High-strength steel sheets, electrogalvanized steel sheets, molten galvanized steel sheets and alloyed molten galvanized steel sheets, and methods for manufacturing the same

Abstract

A high-strength steel sheet having a prescribed composition, a steel structure in which a MA fraction is higher than 0% and 15.0% or less, a fraction of soft α phase structure is 0% or more and 50% or less, and a standard deviation of an equivalent circle diameter of residual austenite is greater than 0.155 μm, in terms of an area ratio in the entire steel structure.

Description

Technical Field

[0001] This invention relates to high-strength steel sheets, electro-galvanized steel sheets, hot-dip galvanized steel sheets, and alloyed hot-dip galvanized steel sheets that can be used in various applications, primarily automotive parts, and methods for manufacturing the same. Background Technology

[0002] Steel sheets (e.g., cold-rolled steel sheets, alloyed galvanized steel sheets, etc.) supplied for automotive parts (e.g., frame parts) require thinner walls to improve fuel efficiency through vehicle body lightweighting, and high strength is required to achieve both thinner walls and ensure part strength. On the other hand, such steel sheets also require excellent machinability to be formed into parts with complex shapes. For example, Patent Document 1 discloses a high-strength steel sheet with excellent machinability and a tensile strength (TS) of 980 MPa or higher. Patent Document 2 discloses a high-strength steel sheet with a tensile strength (TS) of 980 MPa or higher.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent No. 6434348

[0006] Patent Document 2: Japanese Patent Application Publication No. 2018-204058 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] In various applications, primarily automotive parts, high tensile strength (TS) is required, along with an excellent strength-to-ductility balance (TS×EL), a high yield strength ratio (YR), and excellent cavity expansion rate (λ). Furthermore, if uneven deformation occurs under applied external force, cracks may develop from those points; therefore, uniform deformation (uniform elongation, uEL) is also required. In the prior art of Patent Documents 1 and 2, these characteristics are sometimes not fully satisfied.

[0009] Specifically, the requirements for tensile strength, strength-ductility balance, yield strength ratio, uniform elongation, and cavity expansion rate are as follows.

[0010] Regarding tensile strength (TS), a value of 780 MPa or higher is required. Furthermore, to increase the stress it can withstand during use, in addition to high tensile strength (TS), a high yield strength (YS) is also necessary. Additionally, from the perspective of ensuring collision safety, the yield strength of the steel plate also needs to be increased. Therefore, specifically, a yield strength ratio (YR = YS / TS) of 0.65 or higher is required.

[0011] Regarding the total elongation (EL), a value of 21% or higher is required. Furthermore, regarding the balance between strength and ductility, the product of strength (TS) and total elongation (EL) (TS×EL) is required to be 20000 MPa·% or higher. In addition, to ensure the formability of the part during forming, the cavity expansion rate (λ), representing cavity expansion, is required to be 20.0% or higher. Furthermore, a uEL higher than 13% is required to ensure uniform deformation (uniform elongation) under applied external force.

[0012] The present invention is made in view of the following situation, and its object is to provide a high-strength steel sheet, an electro-galvanized steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet, and methods for manufacturing the same, all of which have high tensile strength (TS), yield ratio (YR), uniform elongation (uEL), total elongation (EL), the product of (TS) and total elongation (EL) (TS×EL), and cavity expansion rate (λ).

[0013] Problem-solving methods

[0014] Method 1 of the present invention is a high-strength steel plate, wherein it contains:

[0015] C: Above 0.15% by mass and below 0.35% by mass

[0016] Si: greater than 0% by mass and less than 3.0% by mass

[0017] Mn: ≥1.0% by mass and ≤4.0% by mass

[0018] Al: Above 0% by mass and below 3.0% by mass

[0019] N: greater than 0% by mass and less than 0.01% by mass

[0020] P: Above 0% by mass and below 0.05% by mass

[0021] S: greater than 0% by mass and less than 0.01% by mass, and

[0022] The total content of Si and Al is ≥0.5% by mass and ≤3.0% by mass.

[0023] The balance consists of Fe and unavoidable impurities.

[0024] In steel microstructure, the area percentage within the total steel microstructure is used to measure...

[0025] MA score is above 0% and below 15.0%.

[0026] The fraction of soft α-phase tissue was greater than 0% and less than 50%.

[0027] The standard deviation of the equivalent circle diameter of the retained austenite is greater than 0.155 μm.

[0028] In embodiment 2 of the present invention, the high-strength steel plate according to embodiment 1 further comprises, from

[0029] Cu: above 0% by mass and below 0.50% by mass

[0030] Ni: above 0% by mass and below 0.50% by mass

[0031] Cr: greater than 0% by mass and less than 0.50% by mass

[0032] Mo: above 0% by mass and below 0.50% by mass

[0033] B: Higher than 0% by mass and lower than 0.01% by mass

[0034] V: Higher than 0.005% by mass and lower than 0.05% by mass

[0035] Nb: above 0% by mass and below 0.05% by mass

[0036] Ti: above 0% by mass and below 0.05% by mass

[0037] Ca: above 0% by mass and below 0.05% by mass, and

[0038] REM: One or more selected from the group consisting of more than 0% by mass and less than 0.01% by mass.

[0039] The third embodiment of the present invention is an electro-galvanized steel sheet, wherein an electro-galvanized layer is provided on the surface of the high-strength steel sheet described in embodiment 1 or embodiment 2.

[0040] The fourth embodiment of the present invention is a molten galvanized steel sheet, wherein a molten galvanized layer is present on the surface of the high-strength steel sheet described in embodiment 1 or embodiment 2.

[0041] Method 5 of the present invention is an alloyed molten galvanized steel sheet, wherein an alloyed molten galvanized layer is provided on the surface of the high-strength steel sheet described in Method 1 or Method 2.

[0042] Method 6 of the present invention is a method for manufacturing a high-strength steel plate, comprising the following steps:

[0043] A process for preparing rolled material having the composition described in method 1 or method 2;

[0044] The process of heating the rolled material to a temperature of (Ac3+30℃) or higher for austenitization;

[0045] After austenitization, the temperature range is from (Ms+250℃) to (Ms+110℃) and cooled at an average cooling rate of 15℃ / second or more but less than 200℃ / second. The temperature range is from (Ms+110℃) to (Ms+10℃) and the temperature is held for 10 seconds or more but less than 300 seconds at a cooling rate of 0℃ / second or more but less than 10℃ / second.

[0046] The process of cooling from a temperature above (Ms+10℃) to a cooling stop temperature below Ms℃ after the retention period, at an average cooling rate of 10℃ / second or higher.

[0047] The process of heating from the cooling stop temperature to a reheating temperature in the range of (Ms-100℃)~(Ms+125℃).

[0048] Here, Ac3 and Ms can be calculated by the following formula.

[0049] Ac3 (°C) = 910 - 203 × [C] 1 / 2 -15.2×[Ni]+44.7×[Si]+104×

[0050] [V]+31.5×[Mo]+13.1×[W]-30×[Mn]-11×[Cr]-20×[Cu]+700×[P]+400×[Al]+120×[As]+400×[Ti]

[0051] Ms(℃)=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo]

[0052] Wherein, [] represents the content (mass%) of each element, and the content of elements not included is 0.

[0053] In embodiment 7 of the present invention, the method for manufacturing a high-strength steel plate according to embodiment 6 is wherein, after heating to the reheating temperature, the reheating temperature is maintained for 50 seconds or more and 1200 seconds or less.

[0054] Method 8 of the present invention is a method for manufacturing electro-galvanized steel sheet, wherein electro-galvanizing is performed on the surface of a high-strength steel sheet obtained by the method of method 6 or method 7.

[0055] Method 9 of the present invention is a method for manufacturing a molten galvanized steel sheet, wherein molten galvanizing is performed on the surface of the steel sheet during heating at the reheating temperature described in Method 6 or Method 7.

[0056] Method 10 of the present invention is a method for manufacturing alloyed molten galvanized steel sheet, wherein, after the molten galvanizing described in Method 9, a heat treatment for alloying is performed.

[0057] Invention Effects

[0058] According to embodiments of the present invention, it is possible to provide high-strength steel sheets, electro-galvanized steel sheets, molten galvanized steel sheets, and alloyed molten galvanized steel sheets, as well as methods for manufacturing the same, which all exhibit high levels of tensile strength (TS), yield ratio (YR), uniform elongation (uEL), total elongation (EL), product of (TS) and total elongation (EL) (TS×EL), and cavity expansion rate (λ). Attached Figure Description

[0059] Figure 1 This is a diagram illustrating an example of a heat treatment mode according to an embodiment of the present invention.

[0060] Figure 2 This is a diagram illustrating an example of a heat treatment mode in an embodiment. Detailed Implementation

[0061] Through intensive research, the inventors have discovered that in steel with a specified composition, by ensuring that the steel microstructure (metallic microstructure), in terms of its area percentage in the total steel microstructure, has a MA fraction greater than 0% and less than 15.0%, a soft α-phase microstructure fraction greater than 0% and less than 50%, and a standard deviation of the equivalent circle diameter of retained austenite greater than 0.155 μm, it is possible to obtain high-strength steel sheets, electro-galvanized steel sheets, molten galvanized steel sheets, and alloyed molten galvanized steel sheets with high tensile strength (TS), yield ratio (YR), uniform elongation (uEL), total elongation (EL), the product of (TS) and total elongation (EL) (TS×EL), and cavity expansion rate (λ). In particular, the inventors have found that by ensuring the standard deviation of the equivalent circle diameter of retained austenite is greater than 0.155 μm, both total elongation (EL) and uniform elongation (uEL) can be increased.

[0062] 1. Steel structure

[0063] The following describes the details of the steel structure of the high-strength steel plate according to embodiments of the present invention.

[0064] In the following description of the steel structure, mechanisms by which various properties can be improved by having such a structure will be explained. These are mechanisms considered by the inventors based on currently available knowledge, but it should be noted that they do not limit the technical scope of the present invention.

[0065] (1) MA score: above 0% and below 15.0%

[0066] MA is a hard phase that acts as a void formation site near the parent phase / hard phase interface during deformation. In the embodiments of this invention, MA stands for martensite-austenite constituent, a mixed structure of untempered martensite and retained austenite. If the MA fraction increases, strain concentrates at the parent phase / hard phase interface, making fracture more likely to originate from voids formed near this interface. Furthermore, the formation of MA introduces mobile dislocations, reducing the yield strength ratio (YR).

[0067] Therefore, by taking the area fraction of MA in the total steel structure as 15.0% or less, the yield strength ratio (YR) and the cavity expansion ratio λ can be improved. The MA fraction is preferably 8% or less. Furthermore, the presence of MA causes uneven mechanical properties; therefore, even if the MA fraction does not meet the requirement of 15.0% or less, λ may still be satisfied. However, to consistently obtain the desired λ, the MA fraction needs to be 15.0% or less.

[0068] On the other hand, the retained austenite constituting MA undergoes a TRIP phenomenon, transforming into martensite due to process-induced phase transformation during stamping and other processing, resulting in a high work hardening rate. Therefore, since MA has the effect of increasing uniform elongation (uEL) and total elongation (EL), the MA fraction is higher than 0%. The MA fraction is preferably 3% or more, and more preferably 5% or more.

[0069] (2) Soft α-phase tissue fraction: 0% or more and 50% or less

[0070] While the inclusion of a soft α-phase structure improves ductility, it reduces strength. Furthermore, strain concentration at the parent phase / soft α-phase interface makes fracture more likely, originating from voids formed near this interface. Therefore, by maintaining a soft α-phase fraction of 50% or less in terms of the total steel microstructure area, the yield strength ratio (YR) and cavity expansion rate (λ) can be increased. The soft α-phase fraction is preferably 40% or less, more preferably 30% or less, and most preferably 0%.

[0071] In this specification, the term "α phase" refers to a crystal structure of bcc. For example, the aforementioned "α phase" includes ferrite, pearlite, tempered bainite, unquenched bainite, and untempered martensite contained in MA. The "α phase" can be identified using the EBSD (Electron Back Scatter Diffraction Patterns) method, which utilizes the SEM (Scanning Electron Microscope) crystal analysis method, as described later.

[0072] Furthermore, in this specification, the term "soft α phase," as illustrated in the embodiments described later, refers to grains identified as α phase by EBSD with a GAM (Grain Average Misorientation) value lower than 0.4. GAM is the value obtained by averaging the orientation differences between adjacent pixels within a single grain. Soft α phase structures are considered to have low dislocation density and small orientation differences within a single grain. In embodiments of the present invention, soft structures with low dislocation density as described above are defined as soft α phase structures.

[0073] If the percentage of soft α-phase tissue is 0% or more and less than 50%, then even if it includes tissues other than the soft α-phase in the "α-phase", the effects of the embodiments of the present invention can be achieved.

[0074] (3) Standard deviation of the equivalent circle diameter of retained austenite: greater than 0.155 μm

[0075] Retained austenite undergoes a TRIP phenomenon, transforming into martensite due to process-induced phase transformation during stamping and other machining processes, resulting in high elongation. The stability of retained austenite varies depending on its size; the greater the size diversity of the retained austenite, the longer the work hardening during deformation persists until high strain. Therefore, by ensuring the standard deviation of the equivalent circle diameter of the retained austenite is higher than 0.155 μm, the total elongation (EL) and uniform elongation (uEL) can be improved. The standard deviation of the equivalent circle diameter of the retained austenite is preferably higher than 0.158 μm, more preferably higher than 0.161 μm. On the other hand, if the retained austenite contains excessively diverse sizes, the proportion of large, unstable retained austenite that does not contribute to improving the total elongation (EL) and uniform elongation (uEL) increases, potentially reducing the total elongation (EL) and uniform elongation (uEL). Therefore, the standard deviation of the equivalent circle diameter of the retained austenite is preferably 1.000 μm or less, more preferably 0.500 μm or less.

[0076] Furthermore, excessively large retained austenite may be unstable and does not contribute to improving total elongation (EL) and uniform elongation (uEL). Therefore, the average equivalent circle diameter of the retained austenite is preferably 1.0 μm or less. Also, as described above, in the embodiments of the present invention, MA is a mixed structure of untempered martensite and retained austenite, so the area fraction of retained austenite is less than or equal to the MA fraction.

[0077] (4) Other steel structures

[0078] The steel microstructure of the embodiments of the present invention is not specifically defined for steel microstructures other than MA, soft α-phase microstructures, and retained austenite. However, even if microstructures other than MA are present, the effects of the embodiments of the present invention will still be achieved if the above-described microstructure conditions are met.

[0079] 2. Chemical composition

[0080] The composition of the high-strength steel plate according to embodiments of the present invention will be described below. First, the basic elements will be described, and then the elements that can be optionally added will be described.

[0081] (1) C: Above 0.15% by mass and below 0.35% by mass

[0082] Carbon (C) is an essential element for obtaining the desired microstructure and ensuring properties such as high TS×EL. To effectively perform this function, it needs to contain more than 0.15% by mass. However, amounts exceeding 0.35% by mass are unsuitable for welding and cannot achieve sufficient weld strength. The preferred C content is 0.18% by mass or more, more preferably 0.20% by mass or more. Furthermore, it is preferably 0.30% by mass or less, more preferably 0.25% by mass or less. When the C content is preferably 0.30% by mass or less as described above, welding becomes easier.

[0083] (2) Si: above 0% by mass and below 3.0% by mass

[0084] Si is an effective element for improving resistance to temper softening. Furthermore, Si is also effective in increasing strength through solid solution strengthening. From the viewpoint of effectively utilizing these effects, the Si content is higher than 0% by mass. Preferably, it is 0.02% by mass or more. However, since Si is a ferrite-forming element, a large content impairs hardenability and makes it difficult to ensure high strength. Additionally, it negatively impacts LME sensitivity. Therefore, the Si content is 3.0% by mass or less. Preferably, it is 2.5% by mass or less, more preferably 2.0% by mass or less, and even more preferably 1.95% by mass or less.

[0085] (3) Al: greater than 0% by mass and less than 3.0% by mass

[0086] Al acts as a deoxidizer and also improves the corrosion resistance of steel. To fully realize these effects, the Al content is higher than 0% by mass. Preferably, it is 0.030% by mass or more, more preferably 0.040% by mass or more. However, since Al is a ferrite-forming element, a large content can impair hardenability and make it difficult to ensure high strength. Therefore, the Al content is 3.0% by mass or less. Preferably, it is 2.5% by mass or less, more preferably 2.0% by mass or less, and even more preferably 1.95% by mass or less.

[0087] (4) The total of Si and Al: more than 0.5% by mass and less than 3.0% by mass

[0088] Si and Al both inhibit cementite precipitation and promote the formation of retained austenite. To effectively exert this effect, the combined content of Si and Al needs to be 0.5% by mass or more. Preferably, it is 0.7% by mass or more, more preferably 1.0% by mass or more. However, if the combined content of Si and Al exceeds 3.0% by mass, the deformability of the steel decreases, and the TS×EL decreases. Therefore, the combined content of Si and Al is 3.0% by mass or less. Preferably, it is 2.5% by mass or less, more preferably 2.0% by mass or less, and even more preferably 1.95% by mass or less.

[0089] (5) Mn: ≥1.0% by mass and ≤4.0% by mass

[0090] Mn inhibits ferrite formation. To effectively exert this effect, it needs to contain 1.0% by mass or more. Preferably, it contains 1.5% by mass or more. However, if it exceeds 4.0% by mass, it cannot inhibit bainitic phase transformation and forms relatively coarse retained austenite (i.e., it cannot increase the standard deviation of the equivalent circle diameter of the retained austenite), and it also has an adverse effect on LME sensitivity. Therefore, the Mn content is 4.0% by mass or less. Preferably, it is 3.0% by mass or less, more preferably 2.5% by mass or less, and even more preferably 2.2% by mass or less.

[0091] (6) N: greater than 0% by mass and less than 0.01% by mass

[0092] If the nitrogen content is excessive, the precipitation of nitrides will increase, which will adversely affect toughness. Therefore, the nitrogen content is 0.01% by mass or less. The nitrogen content is preferably 0.008% by mass or less, and more preferably 0.006% by mass or less. Furthermore, when considering costs in steelmaking, the nitrogen content is usually 0.001% by mass or more.

[0093] (7) P: greater than 0% by mass and less than 0.05% by mass

[0094] Phosphorus (P) inevitably exists as an impurity element. If P exceeds 0.05% by mass, the total elongation (EL) and cavity expansion rate (λ) deteriorate. Therefore, the P content is 0.05% by mass or less, preferably 0.03% by mass or less.

[0095] (8) S: greater than 0% by mass and less than 0.01% by mass

[0096] Sulfur (S) inevitably exists as an impurity element. If S content exceeds 0.01%, sulfide inclusions such as MnS are formed, which act as crack initiation points and reduce the cavity expansion rate (λ). Therefore, the S content is 0.01% by mass or less, preferably 0.005% by mass or less.

[0097] (9) Balance

[0098] The balance consists of Fe and unavoidable impurities. As unavoidable impurities, trace elements (e.g., As, Sb, Sn, etc.) introduced due to the condition of raw materials, materials, manufacturing equipment, etc., are permitted. In this embodiment, there is a case where V is unavoidably present at 0.005% by mass or less. That is, in this embodiment, V at 0.005% by mass or less is treated as an unavoidable impurity. Furthermore, elements such as P and S are generally preferred in lower amounts and are therefore unavoidable impurities, but their composition range is separately specified as described above. Therefore, in this specification, the term "unavoidable impurities" constituting the balance refers to elements excluding those whose composition range is separately specified.

[0099] As long as the characteristics of the high-strength steel plate of the embodiments of the present invention can be maintained, it may also contain any other elements. Examples of other elements that can be selectively contained in this way are given below.

[0100] (10) Other elements

[0101] Cu: above 0% by mass and below 0.50% by mass, Ni: above 0% by mass and below 0.50% by mass, Cr: above 0% by mass and below 0.50% by mass, Mo: above 0% by mass and below 0.50% by mass, B: above 0% by mass and below 0.01% by mass, V: above 0.005% by mass and below 0.05% by mass, Nb: above 0% by mass and below 0.05% by mass, Ti: above 0% by mass and below 0.05% by mass, Ca: above 0% by mass and below 0.05% by mass, and REM: above 0% by mass and below 0.01% by mass. Cu, Ni, Cr, Mo and B improve hardenability, prevent ferrite formation, and contribute to the stabilization of austenite and / or the refinement of bainite, thereby improving the strength-ductility balance. It is recommended that the content of Cu, Ni, Cr, and Mo be higher than 0% by mass, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more. The content of B is preferably higher than 0% by mass, more preferably 0.0001% by mass or more, and even more preferably 0.0002% by mass or more. However, if these elements are present in excess, the processability will deteriorate and high costs will result. Therefore, it is recommended that the content of Cu, Ni, Cr, and Mo be lower than 0.50% by mass, more preferably 0.40% by mass or less, and even more preferably 0.30% by mass or less. The content of B is preferably lower than 0.01% by mass, more preferably 0.0075% by mass or less, even more preferably 0.005% by mass or less, and even more preferably 0.003% by mass or less.

[0102] V, Nb, and Ti enhance the precipitation of the parent phase, improving strength without significantly deteriorating ductility, thereby improving the strength-ductility balance. It is recommended that the V content be higher than 0.005% by mass, more preferably 0.01% by mass or more, and even more preferably 0.02% by mass or more. It is recommended that the Nb and Ti contents be higher than 0% by mass, more preferably 0.005% by mass or more, even more preferably 0.01% by mass or more, and even more preferably 0.02% by mass or more. However, if these elements are present in excess, processability deteriorates; therefore, it is desirable that the contents of these elements be limited to, preferably 0.05% by mass or less, more preferably 0.04% by mass or less, and even more preferably 0.03% by mass or less.

[0103] Ca and REM finely disperse inclusions represented by MnS, contributing to the balance of strength and ductility and improving cavitation. Here, REM (rare earth elements) used in embodiments of the present invention can include Sc, Y, lanthanides, etc. It is desirable that the content of Ca and REM is preferably higher than 0% by mass, more preferably 0.001% by mass or more, and even more preferably 0.002% by mass or more. However, even if these elements are excessively present, their effect is saturated, resulting in economic waste; therefore, the content of Ca is preferably 0.05% by mass or less, and the content of REM is preferably 0.01% by mass or less. Furthermore, the content of Ca and REM is more preferably 0.005% by mass or less, respectively.

[0104] 3. Characteristics

[0105] The high-strength steel sheet according to the embodiments of the present invention described above exhibits high levels of TS, YR, uEL, EL, TS×EL, and λ. These characteristics of the high-strength steel sheet according to the embodiments of the present invention will be described in detail below.

[0106] (1) Tensile strength (TS)

[0107] It has a tensile strength (TS) of 780 MPa or higher. This ensures sufficient strength. Preferably, it has a tensile strength of 850 MPa or higher, more preferably 900 MPa or higher, and even more preferably 950 MPa or higher.

[0108] (2) Yield-to-tensile strength ratio (YR)

[0109] It has a yield strength ratio of 0.65 or higher. This, combined with the aforementioned high tensile strength, enables the achievement of high yield strength, allowing for the use of final products obtained through deep drawing or similar processing under high stress. A yield strength ratio of 0.67 or higher is preferred, and more preferably 0.70 or higher is more desirable.

[0110] (3) Total elongation (EL)

[0111] It has a total elongation of 21% or more. This results in excellent processability, such as good stamping formability. Preferably, it is 22% or more, and more preferably 22.6% or more.

[0112] (4) Uniform elongation (uEL)

[0113] It has a uniform elongation of more than 13%. This allows the steel sheet to deform uniformly when an external force is applied. Preferably, it is 13.5% or more, more preferably 15% or more.

[0114] (5) The product of TS and EL (TS×EL)

[0115] The strength-to-elasticity (TS×EL) ratio is 20,000 MPa·% or higher. A high level of strength-to-elasticity balance, simultaneously possessing high strength and high ductility, can be achieved with a TS×EL ratio of 20,000 MPa·% or higher. Preferably, the TS×EL ratio is 21,000 MPa·% or higher, more preferably 22,000 MPa·% or higher.

[0116] (6) Expansion rate (λ)

[0117] It has a cavity expansion rate λ of 20.0% or more. This results in excellent processability, such as stamping formability. Preferably, it is 25.0% or more, and more preferably 29.0% or more.

[0118] As an electro-galvanized steel sheet with an electro-galvanized layer on the surface of the high-strength steel sheet in the embodiments of the present invention, the aforementioned desired properties can also be obtained. The coating amount is not particularly limited; for example, it is 10 to 100 g / m² per side. 2 Left or right is fine.

[0119] As a molten galvanized steel sheet with a molten galvanized layer on the surface of a high-strength steel sheet according to an embodiment of the present invention, the aforementioned desired properties can also be obtained. The coating amount is not particularly limited; for example, it is 10 to 100 g / m² per side. 2 Left or right is fine.

[0120] As an alloyed molten zinc steel sheet with an alloyed molten zinc coating on the surface of the high-strength steel sheet in the embodiments of the present invention, the aforementioned desired characteristics can also be obtained. The coating amount is not particularly limited; for example, it is 10 to 100 g / m² per side. 2 Left or right is fine.

[0121] 4. Manufacturing method

[0122] Next, the manufacturing method of the high-strength steel plate according to the embodiments of the present invention will be described.

[0123] The inventors have discovered that by performing heat treatment, detailed later, on rolled materials having a specified composition, the desired steel microstructure described above can be obtained, resulting in high-strength steel sheets with the aforementioned desired properties. In particular, the inventors have discovered that by appropriately controlling the residence time within a specified temperature range based on the Ms point during the heat treatment, the standard deviation of the equivalent circle diameter of the retained austenite can be controlled to be higher than 0.155 μm, thereby solving the aforementioned problems.

[0124] The details are as follows.

[0125] Figure 1 This is an illustration of a method for manufacturing a high-strength steel plate according to an embodiment of the present invention, particularly a heat treatment process.

[0126] Heat-treated rolled materials are typically manufactured by hot rolling followed by cold rolling. However, this is not a limitation; materials can be manufactured using either hot rolling or cold rolling. Furthermore, the conditions for hot rolling and cold rolling are not particularly restricted.

[0127] (1) Austenitizing treatment

[0128] like Figure 1 As shown in [1] and [2], the rolled material is heated to a temperature of (Ac3 + 30°C) or higher for austenitization. In order to achieve complete austenitization and suppress the formation of soft α-phase structures such as ferrite, it is necessary to heat to (Ac3 + 30°C) or higher. In addition, when the heating temperature in [1] is low, the original γ grain size will be refined and the bainitic phase transformation will be promoted. Here, when the holding temperature in [3] is in an appropriate range, the bainitic phase transformation is easy to carry out, so the result is that the bainitic phase transformation is excessive, and the standard deviation of the equivalent circle diameter of the retained austenite may become smaller. Therefore, from the viewpoint of suppressing it, it is also necessary to heat to (Ac3 + 30°C) or higher. It is preferable to heat to a temperature of (Ac3 + 40°C) or higher. In order to more reliably suppress the coarsening of grains, the heating temperature is preferably (Ac3 + 100°C) or lower. More preferably (Ac3 + 80°C) or lower, and even more preferably (Ac3 + 60°C) or lower. It can be maintained at this heating temperature for 1 second to 1800 seconds.

[0129] Figure 1 Heating to the heating temperature of [1] can be carried out at any heating rate, but as a preferred average heating rate, 1°C / second or more and 20°C / second or less can be listed.

[0130] Here, Ac3 can be calculated according to the following formula (1).

[0131] Ac3 (°C) = 910 - 203 × [C] 1 / 2 -15.2×[Ni]+44.7×[Si]+104×

[0132] [V]+31.5×[Mo]+13.1×[W]-30×[Mn]-11×[Cr]-20×[Cu]+700×[P]+400×[Al]+120×[As]+400×[Ti]…(1)

[0133] In the above formula, [] represents the content (mass%) of each element, and the content of elements not included is 0.

[0134] (Refer to "Leslie Steel Materials Science", translated by Shigeyasu Koda, Maruzen Co., Ltd., 1985, p. 273)

[0135] (2) Cooling and retention within the temperature range of (Ms+10℃) to (Ms+110℃)

[0136] After the austenitization process described above, cooling is performed, such as... Figure 1 As shown in [3], within the temperature range of (Ms+10℃) to (Ms+110℃), the temperature is maintained for more than 10 seconds and less than 300 seconds at a cooling rate of more than 0℃ / second and less than 10℃ / second.

[0137] Cooling is performed at an average cooling rate of at least 15°C / second and less than 200°C / second, between (Ms+250°C) and (Ms+110°C). Maintaining an average cooling rate of 15°C / second or more is to suppress the formation of ferrite during cooling. The average cooling rate is preferably 20°C / second or more, more preferably 25°C / second or more. Furthermore, by maintaining an average cooling rate below 200°C / second, excessive thermal strain caused by rapid cooling can be prevented. The average cooling rate is preferably 150°C / second or less, more preferably 100°C / second or less. Examples of such cooling include... Figure 1 As shown in [2], before the quenching start temperature of (Ms+250℃) or higher, cooling is performed at a low average cooling rate of 0.1℃ / second or higher and 10℃ / second or lower. From the quenching start temperature to the stagnation start temperature of (Ms+110℃) or lower, cooling is performed at an average cooling rate of 20℃ / second or higher and 200℃ / second or lower.

[0138] Within a temperature range of (Ms+10℃) to (Ms+110℃), the object is held for at least 10 seconds and less than 300 seconds at a cooling rate of 0℃ / second to 10℃ / second. In other words, within a temperature range of (Ms+10℃) to (Ms+110℃), the object is placed for at least 10 seconds and less than 300 seconds at a cooling rate of 10℃ / second or less. A cooling rate of 10℃ / second or less is considered... Figure 1 [3] This also includes cases where the temperature is maintained at a certain level (i.e., the cooling rate is 0°C / second).

[0139] This retention partially allows bainite to form. Then, because bainite has a lower carbon solubility limit than austenite, carbon exceeding the solubility limit is expelled. As a result, a carbon-dense austenite region forms around the bainite. This region, after cooling and reheating as described later, becomes slightly coarser retained austenite. The partial formation of this "slightly coarser retained austenite" results in a standard deviation of the equivalent circle diameter of the retained austenite greater than 0.155 μm, which can improve the total elongation (EL) and uniform elongation (uEL).

[0140] If the retention temperature is below (Ms + 10°C), bainite is difficult to form, and therefore it is difficult to form a carbon-thickened austenite region (carbon-thickened region) around the bainite. As a result, the amount of coarse retained austenite is insufficient (i.e., the standard deviation of the equivalent circle diameter of the retained austenite becomes smaller), and the improvement effect of EL and uEL is insufficient. The retention temperature is preferably (Ms + 15°C) or higher, more preferably (Ms + 20°C) or higher. On the other hand, if the retention temperature is above (Ms + 110°C), the soft α-phase structure increases, and YS is insufficient. As a result, YR decreases. The retention temperature is preferably (Ms + 80°C) or lower, more preferably (Ms + 50°C) or lower.

[0141] Furthermore, if the residence time is shorter than 10 seconds, the area of ​​the carbon-thickened region becomes smaller, and the amount of coarse retained austenite is insufficient (i.e., the standard deviation of the equivalent circle diameter of the retained austenite becomes smaller), resulting in insufficient improvement in EL and uEL. The residence time is preferably 20 seconds or more, more preferably 30 seconds or more. On the other hand, if the residence time reaches 300 seconds or more, the carbon-thickened region becomes too large, resulting in coarser retained austenite and coarser MA, thus reducing the cavity expansion rate. The residence time is preferably 200 seconds or less, more preferably 100 seconds or less.

[0142] Furthermore, if the cooling rate during residence exceeds 10°C / second, sufficient bainitic phase transformation will not occur. Consequently, a sufficiently carbon-thickened region cannot be formed, and the amount of coarse retained austenite is insufficient (i.e., the standard deviation of the equivalent circle diameter of the retained austenite becomes smaller). The cooling rate during residence is preferably 8°C / second or less, and from the viewpoint of partially inducing further bainitic phase transformation, the cooling rate during residence is preferably 0°C / second.

[0143] As a preferred embodiment, it can be listed that the temperature range of (Ms+15℃) to (Ms+80℃) is maintained at a cooling rate of less than 8℃ / second for more than 10 seconds, during which time a certain temperature is maintained for 3 to 80 seconds.

[0144] As a more preferred embodiment, for example, the temperature range of (Ms+20°C) to (Ms+50°C) is maintained at a cooling rate of 3°C / second or less for more than 10 seconds, during which time the temperature is maintained at a certain temperature for 5 to 60 seconds.

[0145] Here, Ms(°C) can be calculated according to the following formula (2).

[0146] Ms(℃)=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo]…(2)

[0147] In the above formula, [] represents the content (mass%) of each element, and the content of elements not included is 0.

[0148] (Refer to "Leslie Steel Materials Science", translated and supervised by Nariyasu Koda, Maruzen Co., Ltd., 1985, p. 231)

[0149] (3) Cooling from a temperature above (Ms+10℃) to a cooling stop temperature below Ms℃

[0150] After the above-mentioned delay, such as Figure 1 As shown in [4], from the second cooling start temperature above (Ms+10℃), the temperature is cooled to a cooling stop temperature below Ms℃ at an average cooling rate of 10℃ / second or higher. Through this cooling, the martensitic phase transformation can occur while maintaining the aforementioned carbon-thickened region (austenite region). If the cooling stop temperature is higher than Ms, the amount of MA becomes excessive, and YR and λ decrease. In addition, if the average cooling rate is slower than 10℃ / second, the amount of carbon expelled from bainite during cooling increases, and the carbon that has already been expelled from bainite moves to a farther distance, thus the carbon-thickened region is excessively extended. The carbon-thickened region can be transformed into either retained austenite or untempered martensite, which are the constituent elements of MA, through cooling. Therefore, excessive extension of the carbon-thickened region leads to an excessive increase in retained austenite and untempered martensite, resulting in an excessive MA fraction and a decrease in the cavity expansion rate.

[0151] The cooling stop temperature is preferably (Ms-300°C) or higher and (Ms-40°C) or lower, more preferably (Ms-260°C) or higher and (Ms-200°C) or lower. The average cooling rate is preferably 15°C / second or higher, more preferably 20°C / second or higher. Additionally, if... Figure 1 As shown in [5], it can be held at the cooling stop temperature, and the preferred holding time can be listed as 1 second to 600 seconds. Even if the holding time is long, the effect on characteristics is not significant, but the holding time of more than 600 seconds reduces productivity.

[0152] (4) Reheating to the temperature range of (Ms-100℃)~(Ms+125℃)

[0153] like Figure 1As shown in [6], the temperature is heated from the aforementioned cooling stop temperature to a reheating temperature in the range of (Ms-100°C) to (Ms+125°C). This reheating allows carbon in the martensite to be expelled, promotes the thickening of carbon into the surrounding austenite, and stabilizes the austenite. As a result, the final amount of MA can be increased. If the reheating temperature is below Ms-100°C, the diffusion of carbon is insufficient and a sufficient amount of MA is not obtained, thus reducing TS×EL. On the other hand, if the reheating temperature is above Ms+125°C, carbon precipitates as cementite and a sufficient amount of MA is not obtained, thus reducing TS×EL. The reheating temperature is preferably (Ms-80°C) to (Ms+115°C), and more preferably (Ms-60°C) to (Ms+110°C).

[0154] like Figure 1 As shown in [7], after reaching the reheating temperature, it is preferable to maintain this temperature for a period of 50 seconds or more and 1200 seconds or less. The upper limit of the holding time is more preferably 900 seconds, and even more preferably 600 seconds. There is no particular limitation on the heating rate before the reheating temperature. Also, the term "holding" does not only refer to isothermal holding, but also includes slow cooling and / or heating within the above temperature range.

[0155] Furthermore, reheating, as described in samples No. 1 to 15 below, can also be performed in multiple stages. For example, in maintaining the reheating temperature range, reheating can be divided into three stages for molten zinc plating and alloying molten zinc plating. Even if reheating is performed in multiple stages, it has no effect on the steel microstructure. When reheating in multiple stages, the reheating temperature of each stage can be within the temperature range of (Ms - 100°C) to (Ms + 125°C). In addition, when holding the heating in each stage, it is preferable that the total holding time of each stage is 50 seconds or more and 1200 seconds or less. There is no particular limitation on the heating rate to the reheating temperature of each stage.

[0156] After reheating, as Figure 1 As shown in

[10] , it can be cooled to a temperature below 200°C, such as room temperature. As a preferred average cooling rate to cool to below 200°C, 10°C / second can be cited.

[0157] Through the above processes (1) to (4), the high-strength steel plate of the embodiment of the present invention can be obtained.

[0158] Those skilled in the art who have come into contact with the manufacturing method of the high-strength steel plate of the embodiments of the present invention described above may also obtain the high-strength steel plate of the embodiments of the present invention through trial and error, via a manufacturing method different from the manufacturing method described above.

[0159] The embodiments of this invention focus on steel plates (thin steel plates), but the product form is not particularly limited. After hot rolling or cold rolling, the steel plates that have undergone the above-mentioned heat treatment can also be subjected to coating treatments such as chemical forming, hot-dip galvanizing, electro-galvanizing, alloying hot-dip galvanizing, and vapor deposition, as well as various coatings, coating undercoat treatments, and organic film treatments. When performing electro-galvanizing, hot-dip galvanizing, and alloying hot-dip galvanizing, the following methods are preferred.

[0160] [Electro-zinc plating treatment]

[0161] After the heat treatment described above, the high-strength steel sheet of the embodiment of the present invention, obtained by cooling to room temperature, can be electroplated using conventional methods. For example, electroplating can be performed by immersing the high-strength steel sheet in a zinc solution at 50°C to 60°C while applying an electric current. By performing electroplating, the corrosion resistance of the steel sheet is improved.

[0162] [Melt-galvanizing treatment]

[0163] For example, hot-dip galvanizing can be performed by following conventional methods while maintaining the reheating temperature range described above. Hot-dip galvanizing can be performed by immersing the steel sheet in the reheating holding process for 1 to 10 seconds in a plating solution with the temperature adjusted to the aforementioned reheating temperature range. By performing hot-dip galvanizing, the corrosion resistance of the steel sheet is improved.

[0164] [Alloying molten zinc plating treatment]

[0165] Alloying molten zinc plating, for example, involves forming an alloyed molten zinc layer on the steel sheet surface after molten zinc plating, following conventional methods, while maintaining the reheating temperature range described above. Alloying, for example, involves maintaining the reheating temperature range after the molten zinc plating to obtain the desired alloying. The alloying temperature is not particularly limited, but if the alloying temperature is too low, alloying cannot proceed sufficiently. Preferably, it is 450°C or higher, more preferably 460°C or higher, and even more preferably 480°C or higher. Furthermore, the alloying treatment time is not particularly limited and can be adjusted to obtain the desired alloying. For example, the alloying treatment time is preferably 10 seconds or more and 60 seconds or less. By implementing alloyed molten zinc plating, the corrosion resistance of the steel sheet is improved.

[0166] Example

[0167] 1. Prepare the sample

[0168] Steel with the chemical composition shown in Table 1 below is smelted. Specifically, after one refining in a converter, desulfurization is carried out in the ladle. In addition, if necessary, vacuum degassing is carried out by the RH method after ladle refining. Also, in the chemical composition shown in Table 1, the balance is unavoidable impurities other than iron, N, P, S, and V. In addition, the Ac3 (°C) and Ms (°C) of the steel shown in Table 1 below are values ​​calculated by the above formulas (1) and (2) respectively. In addition, in Table 1, the content of unintentionally added V is described as "''". As shown in Table 1, V is not an element that is intentionally added (i.e., an unavoidable impurity), and the content is 0.005% by mass or less.

[0169] Subsequently, slabs were obtained through continuous casting using conventional methods. After hot rolling, they were then subjected to pickling and cold rolling using conventional methods to produce 1.4mm rolled products. Following this, a plating simulator was used... Figure 2 Heat treatment was performed under the conditions described in Table 2.

[0170] Furthermore, the numbers in Table 2, such as [2] shown in brackets [], correspond to... Figure 2 The processes with the same numbers shown in [ ] are listed in Table 2. Samples No. 1 to 15 are listed as follows: Figure 2 As shown in [7], [8] and [9], these are samples that were reheated in three stages. On the other hand, samples No. 16 and 17 are as follows: Figure 1 The sample shown is the one that was reheated only once.

[0171] Furthermore, in Tables 2 and 3, underlined values ​​indicate values ​​that deviate from the scope of embodiments of the present invention. However, it should be noted that the "-" symbol is not underlined even when it deviates from the scope of embodiments of the present invention.

[0172] Table 1

[0173]

[0174] Table 2

[0175]

[0176] 2. Steel structure

[0177] For each specimen, the MA fraction, the soft α-phase fraction, and the standard deviation of the equivalent circle diameter of the retained austenite were determined as follows. The calculation results are shown in Table 3.

[0178] [MA score]

[0179] By observing the t / 4 portion (t is the plate thickness) of the cross-section etched by nitric acid ethanol etching solution at SEM magnification of 3000x or higher, a straight line with a total length A of 300 μm or more is drawn at any position in the photograph. The intercept lengths of this line intersecting MA are measured, and the sum of these intercept lengths is taken as B. The MA fraction is calculated based on B / A. Furthermore, the MA fraction obtained in this way is an intercept length ratio, but it is equivalent to a volume ratio. Additionally, in embodiments of the present invention, areas that do not contain cementite and appear raised after etching are identified as MA.

[0180] [Partial fraction of soft α-phase tissue]

[0181] The soft α-phase structure was obtained by generating a GAM map using the EBSD method. The EBSD method measures a 40μm × 40μm region in a t / 4 section (t being the plate thickness) of a mirror-polished cross-section with a measurement step size of 0.1μm. From the EBSD analysis results, only regions with a CI value greater than 0.1 are extracted for analysis. Based on the area of ​​the soft α-phase structure with a cumulative GAM value less than 0.4 within the grains identified by EBSD as α-phase (i.e., with a BCC crystal structure), the ratio of this area to the total area is calculated, and the soft α-phase fraction is thus calculated. Furthermore, the soft α-phase fraction obtained in this way is an area ratio, but it is equivalent to a volume ratio.

[0182] [Standard deviation of the equivalent circle diameter of retained austenite]

[0183] The standard deviation of the equivalent circle diameter of the retained austenite was obtained by generating a phase map using the EBSD method. Similar to the fractional measurement of the soft α-phase structure, the EBSD method measures a 40μm × 40μm region at t / 4 of the mirror-polished section (t being the plate thickness) with a measurement interval of 0.1μm. Based on the obtained phase map, the area of ​​each austenitic phase (retained austenite) was calculated, and the equivalent circle diameter of each austenitic phase was determined from this area. The standard deviation of each diameter was calculated and used as the standard deviation of the equivalent circle diameter of the retained austenite.

[0184] 3.Mechanical properties

[0185] [Evaluation of tensile properties]

[0186] The obtained specimens were machined into JIS 5 gauge pieces with a parallel section length of 60 mm. Tensile properties were evaluated using a tensile testing machine with displacement control at a test speed of 10 mm / min. YS, TS, uEL, and EL were measured, and YR and TS×EL were calculated. Two tensile tests were performed under each condition, and the average values ​​were evaluated. The evaluation results are shown in Table 3. In the examples, specimens with TS above 780 MPa, YR above 0.65, uEL above 13%, EL above 21%, and TS×EL above 20000 MPa·%, were evaluated as having high strength and excellent processability.

[0187] [Evaluation of the expansion rate]

[0188] The cavity expansion rate λ is obtained according to JIS Z 2256. A hole with a diameter d0 (d0 = 10 mm) is punched in the test piece, and a punch with a front end angle of 60° is pressed into the hole. The diameter d of the hole is measured when the crack penetrates the thickness of the test piece, and is obtained according to the following formula (3). The calculation results are shown in Table 3. In the examples, λ of 20.0% or more is evaluated as high strength and excellent processability. In Table 3, the test pieces that meet all the evaluation criteria for tensile properties and cavity expansion rate are judged as qualified (〇), and the test pieces that do not meet any evaluation criteria are judged as unqualified (×).

[0189] λ(%)={(d-d0) / d0}×100…(3)

[0190] Table 3

[0191]

[0192] 4. Summary

[0193] The sample specimens of the embodiments that meet the conditions of the present invention, namely specimens No. 1 to 5, 7, 11 and 16, all achieve a tensile strength of 780 MPa or more, a yield strength ratio of 0.65 or more, a TS×EL of 20000 MPa·% or more, a total elongation of 21% or more, a uniform elongation of more than 13% and a cavity expansion rate of 20% or more.

[0194] In contrast, sample No. 6 had a higher MA fraction because its cooling stop temperature was higher than the temperature range below Ms℃. As a result, YR decreased and a sufficient cavity expansion rate λ could not be obtained.

[0195] Samples 8–10, 13–15, and 17 had excessively high soft α-phase fractions due to the austenitizing heating temperatures being lower than (Ac3+30℃), resulting in insufficient YS and reduced YR.

[0196] After austenitization, sample No. 12 remained at a temperature below (Ms+10℃) to (Ms+110℃), resulting in insufficient coarse retained austenite and a reduced standard deviation of the equivalent circle diameter of the retained austenite. Consequently, sufficient uniform elongation and total elongation could not be obtained.

[0197] Furthermore, in addition to the excessively high soft α-phase fraction, sample No. 9 also had a small standard deviation of the equivalent circle diameter of the retained austenite. In addition, although samples No. 13 to 15 had low holding temperatures as mentioned above [3], the standard deviation of the equivalent circle diameter of the retained austenite met the requirements. The inventors currently believe the reasons are as follows.

[0198] As mentioned above, with the bainitic transformation, it is easier to form carbon-thickened regions, thus increasing the standard deviation of the equivalent circle diameter of the retained austenite. However, when the bainitic transformation proceeds excessively, a large amount of carbon-concentrated retained austenite is formed, which can become the reason for the decrease in the standard deviation of the equivalent circle diameter of the retained austenite.

[0199] As mentioned above, when the heating temperature of [1] is low, the original γ particle size is refined and the bainitic phase transformation is promoted. In this case, because carbon-thickened regions are easily formed, the standard deviation of the equivalent circle diameter of the retained austenite increases. Although the holding temperature of [3] is low in samples No. 13 to 15, the standard deviation of the equivalent circle diameter of the retained austenite meets the requirements under this effect.

[0200] Furthermore, as mentioned above, if the heating temperature of [1] is too low, ferrite, which is a type of soft α phase, will form. In this case, the bainitic phase transformation is delayed due to carbon distribution to austenite. In other words, the bainitic phase transformation is fastest within a certain temperature range when the heating temperature of [1] is changed. Moreover, the fastest bainitic phase transformation is considered to occur when the heating temperature of [1] is around 860°C.

[0201] In Sample No. 9, since the heating temperature of [1] above is 860°C, it is considered that the bainitic phase transformation is greatly promoted, and the holding temperature of [3] above is also set at a temperature at which the bainitic phase transformation is easy to carry out. Therefore, in Sample No. 9 (even compared with Sample No. 14), the bainitic phase transformation is excessive, and it can be considered that the standard deviation of the equivalent circle diameter of the retained austenite is smaller.

[0202] Furthermore, although the standard deviation of the equivalent circle diameter of the retained austenite in Sample No. 9 is smaller, the total elongation (EL) and uniform elongation (uEL) are still good. The inventors currently consider the following reasons: As mentioned above, the ductility is improved when the soft α-phase structure is present; therefore, even if the standard deviation of the equivalent circle diameter of the retained austenite is smaller, the total elongation (EL) and uniform elongation (uEL) may still be good. Because Sample No. 9 has a high proportion of soft α-phase structure, the standard deviation of the equivalent circle diameter of the retained austenite is small, but the total elongation (EL) and uniform elongation (uEL) can still be considered good.

[0203] Furthermore, the MA fraction of sample No. 9 meets the requirement (15.0% or less), but λ is outside the requirement (20.0% or more). For samples No. 14 and No. 17, the MA fraction is higher than the requirement (15.0% or less), but λ is within the requirement (20.0% or more). The inventors currently consider the following reasons.

[0204] As mentioned above, the presence of the soft α-phase, similar to that of MA, can easily lead to fracture and may reduce λ. Therefore, sample No. 9, with a soft α-phase fraction outside the specified range (below 50%), is considered to have λ outside the specified range. Furthermore, as mentioned above, the presence of MA causes uneven mechanical properties; even if the MA fraction does not meet the requirement of 15.0%, it may still meet the λ requirement. Therefore, samples No. 14 and No. 17, although having an MA fraction higher than the specified range (below 15.0%), are considered to have λ within the specified range (above 20.0%).

[0205] This application is accompanied by a priority claim based on Japanese Patent Application No. 2020-150167, filed on September 7, 2020. Japanese Patent Application No. 2020-150167 is incorporated herein by reference.

Claims

1. A high-strength steel plate, comprising: C: Higher than 0.15% by mass and lower than 0.35% by mass Si: Above 0% by mass and below 3.0% by mass Mn: ≥1.0% by mass and ≤4.0% by mass Al: Above 0% by mass and below 3.0% by mass N: greater than 0% by mass and less than 0.01% by mass P: greater than 0% by mass and less than 0.05% by mass S: greater than 0% by mass and less than 0.01% by mass, and The total Si and Al content is ≥0.5% by mass and ≤3.0% by mass. The balance includes Fe and unavoidable impurities. The steel microstructure is defined as the area percentage of the total steel microstructure. The MA score is above 0% and below 15.0%. The fraction of soft α-phase tissue was above 0% and below 50%. The standard deviation of the equivalent circle diameter of the retained austenite is greater than 0.155 μm and less than 0.500 μm.

2. The high-strength steel plate according to claim 1, wherein, It also contains one or more elements selected from the following groups: Cu: above 0% by mass and below 0.50% by mass Ni: above 0% by mass and below 0.50% by mass Cr: greater than 0% by mass and less than 0.50% by mass Mo: above 0% by mass and below 0.50% by mass B: Higher than 0% by mass and lower than 0.01% by mass V: Higher than 0.005% by mass and lower than 0.05% by mass Nb: above 0% by mass and below 0.05% by mass Ti: above 0% by mass and below 0.05% by mass Ca: above 0% by mass and below 0.05% by mass, and REM: Above 0% by mass and below 0.01% by mass.

3. An electro-galvanized steel sheet having an electro-galvanized layer on the surface of the high-strength steel sheet as described in claim 1 or claim 2.

4. A molten galvanized steel sheet having a molten galvanized layer on the surface of the high-strength steel sheet as described in claim 1 or claim 2.

5. An alloyed molten galvanized steel sheet, having an alloyed molten galvanized layer on the surface of the high-strength steel sheet as described in claim 1 or claim 2.

6. A method for manufacturing the high-strength steel plate according to claim 1, wherein, The process includes the following steps: A process for preparing rolled material having the composition described in claim 1; The process of heating the rolled material to a temperature of (Ac3 + 30°C) or higher for austenitization; After austenitization, the temperature is cooled at an average cooling rate of 15°C / second or higher and lower than 200°C / second between (Ms+250°C) and (Ms+110°C), and the temperature is held at a cooling rate of 0°C / second or higher and lower than 10°C / second for 30 seconds or more and lower than 100 seconds within the temperature range of (Ms+110°C) to (Ms+10°C). The process of cooling from a temperature of (Ms+10℃) or higher to a cooling stop temperature below Ms℃ at an average cooling rate of 10℃ / second or higher after the retention. The process of heating from the cooling stop temperature to a reheating temperature in the range of (Ms-100℃)~(Ms+125℃) Here, Ac3 and Ms are calculated by the following formula: Ac3⼝910−203×⼻C) 1 / 2 -15.2×Ni)+44.7×Si+104×V+31.5×Mo+13.1×7–30×M n)-11×Cr-20×Cu+700×Al+400×Al+120×As+400×Ti Ms=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo] Wherein, [] represents the content of each element in terms of mass%, and the content of elements not included is 0. The units of Ac3 and Ms are ℃.

7. The method for manufacturing high-strength steel plate according to claim 6, wherein, After heating to the reheating temperature, the reheating temperature is maintained for more than 50 seconds and less than 1200 seconds.

8. A method for manufacturing an electro-galvanized steel sheet, wherein, Electro-galvanizing is performed on the surface of the high-strength steel sheet obtained by the method of claim 6 or claim 7.

9. A method for manufacturing a hot-dip galvanized steel sheet, wherein, When heated at the reheating temperature described in claim 6 or claim 7, the surface of the steel plate is subjected to molten zinc plating.

10. A method for manufacturing alloyed molten galvanized steel sheet, wherein, Following the molten zinc plating as described in claim 9, a heat treatment for alloying is performed.

11. A method for manufacturing the high-strength steel plate according to claim 2, wherein, The process includes the following steps: A process for preparing rolled material having the composition described in claim 2; The process of heating the rolled material to a temperature of (Ac3 + 30°C) or higher for austenitization; After austenitization, the process involves cooling at an average cooling rate of 15°C / second or higher and lower than 200°C / second between (Ms+250°C) and (Ms+110°C), and holding the sample at a cooling rate of 0°C / second or higher and lower than 10°C / second for 30 seconds or more and lower than 100 seconds within the temperature range of (Ms+110°C) to (Ms+10°C). The process of cooling from a temperature above (Ms+10℃) to a cooling stop temperature below Ms℃ at an average cooling rate of 10℃ / second or higher after the retention. The process of heating from the cooling stop temperature to a reheating temperature in the range of (Ms-100℃)~(Ms+125℃) Here, Ac3 and Ms are calculated by the following formula: Ac3⼝910−203×⼻C) 1 / 2 -15.2×Ni)+44.7×Si+104×V+31.5×Mo+13.1×7–30×M n)-11×Cr-20×Cu+700×Al+400×Al+120×As+400×Ti Ms=561-474×[C]-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo] Wherein, [] represents the content of each element in terms of mass%, and the content of elements not included is 0. The units of Ac3 and Ms are ℃.

12. The method for manufacturing high-strength steel plate according to claim 11, wherein, After heating to the reheating temperature, the reheating temperature is maintained for more than 50 seconds and less than 1200 seconds.

13. A method for manufacturing an electro-galvanized steel sheet, wherein, Electro-galvanizing is performed on the surface of the high-strength steel sheet obtained by the method of claim 11 or claim 12.

14. A method for manufacturing a hot-dip galvanized steel sheet, wherein, When heated to the reheating temperature described in claim 11 or claim 12, molten zinc plating is performed on the surface of the steel plate.

15. A method for manufacturing alloyed molten galvanized steel sheet, wherein, Following the molten zinc plating as described in claim 14, a heat treatment for alloying is performed.

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