Steel sheet, steel strip and method for manufacturing the same

Abstract

Provided is a steel sheet and a steel strip having TS of 590 MPa or more and less than 1180 MPa, and excellent stability of mechanical properties (YS, TS, El, λ), and a method for manufacturing the same. A steel sheet having a composition containing C, Si, Mn, P, S, Al, and N in specific amounts, and having, in terms of area ratio, 30.0% to 90.0% of a soft phase and 10.0% to 70.0% of a hard phase, and in the hard phase, 65% or more of the hard phase having [C content in the hard phase] / [C content in the steel sheet] of 1.50 or more and 10% or more of the hard phase having [Mn content in the hard phase] / [Mn content in the steel sheet] of 1.30 or less.

Description

Technical Field

[0001] This invention relates to steel plates and strips with excellent mechanical properties and stability, which are used in various applications such as automobiles and home appliances, and particularly help to improve the yield rate of structural component manufacturing. Background Technology

[0002] To balance improving fuel efficiency and reducing CO2 emissions through thinner, lighter steel sheets used in car bodies, as well as enhancing crash safety, the use of high-strength steel sheets in automobiles is progressing. Furthermore, new and stringent regulations regarding CO2 emissions and crash safety are being introduced. Therefore, to increase body strength, there is an increasing use of high-strength steel sheets, particularly those with a tensile strength (TS) of 590 MPa or higher, for major structural components and reinforcing parts installed on the car's cab frame (hereinafter also referred to as automotive frame structural components).

[0003] As structural components of automobile frames, the application of high-strength steel sheets with TS values ​​of 590MPa to 780MPa is being promoted in impact energy absorption components, such as the left and right longitudinal beams and the rear longitudinal beams. Furthermore, in the passenger compartment frame components, such as the center pillar, which protect occupants, the application of high-strength steel sheets with TS values ​​of 980MPa and above is being promoted. For impact energy absorption components, there is a demand for improved YS, which is related to impact energy absorption performance, and improved deformation performance (E1), which allows the steel sheet to absorb energy through a serpentine deformation without cracking (fracture) during a collision. Moreover, for passenger compartment frame components, strength is further increased, and high-strength steel sheets with a 980MPa grade are gradually being used. Therefore, in addition to the aforementioned YS and TS, there is a demand for improved E1, which represents the in-plane stamping formability of the steel sheet. Furthermore, there is a demand for improved λ, which represents the stamping formability of the steel sheet ends.

[0004] From a design (appearance) perspective, and with the aim of improving load-bearing capacity and body rigidity to ensure crash safety, the shapes of the skeletal structural components of automobile bodies are becoming increasingly complex. In the stamping process of such complex shapes, reducing the fluctuations in the mechanical properties (YS, TS, E1, λ) of the steel sheet (improving the stability of mechanical properties) is crucial for manufacturing automobile bodies without defects.

[0005] Therefore, it is desirable to develop steel plates with excellent mechanical stability.

[0006] For example, Patent Document 1 discloses a method for manufacturing high-strength cold-rolled steel sheets as blanks for automotive parts. When manufacturing high-strength cold-rolled steel sheets with a composite microstructure mainly composed of ferrite and tempered martensite on a continuous annealing line, the continuous annealing line sequentially passes through a heating process, a slow cooling process to the quenching start temperature Tq, a quenching process based on rapid cooling, and a tempering process based on reheating. The method is characterized by immediately measuring the ferrite fraction Vf of the steel sheet by magnetic properties after the quenching process, comparing it with a pre-set target value Vf0 of the ferrite fraction required to achieve the target value of the mechanical properties of the product steel sheet, and adjusting the quenching start temperature Tq so that the deviation ΔVf = Vf - Vf0 of the target value Vf0 of the ferrite fraction approaches 0. This reduces microstructure fluctuations caused by heat treatment temperature variations on the continuous annealing line and the mechanical property fluctuations caused by these microstructure fluctuations.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 5374479 Summary of the Invention

[0010] However, although Patent Document 1 discloses a method for manufacturing high-strength cold-rolled steel sheets, in which a composite microstructure consisting mainly of ferrite and tempered martensite is manufactured on a continuous annealing line, the continuous annealing line sequentially passes through a heating process, a slow cooling process to the quenching start temperature Tq, a quenching process based on rapid cooling, and a tempering process based on reheating. The method is characterized by immediately measuring the ferrite fraction Vf of the steel sheet by magnetic properties after the quenching process, comparing it with a pre-set target value Vf0 of the ferrite fraction required to achieve the target value of the mechanical properties of the product steel sheet, and adjusting the quenching start temperature Tq so that the deviation ΔVf = Vf - Vf0 of the target value Vf0 of the ferrite fraction approaches 0, thereby reducing the microstructure fluctuations caused by heat treatment temperature variations on the continuous annealing line and the mechanical property fluctuations caused by these microstructure fluctuations; however, this method is limited to cold-rolled steel sheets, and sometimes controlling the ferrite fraction alone cannot sufficiently reduce the fluctuations in mechanical properties.

[0011] As stated above, the steel plate and its manufacturing method disclosed in Patent Document 1 may not necessarily reduce the fluctuations in the mechanical properties (YS, TS, E1, λ) of the steel plate.

[0012] That is, there has always been a desire to develop a new technology that can reduce the fluctuations in the mechanical properties of steel plates.

[0013] The present invention was developed in view of the above-mentioned situation, and its purpose is to provide a steel plate and steel strip with a TS of 590MPa or higher and lower than 1180MPa, excellent mechanical properties (YS, TS, E1, λ), and small fluctuations in mechanical properties (YS, TS, E1, λ), that is, excellent stability of mechanical properties, and a method thereof for manufacturing the same.

[0014] It should be noted that in this invention, steel strip refers to a product with a mass of 5 tons or more and a width of 500 mm or more, wound into a coil shape. The length of the steel strip in the longitudinal direction exceeds 10 m. Furthermore, steel plate refers to a portion taken from the steel strip, with a width of 500 mm or more and a length in the longitudinal direction of 10 m or less, preferably less than 1 m.

[0015] Furthermore, in this invention, excellent mechanical properties (YS, TS, E1, λ) mean that the yield stress (YS) and total elongation (E1) measured by tensile test according to JIS Z 2241 (2011), and the expansion ratio (λ) measured by expansion test according to Japan Iron and Steel Federation Standard JFST1001, satisfy the following formulas 1 to 3 based on the tensile strength (TS) measured by the above tensile test.

[0016] Formula 1:

[0017] When 590MPa≤TS<780MPa, 360MPa≤YS

[0018] When 780MPa≤TS<980MPa, 460MPa≤YS

[0019] When 980MPa≤TS<1180MPa, 580MPa≤YS

[0020] Formula 2:

[0021] When 590MPa≤TS<780MPa, 23.0%≤El

[0022] When 780MPa≤TS<980MPa, 17.0%≤El

[0023] When 980MPa≤TS<1180MPa, 11.0%≤El

[0024] Formula 3:

[0025] When 590MPa≤TS<780MPa, 45%≤λ

[0026] When 780MPa≤TS<980MPa, 30%≤λ

[0027] When 980MPa≤TS<1180MPa, 20%≤λ

[0028] Furthermore, in this invention, a steel plate with excellent mechanical property stability refers to a steel plate whose maximum and minimum values ​​of TS, YS, E1, and λ at positions 1 / 4, 1 / 2, and 3 / 4 of the width direction of the steel plate are: ΔTS (maximum value of TS - minimum value of TS), ΔYS (maximum value of YS - minimum value of YS), ΔE1 (maximum value of E1 - minimum value of E1), and Δλ (maximum value of λ - minimum value of λ). These ΔTS, ΔYS, ΔE1, and Δλ satisfy the following equations 4, 6, 8, and 10.

[0029] The standard deviations σTS, σYS, σEl, and σλ obtained from TS, YS, El, and λ at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel plate satisfy the following equations 5, 7, 9, and 11.

[0030] Furthermore, in this invention, a steel strip with excellent mechanical property stability refers to a steel strip at each of the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at each of the 10m front, 10m rear, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip, where the differences between the maximum and minimum values ​​of TS, YS, E1, and λ are: ΔTS (maximum value of TS - minimum value of TS), ΔYS (maximum value of YS - minimum value of YS), ΔE1 (maximum value of E1 - minimum value of E1), Δλ (maximum value of λ - minimum value of λ). These ΔTS, ΔYS, ΔE1, and Δλ satisfy the following equations 4, 6, 8, and 10.

[0031] The standard deviations σTS, σYS, σEl, and σλ obtained from the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, respectively, and from the 10m front, 10m rear, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip, satisfy the following equations 5, 7, 9, and 11.

[0032] Formula 4:

[0033] When 590MPa≤TS<780MPa, ΔTS≤40MPa

[0034] When 780MPa≤TS<980MPa, ΔTS≤50MPa

[0035] When 980MPa≤TS<1180MPa, ΔTS≤60MPa

[0036] Formula 5:

[0037] When 590MPa≤TS<780MPa, σTS≤20MPa

[0038] When 780MPa≤TS<980MPa, σTS≤25MPa

[0039] When 980MPa≤TS<1180MPa, σTS≤30MPa

[0040] Formula 6:

[0041] When 590MPa≤TS<780MPa, ΔYS≤40MPa

[0042] When 780MPa≤TS<980MPa, ΔYS≤50MPa

[0043] When 980MPa≤TS<1180MPa, ΔYS≤60MPa

[0044] Formula 7:

[0045] When 590MPa≤TS<780MPa, σYS≤20MPa

[0046] When 780MPa≤TS<980MPa, σYS≤25MPa

[0047] When 980MPa≤TS<1180MPa, σYS≤30MPa

[0048] Formula 8:

[0049] When 590MPa≤TS<780MPa, ΔEl≤4.0%

[0050] When 780MPa≤TS<980MPa, ΔEl≤4.5%

[0051] When 980MPa≤TS<1180MPa, ΔEl≤5.0%

[0052] Formula 9:

[0053] When 590MPa≤TS<780MPa, σEl≤2.0%

[0054] When 780MPa≤TS<980MPa, σEl≤2.5%

[0055] When 980MPa≤TS<1180MPa, σEl≤3.0%

[0056] Formula 10:

[0057] When 590MPa≤TS<780MPa, Δλ≤25%.

[0058] When 780MPa≤TS<980MPa, Δλ≤25%.

[0059] When 980MPa≤TS<1180MPa, Δλ≤25%

[0060] Formula 11:

[0061] When 590MPa≤TS<780MPa, σλ≤7%

[0062] When 780MPa≤TS<980MPa, σλ≤7%

[0063] When 980MPa≤TS<1180MPa, σλ≤7%

[0064] The inventors conducted in-depth research to solve the above-mentioned problems and obtained the following insights.

[0065] (1) With the specified composition, the area fraction of the soft phase (selected from one or more of unrecrystallized ferrite, recrystallized ferrite, phase transformation ferrite, epitaxial ferrite and bainitic ferrite) is controlled to be below 90.0%, and the area fraction of the hard phase (selected from one or more of fresh martensite and tempered martensite) is controlled to be above 10.0%, thereby ensuring a TS of 590 MPa or above.

[0066] (2) With a specified composition, the area fraction of the soft phase (selected from one or more of unrecrystallized ferrite, recrystallized ferrite, phase transformation ferrite, epitaxial ferrite and bainitic ferrite) is controlled at 30.0% or more, and the area fraction of the hard phase (selected from one or more of fresh martensite and tempered martensite) is controlled at 70.0% or less, thereby becoming a TS with a strength of less than 1180 MPa.

[0067] (3) By controlling the tissue fraction of soft phase and hard phase as described above with the specified composition, the following formulas 1 to 3 can be achieved.

[0068] Formula 1:

[0069] When 590MPa≤TS<780MPa, 360MPa≤YS

[0070] When 780MPa≤TS<980MPa, 460MPa≤YS

[0071] When 980MPa≤TS<1180MPa, 580MPa≤YS

[0072] Formula 2:

[0073] When 590MPa≤TS<780MPa, 23.0%≤El

[0074] When 780MPa≤TS<980MPa, 17.0%≤El

[0075] When 980MPa≤TS<1180MPa, 11.0%≤El

[0076] Formula 3:

[0077] When 590MPa≤TS<780MPa, 45%≤λ

[0078] When 780MPa≤TS<980MPa, 30%≤λ

[0079] When 980MPa≤TS<1180MPa, 20%≤λ

[0080] (4) Furthermore, by controlling the hard phase in the above-mentioned hard phase to have a hard phase content of 65% or more in terms of area ratio of [C content (mass%) in the hard phase] / [C content (mass%) in the steel] and a hard phase content of 10% or more in terms of area ratio of [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel] of 1.30 or less, the fluctuation of mechanical properties (YS, TS, E1, λ) can be reduced (improving the stability of mechanical properties).

[0081] (5) In the annealing and homogenization process, a microstructure containing ferrite (unrecrystallized ferrite and recrystallized ferrite) and austenite is formed. In the subsequent rapid heating process from the annealing and homogenization temperature at an average heating rate of 10°C / second or higher to (annealing and homogenization temperature + 10°C) or higher, due to the relatively small diffusion of the diffusion-substitution element Mn from the ferrite to the austenite, a large amount of austenite mainly composed of the diffusion of the intrusive element C is generated and grown. The region where austenite is generated and grown mainly by the diffusion of C has a high C and low Mn composition. Therefore, as the final microstructure formed after the subsequent cooling process, it becomes a hard phase with a high C and low Mn composition. This hard phase with a high C and low Mn composition is a new microstructure achieved through the above-mentioned annealing and homogenization process to the above-mentioned rapid heating process. By generating an appropriate amount of this hard phase with a high C and low Mn composition and controlling the microstructure ratio of hard phase and soft phase, steel plates with excellent mechanical property stability can be manufactured. That is, the differences between the maximum and minimum values ​​of TS, YS, El, and λ at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel plate: ΔTS (maximum value of TS - minimum value of TS), ΔYS (maximum value of YS - minimum value of YS), ΔEl (maximum value of El - minimum value of El), and Δλ (maximum value of λ - minimum value of λ), satisfy the following equations 4, 6, 8, and 10, and,

[0082] The standard deviations σTS, σYS, σEl, and σλ obtained from TS, YS, El, and λ at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel plate satisfy the following equations 5, 7, 9, and 11.

[0083] Furthermore, as a steel strip, at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction, the differences between the maximum and minimum values ​​of TS, YS, El, and λ at the 10m front, 10m rear, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip: ΔTS (maximum value of TS - minimum value of TS), ΔYS (maximum value of YS - minimum value of YS), ΔEl (maximum value of El - minimum value of El), and Δλ (maximum value of λ - minimum value of λ) satisfy the following equations 4, 6, 8, and 10, and...

[0084] The standard deviations σTS, σYS, σEl, and σλ obtained from the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, respectively, and from the 10m front, 10m rear, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip, satisfy the following equations 5, 7, 9, and 11.

[0085] Formula 4:

[0086] When 590MPa≤TS<780MPa, ΔTS≤40MPa

[0087] When 780MPa≤TS<980MPa, ΔTS≤50MPa

[0088] When 980MPa≤TS<1180MPa, ΔTS≤60MPa

[0089] Formula 5:

[0090] When 590MPa≤TS<780MPa, σTS≤20MPa

[0091] When 780MPa≤TS<980MPa, σTS≤25MPa

[0092] When 980MPa≤TS<1180MPa, σTS≤30MPa

[0093] Formula 6:

[0094] When 590MPa≤TS<780MPa, ΔYS≤40MPa

[0095] When 780MPa≤TS<980MPa, ΔYS≤50MPa

[0096] When 980MPa≤TS<1180MPa, ΔYS≤60MPa

[0097] Formula 7:

[0098] When 590MPa≤TS<780MPa, σYS≤20MPa

[0099] When 780MPa≤TS<980MPa, σYS≤25MPa

[0100] When 980MPa≤TS<1180MPa, σYS≤30MPa

[0101] Formula 8:

[0102] When 590MPa≤TS<780MPa, ΔEl≤4.0%

[0103] When 780MPa≤TS<980MPa, ΔEl≤4.5%

[0104] When 980MPa≤TS<1180MPa, ΔEl≤5.0%

[0105] Formula 9:

[0106] When 590MPa≤TS<780MPa, σEl≤2.0%

[0107] When 780MPa≤TS<980MPa, σEl≤2.5%

[0108] When 980MPa≤TS<1180MPa, σEl≤3.0%

[0109] Formula 10:

[0110] When 590MPa≤TS<780MPa, Δλ≤25%.

[0111] When 780MPa≤TS<980MPa, Δλ≤25%.

[0112] When 980MPa≤TS<1180MPa, Δλ≤25%

[0113] Formula 11:

[0114] When 590MPa≤TS<780MPa, σλ≤7%

[0115] When 780MPa≤TS<980MPa, σλ≤7%

[0116] When 980MPa≤TS<1180MPa, σλ≤7%

[0117] This disclosure is based on the above insights. That is, the key points of this disclosure are as follows.

[0118] [1] A steel plate having the following composition, having a tensile strength of 590 MPa or more and less than 1180 MPa.

[0119] The composition, by mass%, contains C: 0.030%–0.250%, Si: 0.01%–2.50%, Mn: 1.30% or more and less than 3.50%, P: 0.001%–0.100%, S: less than 0.0200%, Al: 0.010%–2.000%, N: less than 0.0100%, with the balance consisting of Fe and unavoidable impurities.

[0120] In the microstructure at 1 / 4 of the steel plate thickness,

[0121] The area fraction of the soft phase is 30.0%–90.0%, and,

[0122] The area fraction of the hard phase is 10.0%–70.0%.

[0123] In the aforementioned hard phase,

[0124] The hard phase having a C content (mass%) in the hard phase of the plate of steel and a C content (mass%) of 1.50 or higher, and a hard phase comprising 65% or more by area, and...

[0125] The hard phase with a ratio of [Mn content in the hard phase (mass%)] / [Mn content in the steel plate (mass%)] of less than 1.30 has a content of more than 10% by area.

[0126] Furthermore, when the tensile strength is set to TS, the yield stress to YS, the total elongation to E1, and the porosity to λ,

[0127] At positions 1 / 4, 1 / 2, and 3 / 4 of the width of the steel plate, the following equations 1 to 3 are satisfied.

[0128] Furthermore, at the measurement positions of TS, YS, El, and λ at the 1 / 4, 1 / 2, and 3 / 4 positions along the width direction of the steel plate, the differences between their respective maximum and minimum values ​​(maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El), and (maximum value of λ - minimum value of λ), i.e., ΔTS, ΔYS, ΔEl, and Δλ, successively satisfy the following equations 4, 6, 8, and 10.

[0129] Furthermore, the standard deviations σTS, σYS, σEl, and σλ obtained from the measurement positions at 1 / 4, 1 / 2, and 3 / 4 of the width direction of the steel plate, respectively, satisfy Equations 5, 7, 9, and 11.

[0130] Formula 1:

[0131] When 590MPa≤TS<780MPa, 360MPa≤YS

[0132] When 780MPa≤TS<980MPa, 460MPa≤YS

[0133] When 980MPa≤TS<1180MPa, 580MPa≤YS

[0134] Formula 2:

[0135] When 590MPa≤TS<780MPa, 23.0%≤El

[0136] When 780MPa≤TS<980MPa, 17.0%≤El

[0137] When 980MPa≤TS<1180MPa, 11.0%≤El

[0138] Formula 3:

[0139] When 590MPa≤TS<780MPa, 45%≤λ

[0140] When 780MPa≤TS<980MPa, 30%≤λ

[0141] When 980MPa≤TS<1180MPa, 20%≤λ

[0142] Formula 4:

[0143] When 590MPa≤TS<780MPa, ΔTS≤40MPa

[0144] When 780MPa≤TS<980MPa, ΔTS≤50MPa

[0145] When 980MPa≤TS<1180MPa, ΔTS≤60MPa

[0146] Formula 5:

[0147] When 590MPa≤TS<780MPa, σTS≤20MPa

[0148] When 780MPa≤TS<980MPa, σTS≤25MPa

[0149] When 980MPa≤TS<1180MPa, σTS≤30MPa

[0150] Formula 6:

[0151] When 590MPa≤TS<780MPa, ΔYS≤40MPa

[0152] When 780MPa≤TS<980MPa, ΔYS≤50MPa

[0153] When 980MPa≤TS<1180MPa, ΔYS≤60MPa

[0154] Formula 7:

[0155] When 590MPa≤TS<780MPa, σYS≤20MPa

[0156] When 780MPa≤TS<980MPa, σYS≤25MPa

[0157] When 980MPa≤TS<1180MPa, σYS≤30MPa

[0158] Formula 8:

[0159] When 590MPa≤TS<780MPa, ΔEl≤4.0%

[0160] When 780MPa≤TS<980MPa, ΔEl≤4.5%

[0161] When 980MPa≤TS<1180MPa, ΔEl≤5.0%

[0162] Formula 9:

[0163] When 590MPa≤TS<780MPa, σEl≤2.0%

[0164] When 780MPa≤TS<980MPa, σEl≤2.5%

[0165] When 980MPa≤TS<1180MPa, σEl≤3.0%

[0166] Formula 10:

[0167] When 590MPa≤TS<780MPa, Δλ≤25%.

[0168] When 780MPa≤TS<980MPa, Δλ≤25%.

[0169] When 980MPa≤TS<1180MPa, Δλ≤25%

[0170] Formula 11:

[0171] When 590MPa≤TS<780MPa, σλ≤7%

[0172] When 780MPa≤TS<980MPa, σλ≤7%

[0173] When 980MPa≤TS<1180MPa, σλ≤7%.

[0174] [2] According to the steel plate described in [1] above, wherein the above composition further contains, by mass%, the following components: Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less. The following are at least one of the following: Zn: less than 0.0200%, Co: less than 0.0200%, Zr: less than 0.1000%, Ca: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0500%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

[0175] [3] The steel plate according to [1] or [2] above, wherein a coating is provided on the surface of the steel plate.

[0176] The above coating is any one of hot-dip galvanized coating, alloyed hot-dip galvanized coating, electro-galvanized coating, and hot-dip aluminized coating.

[0177] [4] A steel strip having the following composition, having a tensile strength of 590 MPa or more and less than 1180 MPa.

[0178] The composition, by mass%, contains C: 0.030%–0.250%, Si: 0.01%–2.50%, Mn: ≥1.30% and <3.50%, P: 0.001%–0.100%, S: <0.0200%, Al: 0.010%–2.000%, N: <0.0100%, with the balance consisting of Fe and unavoidable impurities.

[0179] In the microstructure at the 1 / 4 position of the steel strip thickness,

[0180] The area fraction of the soft phase is 30.0%–90.0%, and,

[0181] The area fraction of the hard phase is 10.0%–70.0%.

[0182] In the aforementioned hard phase,

[0183] The hard phase having a C content (mass%) in the hard phase of the strip being greater than or equal to 1.50 and a C content (mass%) in the steel strip being greater than or equal to 65% by area, and...

[0184] The hard phase with a ratio of [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel strip] of less than 1.30 has a surface area ratio of more than 10%.

[0185] Furthermore, when the tensile strength is set to TS, the yield stress to YS, the total elongation to E1, and the porosity to λ,

[0186] At positions 1 / 4, 1 / 2, and 3 / 4 of the width of the steel strip,

[0187] At positions 10m from the beginning, 10m from the end, 1 / 4, 1 / 2, and 3 / 4 of the length of the steel strip, the following equations 1 to 3 must be satisfied.

[0188] Furthermore, at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip,

[0189] At positions 10m from the front end, 10m from the rear end, 1 / 4 position, 1 / 2 position, and 3 / 4 position along the length of the steel strip, the differences between the maximum and minimum values ​​of TS, YS, El, and λ (maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El), and (maximum value of λ - minimum value of λ), i.e., ΔTS, ΔYS, ΔEl, and Δλ, respectively, satisfy Equations 4, 6, 8, and 10.

[0190] Furthermore, at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip,

[0191] The standard deviations σTS, σYS, σEl, and σλ obtained from TS, YS, E1, and λ at the positions 10m from the front end, 10m from the rear end, 1 / 4 position, 1 / 2 position, and 3 / 4 position along the length of the steel strip respectively satisfy the following equations 5, 7, 9, and 11 respectively.

[0192] Formula 1:

[0193] When 590MPa≤TS<780MPa, 360MPa≤YS

[0194] When 780MPa≤TS<980MPa, 460MPa≤YS

[0195] When 980MPa≤TS<1180MPa, 580MPa≤YS

[0196] Formula 2:

[0197] When 590MPa≤TS<780MPa, 23.0%≤El

[0198] When 780MPa≤TS<980MPa, 17.0%≤El

[0199] When 980MPa≤TS<1180MPa, 11.0%≤El

[0200] Formula 3:

[0201] When 590MPa≤TS<780MPa, 45%≤λ

[0202] When 780MPa≤TS<980MPa, 30%≤λ

[0203] When 980MPa≤TS<1180MPa, 20%≤λ

[0204] Formula 4:

[0205] When 590MPa≤TS<780MPa, ΔTS≤40MPa

[0206] When 780MPa≤TS<980MPa, ΔTS≤50MPa

[0207] When 980MPa≤TS<1180MPa, ΔTS≤60MPa

[0208] Formula 5:

[0209] When 590MPa≤TS<780MPa, σTS≤20MPa

[0210] When 780MPa≤TS<980MPa, σTS≤25MPa

[0211] When 980MPa≤TS<1180MPa, σTS≤30MPa

[0212] Formula 6:

[0213] When 590MPa≤TS<780MPa, ΔYS≤40MPa

[0214] When 780MPa≤TS<980MPa, ΔYS≤50MPa

[0215] When 980MPa≤TS<1180MPa, ΔYS≤60MPa

[0216] Formula 7:

[0217] When 590MPa≤TS<780MPa, σYS≤20MPa

[0218] When 780MPa≤TS<980MPa, σYS≤25MPa

[0219] When 980MPa≤TS<1180MPa, σYS≤30MPa

[0220] Formula 8:

[0221] When 590MPa≤TS<780MPa, ΔEl≤4.0%

[0222] When 780MPa≤TS<980MPa, ΔEl≤4.5%

[0223] When 980MPa≤TS<1180MPa, ΔEl≤5.0%

[0224] Formula 9:

[0225] When 590MPa≤TS<780MPa, σEl≤2.0%

[0226] When 780MPa≤TS<980MPa, σEl≤2.5%

[0227] When 980MPa≤TS<1180MPa, σEl≤3.0%

[0228] Formula 10:

[0229] When 590MPa≤TS<780MPa, Δλ≤25%.

[0230] When 780MPa≤TS<980MPa, Δλ≤25%.

[0231] When 980MPa≤TS<1180MPa, Δλ≤25%

[0232] Formula 11:

[0233] When 590MPa≤TS<780MPa, σλ≤7%

[0234] When 780MPa≤TS<980MPa, σλ≤7%

[0235] When 980MPa≤TS<1180MPa, σλ≤7%.

[0236] [5] According to the steel strip described in [4] above, wherein the above composition further contains, by mass%, elements selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, and Mg: 0.0200% or less. The following are at least one of the following: Zn: less than 0.0200%, Co: less than 0.0200%, Zr: less than 0.1000%, Ca: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0500%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

[0237] [6] The steel strip according to [4] or [5] above, wherein a coating is provided on the surface of the steel strip.

[0238] The above coating is any one of hot-dip galvanized coating, alloyed hot-dip galvanized coating, electro-galvanized coating, and hot-dip aluminized coating.

[0239] [7] A method for manufacturing a steel plate, comprising:

[0240] In the hot rolling process, a steel billet having the composition described above [4] or [5] is hot rolled at a coiling temperature of 350℃ to 650℃ after finishing rolling to obtain a hot-rolled steel plate;

[0241] In the annealing and heating process, the hot-rolled steel plate is heated to a temperature of 720℃~860℃ and a holding time of more than 20 seconds.

[0242] In the rapid heating process, the steel plate is rapidly heated from the aforementioned annealing temperature at an average heating rate of 10°C / second or higher to (the aforementioned annealing temperature + 10°C) or higher, such that the ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction after the rapid heating is 25°C or lower; and

[0243] The cooling process is carried out after the rapid heating process described above, under the condition that the average cooling rate from the above-mentioned reached temperature to 550°C is 4°C / second or more.

[0244] Alternatively, a cold rolling process may be included after the hot rolling process and before the annealing and soaking process to cold roll the hot-rolled steel sheet and obtain a cold-rolled steel sheet.

[0245] Alternatively, a plating process may be included after the cooling process to perform any one of the following on the hot-rolled steel sheet or the cold-rolled steel sheet: hot-dip galvanizing, alloying hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing.

[0246] Alternatively, after the above-mentioned cooling process or the above-mentioned plating process, a reheating and holding process may be included, in which the above-mentioned hot-rolled steel sheet or the above-mentioned cold-rolled steel sheet is cooled to a cooling stop temperature of 0°C to 250°C, heated to a temperature range of (the above-mentioned cooling stop temperature + 50°C) to 460°C, and held in the above-mentioned temperature range for a tempering time of 10 seconds to 2000 seconds.

[0247] [8] In the steel plate manufacturing method described in [7] above, the above-mentioned arrival temperature is determined based on the information on the composition of the above-mentioned components and the information on the coiling temperature in the above-mentioned hot rolling process.

[0248] [9] According to the steel plate manufacturing method described above [7], the above-mentioned temperature is determined based on information on the austenite fraction of the steel plate measured in the temperature range of 150°C to 600°C after the above-mentioned rapid heating process.

[0249]

[10] According to the steel plate manufacturing method described above [7], the above-mentioned arrival temperature is determined based on the information of the above-mentioned composition, the information of the coiling temperature in the above-mentioned hot rolling process, and the information of the austenite fraction of the steel plate measured in the temperature range of 150°C to 600°C after the above-mentioned rapid heating process.

[0250]

[11] A method for manufacturing a steel strip, comprising:

[0251] In the hot rolling process, a steel billet having the composition described above [4] or [5] is hot rolled at a coiling temperature of 350℃ to 650℃ after finishing rolling to obtain a hot-rolled steel strip;

[0252] In the annealing and heating process, the hot-rolled steel strip is heated and annealed and heated at an annealing and heating temperature of 720℃~860℃ and a holding time of more than 20 seconds.

[0253] In the rapid heating process, the steel strip is rapidly heated from the aforementioned annealing temperature at an average heating rate of 10°C / second or higher to (the aforementioned annealing temperature + 10°C) or higher, such that the ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction after the rapid heating is 25°C or lower; and

[0254] The cooling process is carried out after the rapid heating process described above, under the condition that the average cooling rate from the above-mentioned reached temperature to 550°C is 4°C / second or more.

[0255] Alternatively, a cold rolling process may be included after the hot rolling process and before the annealing and soaking process to cold roll the hot-rolled steel strip and obtain a cold-rolled steel strip.

[0256] Alternatively, a plating process may be included after the cooling process, to perform any one of the following on the hot-rolled steel strip or the cold-rolled steel strip: hot-dip galvanizing, alloying hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing.

[0257] Alternatively, after the above-mentioned cooling process or the above-mentioned plating process, a reheating and holding process may be included, in which the above-mentioned hot-rolled steel strip or the above-mentioned cold-rolled steel strip is cooled to a cooling stop temperature of 0°C to 250°C, heated to a temperature range of (the above-mentioned cooling stop temperature + 50°C) to 460°C, and held in the above-mentioned temperature range for a tempering time of 10 seconds to 2000 seconds.

[0258]

[12] In the steel strip manufacturing method described in

[11] above, the arrival temperature is determined based on the information on the composition of the above-mentioned components and the information on the coiling temperature in the hot rolling process described above.

[0259]

[13] In the steel strip manufacturing method described in

[11] above, the above-mentioned temperature is determined based on information on the austenite fraction of the steel strip measured in a temperature range of 150°C to 600°C after the above-mentioned rapid heating process.

[0260]

[14] According to the steel strip manufacturing method described in

[11] above, the above-mentioned arrival temperature is determined based on the information on the composition of the above-mentioned components, the information on the coiling temperature in the above-mentioned hot rolling process, and the information on the austenite fraction of the steel strip measured in the temperature range of 150°C to 600°C after the above-mentioned rapid heating process.

[0261] According to the present invention, steel plates and strips with a TS of 590 MPa or higher and lower than 1180 MPa, excellent mechanical properties (YS, TS, E1, λ), and small fluctuations in mechanical properties (YS, TS, E1, λ), i.e., excellent stability of mechanical properties, and their manufacturing methods can be provided. Attached Figure Description

[0262] Figure 1a These are SEM images representing the soft phases of recrystallized ferrite, unrecrystallized ferrite, and bainitic ferrite, as well as the hard phase of fresh martensite.

[0263] Figure 1bThese are SEM images representing recrystallized ferrite in the soft phase and fresh martensite in the hard phase.

[0264] Figure 1c These are SEM images representing the soft phases of recrystallized ferrite, phase transformation ferrite, and epitaxial ferrite, as well as the hard phase of fresh martensite.

[0265] Figure 1d These are SEM images representing the soft phase bainitic ferrite and the hard phase fresh and tempered martensite.

[0266] Figure 2 middle Figure 2 (a) indicates that in Example No. 2 of the present invention (steel B) Figure 4 The tissue at the location of the symbol 8 in the middle (1) and the C distribution map (2) and Mn distribution map (3) obtained by EPMA measurement. Figure 2 (b) indicates that comparative example No. 39 (steel B) is in Figure 4 The tissue at the location of the symbol 8 in the middle (1) and the C distribution map (2) and Mn distribution map (3) obtained by EPMA measurement.

[0267] Figure 3a This indicates that in Example No. 2 of the present invention (steel B) Figure 5 SEM images ((1), reflected electron images) of the measurement location of symbol 8 in the middle, and linear analysis (quantitative) results of C obtained by EPMA measurement (2) and linear analysis (quantitative) results of Mn (3).

[0268] Figure 3b This indicates that comparative example No. 39 (steel B) is in Figure 5 SEM images ((1), reflected electron images) of the measurement location of symbol 8 in the middle, and linear analysis (quantitative) results of C obtained by EPMA measurement (2) and linear analysis (quantitative) results of Mn (3).

[0269] Figure 4 This is a schematic diagram used to illustrate the measurement positions of YS, TS, and El when observing a steel plate from above.

[0270] Figure 5 This is a schematic diagram used to illustrate the measurement positions of YS, TS, and El within the steel strip when viewed from above.

[0271] Figure 6a This is a schematic diagram illustrating an example of a heat treatment mode that includes an annealing and homogenization process (A), a rapid heating process (B), a cooling process (C), and a reheating and holding process (E).

[0272] Figure 6bThis is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), a hot-dip galvanizing process (D1), and a reheating and holding process (E).

[0273] Figure 6c This is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), a hot-dip galvanizing process (D1), an alloying process (D11), and a reheating and holding process (E).

[0274] Figure 6d This is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), an electroplating zinc treatment process (D2), and a reheating and holding process (E).

[0275] Figure 6e This is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), a hot-dip aluminizing process (D3), and a reheating and holding process (E). Detailed Implementation

[0276] The present invention will be described based on the following embodiments.

[0277] [1] Steel plates and strips

[0278] The steel plate of one embodiment of the present invention has the following composition, and its tensile strength is 590 MPa or more and less than 1180 MPa. The composition, by mass%, contains C: 0.030% to 0.250%, Si: 0.01% to 2.50%, Mn: 1.30% or more and less than 3.50%, P: 0.001% to 0.100%, S: 0.0200% or less, Al: 0.010% to 2.000%, N: 0.0100% or less, with the balance being Fe. Composed of unavoidable impurities, in the microstructure at 1 / 4 of the steel plate thickness, the area fraction of the soft phase is 30.0% to 90.0%, and the area fraction of the hard phase is 10.0% to 70.0%. Among the aforementioned hard phases, the hard phase with a [C content (mass%) in the hard phase] / [C content (mass%) in the steel plate] of 1.50 or more constitutes 65% or more in area fraction, and the hard phase with a [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel plate] of 1.30 or more constitutes... The hard phase below is 10% or more in terms of area ratio. Furthermore, when the tensile strength is set as TS, the yield stress as YS, the total elongation as E1, and the hole expansion ratio as λ, the following equations 1 to 3 are satisfied at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate. Furthermore, at each measurement position at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel plate, the difference between the maximum and minimum values ​​of TS, YS, E1, and λ (maximum value of TS - minimum value of TS) is... The values ​​of ΔTS, ΔYS, ΔEl, and Δλ, which are the maximum value of YS minus the minimum value of YS, the maximum value of El minus the minimum value of El, and the maximum value of λ minus the minimum value of λ, respectively, satisfy Equations 4, 6, 8, and 10. Furthermore, the standard deviations of TS, YS, El, and λ obtained from the measurement positions at 1 / 4, 1 / 2, and 3 / 4 of the width direction of the steel plate, namely σTS, σYS, σEl, and σλ, satisfy Equations 5, 7, 9, and 11, respectively.

[0279] Furthermore, the steel strip of one embodiment of the present invention has the following composition, with a tensile strength of 590 MPa or more and less than 1180 MPa. The composition, by mass%, contains C: 0.030% to 0.250%, Si: 0.01% to 2.50%, Mn: 1.30% or more and less than 3.50%, P: 0.001% to 0.100%, S: 0.0200% or less, Al: 0.010% to 2.000%, N: 0.0100% or less, with the balance being Fe and unavoidable impurities. In the microstructure at 1 / 4 of the steel strip thickness, the soft phase... The area fraction is 30.0% to 90.0%, and the area fraction of the hard phase is 10.0% to 70.0%. Among the aforementioned hard phases, the hard phase with a [C content (mass%) in the hard phase] / [C content (mass%) in the steel strip] of 1.50 or more constitutes 65% or more in terms of area fraction, and the hard phase with a [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel strip] of 1.30 or less constitutes 10% or more in terms of area fraction. Furthermore, when the tensile strength is set to TS, the yield stress is set to YS, the total elongation is set to E1, and the hole expansion rate is set to λ, the area fraction in the width direction of the steel strip is... At the 1 / 4, 1 / 2, and 3 / 4 positions, at each position 10m from the front end, 10m from the rear end, 1 / 4, 1 / 2, and 3 / 4 positions along the length direction of the steel strip, the following equations 1 to 3 are satisfied. Furthermore, at the 1 / 4, 1 / 2, and 3 / 4 positions along the width direction of the steel strip, at each position 10m from the front end, 10m from the rear end, 1 / 4, 1 / 2, and 3 / 4 positions along the length direction of the steel strip, the difference between the maximum and minimum values ​​of TS (maximum value of TS - minimum value of TS), (maximum value of YS) and λ are respectively... The minimum value of YS, (maximum value of El - minimum value of El) and (maximum value of λ - minimum value of λ) that are, ΔTS, ΔYS, ΔEl, and Δλ, successively satisfy Equations 4, 6, 8 and 10 below. Furthermore, at the 1 / 4, 1 / 2 and 3 / 4 positions in the width direction of the steel strip, the standard deviations of TS, YS, El, and λ obtained from the front end 10m position, the rear end 10m position, the 1 / 4 position, the 1 / 2 position and the 3 / 4 position in the length direction of the steel strip, respectively, namely σTS, σYS, σEl, and σλ, successively satisfy Equations 5, 7, 9 and 11 below.

[0280] Formula 1:

[0281] When 590MPa≤TS<780MPa, 360MPa≤YS

[0282] When 780MPa≤TS<980MPa, 460MPa≤YS

[0283] When 980MPa≤TS<1180MPa, 580MPa≤YS

[0284] Formula 2:

[0285] When 590MPa≤TS<780MPa, 23.0%≤El

[0286] When 780MPa≤TS<980MPa, 17.0%≤El

[0287] When 980MPa≤TS<1180MPa, 11.0%≤El

[0288] Formula 3:

[0289] When 590MPa≤TS<780MPa, 45%≤λ

[0290] When 780MPa≤TS<980MPa, 30%≤λ

[0291] When 980MPa≤TS<1180MPa, 20%≤λ

[0292] Formula 4:

[0293] When 590MPa≤TS<780MPa, ΔTS≤40MPa

[0294] When 780MPa≤TS<980MPa, ΔTS≤50MPa

[0295] When 980MPa≤TS<1180MPa, ΔTS≤60MPa

[0296] Formula 5:

[0297] When 590MPa≤TS<780MPa, σTS≤20MPa

[0298] When 780MPa≤TS<980MPa, σTS≤25MPa

[0299] When 980MPa≤TS<1180MPa, σTS≤30MPa

[0300] Formula 6:

[0301] When 590MPa≤TS<780MPa, ΔYS≤40MPa

[0302] When 780MPa≤TS<980MPa, ΔYS≤50MPa

[0303] When 980MPa≤TS<1180MPa, ΔYS≤60MPa

[0304] Formula 7:

[0305] When 590MPa≤TS<780MPa, σYS≤20MPa

[0306] When 780MPa≤TS<980MPa, σYS≤25MPa

[0307] When 980MPa≤TS<1180MPa, σYS≤30MPa

[0308] Formula 8:

[0309] When 590MPa≤TS<780MPa, ΔEl≤4.0%

[0310] When 780MPa≤TS<980MPa, ΔEl≤4.5%

[0311] When 980MPa≤TS<1180MPa, ΔEl≤5.0%

[0312] Formula 9:

[0313] When 590MPa≤TS<780MPa, σEl≤2.0%

[0314] When 780MPa≤TS<980MPa, σEl≤2.5%

[0315] When 980MPa≤TS<1180MPa, σEl≤3.0%

[0316] Formula 10:

[0317] When 590MPa≤TS<780MPa, Δλ≤25%.

[0318] When 780MPa≤TS<980MPa, Δλ≤25%.

[0319] When 980MPa≤TS<1180MPa, Δλ≤25%

[0320] Formula 11:

[0321] When 590MPa≤TS<780MPa, σλ≤7%

[0322] When 780MPa≤TS<980MPa, σλ≤7%

[0323] When 980MPa≤TS<1180MPa, σλ≤7%

[0324] First, the composition of the steel plate according to one embodiment of the present invention will be described.

[0325] The composition of steel strip is the same as that of steel plate. The following description of the composition of steel plate can also be used as the composition of steel strip. It should be noted that the unit in the composition is "mass%", but unless otherwise specified, it will be expressed as "%".

[0326] C: 0.030%~0.250%

[0327] C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, and retained austenite, thereby ensuring a total strength (TS) of 590 MPa or higher. However, when the C content is below 0.030%, the area fraction of the soft phases—unrecrystallized ferrite, recrystallized ferrite, transformation ferrite, epitaxial ferrite, and bainitic ferrite—increases excessively, making it difficult to achieve a TS of 590 MPa or higher.

[0328] On the other hand, when the C content exceeds 0.250%, the area ratio of fresh martensite and tempered martensite in the hard phase increases excessively, leading to a decrease in El. Therefore, the C content is set to 0.030% to 0.250%. The C content is preferably 0.050% or more. Furthermore, the C content is preferably 0.130% or less.

[0329] Si: 0.01%~2.50%

[0330] During the heating process to the annealing homogenization temperature and the annealing homogenization process, Si promotes the formation of both unrecrystallized ferrite and recrystallized ferrite in the soft phase. That is, Si is an element that affects the area fraction of the soft phase. Here, when the Si content is below 0.01%, the area fraction of the soft phase decreases, and EL decreases.

[0331] On the other hand, when the Si content exceeds 2.50%, it is difficult to generate and grow a large amount of austenite in a rapid heating process where the temperature is rapidly increased from the annealing soaking temperature at a heating rate of 10°C / second or higher to (annealing soaking temperature + 10°C). Therefore, in the hard phase, it is impossible to achieve a hard phase with a [C content (mass%) in the hard phase] / [C content (mass%) in the steel] of 1.50 or higher in terms of area ratio exceeding 65%, and it is also impossible to achieve a hard phase with a [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel] of 1.30 or lower in terms of area ratio exceeding 10%, making it difficult to reduce fluctuations in the mechanical properties (YS, TS, E1, λ) of the steel sheet (making it difficult to improve the stability of mechanical properties). Therefore, the Si content is set to 0.01% to 2.50%. The Si content is preferably 0.10% or higher. Furthermore, the Si content is preferably 1.80% or lower.

[0332] Mn: Above 1.30% and below 3.50%

[0333] Mn is an element that can adjust the area ratio of fresh martensite and tempered martensite in the hard phase. Here, when the Mn content is less than 1.30%, the area ratio of unrecrystallized ferrite, recrystallized ferrite, phase transformation ferrite and epitaxial ferrite in the soft phase increases, making it difficult for TS to reach above 590 MPa.

[0334] On the other hand, when the Mn content reaches 3.50% or more, the martensitic transformation initiation temperature (hereinafter also referred to as the Ms point or Ms) decreases, making it difficult to obtain a hard phase composed of high C and low Mn. Therefore, it is difficult to reduce the fluctuations in the mechanical properties (YS, TS, E1, λ) of the steel sheet (making it difficult to improve the stability of mechanical properties). Therefore, the Mn content is set to 1.30% or more and less than 3.50%. The Mn content is preferably 1.60% or more. Furthermore, the Mn content is preferably 3.00% or less.

[0335] P: 0.001%~0.100%

[0336] P is an element that has a solid solution strengthening effect and improves the TS and YS of steel plates. To achieve this effect, the P content is set to 0.001% or higher.

[0337] On the other hand, when the phosphorus (P) content exceeds 0.100%, P will segregate at the original austenite grain boundaries, causing grain boundary embrittlement. Therefore, during tensile testing, voids will form and cracks will propagate along the original austenite grain boundaries, making it impossible to obtain the desired E1. Therefore, the P content is set to 0.001% to 0.100%. The P content is preferably 0.030% or less.

[0338] S: Below 0.0200%

[0339] Sulfur (S) exists in steel in the form of sulfides. In particular, when the S content exceeds 0.0200%, during tensile testing, voids and crack propagation originate from these sulfides, resulting in poor local elongation and failing to achieve the desired elongation (E). Therefore, the S content is set to 0.0200% or less. Preferably, the S content is 0.0080% or less. It should be noted that while there is no specific lower limit for the S content, from a production technology perspective, the S content is preferably 0.0001% or more.

[0340] Al: 0.010%~2.000%

[0341] Al promotes the formation of both unrecrystallized and recrystallized ferrite in the soft phase during the heating process to the annealing homogenization temperature and during the annealing homogenization process. That is, Al is an element that affects the area fraction of the soft phase. Here, when the Al content is below 0.010%, the area fraction of the soft phase decreases, and the Al content also decreases.

[0342] On the other hand, when the Al content exceeds 2.000%, it is difficult to generate and grow a large amount of austenite in a rapid heating process where the temperature is rapidly increased from the annealing soaking temperature at a heating rate of 10°C / second or higher to (annealing soaking temperature + 10°C). Therefore, in the hard phase, it is impossible to achieve a hard phase with a [C content (mass%) in the hard phase] / [C content (mass%) in the steel] of 1.50 or higher in terms of area ratio exceeding 65%, and it is also impossible to achieve a hard phase with a [Mn content (mass%) in the hard phase] / [Mn content (mass%) in the steel] of 1.30 or lower in terms of area ratio exceeding 10%, making it difficult to reduce fluctuations in the mechanical properties (YS, TS, E1, λ) of the steel sheet (making it difficult to improve the stability of mechanical properties). Therefore, the Al content is set to 0.010% to 2.000%. The Al content is preferably 0.015% or higher. Furthermore, the Al content is preferably 1.000% or lower.

[0343] N: below 0.0100%

[0344] Nitrogen (N) exists in steel in the form of nitrides. In particular, when the N content exceeds 0.0100%, during tensile testing, voids and crack propagation originate from these nitrides, resulting in poor local elongation and the undesirable elongation (E). Therefore, the N content is set to 0.0100% or less. Furthermore, the N content is preferably 0.0050% or less. It should be noted that while there is no specific lower limit for the N content, from a production technology perspective, the N content is preferably 0.0005% or more.

[0345] The basic composition of a steel plate according to one embodiment of the present invention has been described above. However, the steel plate according to one embodiment of the present invention has a composition containing the above-described basic components, with the balance being Fe (iron) and unavoidable impurities. Here, the base steel plate of the high-strength steel strip according to one embodiment of the present invention preferably has a composition containing the above-described basic components, with the balance being Fe and unavoidable impurities. In addition to the above-described basic components, the steel plate according to one embodiment of the present invention may also contain at least one component selected from any of the components shown below. It should be noted that the effects of the present invention can be obtained as long as any of the components shown below are contained in amounts lower than the upper limits shown below, therefore the lower limit is not specifically set. It should be noted that when any of the following elements are contained in amounts lower than the preferred lower limit values ​​described below, the element is considered to be contained as an unavoidable impurity.

[0346] Selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: At least one of the following: less than 0.0200%, Zr: less than 0.1000%, Ca: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0500%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

[0347] Nb: below 0.200%

[0348] Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Nb content is preferably 0.001% or more. More preferably, the Nb content is 0.005% or more.

[0349] On the other hand, when the Nb content exceeds 0.200%, a large number of coarse precipitates and inclusions may be generated. In this case, the coarse precipitates and inclusions become the starting point for voids and cracks during tensile testing, thus failing to obtain good local elongation and potentially failing to achieve the desired E1. Therefore, when Nb is present, the Nb content is preferably 0.200% or less. More preferably, the Nb content is 0.060% or less.

[0350] Ti: below 0.200%

[0351] Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Ti content is preferably 0.001% or more. More preferably, the Ti content is 0.005% or more.

[0352] On the other hand, when the Ti content exceeds 0.200%, a large number of coarse precipitates and inclusions may be generated. In this case, the coarse precipitates and inclusions become the starting point for voids and cracks during tensile testing, thus failing to obtain good local elongation and potentially failing to achieve the desired EL. Therefore, in the case of Ti content, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0353] V: Below 0.200%

[0354] Similar to Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. More preferably, the V content is 0.005% or more.

[0355] On the other hand, when the V content exceeds 0.200%, a large number of coarse precipitates and inclusions may be generated. In this case, the coarse precipitates and inclusions become the starting point for voids and cracks during tensile testing, thus failing to obtain good local elongation and potentially failing to achieve the desired E1. Therefore, when V is present, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

[0356] B: Below 0.0100%

[0357] Boron (B) is an element that improves hardenability by segregating at austenite grain boundaries. Furthermore, B controls the formation of phase transformation ferrite and epitaxial ferrite in the soft phase during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. More preferably, the B content is 0.0002% or more.

[0358] On the other hand, when the boron content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling. Furthermore, it is difficult to ensure that the area fraction of the soft phase is above 30.0%, which may prevent obtaining good local elongation and potentially prevent achieving the desired elongation. Therefore, when boron is present, the boron content is preferably 0.0100% or less. More preferably, the boron content is 0.0050% or less.

[0359] Cr: less than 1.000%

[0360] Cr is an element that improves hardenability. By adding Cr, an appropriate amount of fresh martensite and tempered martensite hard phases are generated, thus improving TS and YS. To obtain this effect, the Cr content is preferably 0.0005% or more. Furthermore, the Cr content is more preferably 0.010% or more.

[0361] On the other hand, when the Cr content exceeds 1.000%, the area fraction of the hard phase increases. During tensile testing, voids are generated and cracks propagate starting from the fresh martensite of the hard phase, which may prevent the achievement of good local elongation and the desired E1. Therefore, in the case of Cr, the Cr content is preferably 1.000% or less. Furthermore, the Cr content is more preferably 0.800% or less.

[0362] Ni: below 1.000%

[0363] Ni is an element that improves hardenability; therefore, by adding Ni, an appropriate amount of fresh martensite and tempered martensite hard phases are generated, thus improving TS and YS. To obtain this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more.

[0364] On the other hand, when the Ni content exceeds 1.000%, the area fraction of the hard phase increases. During tensile testing, voids are generated and cracks propagate starting from the fresh martensite of the hard phase, which may prevent the achievement of good local elongation and the desired E1. Therefore, in the case of Ni, the Ni content is preferably 1.000% or less. More preferably, the Ni content is 0.800% or less.

[0365] Mo: 1.000% or less

[0366] Mo is an element that improves hardenability; therefore, by adding Mo, an appropriate amount of fresh martensite and tempered martensite hardenable phases are generated, thereby improving TS and YS. To obtain this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more.

[0367] On the other hand, when the Mo content exceeds 1.000%, the area fraction of the hard phase increases. During tensile testing, voids are generated and cracks propagate starting from the fresh martensite of the hard phase, which may prevent the achievement of good local elongation and the desired E1. Therefore, when Mo is present, the Mo content is preferably 1.000% or less. More preferably, the Mo content is 0.500% or less, further preferably 0.450% or less, and even more preferably 0.400% or less.

[0368] Sb: below 0.200%

[0369] Sb is an effective element for suppressing carbon diffusion near the surface of the steel plate during annealing and controlling the formation of a soft layer near the surface. When the soft layer near the surface of the steel plate increases excessively, it may be difficult to achieve a total strength (TS) of 590 MPa or higher. Therefore, the Sb content is preferably 0.002% or higher, and more preferably 0.005% or higher.

[0370] On the other hand, when the Sb content exceeds 0.200%, a soft layer will not form near the surface of the steel plate, and the variation in surface hardness within the steel plate may lead to increased fluctuations in TS and YS. Therefore, in the case of Sb content, the Sb content is preferably 0.200% or less. More preferably, the Sb content is 0.020% or less.

[0371] Sn: below 0.200%

[0372] Like Sb, Sn is an effective element for suppressing carbon diffusion near the surface of the steel plate during annealing and controlling the formation of a soft layer near the surface. When the soft layer near the surface of the steel plate increases excessively, it may be difficult to achieve a TS (steel strength) of 590 MPa or higher. Therefore, the Sn content is preferably 0.002% or more, and more preferably 0.005% or more.

[0373] On the other hand, when the Sn content exceeds 0.200%, a soft layer will not form near the surface of the steel plate, and the surface hardness variation within the steel plate may lead to increased fluctuations in TS and YS. Therefore, in the case of Sn content, the Sn content is preferably 0.200% or less. More preferably, the Sn content is 0.030% or less.

[0374] Cu: below 1.000%

[0375] Cu is an element that improves hardenability. By adding Cu, an appropriate amount of fresh martensite and tempered martensite hard phases are generated, thus improving TS and YS. To obtain this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more.

[0376] On the other hand, when the Cu content exceeds 1.000%, the area fraction of the hard phase may increase excessively. Furthermore, a large number of coarse precipitates and inclusions may be generated. In this case, the excessively generated fresh martensite of the hard phase, along with the coarse precipitates and inclusions, will lead to void formation and crack propagation starting from the fresh martensite of the hard phase during tensile testing, potentially resulting in poor local elongation and failing to achieve the desired E1. Therefore, in the case of Cu content, the Cu content is preferably 1.000% or less. More preferably, the Cu content is 0.300% or less.

[0377] Ta: Below 0.200%

[0378] Like Ti, Nb, and V, Ta forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. Furthermore, Ta partially dissolves in Nb carbides or Nb carbonitrides, forming complex precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. To achieve this effect, the Ta content is preferably 0.001% or more.

[0379] On the other hand, when the Ta content exceeds 0.200%, a large number of coarse precipitates and inclusions may be generated. In this case, during tensile testing, voids and crack propagation may occur starting from the fresh martensite of the hard phase, which may result in poor local elongation and may prevent the desired E1 from being obtained. Therefore, when Ta is present, the Ta content is preferably 0.200% or less. The Ta content is more preferably 0.100% or less.

[0380] W: Below 0.500%

[0381] W is an element that improves hardenability; therefore, by adding W, a large amount of fresh martensite and tempered martensite hard phases are generated, thus improving TS and YS. To obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.010% or more.

[0382] On the other hand, when the W content exceeds 0.500%, the area fraction of the hard phase increases. During tensile testing, voids and crack propagation originate from the fresh martensite of the hard phase, which may prevent the achievement of good local elongation and the desired E1. Therefore, when W is present, the W content is preferably 0.500% or less. More preferably, the W content is 0.450% or less, and even more preferably 0.400% or less.

[0383] Mg: below 0.0200%

[0384] Mg is an effective element for shaping inclusions such as sulfides and oxides into spherical shapes, thereby increasing the local elongation of steel plates. To achieve this effect, the Mg content is preferably 0.0001% or higher.

[0385] On the other hand, when the Mg content exceeds 0.0200%, a large number of coarse precipitates and inclusions may be generated. In this case, excessively coarse precipitates and inclusions will cause void formation and crack propagation starting from the fresh martensite of the hard phase during tensile testing, which may prevent the achievement of good local elongation and the desired E1. Therefore, in the case of Mg, the Mg content is preferably below 0.0200%.

[0386] Zn: below 0.0200%

[0387] Zinc (Zn) is an effective element for shaping inclusions into a spherical form, thereby increasing the local elongation of steel plates. To achieve this effect, the Zn content is preferably 0.0010% or higher.

[0388] On the other hand, when the Zn content exceeds 0.0200%, a large number of coarse precipitates and inclusions may be generated. In this case, excessively coarse precipitates and inclusions will cause void formation and crack propagation starting from the fresh martensite of the hard phase during tensile testing, which may prevent the achievement of good local elongation and the desired E1. Therefore, in the case of Zn, the Zn content is preferably below 0.0200%.

[0389] Co: below 0.0200%

[0390] Like Zn, Co is an effective element for shaping inclusions into a spherical form, thereby increasing the local elongation of steel plates. To achieve this effect, the Co content is preferably 0.0010% or higher.

[0391] On the other hand, when the Co content exceeds 0.0200%, a large number of coarse precipitates and inclusions may be generated. In this case, excessively coarse precipitates and inclusions will cause void formation and crack propagation starting from the fresh martensite of the hard phase during tensile testing, which may prevent the achievement of good local elongation and the desired E1. Therefore, in the case of Co content, the Co content is preferably below 0.0200%.

[0392] Zr: below 0.1000%

[0393] Like Zn and Co, Zr is an effective element for spherizing inclusions and increasing the local elongation of steel plates. To achieve this effect, the Zr content is preferably 0.0010% or higher.

[0394] On the other hand, when the Zr content exceeds 0.1000%, the excessively large precipitates and inclusions, during tensile testing, can lead to void formation and crack propagation starting from the fresh martensite of the hard phase, potentially resulting in poor local elongation and failing to achieve the desired E1. Therefore, in the case of Zr, the Zr content is preferably below 0.1000%.

[0395] Ca: below 0.0200%

[0396] Ca exists in steel as inclusions. When the Ca content exceeds 0.0200%, a large number of coarse inclusions may form. In this case, excessively coarse precipitates and inclusions, during tensile testing, can lead to void formation and crack propagation starting from the fresh martensite of the hard phase, potentially resulting in poor local elongation and failing to achieve the desired E1. Therefore, in the presence of Ca, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0040% or less.

[0397] It should be noted that there is no specific lower limit for the Ca content, but from the perspective of production technology constraints, the Ca content is preferably 0.0005% or higher. Furthermore, from the perspective of production technology constraints, the Ca content is more preferably 0.0010% or higher.

[0398] Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0500%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

[0399] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all effective elements for increasing the local elongation of steel plates. To achieve this effect, the content of each of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is preferably 0.0001% or more.

[0400] On the other hand, when the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM each exceed 0.0200%, or when the content of As exceeds 0.0500%, a large number of coarse precipitates and inclusions may be generated. In this case, excessively coarse precipitates and inclusions, during tensile testing, will generate voids and propagate cracks starting from the fresh martensite of the hard phase, which may prevent the achievement of good local elongation and the desired E1. Therefore, when containing at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM, the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are preferably 0.0200% or less, and the content of As is preferably 0.0500% or less.

[0401] It should be noted that, in this invention, REM refers to the lanthanide elements, namely scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in this invention refers to the total content of one or more elements selected from the above REM. REM is not particularly limited, but is preferably La and / or Ce.

[0402] Unavoidable impurities are those that are unavoidably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permitted to be present to a extent that does not impede the purpose of this invention. Examples of raw materials include iron ore, reduced iron, or waste. Examples of impurities include, for example, oxygen (O).

[0403] Next, the steel structure of a steel plate according to one embodiment of the present invention will be described. The steel structure of the steel strip is also the same as that of the steel plate. The following description of the steel structure of the steel plate can also be taken as the description of the steel structure of the steel strip.

[0404] <Microstructure at 1 / 4 of the steel plate thickness>

[0405] Area fraction of the soft phase: 30.0%–90.0%

[0406] The soft phase referred to herein is selected from one or more of unrecrystallized ferrite, recrystallized ferrite, transformation ferrite, epitaxial ferrite, and bainitic ferrite. The soft phase (selected from one or more of unrecrystallized ferrite, recrystallized ferrite, transformation ferrite, epitaxial ferrite, and bainitic ferrite) is the phase (microstructure) that improves El. To ensure the desired El, the area fraction of the soft phase is set to 30.0% or more. Furthermore, the area fraction of the soft phase is preferably 35.0% or more.

[0407] On the other hand, when the area fraction of the soft phase increases excessively, it becomes difficult to achieve a TS of 590 MPa or higher. Therefore, the area fraction of the soft phase is set to 90.0% or less. Furthermore, the area fraction of the soft phase is preferably 80.0% or less.

[0408] Here, both unrecrystallized ferrite and recrystallized ferrite are ferrites generated during the process of heating to the annealing homogenization temperature and during annealing homogenization.

[0409] Unrecrystallized ferrite refers to ferrite with a crystal orientation similar to that of processed ferrite grains before heating, and containing subgrain boundaries within the grains.

[0410] Recrystallized ferrite refers to ferrite with a different crystal orientation than the processed ferrite grains before heating, and which does not contain subgrain boundaries within the grains.

[0411] Both phase transformation ferrite and epitaxial ferrite are ferrites generated during the process of cooling to 550°C after a rapid heating process.

[0412] Phase transformation ferrite refers to ferrite formed from the nucleation of austenite with a new crystal orientation during the annealing homogenization process (heating and annealing homogenization) and the rapid heating process, and then through the growth of these ferrite grains.

[0413] Epitaxial ferrite refers to ferrite formed by a phase transformation of austenite generated during the annealing and homogenization process (heating and annealing) and the rapid heating process, while having the same crystal orientation as the adjacent unrecrystallized ferrite and recrystallized ferrite.

[0414] Transformation ferrite and epitaxial ferrite are harder than recrystallized ferrite. Furthermore, bainitic ferrite refers to ferrite formed after a rapid heating process, followed by cooling at below 550°C and holding.

[0415] Bainitic ferrite refers to ferrite that is formed from austenite phase transformation during the annealing and rapid heating processes. It has a relatively high dislocation density within the grains and contains fresh martensite or retained austenite, cementite or cementite carbides within the grains.

[0416] Area fraction of the hard phase: 10.0%–70.0%

[0417] The hard phase referred to here is selected from one or more of fresh martensite and tempered martensite. The hard phase (fresh martensite and tempered martensite) is the phase (microstructure) that improves TS and YS. In order to ensure TS of ≥590 MPa and the desired YS, the area fraction of the hard phase is set to 10.0% or more. Furthermore, the area fraction of the hard phase is preferably 20.0% or more.

[0418] On the other hand, when the area fraction of the hard phase increases excessively, it becomes difficult to ensure the desired E1. Therefore, the area fraction of the hard phase is set to 70.0% or less. Furthermore, the area fraction of the soft phase is preferably 65.0% or less.

[0419] It should be noted that the area fraction of the remaining tissue other than the soft and hard phases is preferably 10.0% or less. More preferably, the area fraction of the remaining tissue is 5.0% or less. Furthermore, the area fraction of the remaining tissue may also be 0.0%.

[0420] It should be noted that the remaining tissue is not specifically limited; examples include retained austenite, pearlite, cementite, and other carbides, as well as other precipitates and oxides. It should also be noted that the type of remaining tissue can be confirmed by observation, for example, using SEM (scanning electron microscopy).

[0421] Here, the area ratio of the soft phase (selected from one or more of unrecrystallized ferrite, recrystallized ferrite, phase transformation ferrite, epitaxial ferrite and bainitic ferrite) and the hard phase (selected from one or more of fresh martensite and tempered martensite) is determined at 1 / 4 of the thickness of the steel plate in the following manner.

[0422] Specifically, the specimen was cut with the thickness section (L section) parallel to the rolling direction of the steel plate as the observation surface. Next, the observation surface of the specimen was polished with diamond polishing paste, followed by fine polishing with alumina. Then, the observation surface of the specimen was etched with a 3 vol.% nitric acid alcohol solution to reveal the microstructure. Next, five fields of view (25 μm × 35 μm) were observed at 3000x magnification using SEM, with the observation position at 1 / 4 of the steel plate thickness. From the obtained microstructure images, Adobe Photoshop (Adobe Systems) was used to divide the area of ​​each constituent microstructure—the soft phase (unrecrystallized ferrite, recrystallized ferrite, phase transformation ferrite, epitaxial ferrite, and bainitic ferrite) and the hard phase (fresh martensite and tempered martensite)—by the measured area to calculate the area ratio. This calculation was performed for the five fields of view, and the average of these values ​​was taken as the area ratio of each microstructure.

[0423] The soft phases of unrecrystallized ferrite, recrystallized ferrite, phase transformation ferrite, epitaxial ferrite, and bainitic ferrite, as well as the hard phases of fresh martensite and tempered martensite, are each identified as follows.

[0424] Recrystallized ferrite: An example of recrystallized ferrite is shown in Figure 1a , Figure 1b and Figure 1c (Refer to symbol F2). Recrystallized ferrite is a black region in a massive form. Furthermore, recrystallized ferrite contains almost no subgrain boundaries or carbides.

[0425] Unrecrystallized ferrite: An example of unrecrystallized ferrite is shown in Figure 1a (Refer to symbol F1). Unrecrystallized ferrite is a black region in a massive form. In addition, unrecrystallized ferrite contains a large number of subgrain boundaries and may also contain carbides.

[0426] Phase transformation ferrite: An example of phase transformation ferrite is shown in Figure 1c (Refer to symbol F3). The phase transformation ferrite is a black region adjacent to fresh martensite and retained austenite.

[0427] Epitaxial ferrite: An example of epitaxial ferrite is shown in Figure 1c (Refer to symbol F4). Epitaxial ferrite is a black region that is adjacent to either unrecrystallized ferrite or recrystallized ferrite.

[0428] Bainitic ferrite: An example of bainitic ferrite is shown in Figure 1a and Figure 1d (Refer to symbol F5). Bainitic ferrite is a region that appears as black to dark gray, and its morphology is blocky or irregular. In addition, bainitic ferrite contains carbides, but in relatively small amounts.

[0429] Tempered martensite: An example of tempered martensite is shown in Figure 1d (Refer to symbol TM). Tempered martensite is a grayish region with an irregular shape. Furthermore, tempered martensite contains a relatively large amount of carbides.

[0430] Fresh martensite + retained austenite: An example of fresh martensite and retained austenite is shown in Figure 1a , Figure 1b , Figure 1c and Figure 1d (Refer to symbol FM). Fresh martensite and retained austenite are white to light gray regions with irregular shapes. Furthermore, fresh martensite and retained austenite do not contain carbides.

[0431] Cementite (carbides): Cementite is a white area, appearing as dots or lines. Cementite is contained within tempered martensite (…). Figure 1d In the middle, refer to the symbol TM.

[0432] Residual structure: In addition to the aforementioned retained austenite and cementite (carbides), other morphologies such as pearlite, other precipitates, and oxides can also be listed as residual structures.

[0433] In addition, the area ratio of retained austenite was determined as follows.

[0434] That is, the base steel plate is mechanically ground to 1 / 4 of its thickness in the thickness direction (depth direction), and then chemically ground with oxalic acid to form the observation surface. Next, the observation surface is observed using X-ray diffraction. MoKα rays are used as the incident X-rays. The ratio of the diffraction intensity of each plane (200), (220), and (311) of fcc iron (austenite) to the diffraction intensity of each plane (200), (211), and (220) of bcc iron is calculated, and the volume fraction of retained austenite is calculated from the diffraction intensity ratio of each plane. Then, the retained austenite is considered to be a three-dimensionally uniform distribution, and the volume fraction of retained austenite is used as the area fraction of retained austenite.

[0435] Furthermore, the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the area ratio of fresh martensite + retained austenite calculated as described above.

[0436] [Area percentage of fresh martensite (%)] = [Area percentage of fresh martensite + Area percentage of retained austenite (%)] - [Area percentage of retained austenite (%)]

[0437] Furthermore, the area ratio of the remaining structure is obtained by subtracting the area ratios of the soft phases (unrecrystallized ferrite, recrystallized ferrite, phase transformation ferrite, epitaxial ferrite, and bainitic ferrite) and the hard phases (fresh martensite and tempered martensite) as calculated above from 100.0%.

[0438] [Area fraction of remaining tissue (%)] = 100.0 - [Area fraction of soft phase (%)] - [Area fraction of hard phase (%)]

[0439] In the hard phase, the hard phase with a C content (mass%) in the hard phase / C content (mass%) in the steel plate (strip) of 1.50 or more constitutes 65% or more in terms of area ratio, and the hard phase with a Mn content (mass%) in the hard phase / Mn content (mass%) in the steel plate (strip) of 1.30 or less constitutes 10% or more in terms of area ratio.

[0440] Generally, the hardness of the hard phase (fresh martensite and tempered martensite) is determined by the carbon content (hereinafter referred to as C content) in the hard phase; the higher the C content, the greater the hardness. In the hard phase, when the ratio of [C content in the hard phase] to [C content in the steel plate (strip)] is 1.50 or higher, and this ratio is less than 65%, meaning there are fewer hard phases with sufficient hardness, the hardness tolerance (TS) decreases. Therefore, the hard phase with a [C content (mass%) in the hard phase] to [C content (mass%) in the steel plate (strip)] of 1.50 or higher constitutes more than 65% of the total hard phase by area ratio.

[0441] Furthermore, when the Mn content in the hard phase (hereinafter referred to as Mn content) is low, i.e., when austenite with low Mn content is generated during the rapid heating process, a hard phase with high C and low Mn content will be generated during the subsequent cooling process. When an appropriate amount of this high-C, low-Mn hard phase is present, it is easy to obtain a good porosity and reduce fluctuations in mechanical properties (TS, YS, E1, λ). The detailed mechanism is not yet clear, but it is speculated as follows: In the case of a microstructure composed of a typical high-C, high-Mn hard phase and a soft phase, due to the large hardness difference between the microstructures, voids are easily generated, leading to a decrease in porosity and fluctuations in mechanical properties. In this regard, the high-C, low-Mn hard phase is softer than the typical high-C, high-Mn hard phase, thus reducing the hardness difference between the microstructures, suppressing void generation, and helping to improve porosity and reduce fluctuations in mechanical properties. The above-mentioned effect can be obtained when the hard phase content is 10% or more and the ratio of Mn content (mass%) in the hard phase to Mn content (mass%) in the steel plate (strip) is less than 1.30.

[0442] Here, the ratio of [C content (mass%) in the hard phase] to [C content (mass%) in the steel plate (strip)] and the ratio of [Mn content (mass%) in the hard phase] to [Mn content (mass%) in the steel plate (strip)] are determined as follows.

[0443] The sample was cut with the thickness section (L section) parallel to the rolling direction of the steel plate as the observation surface. The observation surface of the sample was polished with diamond polishing paste, and then finely polished with alumina. Next, the characteristic X-ray intensity and concentration of C and Mn at 1 / 4 of the plate thickness were measured. The characteristic X-ray intensity and concentration of C and Mn were obtained using a field-emission electron probe microanalyzer (FE-EPMA) through mapping analysis (qualitative values) and line analysis (quantitative values). Mapping analysis was performed under the conditions of a measurement range of 16.6 μm × 16.6 μm, 256 measurement points × 256 points, accelerating voltage of 9 kV, and integration time of 200 ms to obtain the characteristic X-ray intensity of C and Mn (counts: qualitative value). Line analysis was performed under the conditions of a measurement length of 16.6 μm, 256 measurement points, accelerating voltage of 9 kV, and integration time of 2 s to obtain the concentration of C and Mn (mass %: quantitative value).

[0444] exist Figure 2 In the diagram, (a) indicates that in Example No. 2 of the present invention (steel B), Figure 5 The tissue at the location of the symbol 8 in the middle (1) and the C distribution map (2) and Mn distribution map (3) obtained by EPMA determination. In addition, in Figure 2 In the text, (b) indicates that comparative example No. 39 (steel B) was used in... Figure 5 The tissue at the location of the symbol 8 in the middle (1) and the C distribution map (2) and Mn distribution map (3) obtained by EPMA measurement.

[0445] also, Figure 3a This indicates that in Example No. 2 of the present invention (steel B) Figure 5 SEM images ((1), reflected electron images) of the measurement location of symbol 8 in the middle, and linear analysis (quantitative) results of C obtained by EPMA measurement (2) and linear analysis (quantitative) results of Mn (3).

[0446] also, Figure 3b This indicates that comparative example No. 39 (steel B) is in Figure 5 SEM images ((1), reflected electron images) of the measurement location of symbol 8 in the middle, and linear analysis (quantitative) results of C obtained by EPMA measurement (2) and linear analysis (quantitative) results of Mn (3).

[0447] exist Figure 2In the middle, the soft phase is Figure 2 (a) of (1) and Figure 2 (b) is the region indicated in white in (1). Furthermore, the soft phase is... Figure 3a (2) and Figure 3b (2) The region with low C content (no peak value) in the region.

[0448] exist Figure 2 In the middle, the hard phase composed of high C and low Mn is... Figure 2 (a) The region shown in gray in (1) is also present. Furthermore, the hard phase composed of high C and low Mn is... Figure 3a The regions in (2) and (3) with high C content and low Mn content (with a C content peak and no Mn content peak).

[0449] exist Figure 2 In the middle, the hard phase composed of high C and high Mn is in Figure 2 (a) of (1) and Figure 2 (b) is the area marked in black in (1). Furthermore, it is in Figure 3a (2) and (3) and Figure 3b The regions in (2) and (3) where both C and Mn contents are high (both have peak values).

[0450] The above mainly describes the composition and structure of the steel plate of the present invention. However, for the steel strip, the steel plate can also be replaced with the steel strip in the above description of the steel plate.

[0451] Next, the mechanical properties of the steel plate and steel strip according to one embodiment of the present invention will be described.

[0452] Tensile strength (TS)

[0453] In one embodiment of the present invention, the tensile strength (TS) of the steel plate and steel strip is 590 MPa or more and less than 1180 MPa.

[0454] Yield stress (YS), total elongation (E1), and porosity (λ)

[0455] Furthermore, the yield stress (YS) and total elongation (E1) of the steel sheet and strip according to one embodiment of the present invention satisfy the following formulas 1 to 2. Thus, desired impact energy absorption performance and stamping formability can be obtained. Furthermore, the hole expansion ratio (λ) of the steel sheet and strip according to one embodiment of the present invention satisfies the following formula 3.

[0456] Formula 1:

[0457] When 590MPa≤TS<780MPa, 360MPa≤YS

[0458] When 780MPa≤TS<980MPa, 460MPa≤YS

[0459] When 980MPa≤TS<1180MPa, 580MPa≤YS

[0460] Formula 2:

[0461] When 590MPa≤TS<780MPa, 23.0%≤El

[0462] When 780MPa≤TS<980MPa, 17.0%≤El

[0463] When 980MPa≤TS<1180MPa, 11.0%≤El

[0464] Formula 3:

[0465] When 590MPa≤TS<780MPa, 45%≤λ

[0466] When 780MPa≤TS<980MPa, 30%≤λ

[0467] When 980MPa≤TS<1180MPa, 20%≤λ

[0468] Furthermore, tensile strength (TS), yield stress (YS), and total elongation (E1) were determined by tensile testing according to JIS Z 2241 (2011), as described later in the examples.

[0469] Furthermore, the expansion ratio (λ) was determined by an expansion test according to the Japan Iron and Steel Federation standard JFST 1001.

[0470] The measurement locations of TS, YS, E1, and λ of the steel plate are shown in the figure. Figure 4 The measurement locations of TS, YS, El, and λ within the steel strip (coil) are shown in the figure. Figure 5 .

[0471] For a steel plate according to one embodiment of the present invention, in Figure 4 The measurement positions shown satisfy TS (above 590 MPa and below 1180 MPa), YS (Equation 1 above), El (Equation 2 above), and λ (Equation 3 above).

[0472] Furthermore, regarding the steel strip in one embodiment of the present invention, in Figure 5 The measurement positions shown satisfy TS (above 590 MPa and below 1180 MPa), YS (Equation 1 above), El (Equation 2 above), and λ (Equation 3 above).

[0473] Fluctuations in mechanical properties (TS, YS, E1, λ)

[0474] The steel plate and steel strip of one embodiment of the present invention satisfy the following formulas 4 to 11.

[0475] In this case, stamping can be performed flawlessly in complex-shaped automotive body frame structural components. In the following formulas (Formulas 4-11), TS represents the steel sheet... Figure 4 The average TS at each measurement location shown (refer to location a of symbol 31, location b of symbol 32, and location c of symbol 33). Furthermore, the TS in the following formulas (Formulas 4-11) is... Figure 5 The average TS values ​​at each measurement location shown (reference symbols 1-15).

[0476] Formula 4:

[0477] When 590MPa≤TS<780MPa, ΔTS≤40MPa

[0478] When 780MPa≤TS<980MPa, ΔTS≤50MPa

[0479] When 980MPa≤TS<1180MPa, ΔTS≤60MPa

[0480] Formula 5:

[0481] When 590MPa≤TS<780MPa, σTS≤20MPa

[0482] When 780MPa≤TS<980MPa, σTS≤25MPa

[0483] When 980MPa≤TS<1180MPa, σTS≤30MPa

[0484] Formula 6:

[0485] When 590MPa≤TS<780MPa, ΔYS≤40MPa

[0486] When 780MPa≤TS<980MPa, ΔYS≤50MPa

[0487] When 980MPa≤TS<1180MPa, ΔYS≤60MPa

[0488] Formula 7:

[0489] When 590MPa≤TS<780MPa, σYS≤20MPa

[0490] When 780MPa≤TS<980MPa, σYS≤25MPa

[0491] When 980MPa≤TS<1180MPa, σYS≤30MPa

[0492] Formula 8:

[0493] When 590MPa≤TS<780MPa, ΔEl≤4.0%

[0494] When 780MPa≤TS<980MPa, ΔEl≤4.5%

[0495] When 980MPa≤TS<1180MPa, ΔEl≤5.0%

[0496] Formula 9:

[0497] When 590MPa≤TS<780MPa, σEl≤2.0%

[0498] When 780MPa≤TS<980MPa, σEl≤2.5%

[0499] When 980MPa≤TS<1180MPa, σEl≤3.0%

[0500] Formula 10:

[0501] When 590MPa≤TS<780MPa, Δλ≤25%.

[0502] When 780MPa≤TS<980MPa, Δλ≤25%.

[0503] When 980MPa≤TS<1180MPa, Δλ≤25%

[0504] Formula 11:

[0505] When 590MPa≤TS<780MPa, σλ≤7%

[0506] When 780MPa≤TS<980MPa, σλ≤7%

[0507] When 980MPa≤TS<1180MPa, σλ≤7%

[0508] It should be noted that for steel plates, ΔTS is in the width direction of the steel plate ( Figure 4 The positions at 1 / 4, 1 / 2, and 3 / 4 of the length (as indicated by reference symbol 21, which is perpendicular to the length direction 22 of the steel plate) Figure 4 The difference between the maximum and minimum values ​​of TS at positions a, b, and c of symbol 31 in sequence (maximum value of TS - minimum value of TS).

[0509] Furthermore, for steel plates, ΔYS is the difference between the maximum and minimum values ​​of YS at the 1 / 4, 1 / 2, and 3 / 4 positions along the width direction of the steel plate (maximum value of YS - minimum value of YS).

[0510] Furthermore, for steel plates, ΔEl is the difference between the maximum and minimum values ​​of El at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel plate (maximum value of El - minimum value of El).

[0511] Furthermore, for steel plates, Δλ is the difference between the maximum and minimum values ​​of λ at the 1 / 4, 1 / 2, and 3 / 4 positions along the width direction of the steel plate (maximum value of λ - minimum value of λ).

[0512] Furthermore, for steel plates, σTS is the standard deviation of TS at the 1 / 4, 1 / 2, and 3 / 4 positions along the width direction of the steel plate.

[0513] Furthermore, for steel plates, σYS is the standard deviation of YS at the 1 / 4, 1 / 2, and 3 / 4 positions along the width direction of the steel plate.

[0514] Furthermore, for steel plates, σEl is the standard deviation of El at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel plate.

[0515] Furthermore, for steel plates, σλ is the standard deviation of λ at the 1 / 4, 1 / 2, and 3 / 4 positions along the width direction of the steel plate.

[0516] Furthermore, for steel strips, ΔTS is in the width direction of the steel strip ( Figure 4 Reference symbol 21 (perpendicular to the length direction 22 of the steel plate) indicates the 1 / 4, 1 / 2, and 3 / 4 positions. Figure 5 Referring sequentially to symbols 31, 32, and 33), from the front end position 23 along the length direction of the steel strip towards the rear end position 24 along the length direction of the steel strip, at a position 10m from the front end of the steel strip along the length direction ( Figure 5 Reference position 41 (symbols 1, 6, and 11), 10m position at the tail end ( Figure 4 Reference position 45 (symbols 5, 10, and 15), 1 / 4 position ( Figure 5 Reference position 42 (symbols 2, 7, and 12), 1 / 2 position ( Figure 4 Reference position 43 (symbols 3, 8, and 13) and 3 / 4 position ( Figure 5 The difference between the maximum and minimum values ​​of TS at each position of reference position 44 (symbols 4, 9, and 14) (maximum value of TS - minimum value of TS).

[0517] Furthermore, for steel strip, ΔYS is the difference between the maximum and minimum values ​​of YS at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at the front 10m, rear 10m, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip (maximum value of YS - minimum value of YS).

[0518] Furthermore, for the steel strip, ΔEl is the difference between the maximum and minimum values ​​of El at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at the front 10m, rear 10m, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip (maximum value of El - minimum value of El).

[0519] Furthermore, for the steel strip, Δλ is the difference between the maximum and minimum values ​​of λ at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at the 10m front, 10m rear, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip (maximum value of λ - minimum value of λ).

[0520] Furthermore, for steel strips, σTS is the standard deviation of TS at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at the 10m front, 10m rear, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip.

[0521] Furthermore, for steel strips, σYS is the standard deviation of YS at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at the front 10m, rear 10m, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip.

[0522] Furthermore, for steel strips, σEl is the standard deviation of El at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at the front 10m, rear 10m, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip.

[0523] Furthermore, for steel strips, σλ is the standard deviation of λ at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the steel strip, and at the front 10m, rear 10m, 1 / 4, 1 / 2, and 3 / 4 positions in the length direction of the steel strip.

[0524] Coating

[0525] Steel plates and strips can have coatings on their surfaces. Types of coatings include hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing. Coatings can be applied to one or both surfaces of the steel plate or strip.

[0526] It should be noted that the zinc coating mentioned here refers to a coating with zinc (Zn) as the main component (Zn content of 50.0% or more), including hot-dip galvanized coatings, alloyed hot-dip galvanized coatings, and electro-galvanized coatings. Furthermore, the aluminum coating mentioned here refers to a coating with aluminum (Al) as the main component (Al content of 50.0% or more), including hot-dip aluminum coatings.

[0527] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, less than 20.0% by mass of Fe, and 0.001% to 1.0% by mass of Al. Furthermore, the hot-dip galvanized layer may optionally contain a total of 0.0% to 3.5% by mass of one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM. Additionally, the Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. It should be noted that the balance other than the aforementioned elements represents unavoidable impurities.

[0528] Furthermore, the alloyed hot-dip galvanized layer preferably consists of, for example, 20% by mass or less of Fe and 0.001% to 1.0% by mass of Al. Additionally, the alloyed hot-dip galvanized layer may optionally contain a total of 0% to 3.5% by mass of one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. Furthermore, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. It should be noted that the balance other than the aforementioned elements represents unavoidable impurities.

[0529] The electroplated zinc layer is preferably composed of, for example, Zn as the main component (Zn content of 70.0% or more). Furthermore, the electroplated zinc layer may arbitrarily contain a total of 0.0% by mass or more and less than 30.0% by mass of one or more elements selected from Fe, Ni, Co, Mn, Cr, Mg, Si, Zr, V, Cu, Pb, Sb, Sn, Ca, Li, Ti, Be, Bi, and REM. It should be noted that the balance other than the aforementioned elements is unavoidable impurities.

[0530] Furthermore, there is no particular limitation on the amount of zinc coating applied per single side, but it is preferably 20 g / m². 2 ~80g / m 2.

[0531] The type of aluminum plating layer is not particularly limited, but as a representative example, it is preferably composed of, for example, Al and less than 50.0% by mass of Fe. Furthermore, the hot-dip aluminum plating layer may arbitrarily contain a total of 0.0% to 25.0% by mass of one or more elements selected from Zn, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM. Moreover, the Fe content of the hot-dip aluminum plating layer is more preferably less than 25.0% by mass. It should be noted that the balance other than the aforementioned elements represents unavoidable impurities.

[0532] There is no particular limitation on the amount of aluminum plating applied to each single side, but it is preferably 20 g / m². 2 ~120g / m 2 .

[0533] It should be noted that the coating adhesion of zinc plating and hot-dip aluminizing is determined in the following manner.

[0534] Specifically, a treatment solution was prepared by adding 0.6 g of a corrosion inhibitor for Fe (“IBIT 700BK”, manufactured by Asahi Chemical Industry Co., Ltd., a registered trademark) to 1 L of a 10% hydrochloric acid aqueous solution. Next, a steel plate used as the test material was immersed in this treatment solution to dissolve the coating. Then, the mass reduction of the test material before and after dissolution was measured, and this value was divided by the surface area of ​​the base steel plate (the surface area of ​​the portion covered by the coating) to calculate the coating adhesion amount (g / m²). 2 ).

[0535] It should be noted that the thickness of the steel plate and steel strip in one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm to 3.5 mm.

[0536] [2] Manufacturing methods of steel plates and strips

[0537] Next, a method for manufacturing a steel plate according to one embodiment of the present invention will be described.

[0538] A method for manufacturing a steel plate according to one embodiment of the present invention includes: a hot rolling process, wherein a steel billet having the above-mentioned composition is hot rolled at a coiling temperature of 350°C to 650°C after finishing rolling to obtain a hot-rolled steel plate; an annealing and homogenizing process, wherein the hot-rolled steel plate is heated to an annealing and homogenizing temperature of 720°C to 860°C and a holding time of 20 seconds or more; a rapid heating process, wherein the steel plate is rapidly heated from the annealing and homogenizing temperature to (annealing and homogenizing temperature + 10°C) or higher at an average heating rate of 10°C / second or higher, such that the ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the rapidly heated steel plate is 25°C or lower; and a cooling process, wherein the steel plate is rapidly heated to a temperature of 1 / 4, 1 / 2, and 3 / 4 positions in the width direction. After the heating process, cooling is performed at an average cooling rate of 4°C / second or higher from the reached temperature to 550°C; or a cold rolling process is further included after the hot rolling process and before the annealing and soaking process to cold roll the hot-rolled steel sheet to obtain a cold-rolled steel sheet; or a plating process is further included after the cooling process to perform any one of hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing on the hot-rolled or cold-rolled steel sheet; or a reheating and holding process is further included after the cooling process or after the plating process to cool the hot-rolled or cold-rolled steel sheet to a cooling stop temperature of 0°C to 250°C, and then heat it to a temperature range of (cooling stop temperature + 50°C) to 460°C, and hold it in this temperature range for a tempering time of 10 seconds to 2000 seconds.

[0539] Furthermore, a method for manufacturing steel strip according to one embodiment of the present invention includes:

[0540] The hot rolling process involves hot rolling a steel billet with the above-mentioned composition at a coiling temperature of 350℃~650℃ after finishing rolling to obtain a hot-rolled steel strip; the annealing and homogenization process involves heating the hot-rolled steel strip and annealing it at an annealing and homogenization temperature of 720℃~860℃ and a holding time of 20 seconds or more; the rapid heating process involves rapidly heating the steel strip from the annealing and homogenization temperature to (annealing and homogenization temperature + 10℃) or higher at an average heating rate of 10℃ / second or higher, such that the ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction of the rapidly heated steel strip is below 25℃; and

[0541] The cooling process is carried out after the rapid heating process described above, under the condition that the average cooling rate from the above-mentioned reached temperature to 550°C is 4°C / second or more.

[0542] Alternatively, a cold rolling process may be included after the hot rolling process and before the annealing and soaking process to cold roll the hot-rolled steel strip and obtain a cold-rolled steel strip.

[0543] Alternatively, a plating process may be included after the cooling process, to perform any one of the following on the hot-rolled steel strip or the cold-rolled steel strip: hot-dip galvanizing, alloying hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing.

[0544] Alternatively, after the above-mentioned cooling process or the above-mentioned plating process, a reheating and holding process may be included, in which the above-mentioned hot-rolled steel strip or the above-mentioned cold-rolled steel strip is cooled to a cooling stop temperature of 0°C to 250°C, and then heated to a temperature range of (the above-mentioned cooling stop temperature + 50°C) to 460°C, and held in the above-mentioned temperature range for a tempering time of 10 seconds to 2000 seconds.

[0545] Figure 6a ~e is a graph showing an example of a heat treatment mode in a method for manufacturing steel plates and strips according to an embodiment of the present invention, with the horizontal axis representing time and the vertical axis representing temperature.

[0546] Specifically, Figure 6a This is a schematic diagram illustrating an example of a heat treatment mode that includes an annealing and homogenization process (A), a rapid heating process (B), a cooling process (C), and a reheating and holding process (E).

[0547] also, Figure 6b This is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), a hot-dip galvanizing process (D1), and a reheating and holding process (E).

[0548] also, Figure 6c This is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), a hot-dip galvanizing process (D1), an alloying process (D11), and a reheating and holding process (E).

[0549] also, Figure 6d This is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), an electroplating zinc treatment process (D2), and a reheating and holding process (E).

[0550] also, Figure 6e This is a schematic diagram illustrating an example of a heat treatment process that includes an annealing and soaking process (A), a rapid heating process (B), a cooling process (C), a hot-dip aluminizing process (D3), and a reheating and holding process (E).

[0551] It should be noted that, in Figure 6a In ~e, the reheating and holding process (E) shows two modes respectively.

[0552] The manufacturing method for steel strip is the same as that for steel plate. It should be noted that the temperature used in the manufacturing process refers to the surface temperature of the steel plate or steel strip. Furthermore, unless otherwise specified, the temperature used in the manufacturing process is the average temperature at the 1 / 4, 1 / 2, and 3 / 4 positions of the width of the steel plate or steel strip.

[0553] In addition, in the steel strip, the temperatures in the hot rolling process, annealing and homogenization process, rapid heating process and cooling process refer to the temperatures at the front 10m position, the rear 10m position, the 1 / 4 position, the 1 / 2 position and the 3 / 4 position along the length of the steel strip.

[0554] In addition, in steel strip, the temperature in processes other than hot rolling, annealing and soaking, rapid heating and cooling (such as plating and reheating) refers to the temperature at the 1 / 2 position along the length of the steel strip.

[0555] In this invention, the smelting method for the steel billet is not particularly limited, and known smelting methods such as converters and electric furnaces are applicable. Furthermore, to prevent macroscopic segregation, the steel billet (slab) is preferably manufactured by continuous casting, but it can also be manufactured by ingot casting or thin slab casting. In addition to the conventional method of temporarily cooling to room temperature and then reheating after billet manufacturing, energy-saving processes such as direct rolling and direct rolling can be used without problems, including loading the billet directly into the heating furnace without cooling to room temperature, or rolling immediately after a slight holding period. When heating the slab, from the viewpoint of dissolving carbides and reducing rolling load, the slab heating temperature is preferably 1100°C or higher. Furthermore, to prevent increased oxide scale loss, the slab heating temperature is preferably 1300°C or lower. It should be noted that the slab heating temperature refers to the surface temperature of the slab. Furthermore, slabs are typically produced into thin slabs through rough rolling under normal conditions. However, when the heating temperature is set low, from the viewpoint of preventing malfunctions during hot rolling, it is preferable to heat the thin slabs using a strip heater or similar device before finishing rolling.

[0556] [Hot rolling process]

[0557] The finishing temperature of hot rolling is preferably above 820℃. This is because when the finishing temperature of hot rolling is below 820℃, the rolling load increases, and the reduction rate of austenite in the non-recrystallized state becomes higher, leading to the development of abnormal structures extending along the rolling direction, which may result in a decrease in the El of the final material.

[0558] Coiling temperature after finishing rolling: 350℃~650℃

[0559] When the coiling temperature after finishing rolling is below 350°C, a large amount of fresh martensite and tempered martensite are generated during the cooling process to room temperature after coiling, resulting in a significant increase in strength at this location. Therefore, the fluctuations in the mechanical properties (YS, TS, E1, λ) of the final steel plate increase. Thus, the coiling temperature after finishing rolling is set to above 350°C. Preferably, the coiling temperature after finishing rolling is above 400°C. When the coiling temperature after finishing rolling exceeds 650°C, a large amount of ferrite is generated during coiling, resulting in a significant decrease in strength at this location. Therefore, the fluctuations in the mechanical properties (YS, TS, E1, λ) of the final steel plate increase. Therefore, the coiling temperature after finishing rolling is set to below 650°C. Preferably, the coiling temperature after finishing rolling is below 600°C.

[0560] It should be noted that during hot rolling, rough-rolled plates can be joined together for continuous finishing rolling. Furthermore, to reduce the rolling load during hot rolling, part or all of the finishing rolling can be performed as lubricated rolling. From the viewpoint of achieving uniformity in steel plate shape and material composition, lubricated rolling is also effective. It should be noted that the coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0561] The hot-rolled steel sheets manufactured in this way are then pickled. Pickling removes oxides from the surface of the steel sheet, which is important for ensuring good chemical conversion properties and coating quality of the final product. Furthermore, pickling can be performed in one go or in multiple stages.

[0562] When cold rolling is required on the hot-rolled and pickled steel sheet obtained as described above, the hot-rolled and pickled sheet can be cold-rolled as is, or it can be cold-rolled after heat treatment. Furthermore, the cold-rolled steel sheet obtained after cold rolling can be pickled arbitrarily. Cold rolling is performed, for example, by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0563] When cold rolling is performed as required, the reduction rate is not particularly limited, but it is preferably below 80%. When the reduction rate exceeds 80%, the steel sheet is prone to shape defects, which may lead to uneven galvanization. It should be noted that the reduction rate mentioned here is the cumulative reduction rate.

[0564] [Annealing and Heating Process]

[0565] exist Figure 6a In the annealing and homogenization process (A) shown in ~e, the hot-rolled steel plate is heated and annealed and homogenized at an annealing and homogenization temperature of 720℃~860℃ and a holding time of more than 20 seconds.

[0566] Annealing temperature: 720℃~860℃

[0567] When the annealing homogenization temperature is below 720℃, the proportion of austenite formation in the homogenization of the two-phase region of ferrite and austenite is insufficient. Therefore, during the annealing homogenization process, austenite with almost no Mn enrichment (close to the Mn content of steel) constitutes the majority. After the subsequent rapid heating process, the phase transformation to transformation ferrite and epitaxial ferrite occurs during the cooling process, failing to generate the desired hard phase (fresh martensite and tempered martensite), and thus failing to obtain a TS of over 590MPa.

[0568] Furthermore, from the perspective of the excessive increase in ferrite area ratio after annealing and homogenization, the desired TS cannot be obtained.

[0569] On the other hand, when the annealing homogenization temperature exceeds 860°C, a large amount of austenite is generated during annealing homogenization, making it impossible to obtain a ferrite area ratio of more than 30.0% and thus failing to obtain the desired El.

[0570] Therefore, the annealing uniform heating temperature is set to 720℃~860℃. Preferably, the annealing uniform heating temperature is 740℃ or higher. Furthermore, preferably, the annealing uniform heating temperature is 830℃ or lower. It should be noted that the annealing uniform heating temperature is the highest temperature reached during the annealing uniform heating process.

[0571] Duration: 20 seconds or more

[0572] When the holding time (also known as annealing holding time or annealing time) is less than 20 seconds, the proportion of austenite generated during the homogenization of the two-phase region of ferrite and austenite is insufficient.

[0573] Therefore, the holding time should be set to 20 seconds or more. Preferably, the holding time should be 30 seconds or more.

[0574] It should be noted that there is no particular upper limit to the holding time, but it is preferably 900 seconds or less, more preferably 400 seconds or less. It should also be noted that the holding time refers to the holding time within the temperature range of (annealing soaking temperature - 30°C) to the annealing soaking temperature. That is, the holding time includes not only the holding time at the annealing soaking temperature, but also the residence time within the temperature range of (annealing soaking temperature - 30°C) to the annealing soaking temperature during heating before reaching the annealing soaking temperature.

[0575] Here, the heat treatment furnace for the annealing and soaking process is preferably a radiant tube furnace. Radiant tube furnaces heat the steel plate through radiant heat from the radiant tubes and furnace walls, resulting in a very large heat source volume and high thermal inertia. Consequently, it is difficult to quickly follow changes in the set temperature, leading to low responsiveness to temperature control commands.

[0576] It should be noted that the annealing soaking temperature is obtained using a thermometer that measures the surface temperature of the steel plate. There are no particular limitations on the method of temperature measurement; for example, a radiation thermometer that senses the infrared radiation emitted by the steel plate is suitable. When using a radiation thermometer, it may be affected by reflected infrared light emitted from the surrounding furnace; therefore, a covering can be placed between the measuring part of the radiation thermometer and the sensing part of the steel plate. Furthermore, since the emissivity of the steel plate surface may be affected, a multi-reflection measurement method utilizing the wedge-shaped space between the conveyor rollers and the steel plate can also be used.

[0577] [Rapid Heating Process]

[0578] After the annealing and heating process (A), Figure 6a In the rapid heating process (B) shown in ~e, the steel plate is rapidly heated from the annealing temperature at an average heating rate of 10°C / second or higher to (annealing temperature + 10°C) or higher, so that the maximum value of the reached temperature - the minimum value of the reached temperature at each of the 1 / 4, 1 / 2 and 3 / 4 positions in the width direction after the rapid heating is below 25°C.

[0579] Rapidly heat to (annealing homogenization temperature + 10°C) or higher under an average heating rate of 10°C or higher.

[0580] In the rapid heating process, the temperature is rapidly increased from the annealing soaking temperature to a final temperature of (annealing soaking temperature + 10°C) or higher. Preferably, the final temperature is (annealing soaking temperature + 20°C) or higher. Furthermore, the final temperature is preferably (annealing soaking temperature + 150°C) or lower.

[0581] When the average heating rate is below 10°C / second, it also promotes the coarsening of austenite grains and the diffusion of Mn from ferrite to austenite, making it difficult to control mechanical properties. Therefore, it is impossible to ensure that the tensile strength (TS), yield stress (YS), total elongation (E1), the difference between the maximum and minimum values ​​of TS (ΔTS), YS (ΔYS), E1 (ΔE1), and λ (Δλ), as well as the standard deviations of TS (σTS), YS (σYS), E1 (σE1), and λ (σλ), all fall within the desired range. Therefore, the average heating rate is set to 10°C / second or higher. The average heating rate is preferably 20°C / second or higher, more preferably 30°C / second or higher. When the average heating rate is 30°C / second or higher, the diffusion of Mn from ferrite to austenite can be suppressed, and the diffusion of C from ferrite to austenite is more likely to occur. Furthermore, the production line length can be further shortened.

[0582] When the average heating rate exceeds 200°C / second, localized high-temperature areas may occur along the width of the plate, making it impossible to maintain uniformity. Therefore, the average heating rate is preferably below 200°C / second. More preferably, it is below 170°C / second, and even more preferably below 150°C / second. The average heating rate is preferably below 120°C / second. When the average heating rate is below 170°C / second, the risk of buckling deformation of the steel plate due to thermal stress can be further reduced.

[0583] Furthermore, from the viewpoint of reducing the risk of buckling deformation of the steel plate due to thermal stress, the average heating rate is preferably 300°C / second or less, and more preferably 150°C / second or less.

[0584] Here, the average heating rate (°C / second) is obtained by dividing the difference (°C) between the annealing soaking temperature (heating start temperature) and the reached temperature during rapid heating by the heating time (seconds) from the annealing soaking temperature to the reached temperature.

[0585] It should be noted that the annealing soaking temperature (heating start temperature) and the reaching temperature used to determine the average heating rate refer to the average temperature at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel strip. Furthermore, the annealing soaking temperature (heating start temperature) and the reaching temperature used to determine the average heating rate refer to the average temperature at the 1 / 2 position along the length of the steel strip, and at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel strip.

[0586] ΔT (maximum temperature reached - minimum temperature reached) of the reached temperature at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the rapidly heated steel plate: below 25℃

[0587] When ΔT exceeds 25℃, the ratio of ferrite and austenite formation in the two-phase region of ferrite and austenite becomes uneven in the width direction of the steel plate. In the subsequent cooling process, the ratio of soft phase and hard phase formation fluctuates, making it impossible for ΔTS, ΔYS, ΔEl, Δλ, σTS, σYS, σEl, and σλ to all fall within the expected range.

[0588] The temperature reached during rapid heating has a significant impact on the final product's YS, TS, E1, and λ. Therefore, for rapid heating, an induction heating (IH) device with high responsiveness to temperature control commands is preferred. The induction heating (IH) device adjusts its output to rapidly heat the steel sheet, keeping the temperature within the range of 725°C to 940°C. Furthermore, when heating in this temperature range, the induction heating (IH) device is preferably of a horizontal type.

[0589] Method for determining the reached temperature in a rapid heating process

[0590] In the rapid heating process, the appropriate temperature reached after rapid heating varies depending on the composition of the steel plate. Therefore, the preferred temperature range for the aforementioned temperature to reach the target temperature is preferably predicted in advance by measurement, calculation, or simulation, and set based on the steel plate temperature during the annealing and soaking process.

[0591] As a method for determining the reached temperature in the rapid heating process, one method can be listed that is based on the information of the composition of the steel plate and the information of the coiling temperature in the hot rolling process.

[0592] Specifically, such as Figure 6a As shown in ~e, a method can be listed for determining the reached temperature in the feedforward control process F based on information about the steel plate's composition and the coiling temperature F0 after finishing in the hot rolling process. Here, information F0 may include not only information about the steel plate's composition and the coiling temperature after finishing in the hot rolling process, but also information about the steel plate's thickness, width, and reduction rate (cumulative reduction rate) during cold rolling.

[0593] In addition, as a method for determining the temperature reached in the rapid heating process, another method can be listed as determining the temperature reached based on information on the austenite fraction of the steel plate measured by a phase transformation rate meter or the like in the temperature range of 150°C to 600°C after the rapid heating process.

[0594] Specifically, such as Figure 6a As shown in ~e, a method can be listed for determining the reached temperature in the feedback control process (G) based on information G0 of the austenite fraction of the steel plate measured by a phase transformation rate meter or the like in the temperature range of 150°C to 600°C.

[0595] Furthermore, as a method for determining the arrival temperature in the rapid heating process, other methods can be listed, such as determining the arrival temperature based on information about the composition of the steel plate, information about the coiling temperature in the hot rolling process, and information about the austenite fraction of the steel plate measured using a phase transformation rate meter or similar device in a temperature range of 150°C to 600°C after the rapid heating process. That is, the arrival temperature can also be determined based on both the processing in the feedforward control process (F) and the processing in the feedback control process (G).

[0596] The reached temperature in the rapid heating process is obtained using a thermometer that measures the surface temperature of the steel plate. There are no particular limitations on the method of temperature measurement; for example, a radiation thermometer that senses the infrared radiation emitted by the steel plate is suitable. When using a radiation thermometer, it may be affected by reflected infrared light emitted from the surrounding furnace; therefore, a cover can be placed between the measuring part of the radiation thermometer and the sensing part of the steel plate. Furthermore, since the emissivity of the steel plate surface may be affected, a multi-reflection measurement method utilizing the wedge-shaped space between the conveyor rollers and the steel plate can also be used. Moreover, scanning radiation thermometers are preferred for temperature measurement at various locations along the width of the steel plate.

[0597] Regarding the aforementioned austenite fraction information, a phase fraction prediction model is constructed, and offline performance data is collected. As performance data, the thermal history and phase fraction of the operating condition dataset obtained under various changes in operating conditions are calculated. Multiple training datasets based on the performance data are then prepared in a storage device or the aforementioned database.

[0598] The machine learning model used to utilize the aforementioned performance data is not limited by its generation method, as long as it can make predictions within the required accuracy for practical application. For example, commonly used methods such as neural networks (including deep learning), decision tree learning, random forests, and support vector regression can be used. Furthermore, ensemble models combining multiple methods can also be used. Further, as a phase fraction prediction model, a machine learning model that binarizes the output can be used. This model does not output the calculated phase fraction value of the steel plate, but rather determines whether it is within a predetermined allowable range of phase fraction (qualified or unqualified). In this case, classification model methods such as k-nearest neighbors or logistic regression can be used. Based on the phase fraction obtained during annealing from the material prediction model and the phase fraction prediction model for obtaining steel plates with excellent mechanical property stability, the operating conditions are adjusted. By implementing such control, the annealing temperature of the steel plate can be adjusted to manufacture products with stable mechanical properties. It should be noted that the material prediction model used can be a physical model obtained not only through offline laboratory experiments or numerical analysis, but also a machine learning model obtained from accumulated manufacturing performance.

[0599] [Cooling Process]

[0600] After the rapid heating process (B), Figure 6a In the cooling process (C) shown in ~e, cooling is performed under the condition that the average cooling rate from the above-mentioned temperature to 550°C is 4°C / second or more.

[0601] Average cooling rate from the temperature reached in the rapid heating process to 550°C: 4°C / second or higher

[0602] The average cooling rate from the temperature reached in the rapid heating process to 550°C is set to 4°C / second or more. Preferably, it is 5°C / second or more. Furthermore, the average cooling rate is preferably 100°C / second or less, more preferably 60°C / second or less.

[0603] Furthermore, when slowly cooled in the intermediate temperature range below 650°C, the fine austenite at the ferrite grain boundaries merges with adjacent ferrite with similar orientations, forming a single ferrite grain. This grain remains within the ferrite grain as isolated, fine, island-like austenite, ultimately increasing the proportion of isolated, fine, island-like hard second phases (martensite + retained austenite) within the ferrite grain. Therefore, the average cooling rate from 650°C to 500°C is preferably 14°C / second or less, more preferably 12°C / second or less.

[0604] Here, the average cooling rate (°C / sec) is obtained by dividing the difference (°C) between the cooling start temperature and the cooling stop temperature of the target temperature range by the cooling time (sec) from the cooling start temperature to the cooling stop temperature.

[0605] It should be noted that the cooling start temperature and cooling stop temperature used to determine the average cooling rate refer to the average temperature at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel plate. Furthermore, the cooling start temperature and cooling stop temperature used to determine the average cooling rate refer to the average temperature at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel strip, and at the following positions along the length of the steel strip: 10m from the front end, 10m from the rear end, 1 / 4, 1 / 2, and 3 / 4 positions.

[0606] Furthermore, the cooling start temperature and cooling stop temperature used to determine the average cooling rate refer to the average temperature at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel strip at the 1 / 2 position along the length of the steel strip.

[0607] The steel plate is preferably cooled within 10 seconds after reaching the temperature reached in the rapid heating process (heating temperature). The time from reaching the temperature reached in the rapid heating process to the start of cooling is preferably within 5 seconds, more preferably within 3 seconds.

[0608] As a cooling method, gas jet cooling, roller cooling, water cooling (water quenching), etc. can be used. The cooling zone can also be divided into multiple areas, different cooling methods can be combined, or the cooling conditions of the same cooling method can be changed to control the heat path of the steel plate during cooling.

[0609] [Plating Process]

[0610] After the cooling process (C), as Figure 6bThe plating process (D) shown in ~e can be applied to hot-rolled steel sheets without a cold rolling process, or cold-rolled steel sheets with a cold rolling process, by any one of the following: hot-dip galvanizing, alloying hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing.

[0611] Figure 6b This is an example of the heat treatment mode when the plating process (D) is a hot-dip galvanizing process (D1).

[0612] Figure 6c This section describes an example of a heat treatment mode when the plating process (D) is an alloying hot-dip galvanizing process. The alloying hot-dip galvanizing process includes a hot-dip galvanizing process (D1) that performs the hot-dip galvanizing process and an alloying process (D11) that performs the alloying process after the hot-dip galvanizing process (D1).

[0613] Figure 6d This is an example of the heat treatment mode when the plating process (D) is the electro-zinc plating process (D2).

[0614] Figure 6e This is an example of the heat treatment mode when the plating process (D) is a hot-dip aluminizing process (D3).

[0615] As mentioned above, in the coating process, zinc plating processes, for example, include hot-dip galvanizing, alloyed hot-dip galvanizing, and electroplating.

[0616] When performing hot-dip galvanizing, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath at 440°C to 500°C, and then adjust the coating adhesion by methods such as gas wiping. The hot-dip galvanizing bath is not particularly limited as long as it forms the composition of the aforementioned zinc layer, but it is preferable to use a bath with, for example, an Al content of 0.10% to 0.23% by mass, with the balance consisting of Zn and unavoidable impurities.

[0617] Furthermore, when performing alloyed hot-dip galvanizing, it is preferable to heat the hot-dip galvanized steel sheet to an alloying temperature of 500°C to 600°C after hot-dip galvanizing according to the above-described procedure, and then perform alloying treatment. When the alloying temperature is below 500°C, the Zn-Fe alloying rate slows down, and alloying may become difficult. On the other hand, when the alloying temperature exceeds 600°C, untransformed austenite will transform into pearlite, making it difficult to achieve a TS of 590 MPa or higher. It should be noted that the alloying temperature is preferably 510°C or higher. Moreover, the alloying temperature is preferably 570°C or lower.

[0618] Furthermore, the coating adhesion amount for both hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) is preferably 20–80 g / m² per single side. 2 It should be noted that the coating adhesion amount can be adjusted by methods such as gas wiping.

[0619] In the case of electroplating zinc, for example, as the electroplating solution, an electroplating solution containing divalent zinc ions at a concentration of 80 g / L in the form of sulfate is used, and the current density is adjusted to 10–80 A / dm³. 2 By controlling the electrolysis time within the specified range, the zinc coating adhesion can be controlled to 20g / m². 2 ~80g / m 2 .

[0620] Furthermore, during hot-dip aluminizing, cold-rolled steel sheets are immersed in an aluminizing bath at 660–730°C for hot-dip aluminizing, and then the coating adhesion is adjusted by methods such as gas wiping. The preferred coating adhesion is 20–120 g / m² per single side. 2 As for the hot-dip aluminizing bath, there is no particular limitation as long as it forms the composition of the aforementioned aluminized layer, but it is preferable to use a bath with, for example, a Zn content of 1.0% to 20.0% by mass, with the balance consisting of Al and unavoidable impurities.

[0621] [Reheating and Holding Process]

[0622] exist Figure 6a In the reheating and holding process (E) shown in ~e, if the plating process is not included, after the cooling process, or if the plating process is included, after the plating process, the hot-rolled steel sheet or cold-rolled steel sheet is cooled to a cooling stop temperature of 0°C to 250°C, heated to a temperature range of (cooling stop temperature + 50°C) to 460°C, and held in this temperature range for a tempering time of 10 seconds to 2000 seconds.

[0623] Reheat holding temperature: (cooling stop temperature + 50°C) ~ 460°C

[0624] Reheating time: 10 seconds to 2000 seconds

[0625] By reheating to (the aforementioned cooling stop temperature + 50°C) and holding for at least 10 seconds, diffusible hydrogen in the steel is released. Furthermore, the area ratio of fresh martensite in the final microstructure can be reduced, ensuring an appropriate amount of tempered martensite. Additionally, the amount of austenite immediately present after the cooling process can be reduced, decreasing the area ratio of retained austenite in the final microstructure. As a result, the desired λ, R / t, ST, and SFmax can be obtained.

[0626] On the other hand, when the reheat holding temperature exceeds 460°C, during plating processes, the galvanizing coating will partially dissolve and adhere to the rollers, making it impossible to obtain steel sheets with uniform galvanizing coating. Furthermore, when the reheat holding time is less than 10 seconds, the desired amount of diffusible hydrogen in the steel cannot be released.

[0627] Furthermore, the steel sheet obtained as described above can be further subjected to quenching and tempering rolling. When the reduction rate of quenching and tempering rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy when forming the steel sheet into components may decrease. Therefore, the reduction rate of quenching and tempering rolling is preferably 2.00% or less. It should be noted that there is no particular limitation on the lower limit of the reduction rate of quenching and tempering rolling, but from the viewpoint of productivity, it is preferably 0.05% or more. In addition, quenching and tempering rolling can be performed on a continuous (online) annealing equipment that performs the above-mentioned processes, or on an equipment that is discontinuous with the annealing equipment that performs the above-mentioned processes (offline). Furthermore, the number of rolling passes in quenching and tempering rolling can be one or more. It should be noted that as long as the same elongation as quenching and tempering rolling can be imparted, rolling can also be performed using a leveling machine or the like.

[0628] While there are no particular limitations on the conditions for other manufacturing methods, from a productivity standpoint, the aforementioned series of processes, including annealing, hot-dip galvanizing, and alloying of the galvanized coating, are preferably performed on a CGL (Continuous Galvanizing Line) that serves as a hot-dip galvanizing production line. After hot-dip galvanizing, wiping can be performed to adjust the coating adhesion. It should be noted that coating conditions other than those described above can be handled using conventional methods.

[0629] The above mainly describes the manufacturing method of the steel plate of the present invention. However, the manufacturing method of the steel strip can also be described by replacing the steel plate with the steel strip in the entire description of the manufacturing method of the steel plate.

[0630] Example

[0631] Steel billets with the composition shown in Table 1 (balance: Fe and unavoidable impurities) were melted in a converter and produced by continuous casting. In Table 1, - indicates the level of unavoidable impurities.

[0632] The obtained steel billet was heated to 1200℃. After heating, the billet was subjected to rough rolling and hot rolling. It was then coiled into coils at the coiling temperatures shown in Table 2 (Table 2-1, Table 2-2) to obtain hot-rolled steel strip. Then, No.1 to No.82 of the obtained hot-rolled steel strip were pickled and cold-rolled (cumulative reduction rate: 63%) to produce cold-rolled steel strips with the plate thicknesses shown in Tables 2 and 3 (Tables 3-1 to 3-10).

[0633] In addition, No. 83 to No. 91 of the obtained hot-rolled steel strips were pickled to produce hot-rolled steel strips (white strips) with the plate thicknesses shown in Tables 2 and 3.

[0634] Then, the obtained cold-rolled or hot-rolled steel strip (white strip) is subjected to annealing and homogenization, rapid heating (IH rapid heating), and cooling processes under the conditions shown in Table 2 to obtain steel strip. Furthermore, the annealing and homogenization, rapid heating, and cooling processes are performed, and if necessary, plating is carried out, followed by a reheating and holding process as needed. The temperatures in Table 2 represent the average values ​​at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel strip.

[0635] Here, in the plating process, hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing are performed to obtain hot-dip galvanized steel strip (GI), alloyed hot-dip galvanized steel strip (GA), electro-galvanized steel strip (EG), and aluminized steel strip (Al). It should be noted that in Table 2, the type of plating process is also indicated as "GI", "GA", "EG", and "Al". When no plating treatment is performed, it is indicated as "CR". In Table 2, for the cases of CR, GI, EG, and Al, since no alloying treatment is performed, the alloying temperature is indicated as -.

[0636] The zinc plating bath temperature used for GI and GA in the above-mentioned plating processes is 470°C when manufacturing either one. The zinc plating coating weight is 45-72 g / m² per side when manufacturing GI. 2 (Double-sided plating), 45g / m² per side during GA manufacturing. 2 (Double-sided plating). It should be noted that the final zinc coating composition of the steel strip (hot-dip galvanized steel strip) contains Fe: 0.1–2.0% by mass in GI, with the balance being Zn and unavoidable impurities. Additionally, in GA, it contains Fe: 8.0–12.0% by mass, with the balance being Zn and unavoidable impurities. Furthermore, during the manufacture of EG, the zinc coating amount is 45 g / m² per side. 2 (Double-sided plating). Furthermore, in EG where the zinc plating layer is a zinc (Zn)-nickel (Ni) plating layer, the Ni content in the zinc plating layer is 9% to 25% by mass (double-sided plating), and the zinc plating adhesion is 45 g / m² per side. 2 (Double-sided plating). Furthermore, during Al manufacturing, the aluminum plating bath temperature is 700℃, and the aluminum plating adhesion is 45g / m² per side. 2 (Double-sided plating).

[0637] Furthermore, after each measurement of the steel strip described below, the steel plate is positioned at 1 / 2 of the length direction of the steel strip ( Figure 5 Using reference symbols 3, 8, and 13 as the center of the length direction (rolling direction), cut out 1m along the length direction.

[0638] Using the obtained steel strip (CR, GI, GA, EG, Al), perform the following steps at each position of the steel strip according to the above guidelines. Figure 5 The steel microstructure was identified at positions marked 1–15. The results are shown in Table 3. In Table 3, the proportions of the steel microstructure are the average values ​​for all measured positions (marked 1–15). Furthermore, the soft phases consist of unrecrystallized ferrite, recrystallized ferrite, transformation ferrite, epitaxial ferrite, and bainitic ferrite; the hard phases consist of fresh martensite and tempered martensite; RA is retained austenite; P is pearlite; and θ is carbides.

[0639] In addition, for steel plates, the same method is used as for steel strips at the 1 / 2 position along the length of the steel strip ( Figure 5 The steel structure is identified by referring to symbols 3, 8, and 13.

[0640] Tensile tests were conducted according to JIS Z 2241 (2011). That is, tensile tests were performed on the obtained steel strips (CR, GI, GA, EG, Al). Figure 5 JIS 5 test pieces were collected at each position marked 1 to 15, with the length direction perpendicular to the rolling direction of the steel strip. Using the collected test pieces, tensile tests were performed at a crosshead speed of 10 mm / min to determine TS, YS, and E1. From the obtained TS, YS, and E1 values ​​at each position marked 1 to 15 within each steel strip (CR, GI, GA, EG, Al), the differences between the maximum and minimum values ​​of TS (ΔTS), YS (ΔYS), and E1 (ΔE1), as well as the standard deviations of TS (σTS), YS (σYS), and E1 (σE1) were calculated.

[0641] Furthermore, the λ values ​​at positions 1 to 15 of the steel strip were measured using a hole-expanding test based on the Japan Iron and Steel Federation standard JFST 1001. Then, the difference between the maximum and minimum values ​​of λ (Δλ) and the standard deviation of λ (σλ) were calculated from the λ values ​​at each position.

[0642] The results are shown in Table 3.

[0643] It should be noted that, as an example of evaluating steel plate fluctuation, the differences between the maximum and minimum values ​​of TS (ΔTS), YS (ΔYS), and El (ΔEl) at the halfway points along the length of the steel strip (marked as 3, 8, 13), as well as the standard deviations of TS (σTS), YS (σYS), and El (σEl), are also recorded. Furthermore, as an evaluation of steel plate fluctuation, the differences between the maximum and minimum values ​​of λ (Δλ) and the standard deviation of λ (σλ) are also recorded.

[0644] TS

[0645] ○ (Qualified): Above 590MPa and below 1180MPa

[0646] × (Unacceptable): Below 590MPa, above 1180MPa

[0647] YS

[0648] ○(passed):

[0649] When 590MPa≤TS<780MPa, 360MPa≤YS

[0650] When 780MPa≤TS<980MPa, 460MPa≤YS

[0651] When 980MPa≤TS<1180MPa, 580MPa≤YS

[0652] × (Unacceptable):

[0653] When 590MPa≤TS<780MPa, 360MPa>YS

[0654] When 780MPa≤TS<980MPa, 460MPa>YS

[0655] When 980MPa≤TS<1180MPa, 580MPa>YS

[0656] El

[0657] ○(passed):

[0658] When 590MPa≤TS<780MPa, 23.0%≤El

[0659] When 780MPa≤TS<980MPa, 17.0%≤El

[0660] When 980MPa≤TS<1180MPa, 11.0%≤El

[0661] × (Unacceptable):

[0662] When 590MPa≤TS<780MPa, 23.0%>El

[0663] When 780MPa≤TS<980MPa, 17.0%>El

[0664] When 980MPa≤TS<1180MPa, 11.0%>El

[0665] λ

[0666] ○(passed):

[0667] When 590MPa≤TS<780MPa, 45%≤λ

[0668] When 780MPa≤TS<980MPa, 30%≤λ

[0669] When 980MPa≤TS<1180MPa, 20%≤λ

[0670] × (Unacceptable):

[0671] When 590MPa≤TS<780MPa, 45%>λ

[0672] When 780MPa≤TS<980MPa, 30%>λ

[0673] When 980MPa≤TS<1180MPa, 20%>λ

[0674] ΔTS

[0675] ○(passed):

[0676] When 590MPa≤TS<780MPa, ΔTS≤40MPa

[0677] When 780MPa≤TS<980MPa, ΔTS≤50MPa

[0678] When 980MPa≤TS<1180MPa, ΔTS≤60MPa

[0679] × (Unacceptable):

[0680] When 590MPa≤TS<780MPa, ΔTS>40MPa

[0681] When 780MPa≤TS<980MPa, ΔTS>50MPa

[0682] When 980MPa≤TS<1180MPa, ΔTS>60MPa

[0683] ΔYS

[0684] ○(passed):

[0685] When 590MPa≤TS<780MPa, ΔYS≤40MPa

[0686] When 780MPa≤TS<980MPa, ΔYS≤50MPa

[0687] When 980MPa≤TS<1180MPa, ΔYS≤60MPa

[0688] × (Unacceptable):

[0689] When 590MPa≤TS<780MPa, ΔYS>40MPa

[0690] When 780MPa≤TS<980MPa, ΔYS>50MPa

[0691] When 980MPa≤TS<1180MPa, ΔYS>60MPa

[0692] ΔEl

[0693] ○(passed):

[0694] When 590MPa≤TS<780MPa, ΔEl≤4.0%

[0695] When 780MPa≤TS<980MPa, ΔEl≤4.5%

[0696] When 980MPa≤TS<1180MPa, ΔEl≤5.0%

[0697] × (Unacceptable):

[0698] When 590MPa≤TS<780MPa, ΔEl>4.0%

[0699] When 780MPa≤TS<980MPa, ΔEl>4.5%

[0700] When 980MPa≤TS<1180MPa, ΔEl>5.0%

[0701] Δλ

[0702] ○(passed):

[0703] When 590MPa≤TS<780MPa, Δλ≤25%.

[0704] When 780MPa≤TS<980MPa, Δλ≤25%.

[0705] When 980MPa≤TS<1180MPa, Δλ≤25%

[0706] × (Unacceptable):

[0707] When 590MPa≤TS<780MPa, Δλ>25%

[0708] When 780MPa≤TS<980MPa, Δλ>25%

[0709] When 980MPa≤TS<1180MPa, Δλ>25%

[0710] σTS

[0711] ○(passed):

[0712] When 590MPa≤TS<780MPa, σTS≤20MPa

[0713] When 780MPa≤TS<980MPa, σTS≤25MPa

[0714] When 980MPa≤TS<1180MPa, σTS≤30MPa

[0715] × (Unacceptable):

[0716] When 590MPa≤TS<780MPa, σTS>20MPa

[0717] When 780MPa≤TS<980MPa, σTS>25MPa

[0718] When 980MPa≤TS<1180MPa, σTS>30MPa

[0719] σYS

[0720] ○(passed):

[0721] When 590MPa≤TS<780MPa, σYS≤20MPa

[0722] When 780MPa≤TS<980MPa, σYS≤25MPa

[0723] When 980MPa≤TS<1180MPa, σYS≤30MPa

[0724] × (Unacceptable):

[0725] When 590MPa≤TS<780MPa, σYS>20MPa

[0726] When 780MPa≤TS<980MPa, σYS>25MPa

[0727] When 980MPa≤TS<1180MPa, σYS>30MPa

[0728] σEl

[0729] ○(passed):

[0730] When 590MPa≤TS<780MPa, σEl≤2.0%

[0731] When 780MPa≤TS<980MPa, σEl≤2.5%

[0732] When 980MPa≤TS<1180MPa, σEl≤3.0%

[0733] × (Unacceptable):

[0734] When 590MPa≤TS<780MPa, σEl>2.0%

[0735] When 780MPa≤TS<980MPa, σEl>2.5%

[0736] When 980MPa≤TS<1180MPa, σEl>3.0%

[0737] σλ

[0738] ○(passed):

[0739] When 590MPa≤TS<780MPa, σλ≤7%

[0740] When 780MPa≤TS<980MPa, σλ≤7%

[0741] When 980MPa≤TS<1180MPa, σλ≤7%

[0742] × (Unacceptable):

[0743] When 590MPa≤TS<780MPa, σλ>7%.

[0744] When 780MPa≤TS<980MPa, σλ>7%

[0745] When 980MPa≤TS<1180MPa, σλ>7%

[0746] The underlined portions in Tables 1, 2, and 3 indicate portions that are outside the appropriate scope of this invention.

[0747]

[0748]

[0749]

[0750]

[0751]

[0752]

[0753]

[0754]

[0755]

[0756]

[0757]

[0758]

[0759]

[0760] As shown in Table 3, in the invention examples, the tensile strength (TS), yield stress (YS), total elongation (E1), expansion ratio (λ), the difference between the maximum and minimum values ​​of TS (ΔTS), the difference between the maximum and minimum values ​​of YS (ΔYS), the difference between the maximum and minimum values ​​of E1 (ΔE1), the difference between the maximum and minimum values ​​of λ (Δλ), the standard deviation of TS (σTS), the standard deviation of YS (σYS), the standard deviation of E1 (σE1), and the standard deviation of λ (σλ) are all qualified.

[0761] On the other hand, in the comparative examples, at least one of the following in the steel strip and steel plate (CR, GI, GA, EG, Al): tensile strength (TS), yield stress (YS), total elongation (E1), expansion ratio (λ), difference between the maximum and minimum values ​​of TS (ΔTS), difference between the maximum and minimum values ​​of YS (ΔYS), difference between the maximum and minimum values ​​of E1 (ΔE1), difference between the maximum and minimum values ​​of λ (Δλ), standard deviation of TS (σTS), standard deviation of YS (σYS), standard deviation of E1 (σE1), and standard deviation of λ (σλ) is insufficient.

[0762] Industrial availability

[0763] According to the present invention, it is possible to manufacture steel sheets and strips with a TS value of 590 MPa or higher and lower than 1180 MPa, exhibiting small fluctuations in mechanical properties (YS, TS, E1, λ), i.e., excellent stability of mechanical properties. By applying the steel sheets and strips with excellent stability of mechanical properties obtained by the method according to the present invention to structural components such as automobile bodies, the yield rate during the stamping of steel sheets in the body manufacturing process, as well as during the welding and assembly of stamped components, is significantly improved, which helps to reduce the environmental impact of automobile manufacturing processes, thus having great industrial application value.

[0764] Symbol Explanation

[0765] F1 Unrecrystallized Ferrite

[0766] F2 recrystallized ferrite

[0767] F3 Phase Transformation Ferrite

[0768] F4 Epitaxial Ferrite

[0769] F5 bainitic ferrite

[0770] FM Fresh Martensite

[0771] TM tempered martensite

[0772] A. Annealing and homogenization process

[0773] B Rapid heating process

[0774] C Cooling process

[0775] D. Plating process

[0776] D1 Hot-dip galvanizing process

[0777] D11 Alloying Process

[0778] D2 Electroplating zinc treatment process

[0779] D3 Hot-dip aluminizing process

[0780] E Reheating and Holding Process

[0781] F Feedforward Control Process

[0782] Information on the composition, thickness, width, reduction rate during cold rolling, and coiling temperature after finishing in hot rolling of F0 steel plate.

[0783] G Feedback control process

[0784] Information on G0 austenite fraction

[0785] a. Position at 1 / 4 of the width of the steel plate

[0786] b. Position at 1 / 2 of the width of the steel plate

[0787] c. Position at 3 / 4 of the width of the steel plate

[0788] 1. The steel strip is positioned 10m from its front end along its length and at 1 / 4 of its width.

[0789] 2. The steel strip at 1 / 4 position in both the length and width directions.

[0790] 3. At the position halfway along the length of the steel strip and at the position one-quarterway along the width.

[0791] 4. At the 3 / 4 position along the length of the steel strip and the 1 / 4 position along the width.

[0792] 5. The steel strip is positioned 10m from its length and 1 / 4 of its width.

[0793] 6. The steel strip is positioned 10m from its front end along its length and at half its width.

[0794] 7. The steel strip at 1 / 4 of its length and 1 / 2 of its width.

[0795] 8. The steel strip at half its length and half its width.

[0796] 9. The steel strip at 3 / 4 of its length and 1 / 2 of its width.

[0797] 10m from the end of the steel strip along its length and at half its width.

[0798] 11. The steel strip is positioned 10m from its front end along its length and at 3 / 4 of its width.

[0799] 12. The steel strip is positioned at 1 / 4 of its length and 3 / 4 of its width.

[0800] 13. The steel strip at the 1 / 2 position in the length direction and the 3 / 4 position in the width direction.

[0801] 14. The steel strip at 3 / 4 position in both the length and width directions.

[0802] 15. The steel strip is positioned 10m from its length and 3 / 4 of its width.

[0803] 21. Width direction of steel plates and strips

[0804] 22. Length direction of steel plates and strips

[0805] 23. Position of the front end along the length of the steel strip

[0806] 24. The position of the tail end along the length of the steel strip.

[0807] 31. 1 / 4 position in the width direction of steel plates and strips.

[0808] 32. The steel plate and strip at half the width.

[0809] 33. 3 / 4 position in the width direction of steel plates and strips.

[0810] 41. Position 10m from the front end of the steel strip along its length.

[0811] 42. 1 / 4 position along the length of the steel strip.

[0812] 43. Position at 1 / 2 of the length of the steel strip. 44. Position at 3 / 4 of the length of the steel strip.

[0813] 45. The steel strip is positioned 10m from its tail end along its length.

[0814] P-line analysis position

[0815] P1 Hard Phase

[0816] P2 Soft Phase

[0817] C0 The carbon content (0.088%) of Examples No.2 (Steel B) and No.39 (Steel B)

[0818] C1 The carbon content (0.088%) of Examples No. 2 (Steel B) and No. 39 (Steel B) is 1.50 times that of C1.

[0819] Areas where C2 [C content at the measurement location] / [C content of the steel] is 1.50 or higher.

[0820] Mn content (2.71%) in Examples No. 2 (Steel B) and No. 39 (Steel B)

[0821] M1 has 1.30 times the Mn content (2.71%) of Examples No. 2 (Steel B) and No. 39 (Steel B).

[0822] For regions where M2 [Mn content at the measurement location] / [Mn content in the steel] is below 1.30.

Claims

1. A steel plate having the following composition, and having a tensile strength of 590 MPa or more and less than 1180 MPa. The composition, by mass%, contains C: 0.030%–0.250%, Si: 0.01%–2.50%, Mn: ≥1.30% and <3.50%, P: 0.001%–0.100%, S: <0.0200%, Al: 0.010%–2.000%, N: <0.0100%, with the balance consisting of Fe and unavoidable impurities. In the microstructure at 1 / 4 of the plate thickness, The area fraction of the soft phase is 30.0%–90.0%, and, The area fraction of the hard phase is 10.0%–70.0%. In the hard phase, The ratio of [C content in the hard phase (by mass%)] to [C content in the steel plate (by mass%)] is 1.50 or higher, and the hard phase content by area is 65% or higher. The ratio of Mn content in the hard phase (by mass%) to Mn content in the steel plate (by mass%) is less than 1.30, and the hard phase content is more than 10% by area. Furthermore, when the tensile strength is set to TS, the yield stress to YS, the total elongation to E1, and the porosity to λ, At positions 1 / 4, 1 / 2, and 3 / 4 of the width of the steel plate, the following equations 1 to 3 are satisfied. Furthermore, at the measurement positions of TS, YS, El, and λ at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel plate, the differences between their respective maximum and minimum values ​​(maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El), and (maximum value of λ - minimum value of λ), i.e., ΔTS, ΔYS, ΔEl, and Δλ, successively satisfy the following equations 4, 6, 8, and 10. Furthermore, the standard deviations σTS, σYS, σEl, and σλ obtained from the measurement positions at 1 / 4, 1 / 2, and 3 / 4 of the width direction of the steel plate, respectively, satisfy Equations 5, 7, 9, and 11. Formula 1: When 590MPa≤TS<780MPa, 360MPa≤YS When 780MPa≤TS<980MPa, 460MPa≤YS When 980MPa≤TS<1180MPa, 580MPa≤YS Formula 2: When 590MPa≤TS<780MPa, 23.0%≤El When 780MPa≤TS<980MPa, 17.0%≤El When 980MPa≤TS<1180MPa, 11.0%≤El Formula 3: When 590MPa≤TS<780MPa, 45%≤λ When 780MPa≤TS<980MPa, 30%≤λ When 980MPa≤TS<1180MPa, 20%≤λ Formula 4: When 590MPa≤TS<780MPa, ΔTS≤40MPa When 780MPa≤TS<980MPa, ΔTS≤50MPa When 980MPa≤TS<1180MPa, ΔTS≤60MPa Formula 5: When 590MPa≤TS<780MPa, σTS≤20MPa When 780MPa≤TS<980MPa, σTS≤25MPa When 980MPa≤TS<1180MPa, σTS≤30MPa Formula 6: When 590MPa≤TS<780MPa, ΔYS≤40MPa When 780MPa≤TS<980MPa, ΔYS≤50MPa When 980MPa≤TS<1180MPa, ΔYS≤60MPa Formula 7: When 590MPa≤TS<780MPa, σYS≤20MPa When 780MPa≤TS<980MPa, σYS≤25MPa When 980MPa≤TS<1180MPa, σYS≤30MPa Formula 8: When 590MPa≤TS<780MPa, ΔEl≤4.0% When 780MPa≤TS<980MPa, ΔEl≤4.5% When 980MPa≤TS<1180MPa, ΔEl≤5.0% Formula 9: When 590MPa≤TS<780MPa, σEl≤2.0% When 780MPa≤TS<980MPa, σEl≤2.5% When 980MPa≤TS<1180MPa, σEl≤3.0% Formula 10: When 590MPa≤TS<780MPa, Δλ≤25%. When 780MPa≤TS<980MPa, Δλ≤25%. When 980MPa≤TS<1180MPa, Δλ≤25% Formula 11: When 590MPa≤TS<780MPa, σλ≤7% When 780MPa≤TS<980MPa, σλ≤7% When 980MPa≤TS<1180MPa, σλ≤7%.

2. The steel plate according to claim 1, wherein, The composition, by mass%, further contains the following components selected from: Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.020% or less. At least one of the following: less than 0%, Co: less than 0.0200%, Zr: less than 0.1000%, Ca: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0500%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

3. The steel plate according to claim 1 or 2, wherein, The steel plate has a coating. The coating is any one of hot-dip galvanized coating, alloyed hot-dip galvanized coating, electroplated zinc coating, and hot-dip aluminized coating.

4. A steel strip having the following composition, and having a tensile strength of 590 MPa or more and less than 1180 MPa. The composition, by mass%, contains C: 0.030%–0.250%, Si: 0.01%–2.50%, Mn: ≥1.30% and <3.50%, P: 0.001%–0.100%, S: <0.0200%, Al: 0.010%–2.000%, N: <0.0100%, with the balance consisting of Fe and unavoidable impurities. In the microstructure at the 1 / 4 position of the steel strip thickness, The area fraction of the soft phase is 30.0%–90.0%, and, The area fraction of the hard phase is 10.0%–70.0%. In the hard phase, The hard phase having a carbon content (in mass% of the hard phase) / (carbon content in the steel strip in mass% of the steel strip) of 1.50 or higher, and an area fraction of 65% or higher, and... The ratio of Mn content in the hard phase (by mass%) to Mn content in the steel strip (by mass%) is less than 1.30, and the hard phase content is more than 10% by area. Furthermore, when the tensile strength is set to TS, the yield stress to YS, the total elongation to E1, and the porosity to λ, At positions 1 / 4, 1 / 2, and 3 / 4 of the width of the steel strip, At positions 10m from the beginning, 10m from the end, 1 / 4, 1 / 2, and 3 / 4 of the length of the steel strip, the following equations 1 to 3 must be satisfied. Furthermore, at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel strip, At positions 10m from the front end, 10m from the rear end, 1 / 4 position, 1 / 2 position, and 3 / 4 position along the length of the steel strip, the differences between the maximum and minimum values ​​of TS, YS, El, and λ (maximum value of TS - minimum value of TS), (maximum value of YS - minimum value of YS), (maximum value of El - minimum value of El), and (maximum value of λ - minimum value of λ), i.e., ΔTS, ΔYS, ΔEl, and Δλ, respectively, satisfy Equations 4, 6, 8, and 10. Furthermore, at the 1 / 4, 1 / 2, and 3 / 4 positions along the width of the steel strip, The standard deviations σTS, σYS, σEl, and σλ obtained from TS, YS, E1, and λ at the positions 10m from the front end, 10m from the rear end, 1 / 4 position, 1 / 2 position, and 3 / 4 position along the length of the steel strip respectively satisfy the following equations 5, 7, 9, and 11 respectively. Formula 1: When 590MPa≤TS<780MPa, 360MPa≤YS When 780MPa≤TS<980MPa, 460MPa≤YS When 980MPa≤TS<1180MPa, 580MPa≤YS Formula 2: When 590MPa≤TS<780MPa, 23.0%≤El When 780MPa≤TS<980MPa, 17.0%≤El When 980MPa≤TS<1180MPa, 11.0%≤El Formula 3: When 590MPa≤TS<780MPa, 45%≤λ When 780MPa≤TS<980MPa, 30%≤λ When 980MPa≤TS<1180MPa, 20%≤λ Formula 4: When 590MPa≤TS<780MPa, ΔTS≤40MPa When 780MPa≤TS<980MPa, ΔTS≤50MPa When 980MPa≤TS<1180MPa, ΔTS≤60MPa Formula 5: When 590MPa≤TS<780MPa, σTS≤20MPa When 780MPa≤TS<980MPa, σTS≤25MPa When 980MPa≤TS<1180MPa, σTS≤30MPa Formula 6: When 590MPa≤TS<780MPa, ΔYS≤40MPa When 780MPa≤TS<980MPa, ΔYS≤50MPa When 980MPa≤TS<1180MPa, ΔYS≤60MPa Formula 7: When 590MPa≤TS<780MPa, σYS≤20MPa When 780MPa≤TS<980MPa, σYS≤25MPa When 980MPa≤TS<1180MPa, σYS≤30MPa Formula 8: When 590MPa≤TS<780MPa, ΔEl≤4.0% When 780MPa≤TS<980MPa, ΔEl≤4.5% When 980MPa≤TS<1180MPa, ΔEl≤5.0% Formula 9: When 590MPa≤TS<780MPa, σEl≤2.0% When 780MPa≤TS<980MPa, σEl≤2.5% When 980MPa≤TS<1180MPa, σEl≤3.0% Formula 10: When 590MPa≤TS<780MPa, Δλ≤25%. When 780MPa≤TS<980MPa, Δλ≤25%. When 980MPa≤TS<1180MPa, Δλ≤25% Formula 11: When 590MPa≤TS<780MPa, σλ≤7% When 780MPa≤TS<980MPa, σλ≤7% When 980MPa≤TS<1180MPa, σλ≤7%.

5. The steel strip according to claim 4, wherein, The composition, by mass%, further contains the following components selected from: Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.020% or less. At least one of the following: less than 0%, Co: less than 0.0200%, Zr: less than 0.1000%, Ca: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0500%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

6. The steel strip according to claim 4 or 5, wherein, The steel strip has a coating on its surface. The coating is any one of hot-dip galvanized coating, alloyed hot-dip galvanized coating, electroplated zinc coating, and hot-dip aluminized coating.

7. A method for manufacturing a steel plate, comprising: The hot rolling process involves hot rolling a steel billet having the composition described in claim 1 or 2 at a coiling temperature of 350°C to 650°C after finishing rolling to obtain a hot-rolled steel plate. In the annealing and homogenization process, the hot-rolled steel sheet is heated to a homogenization temperature of 720℃~860℃ and held for at least 20 seconds. The rapid heating process involves rapidly heating the steel plate from the annealing homogenization temperature to a point above (annealing homogenization temperature + 10°C) at an average heating rate of 10°C / second or higher, ensuring that the ΔT (maximum value of the reached temperature - minimum value of the reached temperature) at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction after this rapid heating is below 25°C. The cooling process is carried out after the rapid heating process, under the condition that the average cooling rate from the reached temperature to 550°C is 4°C / second or more. Alternatively, a cold rolling process may be included after the hot rolling process and before the annealing and soaking process to cold roll the hot-rolled steel sheet and obtain a cold-rolled steel sheet. Alternatively, a plating process may be included after the cooling process to perform any one of the following on the hot-rolled steel sheet or the cold-rolled steel sheet: hot-dip galvanizing, alloying hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing. Alternatively, after the cooling process or the plating process, a reheating and holding process may be included, in which the hot-rolled steel sheet or the cold-rolled steel sheet is cooled to a cooling stop temperature of 0°C to 250°C, heated to a temperature range of (cooling stop temperature + 50°C) to 460°C, and held in the temperature range for a tempering time of 10 seconds to 2000 seconds.

8. The method for manufacturing a steel plate according to claim 7, wherein, The arrival temperature is determined based on information about the composition and the coiling temperature during the hot rolling process.

9. The method for manufacturing a steel plate according to claim 7, wherein, The reached temperature is determined based on information about the austenite fraction of the steel plate measured in the temperature range of 150°C to 600°C after the rapid heating process.

10. The method for manufacturing a steel plate according to claim 7, wherein, The reached temperature is determined based on information about the composition, information about the coiling temperature during the hot rolling process, and information about the austenite fraction of the steel plate measured in the temperature range of 150°C to 600°C after the rapid heating process.

11. A method for manufacturing a steel strip, comprising: In the hot rolling process, a steel billet having the composition described in claim 4 or 5 is hot rolled at a coiling temperature of 350°C to 650°C after finishing rolling to obtain a hot-rolled steel strip. In the annealing and homogenization process, the hot-rolled steel strip is heated and subjected to annealing and homogenization at an annealing and homogenization temperature of 720℃~860℃ for a holding time of more than 20 seconds. The rapid heating process involves rapidly heating the steel strip from the annealing temperature at an average heating rate of 10°C / second or higher to a point above (annealing temperature + 10°C), ensuring that the ΔT (maximum temperature minus minimum temperature) of the reached temperature at the 1 / 4, 1 / 2, and 3 / 4 positions in the width direction after this rapid heating is below 25°C. The cooling process is carried out after the rapid heating process, under the condition that the average cooling rate from the reached temperature to 550°C is 4°C / second or more. Alternatively, a cold rolling process may be included after the hot rolling process and before the annealing and homogenization process to cold roll the hot-rolled steel strip and obtain a cold-rolled steel strip. Alternatively, a plating process may be included after the cooling process to perform any one of the following on the hot-rolled steel strip or the cold-rolled steel strip: hot-dip galvanizing, alloying hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing. Alternatively, after the cooling process or the plating process, a reheating and holding process may be included, in which the hot-rolled steel strip or the cold-rolled steel strip is cooled to a cooling stop temperature of 0°C to 250°C, heated to a temperature range of (cooling stop temperature + 50°C) to 460°C, and held in the temperature range for a tempering time of 10 seconds to 2000 seconds.

12. The method for manufacturing steel strip according to claim 11, wherein, The arrival temperature is determined based on information about the composition and the coiling temperature during the hot rolling process.

13. The method for manufacturing steel strip according to claim 11, wherein, The reached temperature is determined based on information about the austenite fraction of the steel strip measured in the temperature range of 150°C to 600°C after the rapid heating process.

14. The method for manufacturing steel strip according to claim 11, wherein, The reached temperature is determined based on information about the composition, information about the coiling temperature in the hot rolling process, and information about the austenite fraction of the steel strip measured in the temperature range of 150°C to 600°C after the rapid heating process.

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