006] Patent Document 3 lists the following compositional ratios by mass: C: 0.02-0.12%, Si: 0.01-2.00%, Mn: 1.00-3.00%, P: 0.100% or less, S: 0.010% or less, N: 0.010% or less, Al: 0.005-1.000%, Ti: 0.01-0.20%, Nb: 0-0.10%, V: 0-0.100%, Ni: 0-2.00%, Cu: 0-2.00%, Cr: 0-2.00%, Mo: 0-2.00%, W: 0-0.100%, B: 0-0.0100%, REM: 0-0.0 It contains 300% Ca: 0-0.0300% and Mg: 0-0.0300%, with the remainder being Fe and impurities, and the chemical composition satisfies 0.10 ≤ Ti + Nb + V ≤ 0.45, and the microstructure contains 80% or more tempered martensite by volume, with the remainder being one or more of ferrite, pearlite, bainite, fresh martensite, and retained austenite, and the tempered martensite contains Ti precipitates with an equivalent circle diameter of 5 nm or less, with a quantity of 5 × 10 per unit volume. ９ pieces / mm ３A technology is disclosed relating to a hot-rolled steel sheet having a tensile strength of 980 MPa or more with excellent yield ratio and bendability, by ensuring that the sum of the average polar density of the orientation group consisting of {211}<111> to {111}<112> and the polar density of the crystal orientation {110}<001> is 6.0 or less in the surface region, which is the range from the surface to a position 1 / 10 of the sheet thickness.

[0007] Patent Document 4 lists the following by mass%, C: 0.05-0.30%, Si: 0.5-3.0%, Mn: 1.7-3.0%, P: 0.020% or less, S: 0.010% or less, Al: 0.005-1.00%, N: 0.0010-0.0500%, Ti: 0-0.30%, Nb: 0-0.30%, V: 0-0.50%, Cr: 0-3.0%, Mo: 0-3.0%, Ni: 0-5.0%, Cu: 0-3.0%, B: 0-0.0100%, Ca: 0-0.0100%, Mg: 0-0.01 It contains 00%, Zr: 0-0.0500%, and REM: 0-0.0500%, with the remainder being Fe and impurities. The structure consists of, by volume fraction, at least 70% tempered martensite and bainite, at least 8% retained austenite, 0-10% fresh martensite, and 0-10% ferrite. The difference between the maximum and minimum Vickers hardness values, ΔHv, is Hv 50 or less. The zinc plating layer contains less than 7.0% by mass of Fe and 0.5-2.0 g / m². ２ A technology is disclosed relating to a galvanized steel sheet having a tensile strength of 980 MPa or more, which has excellent elongation, hole-expanding properties, and fatigue resistance due to the inclusion of Ni.

[0008] Japanese Patent Publication No. 2012-36497, International Publication No. 2010 / 137317, International Publication No. 2021 / 131876, International Publication No. 2020 / 203943

[0009] However, while the technology in Patent Document 1 exhibits excellent properties, it lacks knowledge or suggestions regarding tensile strength (TS) exceeding 980 MPa, and fatigue resistance has not been considered, leaving room for improvement. The technology in Patent Document 2 yields steel sheets with a TS of 590 MPa that exhibit excellent properties, but hole expansion properties are not considered, leaving room for improvement. The technology in Patent Document 3 does not consider fatigue resistance at all, leaving room for improvement. Furthermore, Patent Document 3 does not disclose the aspect ratio of Ti-containing micro-precipitations with an equivalent circle diameter of 5 nm or less in the microstructure. However, since Patent Document 3 forms the micro-precipitations at a low temperature of 700°C or less, the micro-precipitations are considered to have a large aspect ratio and a large misfit (non-match) with the matrix phase. Since such micro-precipitations tend to form stress fields around them, it is thought that hot-rolled steel sheets obtained by the technology in Patent Document 3 have low toughness, which can lead to problems during steel sheet manufacturing and pressing processes, among other issues unrelated to material properties. Although the technology described in Patent Document 4 has produced a TS980MPa+ grade steel sheet with excellent elongation, hole-expanding properties, and fatigue resistance, it is thought that the low yield ratio is due to the large amount of retained austenite required to achieve the elongation, and there is room for improvement.

[0010] This invention has been made in view of the above circumstances, and aims to provide a high-strength plated steel sheet having an excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties.

[0011] Furthermore, the present invention aims to provide a method for manufacturing the above-mentioned high-strength plated steel sheet.

[0012] Here, in the present invention, "high strength" means that the tensile strength (TS) is 980 MPa or more. In the present invention, "excellent yield ratio" means that the ratio of the yield strength (YS) to TS (YS / TS) is 0.80 or more. The above tensile strength and yield strength can be obtained by a tensile test. For details of the tensile test, reference can be made to the description of the examples described later. In the present invention, "excellent fatigue resistance characteristics" means that the ratio of the fatigue strength to TS (fatigue strength / TS) is 0.55 or more. The above fatigue strength can be obtained by a fatigue test. For details of the fatigue test, reference can be made to the description of the examples described later. In the present invention, "excellent hole expansion property" means that the hole expansion rate is 40% or more. The above hole expansion rate can be obtained by a hole expansion test. For details of the hole expansion test, reference can be made to the description of the examples described later.

[0013] In order to solve the above problems, the present inventors focused on the hard matrix and precipitates therein, and conceived to improve the yield ratio, fatigue resistance characteristics, and hole expansion property of the steel sheet by controlling the form and amount of the precipitates. As a result, the following was found. After adjusting the chemical composition of the steel sheet to a specific range, a structure composed of martensite and bainite is used as the main phase, the area ratio of pearlite is limited to 0 to 2%, and the area ratio of retained austenite is limited to the range of  0 to 6%, and further spherical fine precipitates are dispersed in the main phase. Thereby, excellent yield ratio, excellent fatigue resistance characteristics, and excellent hole expansion property can be obtained. In addition, in order to obtain the structure and precipitates, it was found that it is necessary to transform the precipitates into bainite or martensite after once arranging them in austenite single phase, and the present invention has been completed.

[0014] The present invention has the following gist. [1] Having a plating layer on at least one side of a steel sheet, the steel sheet has, in mass %, C: 0.05 to 0.15%, Si: 0.01 to 2.0%, Mn: 1.5 to 3.5%, P: 0.100% or less, S: 0.020% or less, Al: 1.0% or less, and Ti: 0.050 to 0.20%, and the balance consists of Fe and inevitable impurities. The steel structure of the steel sheet has a total area ratio of martensite and bainite of 80 to 100%, a perlite area ratio of 0 to 2%, a retained austenite area ratio of 0 to 6%, and the number density of precipitate A with a particle size of 3 to 20 nm and an aspect ratio of 1.0 to 2.0 present in martensite or bainite is 1.0×10 ３ to 10.0×10 ３ pieces / μm ２ and the number density of the precipitate group defined by the following requirement 1 and requirement 2 is 1.0 piece / μm ２The following is a high-strength plated steel sheet. Requirement 1: The number n of precipitates A that form a precipitate group is 6 ≤ n ≤ 40. Requirement 2: For any precipitate i and precipitate j among precipitates A, precipitate j whose centroid-to-centroid distance from precipitate i satisfies the following equation (1) shall be considered to belong to the same precipitate group as precipitate i. Lij ≤ 1.5Di + 1.5Dj ... (1) Here, in equation (1), Lij is the centroid-to-centroid distance (nm) between precipitate i and precipitate j, Di is the particle size (nm) of precipitate i, and Dj is the particle size (nm) of precipitate j. [2] The high-strength plated steel sheet according to [1], wherein the component composition further comprises, in mass%, one or more selected from Nb: 0.005 to 0.20%, Cr: 0.005 to 2.0%, Cu: 0.05 to 4.0%, Ni: 0.005 to 2.0%, Mo: 0.05 to 2.0%, V: 0.05 to 1.0%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, and Sn: 0.0010 to 0.50%. [3] A steel material heating step of heating a steel material having the component composition described in [1] or [2] above to 1150°C or higher; a hot rolling step of roughly rolling the steel material after the steel material heating step, then finishing rolling it under conditions where the final pass temperature is 850 to 1000°C, then cooling it in the temperature range up to 500°C at an average cooling rate of 50°C / s or more, and winding it up to 500°C; a pickling step of pickling the steel sheet after the hot rolling step; an annealing step of heating the steel sheet after the pickling step in the temperature range of 550 to 850°C at an average heating rate of 20°C / s or more, holding it in the temperature range of 850 to 1000°C for 10 seconds or less, and then cooling it in the temperature range up to 550°C at an average cooling rate of 10°C / s or more. A method for manufacturing a high-strength plated steel sheet, comprising a heat treatment step of applying a plating treatment to the steel sheet after the annealing step, or applying a plating alloying treatment after the plating treatment, and then applying a cooling treatment in which the cooling stop temperature is 400°C or lower, wherein in the heat treatment step, the residence time in the temperature range of 500 to 550°C is 60 s or less.

[0015] According to the present invention, it is possible to provide a high-strength plated steel sheet having an excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties, as well as a method for manufacturing the high-strength plated steel sheet.

[0016] The high-strength plated steel sheet of the present invention is suitable as a material for automobile parts. By using the high-strength plated steel sheet of the present invention as a material for automobile parts, products such as high-strength automobile parts can be obtained while reducing processing cracks.

[0017] Hereinafter, the high-strength plated steel sheet and the manufacturing method of the high-strength plated steel sheet according to an embodiment of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments.

[0018] <High-strength plated steel sheet> The high-strength plated steel sheet according to this embodiment is a plated steel sheet that can be manufactured in a plating line after hot rolling and pickling. The type of plating may be any. Also, the high-strength plated steel sheet according to this embodiment preferably has a plate thickness of 0.6 mm or more and 4.0 mm or less. The plate thickness may be 1.0 mm or more. Also, the plate thickness may be 3.0 mm or less. When used as a material for automobile parts, the plate thickness is more preferably 1.0 mm or more and 3.0 mm or less. Also, the plate width of the high-strength plated steel sheet according to this embodiment is preferably 500 mm or more and 1800 mm or less. The plate width is more preferably at least 700 mm. Also, the plate width is more preferably 1400 mm or less.

[0019] The high-strength plated steel sheet of the present invention has a plating layer on at least one side of a steel sheet (base steel sheet). The steel sheet (base steel sheet) has a specific component composition and a specific steel structure. Here, the component composition and the steel structure of the steel sheet will be described in this order.

[0020] First, the component composition of the steel sheet of the present invention will be described. Note that "%" representing the content of the component composition means "mass%".

[0021] The component composition of the steel sheet of the present invention is, in mass%, C: 0.05 to 0.15%, Si: 0.01 to 2.0%, Mn: 1.5 to [3.5%], P: 0.100% or less, S: 0.020% or less, Al: 1.0% or less, and Ti: 0.050 to 0.20%, and the balance consists of Fe and inevitable impurities.

[0022] It should be noted that the "%" in the original text is assumed to be "mass%" for the purpose of translation. Also, there seems to be a possible error in the "Mn: 1.5 to [3.5%]" in the original text, which is presented as it is here. If this is an incorrect value, it should be corrected in the original text for a more accurate translation.C: 0.05-0.15% C is an effective element for increasing TS by generating and strengthening bainite and martensite. If the C content is less than 0.05%, such effects cannot be sufficiently obtained, and a TS of 980 MPa or higher cannot be obtained. On the other hand, if the C content exceeds 0.15%, the hardening of martensite becomes significant, and the excellent hole-expanding properties of the present invention cannot be obtained. Therefore, the C content should be 0.05-0.15%. From the viewpoint of obtaining a TS of 980 MPa or higher more stably, the C content is preferably 0.06% or higher. From the viewpoint of improving hole-expanding properties, the C content is preferably 0.14% or less, and more preferably 0.12% or less.

[0023] Si: 0.01-2.0% Si is an effective element for increasing TS by solid solution strengthening of steel and suppressing tempering softening of martensite. To obtain such effects, the Si content must be 0.01% or more. On the other hand, if the Si content exceeds 2.0%, excess ferrite remains, and the steel structure of the present invention cannot be obtained. Therefore, the Si content should be 0.01-2.0%. The Si content is preferably 0.1% or more, more preferably 0.2% or more. Furthermore, the Si content is preferably 1.5% or less, more preferably 1.0% or less.

[0024] Mn: 1.5-3.5% Mn is an effective element for increasing the yield ratio and TS by generating martensite and bainite. If the Mn content is less than 1.5%, these effects are not sufficiently obtained, and polygonal ferrite and pearlite are formed, making it impossible to obtain the steel structure of the present invention. On the other hand, if the Mn content exceeds 3.5%, inclusions increase or the steel becomes brittle, making it impossible to obtain the excellent fatigue resistance and excellent hole expansion properties of the present invention. Therefore, the Mn content should be 1.5-3.5%. From the viewpoint of stably obtaining bainite and martensite, the Mn content is preferably 1.6% or more, and more preferably 1.8% or more. Also from the above viewpoint, the Mn content is preferably 3.2% or less, and more preferably 2.9% or less.

[0025] P: 0.100% or less (excluding 0%) Since excessive P content reduces hole-expanding properties, it is preferable to reduce the P content as much as possible. In this invention, a P content of up to 0.100% is acceptable. Therefore, the P content should be 0.100% or less. Preferably, the P content should be 0.030% or less. No lower limit is specifically defined. The P content may be greater than 0%, but since a P content of less than 0.001% leads to a decrease in production efficiency, a P content of 0.001% or more is preferred.

[0026] S: 0.020% or less (excluding 0%) Since sulfur reduces fatigue resistance and hole expansion properties, it is preferable to reduce the sulfur content as much as possible. In this invention, an sulfur content of up to 0.020% is acceptable. Therefore, the sulfur content shall be 0.020% or less. The sulfur content shall preferably be 0.0050% or less, and more preferably 0.0020% or less. No lower limit is particularly specified. The sulfur content may be greater than 0%, but since an sulfur content of less than 0.0002% leads to a decrease in production efficiency, an sulfur content of 0.0002% or more is preferred.

[0027] Al: 1.0% or less (excluding 0%) Al acts as a deoxidizing agent and is preferably added in the deoxidation process. The Al content may be greater than 0%, but from the viewpoint of using it as a deoxidizing agent, an Al content of 0.01% or more is preferable. On the other hand, if a large amount of Al is included, a large amount of polygonal ferrite will be formed, and the steel structure of the present invention cannot be obtained. In the present invention, an Al content of up to 1.0% is permissible. Therefore, the Al content should be 1.0% or less. The Al content should preferably be 0.50% or less, and more preferably 0.30% or less.

[0028] Ti: 0.050-0.20% Ti is an element necessary for the formation of precipitates (also referred to as precipitate A in this invention) with a particle size of 3-20 nm and an aspect ratio of 1.0-2.0 in the steel structure of the present invention. When the Ti content is less than 0.050%, the number density of precipitate A is 1.0 × 10⁻⁶. ３ pieces / μm ２ The above-mentioned steel structure cannot be obtained. On the other hand, when the Ti content exceeds 0.20%, the number density of precipitate A is 10.0 × 10 ３ pieces / μm ２The following, and the number density of a specified precipitate group: 1.0 particles / μm ２ The following steel structures cannot be obtained. Therefore, the Ti content should be 0.050 to 0.20%. Preferably, the Ti content should be 0.070% or more. Furthermore, preferably, the Ti content should be 0.160% or less, more preferably 0.140% or less, and even more preferably 0.120% or less.

[0029] The remainder may consist of Fe and unavoidable impurities. In this invention, unavoidable impurities may include N in an amount of 0.01% or less. The N content is preferably 0.008% or less. On the other hand, there is no particular lower limit for the N content, but since it is difficult to make the N content 0%, the N content may be greater than 0%.

[0030] The above describes the basic component composition of the steel sheet (base steel sheet) of the present invention. In the present invention, the component composition of the steel sheet may further optionally contain the following components.

[0031] One or more elements selected from Nb: 0.005–0.20%, Cr: 0.005–2.0%, Cu: 0.05–4.0%, Ni: 0.005–2.0%, Mo: 0.05–2.0%, V: 0.05–1.0%, B: 0.0002–0.0050%, Ca: 0.0001–0.0050%, REM: 0.0001–0.0050%, Sb: 0.0010–0.10%, and Sn: 0.0010–0.50%.

[0032] Nb is an effective element that forms precipitates and contributes to improving yield ratio and fatigue resistance. To obtain such effects, when Nb is included, it is preferable that the Nb content be 0.005% or more. On the other hand, if the Nb content exceeds 0.20%, the precipitates may become coarser, which may lead to a decrease in yield ratio and fatigue resistance. Therefore, when Nb is included, it is preferable that the Nb content be 0.20% or less. More preferably, the Nb content is 0.010% or more. Furthermore, more preferably, the Nb content is 0.10% or less, and even more preferably 0.050% or less.

[0033] Cr, Cu, Ni, Mo, and V are effective elements that contribute to high strength by generating martensite. To obtain such effects, when Cr, Cu, Ni, Mo, and V are included, it is preferable that the content of each is above the lower limit value. On the other hand, if the content of each of Cr, Cu, Ni, Mo, and V exceeds the upper limit value, the steel may become brittle, and the excellent hole expansion ratio of the present invention may not be obtained. For this reason, when Cr, Cu, Ni, Mo, and V are included, it is preferable that the content of each is within the above range. The Cr content is more preferably 0.10% or more. The Cr content is also more preferably 0.6% or less. The Cu content is more preferably 0.10% or more. The Cu content is also more preferably 0.6% or less, and even more preferably 0.3% or less. The Ni content is more preferably 0.10% or more. The Ni content is also more preferably 0.6% or less. The Mo content is more preferably 0.10% or more. Furthermore, the Mo content is more preferably 0.6% or less, and even more preferably 0.3% or less. The V content is more preferably 0.10% or more. Furthermore, the V content is more preferably 0.3% or less, and even more preferably 0.2% or less.

[0034] B is an effective element that enhances the hardenability of steel plates, promotes martensite formation, and contributes to increased strength and improved hole-expandability. To obtain these effects, when B is included, it is preferable that the B content be 0.0002% or more. On the other hand, if the B content exceeds 0.0050%, the amount of B-based compounds increases, the hardenability decreases, and the steel structure of the present invention may not be obtained. Therefore, when B is included, it is preferable that the B content be 0.0002 to 0.0050%. The B content is more preferably 0.0005% or more. Furthermore, the B content is more preferably 0.0040% or less.

[0035] Ca and REM (rare earth metals) are elements that are effective in improving bendability by controlling the morphology of inclusions. To obtain this effect, when Ca and REM are included, it is preferable that their respective contents be 0.0001% or more. On the other hand, if the Ca and REM contents each exceed 0.0050%, the amount of inclusions may increase and the hole-expanding properties may deteriorate. Therefore, when Ca and REM are included, it is preferable that the Ca and REM contents be Ca: 0.0001 to 0.0050% and REM: 0.0001 to 0.0050%. The Ca content is more preferably 0.0005% or more. Furthermore, the Ca content is more preferably 0.0030% or less. The REM content is more preferably 0.0005% or more. Furthermore, the REM content is more preferably 0.0030% or less. Note that REM is a collective term for 17 elements in total, including Sc, Y, and lanthanide elements. REM can contain one or more of these 17 elements. Furthermore, the REM content referred to here is the total content of these elements.

[0036] Sb is an element that effectively suppresses the decline in steel's strength and fatigue resistance by inhibiting denitrification, deboration, etc. To obtain such effects, it is preferable that the Sb content be 0.0010% or more when Sb is included. On the other hand, if the Sb content exceeds 0.10%, it may lead to embrittlement of the steel plate. Therefore, when Sb is included, it is preferable that the Sb content be between 0.0010% and 0.10%. More preferably, the Sb content is 0.0050% or more. Furthermore, it is even more preferable that the Sb content be 0.050% or less.

[0037] Sn is an element that is effective in suppressing pearlite formation and preventing a decrease in steel strength. To obtain this effect, when Sn is included, it is preferable that the Sn content be 0.0010% or more. On the other hand, if the Sn content exceeds 0.50%, the steel sheet may become brittle, leading to a decrease in hole-expanding properties. Therefore, when Sn is included, it is preferable that the Sn content be between 0.0010% and 0.50%. More preferably, the Sn content is 0.0050% or more. Furthermore, it is even more preferable that the Sn content be 0.10% or less, and even more preferably 0.050% or less.

[0038] Furthermore, even if the content of Nb, Cr, Cu, Ni, Mo, V, B, Ca, REM, Sb, and Sn is below the above lower limit, the effects of the present invention will not be impaired. Therefore, if the content of these components is below the above lower limit, these elements will be treated as unavoidable impurities.

[0039] Next, the microstructure of the steel sheet of the present invention will be described. The microstructure of the steel sheet of the present invention has a total area ratio of martensite and bainite of 80 to 100%, an area ratio of pearlite of 0 to 2%, and an area ratio of retained austenite of 0 to 6%. In addition, the number density of precipitate A present in the martensite or bainite, with a particle size of 3 to 20 nm and an aspect ratio of 1.0 to 2.0, is 1.0 × 10⁻⁶. ３ ~10.0 x 10 ３ pieces / μm ２ Furthermore, the number density of a predetermined precipitate group is 1.0 particles / μm ２ The following applies:

[0040] Total area ratio of martensite and bainite: 80-100% In this invention, in order to ensure high TS, excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties, the main phase is a structure consisting of hard martensite and bainite. Note that precipitates in the steel structure of the steel sheet of this invention have little effect on the TS of the steel sheet, and high TS is achieved by strengthening the structure with hard martensite and bainite. If the total area ratio of martensite and bainite to the entire steel structure of the steel sheet is less than 80%, at least one of the high TS, excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties of this invention cannot be obtained. Therefore, the total area ratio of martensite and bainite is set to 80-100%. The above total area ratio is preferably 85% or more, and more preferably 90% or more.

[0041] Perlite area ratio: 0-2% Perlite has a large elastic strain and reduces hole-expanding properties, so it is preferable to reduce it as much as possible, but in this invention, up to 2% is permissible. If the perlite area ratio exceeds 2%, the excellent hole-expanding properties of this invention cannot be obtained. Therefore, the perlite area ratio should be 0-2%. The perlite area ratio is preferably 0-1%, and more preferably 0%.

[0042] Area ratio of retained austenite: 0-6% Retained austenite reduces the yield ratio and hole-expanding properties by lowering the YS ratio, so it is preferable to reduce it as much as possible. However, in the present invention, up to 6% is acceptable for the purpose of improving ductility, etc. If the area ratio of retained austenite exceeds 6%, the excellent yield ratio and excellent hole-expanding properties of the present invention cannot be obtained. Therefore, the area ratio of retained austenite is set to 0-6%. The area ratio of retained austenite is preferably 0-4%, more preferably 0-3%, and even more preferably 0-2%.

[0043] The number density of precipitate A present in martensite or bainite, with a particle size of 3-20 nm and an aspect ratio of 1.0-2.0, is 1.0 × 10⁻¹⁴. ３ ~10.0 x 10 ３ pieces / μm ２ In a hard martensite or bainite, precipitate A with a particle size of 3-20 nm and an aspect ratio of 1.0-2.0 is placed in a 1.0 × 10⁻¹⁴ layer. ３ pieces / μm ２ By arranging the precipitates as described above, it is possible to achieve both the excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties of the present invention. Although the detailed mechanism is not clear, it is thought that precipitate A hinders dislocation movement, thereby influencing the increase in yield ratio and the suppression of fatigue crack initiation. Furthermore, it is thought that precipitate A, due to its small aspect ratio, has a small misfit with the matrix phase around the precipitate, thus achieving both an increase in yield ratio, suppression of fatigue crack initiation, and high hole-expanding properties. On the other hand, the number density of precipitate A is 10.0 × 10⁻⁶. ３ pieces / μm ２Beyond this point, the decrease in pore-expanding properties becomes significant. Therefore, the number density of precipitate A present in martensite or bainite, with a particle size of 3 to 20 nm and an aspect ratio of 1.0 to 2.0, is 1.0 × 10⁻⁶. ３ ~10.0 x 10 ３ pieces / μm ２ The number density of precipitate A is preferably 1.5 × 10⁻⁶. ３ pieces / μm ２ The above is preferable, and more preferably 2.0 × 10 ３ pieces / μm ２ The above is complete. Furthermore, the number density of precipitate A is preferably 8.0 × 10⁻⁶. ３ pieces / μm ２ The following, more preferably 6.0 × 10 ３ pieces / μm ２ The following applies. Note that most precipitates in the steel structure of the present invention have an aspect ratio of 1.0 to 2.0, and hardly any precipitates with an aspect ratio greater than 2.0 are present. If the main precipitates are those with an aspect ratio greater than 2.0 and large misfit strains, it becomes difficult to achieve both of the above properties (excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties). Furthermore, low toughness can lead to problems such as fracture during steel plate manufacturing or press working, making it more susceptible to issues unrelated to material properties. Therefore, in the present invention, precipitates with an aspect ratio greater than 2.0 are reduced as much as possible.

[0044] Number density of precipitate groups defined in requirements 1 and 2 below: 1.0 particles / μm ２The following is requirement 1: The number n of precipitates A forming a precipitate group is 6 ≤ n ≤ 40. Requirement 2: For any precipitates i and j among precipitates A, precipitate j whose centroid-to-centroid distance from precipitate i satisfies the following equation (1) shall be considered to belong to the same precipitate group as precipitate i. Lij ≤ 1.5Di + 1.5Dj ... (1) Here, in equation (1), Lij is the centroid-to-centroid distance (nm) between precipitate i and precipitate j, Di is the particle size (nm) of precipitate i, and Dj is the particle size (nm) of precipitate j. The above precipitate group is formed by the generation and aggregation of precipitates A during the heating process of the annealing, and by setting these to a predetermined number density, the excellent fatigue resistance characteristics of the present invention can be obtained. Although the detailed mechanism is not clear, it is thought that when precipitate A aggregates, dislocation movement in the non-aggregated region becomes easier, and the fatigue crack initiation suppression effect of precipitate A is reduced. Therefore, the number density of the precipitate group defined in requirements 1 and 2 above is 1.0 particles / μm ２ The following applies: The number density of the precipitate group is preferably 0.7 particles / μm ２ The following, more preferably 0.3 particles / μm ２ The following applies: The lower limit of the number density of the precipitate group is not limited. The number density of the precipitate group is 0 particles / μm ２ That's fine.

[0045] As described above, precipitates i and j that satisfy formula (1) belong to the same precipitate group. To explain in more detail, for example, if we consider any precipitate k from precipitate A that is different from precipitates i and j, then precipitates i and j satisfy formula (1), and precipitate k satisfies one or more of the following formulas (2) and (3), then precipitates i, j, and k belong to the same precipitate group. Lik ≤ 1.5Di + 1.5Dk ... (2) Ljk ≤ 1.5Dj + 1.5Dk ... (3) Here, in equation (2), Lik is the distance between the centroids of precipitate i and precipitate k (nm), Di is the particle size of precipitate i (nm), and Dk is the particle size of precipitate k (nm). In equation (3), Ljk is the distance between the centroids of precipitate j and precipitate k (nm), Dj is the particle size of precipitate j (nm), and Dk is the particle size of precipitate k (nm).

[0046] Furthermore, structures other than the martensite, bainite, pearlite, and retained austenite mentioned above are ferrite. The total area percentage of structures other than martensite and bainite (main phase) should be 20% or less. The mechanical properties of the present invention can be achieved if the total area percentage of structures other than the main phase is 20% or less.

[0047] In this invention, the area ratio of each of the above-described structures can be measured by the method described in the examples below. The above-described steel structures are those at the 1 / 4 thickness position, as also described in the examples.

[0048] The high-strength plated steel sheet of the present invention comprises a plating layer on at least one side of the steel sheet (base steel sheet). The high-strength plated steel sheet of the present invention may have a plating layer on only one side of the steel sheet, or it may have a plating layer on both sides of the steel sheet. The type of plating layer is not particularly limited and may be, for example, a hot-dip galvanized layer or an electroplated layer. The plating layer may also be an alloyed plating layer. A zinc plating layer is preferred as the plating layer. The zinc plating layer may contain Al or Mg.

[0049] Furthermore, the composition of the plating layer is not particularly limited and can be, for example, a known composition. In the case of a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, the composition of the plating layer can be, for example, one containing Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further containing one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0% by mass or more and 3.5% by mass or less, with the remainder being Zn and unavoidable impurities. Also, the amount of plating adhesion is not particularly limited, but for example, the amount of plating adhesion per side can be 20 to 80 g / m². ２ It can be done this way.

[0050] The tensile strength of the high-strength plated steel sheet of the present invention is 980 MPa or higher. While there is no particular upper limit to the tensile strength, one example is 1470 MPa or lower. The yield ratio of the high-strength plated steel sheet of the present invention is 0.80 or higher. Preferably, the yield ratio is 0.82 or higher. While there is no particular upper limit to the yield ratio, one example is 0.96 or lower. The fatigue resistance of the high-strength plated steel sheet of the present invention is σw / TS of 0.55 or higher in the fatigue test described later. The hole-expanding properties of the high-strength plated steel sheet of the present invention are 40% or higher in the hole-expanding properties test described later.

[0051] <Method for Manufacturing High-Strength Plated Steel Sheet> The high-strength plated steel sheet of the present invention has a steel material heating step in which a steel material having the above component composition is heated to 1150°C or higher. The steel material after the steel material heating step is roughly rolled, then finish rolled under conditions where the final pass temperature is 850 to 1000°C, then cooled in the temperature range up to 500°C at an average cooling rate of 50°C / s or more, and wound up at 500°C or lower in a hot rolling step. The steel sheet after the hot rolling step is pickled in a pickling step. The steel sheet after the pickling step is heated in the temperature range of 550 to 850°C at an average heating rate of 20°C / s or more, held in the temperature range of 850 to 1000°C for 10 seconds or less, and then cooled in the temperature range up to 550°C at an average cooling rate of 10°C / s or more in an annealing step. Furthermore, the process includes a heat treatment step in which the steel sheet after the annealing step is plated, or further plated alloyed after the plating step is performed, and then a cooling treatment is performed with a cooling stop temperature of 400°C or lower. In the heat treatment step, the residence time in the temperature range of 500 to 550°C is set to 60 seconds or less.

[0052] The method for manufacturing high-strength plated steel sheets according to the present invention will be described in detail below. The temperatures mentioned above refer to the temperature (surface temperature) of the steel material (slab, etc.) or the center of the width of the steel sheet, and the average cooling rate and average heating rate mentioned above refer to the average cooling rate and average heating rate of the center of the width of the steel sheet, respectively. These temperatures can be measured using a radiation thermometer or the like. Also, X １ From °C (cooling start temperature) to X ２ °C (X １ >X ２The average cooling rate in the temperature range of (X １ ℃-X ２ (℃) / (X １ From ℃ to X ２ It is determined by the cooling time (s) to reach °C. １ From ℃ to Y ２ ℃ (Y １ <Y ２ The average heating rate in the temperature range of (Y) is ２ ℃-Y １ (℃) / (Y １ From ℃ to Y ２ It is determined by the heating time (s) to reach °C.

[0053] First, steel having the above-mentioned component composition is melted using known methods such as converters, electric furnaces, and vacuum melting furnaces, and then cast using known methods such as continuous casting or ingot-fragmentation to obtain steel material (cast slabs, etc.).

[0054] Heating temperature of steel material: 1150°C or higher In the steel material heating process, the steel material having the above component composition is heated to 1150°C or higher. If the heating temperature of the steel material is less than 1150°C, the precipitate will not be sufficiently dissolved, and the predetermined precipitate (precipitate A) will not be obtained. Therefore, the heating temperature of the steel material should be 1150°C or higher. Preferably, the heating temperature of the steel material should be 1170°C or higher, more preferably 1200°C or higher, and even more preferably 1250°C or higher. There is no upper limit specified, but if the heating temperature of the steel material exceeds 1350°C, it may cause damage to the furnace, so it is preferable that the heating temperature of the steel material be 1350°C or lower.

[0055] Final pass temperature: 850 to 1000°C The steel material after the heating process is roughly rolled, and then finish-rolled under conditions where the final pass temperature is 850 to 1000°C. The conditions for rough rolling are not particularly limited, and known conditions can be applied, for example. If the final pass temperature of the finish rolling is less than 850°C, excessive rolling strain remains, causing ferrite and pearlite to form in the steel sheet (hot-rolled sheet) after finish rolling, as well as the formation and coarsening of precipitates, making it impossible to obtain the steel structure of the present invention after annealing. On the other hand, if the final pass temperature of the finish rolling exceeds 1000°C, the amount of coarse precipitates formed during hot rolling becomes excessive and remains even after the subsequent annealing process, making it impossible to obtain the predetermined precipitate (precipitate A). Therefore, the final pass temperature of the finish rolling is set to 850 to 1000°C. The final pass temperature is preferably 870°C or higher, and more preferably 890°C or higher. Furthermore, the final pass temperature is preferably 980°C or lower, and more preferably 960°C or lower. The final pass temperature refers to the temperature at the exit of the final pass (final rolling pass).

[0056] Average cooling rate in the temperature range up to 500°C: 50°C / s or more If the cooling rate in the temperature range up to 500°C is slow, excessive ferrite, retained austenite, etc. will be formed, or precipitates will be formed and coarsened, making it impossible to obtain the steel structure of the present invention. Therefore, the average cooling rate in the temperature range from immediately after the final pass rolling of finish rolling up to 500°C should be 50°C / s or more. The average cooling rate should preferably be 60°C / s or more, and more preferably 70°C / s or more. Furthermore, there is no particular upper limit to the average cooling rate, but as an example, from the viewpoint of the shape stability of the steel sheet, the average cooling rate should preferably be 500°C / s or less.

[0057] Winding temperature: 500°C or less. If the winding temperature exceeds 500°C, ferrite and pearlite will form, or precipitates will form and coarse, making it impossible to obtain the steel structure of the present invention. Therefore, the winding temperature should be 500°C or less. Preferably, the winding temperature should be 450°C or less, and more preferably 400°C or less. There is no lower limit, but since special equipment is required to keep it below room temperature, the winding temperature should preferably be room temperature or higher. For example, room temperature is 25°C.

[0058] Next, the steel sheet (hot-rolled sheet) after the hot-rolling process is subjected to pickling (pickling process). The pickling conditions in the pickling process are not particularly limited, and known conditions can be applied, for example.

[0059] Average heating rate in the 550-850°C temperature range: 20°C / s or more Next, in the annealing process, the steel sheet after the pickling process is heated in the 550-850°C temperature range at an average heating rate of 20°C / s or more, held in the 850-1000°C temperature range for 10 seconds or less, and then cooled in the temperature range up to 550°C at an average heating rate of 10°C / s or more. If the average heating rate in the 550-850°C temperature range is less than 20°C / s, aggregation of precipitates will occur and the predetermined number density of precipitate groups cannot be obtained. Therefore, the average heating rate in the 550-850°C temperature range should be 20°C / s or more. The average heating rate is preferably 30°C / s or more, more preferably 50°C / s or more. Furthermore, there is no particular upper limit to the average heating rate, but as an example, the average heating rate is preferably 500°C / s or less.

[0060] Annealing temperature: 850 to 1000°C After heating in the above temperature range of 550 to 850°C at an average heating rate of 20°C / s or more, the annealing temperature is set to the range of 850 to 1000°C, and annealing is performed while holding the material in this temperature range for 10 seconds or less. In this invention, by setting the annealing temperature to 850°C or higher, the microstructure during annealing is made into a single austenite phase while reducing fine precipitates with large misfits, and spherical precipitates with small misfits (precipitate A) can be obtained. If the annealing temperature is less than 850°C, this effect cannot be sufficiently obtained, and fine precipitates or precipitates with an aspect ratio greater than 2.0 and large misfits are generated, making it impossible to obtain the predetermined precipitate A or precipitate group. On the other hand, if the annealing temperature exceeds 1000°C, the coarsening of the precipitates becomes significant, making it impossible to obtain the predetermined precipitate A or precipitate group. Therefore, the annealing temperature is set to 850 to 1000°C. The annealing temperature is preferably 870°C or higher, and more preferably 890°C or higher. Furthermore, the annealing temperature is preferably 980°C or lower, and more preferably 960°C or lower.

[0061] Holding time in the temperature range of 850 to 1000°C: 10 s or less. If the holding time in the temperature range of 850 to 1000°C exceeds 10 s, the coarsening of the precipitates becomes significant, and the predetermined number density of precipitate A or precipitate group cannot be obtained. Therefore, the holding time in the temperature range of 850 to 1000°C should be 10 s or less. The holding time is preferably 7 s or less, more preferably 5 s or less, and even more preferably 3 s or less. There is no particular lower limit, but since ferrite formation may occur if the holding time is 0.1 s or less, it is preferable that the holding time be greater than 0.1 s.

[0062] Average cooling rate in the temperature range up to 550°C: 10°C / s or more Next, the temperature range from the start of cooling after holding up to 550°C is cooled at an average cooling rate of 10°C / s or more. If the average cooling rate in the temperature range up to 550°C is less than 10°C / s, excessive amounts of ferrite, pearlite, retained austenite, etc. will be formed, or precipitates will become coarser, and the steel structure of the present invention cannot be obtained. Therefore, the average cooling rate in the temperature range up to 550°C should be 10°C / s or more. The average cooling rate is preferably 20°C / s or more, more preferably 30°C / s or more, and even more preferably 50°C / s or more. There is no particular upper limit specified for the average cooling rate, but if it is 500°C / s or more, problems such as instability of the plate shape are likely to occur, so it is preferable that the average cooling rate be less than 500°C / s.

[0063] Next, the steel sheet after the annealing process is subjected to plating, or optionally further plating alloying treatment after plating, followed by a cooling treatment with a cooling stop temperature of 400°C or lower (heat treatment process). In this heat treatment process, the residence time in the temperature range of 500 to 550°C is set to 60 seconds or less. The conditions for the plating and plating alloying treatments are not particularly limited, and known conditions can be applied, for example. As the plating treatment, hot-dip galvanizing can be applied, for example, by immersing the steel sheet (base steel sheet) in a zinc plating bath at 440 to 550°C. The zinc plating bath consists of Zn, Al, and unavoidable impurities, and its composition is not particularly specified, but as an example, the Al concentration in the bath can be 0.001% by mass or more and 1.0% by mass or less. Furthermore, the plating alloying treatment can be performed, for example, by heating the steel sheet after hot-dip galvanizing to an alloying temperature of 450 to 550°C.

[0064] Residence time in the 500-550°C temperature range during the heat treatment process: 60 s or less. In the heat treatment process, the residence time in the 500-550°C temperature range is set to 60 s or less. If the residence time in the 500-550°C temperature range exceeds 60 s, it will lead to the formation of pearlite and coarsening of precipitates, making it impossible to obtain the steel structure of the present invention. Therefore, the residence time in the 500-550°C temperature range is set to 60 s or less. The residence time is preferably 50 s or less, more preferably 40 s or less, and even more preferably 30 s or less. Furthermore, the lower limit of the residence time is not particularly limited, but as an example, the residence time is 1 s or more. Note that the residence time in the 500-550°C temperature range during the heat treatment process includes the time spent in the 500-550°C temperature range during the heat treatment process after the annealing process. In other words, for example, if a plating treatment or a plating alloying treatment is performed in the aforementioned temperature range, the residence time includes the time spent in the aforementioned temperature range during the plating treatment or plating alloying treatment. Furthermore, if there is any time spent in the aforementioned temperature range before the plating treatment or after the plating treatment or plating alloying treatment, that time is included in the residence time.

[0065] Cooling stop temperature: After plating treatment, or if plating alloying treatment is performed after plating treatment, a cooling treatment is performed after the plating alloying treatment, with a cooling stop temperature of 400°C or lower. If the cooling stop temperature exceeds 400°C, excessive pearlite formation may occur, making it impossible to obtain the steel structure of the present invention. Therefore, the cooling stop temperature should be 400°C or lower. Preferably, the cooling stop temperature should be 350°C or lower. Furthermore, there is no particular lower limit to the cooling stop temperature, but as an example, the cooling stop temperature should be 0°C or higher.

[0066] Furthermore, in the above cooling process, it is preferable that the average cooling rate in the temperature range up to 400°C be 0.05°C / s or higher. When the average cooling rate in the temperature range up to 400°C is 0.05°C / s or higher, it becomes easier to suppress the excessive formation of pearlite, and the steel structure of the present invention is easier to obtain. The average cooling rate in the aforementioned temperature range is more preferably 0.10°C / s or higher, and even more preferably 0.50°C / s or higher. Furthermore, there is no particular upper limit to the average cooling rate in the aforementioned temperature range, but as an example, the average cooling rate in the aforementioned temperature range is 1000°C / s or lower.

[0067] The high-strength plated steel sheet of the present invention can be manufactured by the above process. While there are no particular limitations other than the conditions of the manufacturing method described above, it is preferable to adjust the conditions as appropriate, as follows. For example, a post-heat treatment can be optionally performed after the heat treatment process (post-heat treatment process). The post-heat treatment may involve reheating to a temperature range of 450°C or lower and holding at that temperature range for 1.0 to 600,000 seconds. By setting the reheating temperature to 450°C or lower, the risk of damaging the plating quality can be reduced. Therefore, the reheating temperature is preferably 450°C or lower, and more preferably 400°C or lower. While there are no particular limitations on the lower limit of the reheating temperature, if a long-term aging treatment is performed for material adjustment or dehydrogenation, it is preferable to perform the post-heat treatment at a reheating temperature of approximately 25 to 100°C for 8,600 to 600,000 seconds. However, if rolling with a reduction ratio exceeding 5% is performed between the hot rolling process and the annealing process, coarse grains may be generated, or precipitates may change, making it impossible to obtain the steel structure of the present invention. Therefore, rolling with such a reduction ratio, such as cold rolling, is not permitted. Furthermore, temper rolling can be performed during the heat treatment process (after the plating process, or after the plating alloying process if plating alloying is performed after the plating process), or after the heat treatment process, or both, when the steel sheet temperature is 50°C or lower. The elongation ratio of the temper rolling is preferably 5% or less, more preferably 2% or less, and even more preferably 1% or less.

[0068] The present invention will be further described below based on the following examples. However, the present invention is not limited to the following examples.

[0069] Steel with the component composition shown in Table 1 was melted in a vacuum melting furnace to produce steel material (slabs). Subsequently, these steel material (slabs) were heated under the conditions shown in Table 2 (steel material heating process), followed by hot rolling, annealing, and heat treatment processes to obtain plated steel sheets. Between the hot rolling and annealing processes, scale was removed from the surface of the steel sheets by pickling (pickling process). In the hot rolling process, the total number of passes for finish rolling was set to seven. For some samples, a post-heat treatment process was performed after the heat treatment process. After the heat treatment process, or after the post-heat treatment process if one was performed, the steel sheets were temper-rolled at room temperature with an elongation rate of 0.3%. Note that blank spaces (indicated by -) in Table 1 indicate that the substance was intentionally omitted, and this includes not only cases where it is not contained (0%), but also cases where it is inevitably contained.

[0070] Using the obtained plated steel sheets, the microstructure, tensile properties, fatigue resistance, and hole expansion properties were evaluated according to the following test methods.

[0071] <Tissue Observation> (Area Percentage of Each Tissue) The area percentages of martensite, bainite, and pearlite refer to the proportion of the area of ​​each tissue in the observed area. Samples were cut from the obtained plated steel sheet, the thickness cross section parallel to the rolling direction was polished, and then etched with 3 vol% nital. Three fields of view were taken at a magnification of 1500x using an SEM (scanning electron microscope) at a position 1 / 4 of the sheet thickness. The area percentage of each tissue was determined from the obtained secondary electron image data using Image-Pro from Media Cybernetics, and the average area percentage of the fields of view was taken as the area percentage of each tissue.

[0072] In the image data, upper bainite is distinguished as black or dark gray portions of a black or dark gray structure containing carbides or martensite with linear interfaces. Lower bainite is distinguished as black, dark gray, gray, or light gray containing oriented carbides. Martensite is distinguished as black, dark gray, gray, or light gray containing carbides in multiple orientations, or as white or light gray without carbides. Retained austenite is distinguished as white or light gray without carbides.

[0073] Since martensite and retained austenite may be indistinguishable, the area ratio of martensite was determined by dividing the area ratio of retained austenite, obtained by the method described later, by the total area ratio of martensite and retained austenite obtained from the SEM image. In this invention, martensite may be any type of martensite, such as fresh martensite, auto-tempered martensite, or tempered martensite. Similarly, bainite may be any type of bainite, such as upper bainite, lower bainite, or tempered bainite. The stronger the degree of tempering, the darker the contrast image of the substrate becomes; therefore, the above substrate color is merely a guideline. In this invention, the amount of carbides and the microstructure morphology were considered comprehensively, and the substrate was classified into one of the structures with similar characteristics, including those described later. The carbides are white, dot-like or linear.

[0074] Perlite can be distinguished as a layered or partially discontinuous layered structure of black and white. In addition, although not basically included in the present invention, ferrite can be distinguished as a black structure, a dark gray structure without carbides or with a small amount of carbides, or a dark gray structure without a linear interface with martensite.

[0075] The area ratio of retained austenite was determined as follows: After grinding the plated steel sheet to a position of 1 / 4 of the sheet thickness + 0.1 mm, the surface was further polished by 0.1 mm using chemical polishing and used as the measurement surface. For the measurement surface, the integrated reflectance intensity of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron (ferrite) was measured using an X-ray diffractometer with Mo Kα1 rays. The volume fraction was determined from the intensity ratio of the integrated reflectance intensity from each plane of fcc iron to the integrated reflectance intensity from each plane of bcc iron, and this was taken as the area ratio of retained austenite.

[0076] Using the area percentages of each tissue obtained, the total area percentage of martensite and bainite, the area percentage of pearlite, the area percentage of retained austenite, and the total area percentage of other tissues were determined. The area percentages of each tissue are shown in Table 3. In Table 3, "V(M)" means the area percentage of martensite (%), "V(B)" means the area percentage of bainite (%), "V(M+B)" means the total area percentage of martensite and bainite (%), "V(P)" means the area percentage of pearlite (%), "V(R)" means the area percentage of retained austenite (%), and "V(O)" means the total area percentage of other tissues (%).

[0077] The particle size of the precipitates was determined by imaging four fields of view in a 200 nm x 200 nm area at a position 1 / 4 of the plate thickness using the TEM replica method. For each field of view, the area of ​​each precipitate was determined for precipitates with an aspect ratio (long axis length / short axis length) of 1.0 to 2.0. The area of ​​each precipitate was then converted to an equivalent circle diameter to determine the particle size of each precipitate. The number of precipitates with a particle size of 3 to 20 nm in each field of view was counted, and the number density of precipitates in each field of view was calculated by dividing by the area of ​​each field of view. The average value of these values ​​was then used as the number density of precipitates with a particle size of 3 to 20 nm and an aspect ratio of 1.0 to 2.0 (precipitate A).

[0078] The number density of precipitate groups was determined as follows: For the precipitates (precipitate A) in each field of view obtained above, with a particle size of 3 to 20 nm and an aspect ratio of 1.0 to 2.0, the distance between the centroids of each precipitate was determined, and precipitate groups that satisfy requirement 2 were identified. The number of precipitates n in each precipitate group was determined, and the number of precipitate groups where n satisfies requirement 1 was determined. The number of precipitate groups in each field of view was calculated by dividing the number of precipitate groups in each field of view by the area of ​​each field of view, and the average value of these was taken as the number density of the precipitate groups. The distance between the centroids of each precipitate was determined using WinROOF2015 manufactured by Mitani Corporation. Requirement 1: The number n of precipitates A forming a precipitate group is 6 ≤ n ≤ 40. Requirement 2: For any two precipitates i and j among precipitate A, precipitate j whose centroid-to-centroid distance from precipitate i satisfies the following equation (1) shall be considered to belong to the same precipitate group as precipitate i. Lij ≤ 1.5Di + 1.5Dj ... (1) Here, in equation (1), Lij is the centroid-to-centroid distance (nm) between precipitate i and precipitate j, Di is the particle size (nm) of precipitate i, and Dj is the particle size (nm) of precipitate j. The number density of the obtained precipitate A and the number density of the precipitate group are shown in Table 3.

[0079] <Tensile Test> The tensile properties were evaluated by tensile testing. From the obtained plated steel sheet, JIS No. 5 tensile test specimens (JIS Z 2241:2011) were taken in a direction parallel to the rolling direction, and the strain rate was 10 －３ Tensile tests were conducted in accordance with the provisions of JIS Z 2241:2011, with a value of / s, to determine YS and TS. The yield ratio YR was calculated from YS / TS. In this invention, a TS of 980 MPa or higher was evaluated as passing (having high strength), and a yield ratio of 0.80 or higher was evaluated as passing (having an excellent yield ratio).

[0080] <Fatigue Test> Fatigue resistance was evaluated by a planar bending fatigue test. From the obtained plated steel sheet, a planar bending fatigue test specimen with a width of 30 mm, a length of 90 mm, and a radius of R40 mm was taken with the longitudinal side facing the rolling direction. A planar bending fatigue test was then performed under conditions of a stress ratio of -1 and a frequency of 25 Hz. The fatigue strength σw after 200,000 cycles was determined, and a σw / TS of 0.55 or higher was evaluated as passing (having excellent fatigue resistance).

[0081] <Hole Expansion Test> Hole expansion properties were evaluated by a hole expansion test. A test piece measuring 100 mm in width and 100 mm in length was taken from the obtained plated steel sheet. In accordance with JIS Z 2256:2010, a hole with a diameter of 10 mm was punched with a clearance of 12 ± 1%. The punched hole was expanded using a 60° conical punch, and the test was terminated and the load removed when one of the cracks on the end face of the punched hole penetrated the thickness of the sheet. Initial hole diameter D of the punched hole ０ and hole diameter D after testing ｆ The hole expansion ratio λ (%) was calculated from the following formula. In this invention, a hole expansion ratio of 40% or more was evaluated as acceptable (having excellent hole expansion properties). λ = {(D ｆ -D ０ ) / D ０ Table 3 shows the results of various evaluations.

[0082]

[0083]

[0084]

[0085] As shown in Table 3, all of the examples of the present invention are high-strength plated steel sheets with excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties. On the other hand, the comparative examples that fall outside the scope of the present invention do not obtain one or more of the desired strength, yield ratio, fatigue resistance, and hole-expanding properties.

[0086] According to the present invention, a high-strength plated steel sheet with a TS of 980 MPa or higher, possessing an excellent yield ratio, excellent fatigue resistance, and excellent hole-expanding properties, can be obtained. By using the high-strength plated steel sheet of the present invention for automotive parts, it is possible to significantly contribute to improving the collision safety and fuel efficiency of automobiles.

Claims

1. A steel sheet having a plating layer on at least one side, wherein the steel sheet has a composition by mass% of: C: 0.05-0.15%, Si: 0.01-2.0%, Mn: 1.5-3.5%, P: 0.100% or less, S: 0.020% or less, Al: 1.0% or less, and Ti: 0.050-0.20%, with the remainder being Fe and unavoidable impurities, the steel structure of the steel sheet having a total area ratio of martensite and bainite of 80-100%, an area ratio of pearlite of 0-2%, an area ratio of retained austenite of 0-6%, and a number density of precipitates A present in the martensite or bainite with a particle size of 3-20 nm and an aspect ratio of 1.0-2.0 of 1.0 × 10⁻¹⁴ ３ ~10.0 x 10 ３ pieces / μm ２ Furthermore, the number density of the precipitate group defined by requirements 1 and 2 below is 1.0 particles / μm ２ The following is a high-strength plated steel sheet. Requirement 1: The number n of precipitates A that form a precipitate group is 6 ≤ n ≤ 40. Requirement 2: For any precipitate i and precipitate j among precipitates A, precipitate j whose centroid-to-centroid distance from precipitate i satisfies the following equation (1) shall be considered to belong to the same precipitate group as precipitate i. Lij ≤ 1.5Di + 1.5Dj ... (1) Here, in equation (1), Lij is the centroid-to-centroid distance (nm) between precipitate i and precipitate j, Di is the particle size (nm) of precipitate i, and Dj is the particle size (nm) of precipitate j.

2. The high-strength plated steel sheet according to claim 1, wherein the component composition further comprises, in mass%, one or more selected from Nb: 0.005 to 0.20%, Cr: 0.005 to 2.0%, Cu: 0.05 to 4.0%, Ni: 0.005 to 2.0%, Mo: 0.05 to 2.0%, V: 0.05 to 1.0%, B: 0.0002 to 0.0050%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Sb: 0.0010 to 0.10%, and Sn: 0.0010 to 0.50%.

3. A steel material heating step of heating a steel material having the component composition described in claim 1 or 2 to 1150°C or higher; a hot rolling step of roughly rolling the steel material after the steel material heating step, then finishing rolling it under conditions where the final pass temperature is 850 to 1000°C, then cooling it in the temperature range up to 500°C at an average cooling rate of 50°C / s or higher, and winding it up to 500°C; a pickling step of pickling the steel sheet after the hot rolling step; an annealing step of heating the steel sheet after the pickling step in the temperature range of 550 to 850°C at an average heating rate of 20°C / s or higher, holding it in the temperature range of 850 to 1000°C for 10 seconds or less, and then cooling it in the temperature range up to 550°C at an average cooling rate of 10°C / s or higher. A method for manufacturing a high-strength plated steel sheet, comprising a heat treatment step of applying a plating treatment to the steel sheet after the annealing step, or applying a plating alloying treatment after the plating treatment, and then applying a cooling treatment in which the cooling stop temperature is 400°C or lower, wherein in the heat treatment step, the residence time in the temperature range of 500 to 550°C is 60 s or less.

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