Steel sheet, member, and method for manufacturing the same

Abstract

The present invention provides a steel sheet having high strength, high ductility, excellent stretch flange formability, and good chemical conversion treatment properties, a member, and a method for manufacturing the same. A steel sheet in which a composition containing C, Si, Mn, P, S, sol. Al, N in specific amounts in mass% is set, the total of polygonal ferrite, upper bainite, residual γ, fresh martensite, tempered martensite, and lower bainite, a remainder structure is set to a specific ratio, the number of fresh martensite particles and residual γ particles having an equivalent circle diameter of less than 1.2 μm is set to a specific ratio, and the number of fresh martensite particles and residual γ particles having an equivalent circle diameter of 1.2 μm or more and an aspect ratio of 2.5 or more is set to a specific ratio.

Description

Technical Field

[0001] This invention relates to steel sheets suitable for use in stamped products with complex shapes after stamping processes in automobiles, home appliances, etc., and having excellent chemical conversion properties, as well as components using the steel sheets and methods for manufacturing them. Background Technology

[0002] Against the backdrop of increasingly stringent global CO2 emission restrictions, there is a growing demand for higher-strength steel sheets in automobiles to reduce vehicle weight. For body and seat components, there is a growing trend towards using high-strength steel sheets with a strength of 590MPa or higher, moving away from the existing 440MPa cold-rolled steel sheets. However, increasing the strength of steel sheets typically reduces their ductility, flangeability, and other stamping formability properties, making them more prone to cracking during stamping and reducing the freedom of shape creation. Therefore, their application is limited to simple-shaped components. Thus, to apply high-strength steel sheets to complex-shaped components, it is crucial to increase the strength of the steel sheets while maintaining or improving formability.

[0003] Against this backdrop, as a technology to improve the ductility of steel sheets, TRIP steel was developed, which disperses retained austenite (retained γ) in the microstructure of the steel sheet.

[0004] For example, Patent Document 1 discloses a manufacturing method based on isothermal quenching (cooling from a single-phase annealing temperature or a two-phase annealing temperature to the bainitic transformation temperature, isothermally holding the solution, thereby utilizing the bainitic transformation during isothermal holding or cooling to form residual γ). The method discloses that a steel plate containing C: 0.10–0.45%, Si: 0.5–1.8%, and Mn: 0.5–3.0% is subjected to aging treatment at a temperature range of 350–500°C for 1–30 minutes after annealing to form residual γ, resulting in a TS of 80 kgf / mm². 2 The above and TS×EL: 2500kgf / mm 2 • More than % of steel plates have high ductility.

[0005] Patent document 2 discloses a method of controlling the microstructure of a steel sheet containing C: 0.10-0.25%, Si: 1.0-2.0%, and Mn: 1.5-3.0% after annealing at a rate of 10°C / second or higher to 450-300°C and holding it for 180-600 seconds, so as to obtain a steel sheet with excellent ductility (E1) and elongation flangeability (λ).

[0006] In the above technologies, the steel sheet contains a high amount of Si to promote efficient carbon enrichment to the untransformed γ phase. On the other hand, thin steel sheets used for stamping components are subsequently painted and assembled into automobiles, etc., therefore, chemical conversion treatment is performed to impart good paintability to the steel sheet. During chemical conversion treatment, in the presence of an oxide film on the surface of the steel sheet, the crystalline particles adhering through the chemical conversion treatment become uneven, contributing to the deterioration of paintability. Therefore, pickling is typically performed in a continuous annealing furnace used in the manufacture of thin steel sheets as a pretreatment to improve chemical conversion treatability. However, in steel sheets with high Si content, the Si-containing surface oxide layer, which cannot be removed by pickling, causes a significant deterioration in chemical conversion treatability.

[0007] To address this problem, for example, Patent Document 3 discloses a process of continuously immersing the steel plate in a mixed acid solution containing an oxidizing first acid and a non-oxidizing second acid for pickling, followed by continuous immersion in an acid solution containing a non-oxidizing third acid for re-pickling. This process enables steel plates with high Si content to have excellent chemical conversion treatment properties.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Publication No. 6-35619

[0011] Patent Document 2: Japanese Patent No. 4411221

[0012] Patent Document 3: Japanese Patent No. 6041079 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] Simple-shaped parts can be formed by stamping simply by achieving uniform elongation, but for complex-shaped parts, local elongation is also important. However, while the conventional TRIP steel described in Patent Document 1 has excellent E1, it has the problem of very low elongation flange formability.

[0015] In the technology described in Patent Document 2, bainitic ferrite is mainly utilized as the microstructure, with less ferrite. Therefore, although the elongated flange has excellent formability, its ductility may not be high. Furthermore, it involves steel sheets with low yield ratios, making it difficult to apply to vehicle body frame components and energy-absorbing components. Additionally, Patent Documents 1 and 2 do not consider chemical conversion treatment properties, which can be assumed to deteriorate due to Si content or annealing conditions.

[0016] The technology described in Patent Document 3 achieves a steel sheet with high ductility and excellent elongation flange properties by utilizing the retention of the upper bainitic phase transformation during cooling after annealing, and subsequent Q&P treatment and reheating of the bainitic phase transformation. However, it does not demonstrate an improvement in the local elongation rate that simultaneously meets the requirements of bending formability and bulging properties for difficult-to-form parts. Furthermore, promoting the distribution of carbon from the martensite formed through Q&P treatment to the untransformed γ phase, which contains a large amount of Si, necessitates the pickling technology described in Patent Document 3. Therefore, this requires the construction of additional pickling equipment or increases operating costs; thus, it is desirable to establish other technologies.

[0017] Therefore, in the existing technology, the technology for steel sheets that ensures high ductility and excellent elongation flange formability, while also possessing excellent chemical conversion treatment properties, is not yet sufficient.

[0018] This invention was made to solve such problems, and aims to provide a steel plate, component, and method thereof with a tensile strength of 590 MPa or more, high ductility and excellent elongation flange formability, and good chemical conversion treatment properties.

[0019] Here, a tensile strength of 590 MPa or more means that for a JIS 5 tensile test piece with a tensile direction perpendicular to the rolling direction, the crosshead speed is set to 10 mm / min, and the tensile strength is 590 MPa or more by passing a tensile test according to JIS Z 2241 (2011).

[0020] In addition, high ductility means that for a JIS 5 tensile test piece with a tensile direction perpendicular to the rolling direction, with the crosshead speed set at 10 mm / min, the tensile strength (TS) × total elongation (T.E1) is ≥22000 MPa·% or more through a tensile test according to JIS Z 2241 (2011).

[0021] In addition, excellent extension flange formability means that, through the hole expansion test according to JFST 1001 (Japan Iron and Steel Federation Standard), it meets the following (A1) or (A2).

[0022] (A1) The porosity λ is above 60% when the tensile strength is above 590MPa and below 780MPa.

[0023] (A2) When the tensile strength is above 780 MPa, the porosity λ is above 35%.

[0024] In addition, good chemical conversion treatment properties refer to the ability to process steel plates at a rate of 20–35 A / dm. 2The current density was used to perform sulfuric acid electrolytic pickling for 2 seconds, followed by degreasing (treatment temperature 40℃, treatment time 120 seconds, spray degreasing), surface conditioning (pH 9.5, treatment temperature room temperature, treatment time 20 seconds), and then chemical conversion treatment was performed using zinc phosphate chemical conversion treatment solution (chemical conversion treatment solution temperature 35℃, treatment time 120 seconds). There were no surfaces without chemical conversion film formation.

[0025] Methods for solving problems

[0026] The inventors have conducted in-depth research on a method for achieving high ductility and excellent elongation flange formability even with steel sheet compositions containing low Si content, and have reached the following conclusions. Here, although not specifically limited, "low Si content" refers to a Si content of less than 1.60% by mass.

[0027] In isothermal quenching, carbon partitioning occurs towards the untransformed austenite through bainitic transformation near 400℃ until the T0 composition, where the free energies of the fcc and bcc phases are equal, ceases. Therefore, the coarse and thermally unstable untransformed austenite transforms into hard martensite or mechanically unstable residual γ upon final cooling, resulting in deteriorated elongation flange properties. Thus, it is generally difficult to simultaneously achieve ductility and elongation flange properties in isothermal quenching.

[0028] The inventors conducted an in-depth study on the heating process before annealing and found that by controlling the cold rolling conditions, steel composition and heating conditions, a soft ferrite structure is formed and adjacent acicular austenite is formed during the heating process through recrystallization. This acicular austenite helps carbon distribution and the formation of retained austenite in the microstructure formation during the cooling process.

[0029] In addition, given that the lower the bainitic transformation temperature, the wider the T0 composition expands into the high-carbon region, a two-stage isothermal quenching process was investigated, involving both the high-temperature and low-temperature sides. This further promotes carbon distribution and improves uniform elongation. Furthermore, it was found that by allowing the bainitic transformation to proceed to the later stages without halting the transformation, the amount and size of the coarse and hard fresh martensite formed during final cooling are reduced. Moreover, due to the bainitic transformation of acicular austenite, residual austenite with a high aspect ratio and high processing stability can be obtained. This resulted in the suppression of stress concentration during stamping, inhibiting void formation and thus improving local elongation. Even when such acicular austenite transforms into hard fresh martensite during final cooling, it does not deteriorate porosity.

[0030] Steel sheets manufactured based on the above principles can simultaneously improve both uniform elongation and local elongation, even in alloy designs with reduced Si content. Therefore, costly pickling processes required to impart chemical conversion treatability through Si reduction are eliminated, resulting in significant cost reductions. It should be noted that the chemical conversion treatability described here refers to characteristics where, under normal pickling conditions, both adhesion and uniformity are sufficient for coating. For example, normal pickling processes include sulfuric acid pickling, and the pickling method is not limited.

[0031] As mentioned above, it was found that even steel sheets with low Si content exhibit excellent ductility and elongation flange properties. This is based on the following points.

[0032] (i) In the cold rolling process, cold-rolled steel sheets with developed rolling preferred orientation (texture) and Rotated Cube orientation (having {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation and {100} are manufactured by suppressing the development of shear texture by cold rolling with a first-pass cold rolling ratio (reduction) of 5% or more and less than 25%. <011> (Cold-rolled steel sheet in which the total area percentage of the oriented microstructure relative to the total microstructure of the bcc phase is 35% or more and 75% or less).

[0033] (ii) In the annealing process, during the heating process before the soaking and holding, the heating rate (average heating rate) from 500°C to Ac1 is set to 15°C / second or less, so that the cold-rolled structure in the cold-rolled steel sheet with a rolling rate of more than 30% can be fully recrystallized, and the recrystallized texture is developed.

[0034] (iii) Subsequently, at temperatures above Ac1, the austenite (γ) transformed from the phase transformation nucleates from the grain boundaries of the recrystallized bcc phase or the residual carbides, but has a specific crystal orientation relative to the surrounding bcc phase. Therefore, the interface matching degree is high, accompanied by delayed grain growth due to interface movement, but in order to approach the equilibrium state, some interfaces preferentially move to form acicular austenite (acicular γ). In order to make good use of this acicular γ, the annealing temperature is set to two-phase annealing, and annealing is performed at an annealing temperature T that satisfies (T-Ac1) / (Ac3-Ac1)<1.0.

[0035] (iv) During the cooling process after the homogenization holding in the annealing process, isothermal holding (first holding) is performed in the temperature range of 400 to 550°C, thereby transforming the acicular γ phase into upper bainite with fewer precipitates and forming untransformed austenite with high solid solution C content (untransformed γ).

[0036] In addition, through two-stage isothermal quenching treatment on both the high-temperature and low-temperature sides, carbon distribution is efficiently achieved without halting the bainitic phase transformation. Furthermore, acicular austenite is formed in the microstructure before cooling. As a result, retained austenite with a high aspect ratio and high processing stability can be formed after the bainitic phase transformation. Consequently, it is possible to manufacture steel sheets that simultaneously achieve excellent uniform elongation and local elongation, without cracking even during stamping processes involving both bulging and extended flange forming, enabling applications in complex formed products.

[0037] Thus, by utilizing the acicular austenite formed during the heating process and the bainitic phase transformation in both high-temperature and low-temperature stages, excellent uniform elongation and local elongation can be obtained simultaneously. As a result, even steels with a low Si content can produce steel sheets that balance high ductility and excellent elongation flange formability, and steel sheets with improved chemical conversion properties can also be obtained.

[0038] This invention is based on the above insights and specifically provides the following solutions.

[0039] [1] A steel plate having, by mass percent, the following composition: C: 0.06 to 0.24%, Si: 0.4% or more and less than 1.60%, Mn: 1.5 to 3.2%, P: less than 0.02%, S: less than 0.01%, sol.Al: less than 1.0%, N: less than 0.015%, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities.

[0040] The steel plate has a microstructure with an area ratio of polygonal ferrite of 20% to 85%, an area ratio of upper bainite of 9% to 45%, a volume ratio of retained austenite of 3% to 15%, an area ratio of fresh martensite of 3% to 15%, a combined area ratio of tempered martensite and lower bainite of 50% or less (including 0%), and an area ratio of the remaining microstructure of 5% or less.

[0041] The total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of less than 1.2 μm accounts for more than 50% of the total number of fresh martensite particles and retained austenite particles.

[0042] Furthermore, the total number of fresh martensite particles and retained austenite particles with an aspect ratio of 2.5 or higher and an equivalent circle diameter of 1.2 μm or higher is 40% or higher relative to the total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of 1.2 μm or higher.

[0043] Si / Mn < 0.50… Equation (1)

[0044] In Equation (1), Si and Mn represent the Si content (mass%) and Mn content (mass%), respectively.

[0045] [2] The steel plate according to [1] above, wherein, as the above-mentioned component composition, it further contains, by mass %, one or more of the following: Nb: less than 0.2%, Ti: less than 0.2%, V: less than 0.2%, B: less than 0.01%, Cu: less than 0.2%, Ni: less than 0.2%, Cr: less than 0.4%, and Mo: less than 0.15%.

[0046] [3] The steel plate according to [1] or [2] above, wherein, as a component of the above composition, it further contains, by mass %, one or more of the following: Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.10% or less, Sb: 0.10% or less, REM: 0.0050% or less.

[0047] [4] A component made of steel plate as described in any one of [1] to [3] above.

[0048] [5] A method for manufacturing a steel plate, comprising:

[0049] In the cold rolling process, a steel billet having the composition described in any one of [1] to [3] above is subjected to hot rolling and pickling, and the resulting hot-rolled steel sheet is subjected to cold rolling treatment to obtain a cold-rolled steel sheet; and

[0050] The annealing process involves annealing the aforementioned cold-rolled steel sheet to obtain the steel sheet.

[0051] The cold rolling process described above includes the following cold rolling treatment:

[0052] By setting the cumulative cold rolling rate to 30-85% and the reduction rate of the first pass to 5% or more but less than 25%, we obtain materials with {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation, and {100} orientation. <011> For the aforementioned cold-rolled steel sheets, the total area percentage of the oriented microstructure relative to the total microstructure of the bcc phase is 35% or more and 75% or less.

[0053] The annealing process described above includes the annealing treatment as follows:

[0054] For the aforementioned cold-rolled steel sheet, the average heating rate within the temperature range of 500℃ and below Ac1 is set to 0.5~15℃ / second, and heating continues until the annealing temperature T below 840℃ satisfies 0.5≤(T-Ac1) / (Ac3-Ac1)<1.0.

[0055] After heating, the material is held in a furnace atmosphere with a dew point Td between -50°C and -30°C at the aforementioned annealing temperature T, thereby obtaining a acicular austenite structure with a number density of 5 acicular atoms / 1000 μm. 2 The above steel plates,

[0056] Next, a first cooling process is implemented, in which the average cooling rate within the temperature range of 750–550°C is set to 6.0°C / second or higher, and the temperature is cooled to a first cooling stop temperature Tc1 below 550°C but above 400°C.

[0057] After the first cooling, a first holding period is implemented, maintaining the temperature at the first cooling stop temperature Tc1 for more than 25 seconds.

[0058] After the first holding period, a second cooling process is implemented, cooling to a second cooling stop temperature Tc2 that is below the first cooling stop temperature Tc1 and below 450°C and above 300°C.

[0059] The second holding period is implemented at the aforementioned second cooling stop temperature Tc2 for 20 to 3000 seconds.

[0060] After the second cooling is completed, a third cooling process is implemented.

[0061] [6] A method for manufacturing a component, comprising a step of forming a component by performing at least one of forming or joining processes on a steel plate as described in any one of [1] to [3].

[0062] Invention Effects

[0063] According to the present invention, steel plates, components, and methods thereof are provided that have tensile strength of 590 MPa or more, achieve high ductility and excellent elongation flange formability, and have good chemical conversion treatment properties.

[0064] The steel sheet of the present invention is suitable for use in stamped products with complex shapes after stamping processes in automobiles, home appliances and the like. Attached Figure Description

[0065] Figure 1 This is a SEM image showing the microstructure after final cooling (third cooling in the annealing process) and the microstructure after water cooling following holding at temperature T, in which acicular austenite (acicular γ) was observed.

[0066] Figure 2 This is a schematic diagram of acicular austenite (acicular γ), illustrating the definition of the aspect ratio of acicular γ. Detailed Implementation

[0067] The present invention will now be described in detail. It should be noted that the present invention is not limited to the following embodiments.

[0068] The steel plate of the present invention is a steel plate as described below, having, by mass percent, the following composition: C: 0.06 to 0.24%, Si: 0.4% or more and less than 1.60%, Mn: 1.5 to 3.2%, P: less than 0.02%, S: less than 0.01%, sol.Al: less than 1.0%, N: less than 0.015%, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities; having an area fraction of polygonal ferrite of 20% or more and less than 85%, an area fraction of upper bainite of 9% or more and less than 45%, a volume fraction of retained austenite of 3% or more and less than 15%, and an area fraction of fresh martensite of The microstructure is defined as follows: the area ratio of tempered martensite to lower bainite is 3% to 15% or less, the total area ratio of tempered martensite and lower bainite is 50% or less (including 0%), and the area ratio of the remaining microstructure is 5% or less. The total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of less than 1.2 μm is 50% or more relative to the total number of fresh martensite particles and retained austenite particles. Furthermore, the total number of fresh martensite particles and retained austenite particles with an aspect ratio of 2.5 or more and an equivalent circle diameter of 1.2 μm or more is 40% or more relative to the total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of 1.2 μm or more.

[0069] Si / Mn < 0.50… Equation (1)

[0070] In Equation (1), Si and Mn represent the Si content (mass%) and Mn content (mass%), respectively.

[0071] The steel plate of the present invention will be described below in the order of composition and steel structure.

[0072] The steel plate of the present invention contains the following components. In the following description, the unit "%" for the content of the components refers to "mass %".

[0073] C: 0.06~0.24%

[0074] C is included from the viewpoint of achieving the desired range of hardenability, martensite strength, and residual γ volume fraction in the steel sheet. When the C content is less than 0.06%, the strength and ductility of the steel sheet cannot be sufficiently guaranteed; therefore, the C content is set to 0.06% or more. The C content is preferably 0.08% or more, more preferably 0.10% or more. When the C content exceeds 0.24%, the toughness of the weld deteriorates. Furthermore, when the C content exceeds 0.24%, the desired area fraction of fresh martensite cannot be achieved. Therefore, the C content is set to 0.24% or less. From the viewpoint of improving ductility and the toughness of the spot weld, the C content is preferably set to 0.21% or less. From the viewpoint of further improving the toughness of the spot weld, the C content is more preferably set to 0.20% or less.

[0075] Si: 0.4% or more and less than 1.60%

[0076] Si is included from the viewpoint of achieving high strength in ferrite structure and improving ductility by stabilizing residual γ-rays through suppressing carbide formation in martensite or bainite. From these viewpoints, the Si content is set to 0.4% or more. From the viewpoint of improving ductility, the Si content is preferably set to 0.5% or more. The Si content is more preferably 0.6% or more. When the Si content is 1.60% or more, the chemical conversion treatability deteriorates significantly. Therefore, the Si content is set to less than 1.60%. Preferably, the Si content is 1.30% or less, more preferably 1.20% or less. Further preferably, the Si content is less than 1.0%.

[0077] Mn: 1.5–3.2%

[0078] Mn ensures the specified hardenability, suppresses ferrite phase transformation, and ensures the desired area ratio of tempered martensite and / or bainite, thereby ensuring strength. Furthermore, Mn enriches in γ during annealing in the ferrite / γ two-phase region, lowering the Ms point of untransformed γ and stabilizing the residual γ, thus improving ductility. Additionally, Mn, like Si, suppresses the formation of carbides in bainite, thus improving ductility. Furthermore, Mn increases the volume fraction of residual γ, further improving ductility. From these aspects, Mn is an important element in this invention. To obtain these effects, the Mn content is set to 1.5% or more. From the viewpoint of improving hardenability, the Mn content is preferably set to 1.7% or more. The Mn content is more preferably set to 1.9% or more. On the other hand, when the Mn content exceeds 3.2%, the bainite phase transformation is significantly delayed, making it difficult to ensure high ductility. Furthermore, when the Mn content exceeds 3.2%, it is difficult to suppress the formation of bulky, coarse γ, and the formability of the extended flange also deteriorates. Therefore, the Mn content is set to 3.2% or less. From the viewpoint of promoting bainitic phase transformation and ensuring high ductility, the Mn content is preferably set to 3.0% or less, and more preferably to 2.8% or less.

[0079] Si / Mn < 0.50… Equation (1)

[0080] The surface oxides of steel sheets that significantly deteriorate chemical conversion treatability are Si-based oxides. Therefore, in order to form Mn-containing oxides that are readily soluble in acid, the Si / Mn ratio is set to be less than 0.50. That is, in this invention, as formula (1), Si / Mn is set to < 0.50. Here, in formula (1), Si and Mn represent the Si content (mass%) and Mn content (mass%), respectively. If it is within this range, chemical conversion treatability can be achieved within a dew point range of -50°C to -30°C. Preferably, the Si / Mn ratio is 0.40 or less, more preferably 0.35 or less.

[0081] P: below 0.02%

[0082] Phosphorus (P) is an element that strengthens steel, but a high P content deteriorates spot weldability. Therefore, the P content is set to 0.02% or less. From the viewpoint of improving spot weldability, the P content is preferably set to 0.01% or less. It should be noted that P can be absent, but from the viewpoint of manufacturing cost, the P content is preferably set to 0.001% or more.

[0083] S: below 0.01%

[0084] S has the effect of improving the peeling properties of oxide scale during hot rolling and suppressing nitriding during annealing, but S is also an element that deteriorates local elongation in addition to spot weldability. To suppress these deteriorations, the S content is set to 0.01% or less. In this invention, the contents of C, Si, and Mn are high, so spot weldability is easily deteriorated. From the viewpoint of improving spot weldability, the S content is preferably set to 0.0020% or less, and more preferably to less than 0.0010%. It should be noted that S can also be absent, but from the viewpoint of manufacturing cost, the S content is preferably set to 0.0001% or more.

[0085] sol.Al: Less than 1.0%

[0086] Al is included for deoxidation purposes or to stabilize residual γ as a substitute for Si. The lower limit of sol.Al is not specifically defined, but for stable deoxidation, it is preferably set to 0.01% or more. On the other hand, when the sol.Al content is 1.0% or more, the strength of the raw material decreases drastically, and its chemical conversion treatability deteriorates. Furthermore, a large amount of aluminum oxides is generated during steelmaking, resulting in significantly poor bending properties. Therefore, the sol.Al content is set to less than 1.0%. To obtain high strength, the sol.Al content is preferably set to less than 0.50%, and more preferably to 0.10% or less.

[0087] N: less than 0.015%

[0088] Nitrogen (N) forms nitrides such as boron (BN), alnitride (AlN), and nitride (TiN) in steel, reducing its thermal ductility and surface quality. Furthermore, in steels containing boron (B), the formation of boron (BN) negates its beneficial effects. When the N content is 0.015% or higher, surface quality deteriorates significantly. Therefore, the N content is set to be less than 0.015%. It should be noted that N can be absent, but from a manufacturing cost perspective, the N content is preferably set to 0.0001% or higher.

[0089] The balance other than those mentioned above consists of Fe and unavoidable impurities. The steel sheet of the present invention preferably has a composition containing the above-mentioned basic components, with the balance consisting of Fe and unavoidable impurities.

[0090] In the composition of the steel plate of the present invention, in addition to the above-mentioned components, one or two of the following (A) and (B) may be appropriately contained as optional elements, replacing the above-mentioned Fe and a portion of the unavoidable impurities.

[0091] (A) Selected by mass% from one or more of the following: Nb: less than 0.2%, Ti: less than 0.2%, V: less than 0.2%, B: less than 0.01%, Cu: less than 0.2%, Ni: less than 0.2%, Cr: less than 0.4%, and Mo: less than 0.15%.

[0092] (B) Selected by mass% from one or more of the following: Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.10% or less, Sb: 0.10% or less, REM: 0.0050% or less, and Nb: 0.2% or less.

[0093] From the viewpoint of improving the defect resistance of spot welds by refining the microstructure, the addition of Nb is preferred. Furthermore, Nb can be included to achieve the effects of refining the steel microstructure and increasing strength, promoting bainitic phase transformation through grain refinement, improving bending properties, and enhancing resistance to delayed fracture. While no specific lower limit is specified for achieving these effects, the Nb content is preferably 0.002% or more. More preferably, it is 0.004% or more, and even more preferably 0.010% or more. However, when a large amount of Nb is present, precipitation strengthening becomes excessive, reducing ductility. Additionally, it leads to increased rolling load and deterioration of castability. Therefore, when Nb is included, the Nb content is set to 0.2% or less. The Nb content is preferably 0.1% or less, more preferably 0.05% or less, and even more preferably 0.03% or less.

[0094] Ti: below 0.2%

[0095] From the viewpoint of refining the microstructure and thus improving the defect resistance of the spot weld, the addition of Ti is preferred. Furthermore, Ti has the effect of fixing N in the steel as TiN, thereby improving hot ductility and hardenability of B. To obtain these effects, there is no particular lower limit, but it is preferable to set the Ti content to 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is further preferably 0.008% or more, and more preferably 0.010% or more. On the other hand, when the Ti content exceeds 0.2%, it leads to an increase in rolling load and a decrease in ductility due to an increase in precipitation strengthening; therefore, in the case of Ti content, the Ti content is set to 0.2% or less. The Ti content is preferably 0.1% or less, and more preferably 0.05% or less. To ensure high ductility, the Ti content is further preferably set to 0.03% or less.

[0096] V: Below 0.2%

[0097] The presence of V is desirable for improving the hardenability of steel, suppressing carbide formation in martensite and upper / lower bainite, refining the microstructure, and improving resistance to delayed fracture by precipitating carbides. While no specific lower limit is specified for achieving these effects, the V content is preferably set to 0.003% or more. More preferably, it is 0.005% or more, and even more preferably 0.010% or more. On the other hand, a large amount of V significantly deteriorates castability; therefore, when V is present, the V content is set to 0.2% or less. The V content is preferably 0.1% or less. More preferably, it is 0.05% or less, and even more preferably 0.03% or less.

[0098] B: Below 0.01%

[0099] Boron (B) has the advantage of readily forming tempered martensite and / or bainite with a specified area ratio. Furthermore, the residual B from solid solution improves resistance to delayed fracture. To achieve this effect, the B content is preferably set to 0.0002% or more. More preferably, the B content is 0.0005% or more. Even more preferably, the B content is 0.0010% or more. On the other hand, when the B content exceeds 0.01%, not only does its effect saturate, but it also leads to a significant decrease in thermal ductility and the formation of surface defects. Therefore, in cases where B is present, the B content is set to 0.01% or less. Preferably, the B content is 0.0050% or less. More preferably, the B content is 0.0030% or less.

[0100] Cu: less than 0.2%

[0101] Cu improves the corrosion resistance of automobiles under operating conditions. Furthermore, the corrosion products of Cu coat the surface of the steel sheet, inhibiting the penetration of hydrogen into the steel. Cu is an element mixed in when utilizing scrap steel as raw material; by allowing the inclusion of Cu, recycled materials can be utilized as raw materials, reducing manufacturing costs. Therefore, no specific lower limit is specified, but from the viewpoint of improving resistance to delayed fracture, the Cu content is preferably 0.05% or more. The Cu content is more preferably 0.10% or more. On the other hand, excessive Cu content leads to surface defects; therefore, when Cu is present, the Cu content is set to 0.2% or less.

[0102] Ni: below 0.2%

[0103] Like Cu, Ni is an element that improves corrosion resistance. Furthermore, Ni helps suppress the formation of surface defects that are prone to occur when Cu is present. While no specific lower limit is specified for achieving this effect, the Ni content is preferably set to 0.01% or more. More preferably, it is 0.04% or more, and even more preferably 0.06% or more. On the other hand, excessive Ni content leads to uneven oxide scale formation in the furnace, which in turn causes surface defects. It also increases costs. Therefore, in the case of Ni, the Ni content is set to 0.2% or less.

[0104] Cr: less than 0.4%

[0105] Cr can be included to improve the hardenability of steel and suppress the formation of carbides in martensite and upper / lower bainite. While no specific lower limit is specified for achieving these effects, the Cr content is preferably set to 0.01% or more. More preferably, it is 0.03% or more, and even more preferably 0.06% or more. On the other hand, excessive Cr content deteriorates pitting corrosion resistance; therefore, when Cr is present, the Cr content is set to 0.4% or less.

[0106] Mo: 0.15% or less

[0107] Mo can be included to improve the hardenability of steel and suppress the formation of carbides in martensite and upper / lower bainite. To achieve these effects, the Mo content is preferably set to 0.01% or more. More preferably, the Mo content is 0.03% or more, and even more preferably 0.06% or more. On the other hand, Mo significantly deteriorates the chemical transformation processability of cold-rolled steel sheets; therefore, when Mo is included, the Mo content is set to 0.15% or less.

[0108] Mg: less than 0.0050%

[0109] Mg fixes oxygen in the form of MgO, which helps improve the resistance to delayed fracture. Therefore, the Mg content is preferably set to 0.0002% or more. More preferably, the Mg content is 0.0004% or more, and even more preferably 0.0006% or more. On the other hand, adding a large amount of Mg deteriorates surface quality and flexibility; therefore, when Mg is present, the Mg content is set to 0.0050% or less. The Mg content is preferably 0.0025% or less, and more preferably 0.0010% or less.

[0110] Ca: below 0.0050%

[0111] Ca fixes S in the form of CaS, which helps improve flexibility and resistance to delayed fracture. Therefore, the Ca content is preferably set to 0.0002% or more. The Ca content is more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, when Ca is added in large quantities, it deteriorates the surface quality and flexibility. Therefore, when Ca is present, the Ca content is set to 0.0050% or less. The Ca content is preferably 0.0035% or less, and more preferably 0.0020% or less.

[0112] Sn: less than 0.10%

[0113] Sn inhibits oxidation and nitriding of the steel plate surface layer, thus suppressing the resulting decrease in the content of carbon (C) and boron (B) in the surface layer. Furthermore, by suppressing the aforementioned decrease in the content of C and B, ferrite formation in the steel plate surface layer is suppressed, resulting in increased strength and improved fatigue resistance. From this perspective, the Sn content is preferably set to 0.002% or more. More preferably, the Sn content is 0.004% or more, and even more preferably 0.006% or more. More preferably, the Sn content is 0.008% or more, and even more preferably 0.01% or more.

[0114] On the other hand, when the Sn content exceeds 0.10%, castability deteriorates. Furthermore, Sn segregates at the original γ grain boundaries, worsening resistance to delayed fracture. Therefore, in cases where Sn is present, the Sn content is set to 0.10% or less. The Sn content is preferably 0.04% or less, more preferably 0.03% or less.

[0115] Sb: below 0.10%

[0116] Sb suppresses oxidation and nitriding of the steel plate surface layer, thereby suppressing the decrease in C and B content in the surface layer. Furthermore, by suppressing the aforementioned decrease in C and B content, ferrite formation in the steel plate surface layer is suppressed, resulting in increased strength and improved fatigue resistance. From this perspective, the Sb content is preferably set to 0.002% or more. More preferably, it is 0.004% or more, and even more preferably 0.006% or more. On the other hand, when the Sb content exceeds 0.10%, castability deteriorates, and segregation at the original γ grain boundaries worsens the resistance to delayed fracture. Therefore, in the case of Sb content, the Sb content is set to 0.10% or less. Preferably, it is 0.04% or less, and more preferably 0.03% or less.

[0117] REM: below 0.0050%

[0118] REM (Resin M) is an element that suppresses the adverse effects of sulfides on the formability of extended flanges by spherizing the shape of sulfides, thereby improving the formability of extended flanges. To achieve these effects, it is preferable to set the REM content to 0.0005% or more. More preferably, the REM content is 0.0010% or more, and even more preferably 0.0020% or more.

[0119] On the other hand, when the REM content exceeds 0.0050%, the improvement effect on the formability of the extended flange becomes saturated. Therefore, in the case of REM, the REM content is set to 0.0050% or less.

[0120] It should be noted that, in this invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM concentration in this invention refers to the total content of one or more elements selected from the aforementioned REMs.

[0121] When the optional components described above are contained in amounts below the lower limit, the presence of optional elements in amounts below the lower limit does not impair the effects of the present invention. Therefore, when the optional elements described above are contained in amounts below the lower limit, these optional elements are contained as unavoidable impurities.

[0122] Next, the steel structure of the steel plate of the present invention will be described.

[0123] Area ratio of polygonal ferrite: 20% or more and 85% or less

[0124] To ensure high ductility, the polygonal ferrite content is set to 20% or more in terms of area ratio. Preferably, the polygonal ferrite content is 25% or more, more preferably 30% or more. More preferably, the polygonal ferrite content is 35% or more, more preferably 40% or more.

[0125] On the other hand, in order to obtain the specified strength, the polygonal ferrite content is set to 85% or less in terms of area ratio. More preferably, it is 82% or less. More preferably, it is 80% or less, and even more preferably 78% or less.

[0126] Area ratio of upper bainite: ≥9% and ≤45%

[0127] Upper bainite is bainite with low carbide precipitation. Upper bainite can be utilized to distribute carbon in the surrounding untransformed γ phase and to form residual γ phase with high processing stability. Furthermore, upper bainite has a hardness intermediate between ferrite and martensite; by forming a structure with this intermediate hardness, local elongation is increased. Therefore, for strength levels with a tensile strength (TS) of 590 MPa or higher, an upper bainite content of 9% or more is required. Therefore, the upper bainite content is set to 9% or more in terms of area ratio. Preferably, the upper bainite area ratio is 12.0% or more, more preferably 15.0% or more. On the other hand, in the present invention, which maintains the structure at a two-phase temperature, considering the formation of a large amount of ferrite, the upper bainite area ratio is set to 45% or less to suppress strength reduction. Upper bainite content is preferably 38% or less, more preferably 30% or less.

[0128] Volume fraction of retained austenite (retained γ): ≥3% and ≤15%

[0129] To ensure high ductility, the residual γ is set to 3% or more in volume fraction relative to the overall steel structure. The volume fraction (residual γ amount) of the residual γ is preferably 3.0% or more, more preferably 5% or more, and even more preferably 7% or more. This residual γ amount includes both residual γ generated adjacent to upper bainite and residual γ generated adjacent to martensite and lower bainite. Excessive increase in the residual γ amount leads to a decrease in strength and a significant reduction in the formability of the extended flange. Therefore, the volume fraction of the residual γ is set to 15% or less. The volume fraction of the residual γ is preferably 13% or less. Furthermore, "volume fraction" can be considered as "area fraction".

[0130] Area fraction of fresh martensite: 3% or more and 15%

[0131] Fresh martensite is a microstructure that reduces local elongation. By forming fresh martensite within a range that does not deteriorate flexibility or porosity, strength can be improved. From this perspective, the area ratio of fresh martensite is set to a lower limit of 3% and an upper limit of 15%.

[0132] The combined area ratio of tempered martensite and lower bainite: less than 50% (including 0%).

[0133] In this invention, tempered martensite and lower bainite are formed by retaining the austenite on the low-temperature side of two-stage austenite tempering.

[0134] Compared to upper bainite with less carbide precipitation, tempered martensite and lower bainite, which precipitate carbides in the microstructure, suppress carbon distribution to untransformed γ. However, tempered martensite and lower bainite lead to carbon enrichment to untransformed γ due to the expansion of the T0 composition at low temperatures, or consequently reduce the amount of fresh martensite upon final cooling. Therefore, it is necessary to control these microstructures to obtain residual γ with high processing stability.

[0135] Based on the steel composition and austenite tempering temperature, the low-temperature side of the austenite tempering in the above two stages is maintained below the Ms point. Therefore, a portion of the untransformed γ undergoes martensitic transformation and is then tempered through subsequent holding. When the combined area ratio of tempered martensite and lower bainite exceeds 50%, carbide precipitation is promoted, the necessary residual γ is not obtained, and the strength becomes excessively high, thus failing to achieve the desired ductility. Therefore, in this invention, the combined area ratio of tempered martensite and lower bainite is set to 50% or less. Preferably, their combined area ratio is 40% or less, more preferably 35% or less. Their combined area ratio is further preferably 30% or less, and even more preferably 25% or less.

[0136] On the other hand, if the area ratios of polygonal ferrite, upper bainite, residual γ and fresh martensite can be controlled within the desired range, the total area ratios of tempered martensite and lower bainite can be 0.

[0137] Area ratio of excess tissue: less than 5%

[0138] The remaining microstructure is a microstructure other than polygonal ferrite, upper bainite, retained austenite, fresh martensite, tempered martensite, and lower bainite; for example, pearlite can be listed. If a pearlite microstructure forms, efficient carbon distribution is hindered, and the formation of retained γ is suppressed, thus reducing ductility. Furthermore, in the annealing process involving two-stage isothermal quenching, a pearlite phase transformation occurs from austenite (γ) in a portion of the microstructure, depending on the steel composition and the austenite tempering temperature. In this invention, if the area ratio of the remaining microstructure is 5% or less, its impact on the material can be ignored; therefore, the upper limit for the area ratio of the remaining microstructure is set to 5%. The area ratio of the remaining microstructure is preferably 5.0% or less. The area ratio of the remaining microstructure can also be 0%.

[0139] The ratio of the total number of fresh martensite particles and retained γ particles with an equivalent circle diameter less than 1.2 μm to the total number of fresh martensite particles and retained austenite particles is greater than 50%.

[0140] Fresh martensite particles and residual γ particles with an equivalent circle diameter of less than 1.2 μm are microstructures that are not prone to stress concentration during local deformation, do not contribute to the formation of voids, and therefore do not deteriorate local ductility and porosity.

[0141] If the total number of fresh martensite particles and residual γ particles with an equivalent circle diameter of less than 1.2 μm is more than 50% of the total number of fresh martensite particles and residual γ particles, then excellent local elongation and porosity can be obtained in this invention.

[0142] Therefore, in this invention, the ratio of the total number of fresh martensite particles and retained γ particles with an equivalent circle diameter of less than 1.2 μm to the total number of fresh martensite particles and retained austenite particles is set to 50% or more. That is, it satisfies the following formula (A).

[0143] 100 × (total number of fresh martensite particles and residual γ particles with an equivalent circle diameter less than 1.2 μm) / (total number of fresh martensite particles and residual γ particles) ≥ 50 (%) ... Equation (A)

[0144] Preferably, the proportion on the left side of the above formula (A) is 55% or more.

[0145] It should be noted that, in order to obtain the above-mentioned structure, upper bainite, tempered martensite, and lower bainite can be formed in the structure through a two-stage austenitic tempering process.

[0146] The ratio of the total number of fresh martensite particles and retained gamma particles with an aspect ratio of 2.5 or higher and an equivalent circle diameter of 1.2 μm or higher to the total number of fresh martensite particles and retained gamma particles with an equivalent circle diameter of 1.2 μm or higher is 40% or higher.

[0147] In fresh martensite particles and / or residual γ particles with an equivalent circle diameter of 1.2 μm or more, by increasing the aspect ratio of fresh martensite particles and / or residual γ particles, stress concentration during local deformation can be reduced, void formation can be suppressed, thereby improving local ductility and porosity.

[0148] For such fresh martensite particles and / or residual γ particles, the area ratio can be increased by causing the needle-like γ particles, which are surrounded by a soft ferrite structure and formed during heating, to undergo a bainitic phase transformation during subsequent cooling. In this invention, the desired formability can be achieved by ensuring that particles with an equivalent circle diameter of 1.2 μm or more and an aspect ratio of 2.5 or more account for 40% or more of the total number of fresh martensite particles and residual γ particles with an equivalent circle diameter of 1.2 μm or more.

[0149] Therefore, in this invention, the ratio of the total number of fresh martensite particles and retained austenite particles with an aspect ratio of 2.5 or higher and an equivalent circle diameter of 1.2 μm or higher to the total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of 1.2 μm or higher is set to 40% or higher. That is, together with the above formula (A), the following formula (B) is further satisfied. Preferably, the ratio on the left side of the following formula (B) is 45% or higher.

[0150] 100 × (total number of fresh martensite particles and residual γ particles with an aspect ratio of 2.5 or higher and an equivalent circle diameter of 1.2 μm or higher) / (number of fresh martensite particles and residual γ particles with an equivalent circle diameter of 1.2 μm or higher) ≥ 40… Equation (B)

[0151] The microstructure of the obtained steel plate was determined by the following method.

[0152] Determination of the area ratio of steel microstructure

[0153] The specimen was cut from the steel plate using a section perpendicular to the steel plate surface and parallel to the rolling direction as the observation plane. The thick section was then etched and visualized using a 1% (v / v) nitric acid ethanol solution. The image was then magnified 2000 times using a scanning electron microscope (SEM) at a depth of 3000 μm in the t / 4 section of the plate. 2 Tissue photographs were taken of the above-mentioned areas. Then, the following items (i) to (iv) were measured respectively. It should be noted that t represents the plate thickness and w represents the plate width.

[0154] (i) Polygonal ferrite and upper bainite

[0155] Both polygonal ferrite (recrystallized F) and upper bainite (UB) appear gray in SEM images, but can be identified by their shape. An example of a SEM image is shown alongside an SEM image of a microstructure held at temperature T and then water-cooled. Figure 1 middle. Figure 1 The area indicated by the dashed line in (a) represents a needle-like γ-structure formed by homogenization and holding at an annealing temperature T up to the scope of this invention during the annealing process. Upper bainite (UB) forms within this structure, surrounded by high aspect ratio residual γ-structure or fresh martensite (M). The same structure was also observed in the blocky γ-structure formed by homogenization and holding at an annealing temperature T. The area ratios of polygonal ferrite and upper bainite were determined using a point-based method according to ASTM E562-11 (2014). The area ratios of polygonal ferrite and upper bainite were the average values ​​of measurements taken at five locations.

[0156] (ii) Fresh martensite and residual γ

[0157] Both fresh martensite and residual γ appear white in SEM images and are indistinguishable. Therefore, residual γ was measured separately using the method described later. Additionally, the area ratio of fresh martensite and residual γ was determined from SEM images using a point-based method according to ASTM E562-11 (2014). The area ratio of fresh martensite was then determined by subtracting the area ratio of residual γ, as determined by the method described later, from this total area ratio. The total area ratio of fresh martensite and residual γ was determined using a point-based method, and the area ratio of fresh martensite was set as the value obtained by subtracting the volume fraction of residual γ, as determined by the method described later, from the average value of the measurements taken at five locations.

[0158] (iii) Tempered martensite and / or lower bainite

[0159] Tempered martensite and lower bainite are carbide-containing microstructures observed as fine, white particles in SEM images. While they can be distinguished at a more microscopic scale, they are difficult to differentiate in SEM images. Therefore, in this invention, tempered martensite and lower bainite are defined as a single microstructure, and the combined area ratio of tempered martensite and lower bainite is determined using a point-based algorithm according to ASTM E562-11 (2014). The average value of measurements from five locations is taken as the combined area ratio of tempered martensite and lower bainite.

[0160] (iv) Excess Fabric

[0161] The area ratio of the remaining structure is defined as the area ratio of polygonal ferrite, upper bainite, fresh martensite, residual γ, tempered martensite, and lower bainite determined by the above method, which is subtracted from 100%.

[0162] Determination of the volume fraction of residual γ

[0163] After grinding the steel plate to 1 / 4 of its thickness, it is further ground by 0.1 mm using chemical grinding. The resulting surfaces are then subjected to X-ray diffraction using Kα rays of Mo to measure the integrated reflection intensity of the (200), (220), and (311) surfaces of FCC iron (γ) and the (200), (211), and (220) surfaces of BCC iron (ferrite). The volume fraction of residual γ is determined by the intensity ratio of the integrated reflection intensity from each surface of FCC iron (γ) to the integrated reflection intensity from each surface of BCC iron (ferrite). In this invention, the volume fraction of residual γ can be used as the area fraction of residual γ.

[0164] Equivalent circle diameter and aspect ratio of fresh martensite particles and / or residual gamma particles

[0165] The specimen was cut from the steel plate with the cross-section parallel to the rolling direction as the observation plane. The microstructure of the thick section was revealed by corrosion using Lepera etching solution. A laser microscope (LM) was used to magnify the specimen to 1000x at a depth of 10000 μm on the t / 4 section of the plate. 2 The above areas were photographed.

[0166] Lepera etching is a color etching process that extracts fresh martensite particles and / or residual γ particles by representing fresh martensite and / or residual γ particles with white contrast, performs image analysis, and thereby determines the equivalent circle diameter and aspect ratio of the fresh martensite particles and / or residual γ particles.

[0167] Of all the particles obtained, those with an equivalent circle diameter of less than 1.2 μm were selected as the target particles, and their number was measured to calculate the proportion of the total number of particles.

[0168] In addition, among all the particles obtained, particles with an equivalent circle diameter of 1.2 μm or more were selected as the target, and the number of particles with an aspect ratio of 2.5 or more was measured. The proportion of particles with an aspect ratio of 2.5 or more and an equivalent circle diameter of 1.2 μm or more to all particles with an equivalent circle diameter of 1.2 μm or more was calculated.

[0169] Next, one embodiment of the method for manufacturing the steel plate of the present invention will be described in detail. It should be noted that, unless otherwise specified, the temperatures shown below for heating or cooling the steel billet (steel raw material), steel plate, etc. refer to the surface temperature of the steel billet (steel raw material), steel plate, etc.

[0170] The method for manufacturing a steel sheet according to the present invention includes: a cold rolling process in which a hot-rolled steel sheet having the above-mentioned composition is subjected to hot rolling and pickling, and the resulting hot-rolled steel sheet is subjected to cold rolling treatment to obtain a cold-rolled steel sheet; and an annealing process in which the cold-rolled steel sheet is subjected to annealing treatment to obtain a steel sheet. The cold rolling process includes the following cold rolling treatment: setting the cumulative cold rolling rate to 30-85%, setting the reduction rate of the first pass to 5% or more and less than 25%, thereby obtaining a steel sheet having {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation, and {100} <011> For cold-rolled steel sheets where the total area percentage of oriented microstructure relative to the total bcc phase microstructure is 35% to 75%, the annealing process includes the following annealing treatment: For cold-rolled steel sheets, an average heating rate of 0.5 to 15°C / second is set within a temperature range of 500°C to 1, and the sheets are heated to an annealing temperature T below 840°C where 0.5 ≤ (T-Ac1) / (Ac3-Ac1) < 1.0. After this heating, the sheets are held at the annealing temperature T in a furnace atmosphere with a dew point Td of -50°C to -30°C, thereby obtaining an acicular austenite microstructure with a number density of 5 acicular austenite particles per 1000 μm. 2 The steel plate is then subjected to a first cooling process, in which the average cooling rate within the temperature range of 750 to 550°C is set to 6.0°C / second or higher, cooling to a first cooling stop temperature Tc1 below 550°C and above 400°C. After the first cooling, the plate is held at the first cooling stop temperature Tc1 for at least 25 seconds. After the first holding, the plate is cooled to a second cooling stop temperature Tc2 below the first cooling stop temperature Tc1 and below 450°C and above 300°C. The plate is held at the second cooling stop temperature Tc2 for 20 to 3000 seconds. After the second holding, a third cooling process is performed.

[0171] The following is a description of each process.

[0172] Hot rolling process

[0173] In this invention, hot rolling in the hot rolling process includes methods such as reheating and rolling a steel billet with the above-mentioned composition, directly rolling a continuously cast steel billet without heating, and rolling a continuously cast steel billet after a short-time heat treatment. Hot rolling can be carried out using conventional methods. For example, the billet heating temperature can be set to 1100°C or higher and 1300°C or lower, the soaking time can be set to 20-30 minutes, the finishing rolling temperature can be set to 1100°C or higher and 1300°C or lower than the Ar3 phase transformation point, and the coiling temperature can be set to 400-720°C. From the viewpoint of suppressing thickness variation and stably ensuring high strength, the coiling temperature is preferably set to 430-620°C.

[0174] There are no particular limitations on the smelting method used to manufacture the aforementioned steel billets (steel raw materials), and well-known smelting methods such as converters and electric furnaces can be used. Alternatively, a vacuum degassing furnace can be used for secondary refining.

[0175] Pickling process

[0176] Pickling is a process in which hot-rolled steel sheets are pickled after the hot-rolling process. There are no particular limitations on the pickling conditions; the pickling conditions from well-known manufacturing methods can be used.

[0177] cold rolling process

[0178] Cumulative cold rolling rate: 30-85%

[0179] When the reduction rate (cumulative cold rolling rate) during cold rolling is less than 30%, recrystallization cannot be sufficiently promoted, and the formation of needle-like γ-rays mentioned in this invention cannot be sufficiently achieved. Furthermore, the microstructure becomes uneven after the annealing process. Therefore, the reduction rate (cumulative cold rolling rate) during cold rolling needs to be in the range of 30% or more, preferably 40% or more, and more preferably 50% or more. On the other hand, from the viewpoint of cold rolling load or, further, from the viewpoint of material properties, the reduction rate (cumulative cold rolling rate) should be 85% or less.

[0180] In the cold rolling process, there is no particular limit to the number of passes; for example, it can be set to 5 passes. The cumulative cold rolling rate (thickness reduction rate) is calculated as (1 - (thickness after cold rolling (after the final pass) / thickness before cold rolling) × 100).

[0181] First pass reduction rate: 5% or more but less than 25%

[0182] From an operational point of view, the reduction rate of the first pass is set to be 5% or more. On the other hand, when the reduction rate of the first pass is 25% or more, the sheet temperature during the first pass is low, thus imparting shear strain to the cold-rolled material, resulting in underdeveloped texture and the absence of needle-like γ-rays. Therefore, the reduction rate of the first pass is set to be 5% or more and less than 25%.

[0183] It should be noted that the reduction rate (thickness reduction rate) of the first pass refers to (1 - (thickness of the plate after the first pass of cold rolling) / (thickness of the plate before cold rolling)) × 100.

[0184] The rolling temperature (plate temperature) for the first pass is preferably 20°C or higher and 40°C or lower. It should be noted that the rolling temperature for the first pass is determined by measuring the surface of the steel plate after one pass using a radiation thermometer on areas without lubricating oil. When the rolling temperature for the first pass is below 20°C or above 40°C, the desired texture may not be developed, and needle-like γ-rays may not form. Therefore, the rolling temperature for the first pass is preferably 20°C or higher and 40°C or lower.

[0185] The microstructure of cold-rolled steel sheet after cold rolling: It has {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation, and {100}... <011> The total area fraction of the oriented tissue relative to the total area fraction of the bcc phase is more than 35% and less than 75%.

[0186] The needle-like γ-rays have a specific crystal orientation relationship with the ferrite surrounding their nucleation sites (Near Kurdjumov-Sachs relationship).

[0187] In the structure of a cold-rolled steel sheet after cold rolling, by controlling the total area fraction of the structure having predetermined orientations including {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation and {100}<011> orientation to be not less than a certain amount relative to the entire structure of the bcc phase, reverse transformed austenite (γ) having the above predetermined orientations is easily formed between surrounding ferrite grains, and as a result, a large amount of acicular γ is formed. To form a desired amount of acicular γ, the total area fraction of the structure having {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation and {100}<011> orientation relative to the entire structure of the bcc phase needs to be 35% or more, preferably 40% or more. On the other hand, when the total area fraction of the structure having {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation and {100}<011> orientation relative to the entire structure of the bcc phase exceeds 75%, material anisotropy of the steel sheet occurs. Therefore, the total area fraction of the structure having the above predetermined orientations relative to the entire structure of the bcc phase is set to 75% or less, preferably 70% or less, and more preferably 65% or less.

[0188] In the present invention, by subjecting a hot-rolled steel sheet having the above-described composition to cold rolling treatment at a cold rolling reduction of 30 to 85%, and setting the reduction ratio of the first pass to 5% or more and less than 25%, the ratio of the total area fraction of the structure having the above predetermined orientations to the area fraction of the entire bcc phase can be adjusted to a desired range.

[0189] Method for measuring texture of cold-rolled structure

[0190] A measurement specimen with a cross-section parallel to the rolling direction as the measurement surface is cut from the cold-rolled steel sheet after the cold rolling process. After mechanically or electrolytically polishing the measurement surface, an area of 80000μm 2 or larger is measured by SEM-EBSD method (measurement conditions: WD: 20 mm, accelerating voltage: 20 kV). The area fraction of the bcc phase structure with {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation and {100}<011> orientation with respect to rolling {ND surface}<RD direction> is quantified, and the ratio of the area fraction to the area fraction of the bcc phase with all orientations is calculated, thereby evaluating the texture of the cold-rolled steel sheet.

[0191] Annealing process

[0192] The annealing process of the present invention is as follows: For the cold-rolled steel sheet after the above-mentioned cold rolling process, the average heating rate (HR1) in the temperature range of 500°C or higher and Ac1 or lower is set to 0.5 to 15°C / second, and the heating is carried out until the annealing temperature T is below 840°C and satisfies 0.5 ≤ (T-Ac1) / (Ac3-Ac1) < 1.0. After this heating, the sheet is held at the above-mentioned annealing temperature T in a furnace atmosphere with a dew point Td of -50°C or higher and -30°C, thereby obtaining an acicular austenite structure with a number density of 5 acicular austenite particles / 1000 μm. 2 The steel plate is then subjected to a first cooling process, in which the average cooling rate within the temperature range of 750 to 550°C is set to 6.0°C / second or higher, cooling to a first cooling stop temperature Tc1 below 550°C and above 400°C. After the first cooling, the plate is held at the first cooling stop temperature Tc1 for at least 25 seconds. After the first holding, the plate is cooled to a second cooling stop temperature Tc2 below the first cooling stop temperature Tc1 and below 450°C and above 300°C. The plate is held at the second cooling stop temperature Tc2 for 20 to 3000 seconds. After the second holding, a third cooling process is performed.

[0193] Average heating rate for the temperature range above 500℃ and below Ac1: 0.5~15℃ / second or less

[0194] In this invention, by heating a cold-rolled sheet having the structure described above after the cold rolling process at an appropriate heating rate, recrystallization is sufficiently promoted. Then, by heating to or holding at temperature T, needle-like γ-phases are formed. Therefore, an average heating rate of 15°C / second or less is set within a temperature range of Ac1 or less where a γ-phase transformation does not occur above 500°C. The average heating rate is preferably 10°C / second or less.

[0195] Furthermore, from an operational point of view, the average heating rate is set to 0.5°C / second or higher. Preferably, the average heating rate is 1.0°C / second or higher, and more preferably 1.5°C / second or higher.

[0196] Here, the average heating rate (°C / sec) is calculated from ((Ac1(°C) - 500°C) / (heating time (sec) from 500°C to Ac1(°C)).

[0197] The annealing temperature T is maintained at a temperature below 840℃ and satisfying 0.5 ≤ (T - Ac1) / (Ac3 - Ac1) < 1.0.

[0198] After heating, the furnace is kept in a homogenized state at an annealing temperature T in a furnace atmosphere with a dew point Td between -50°C and -30°C.

[0199] In this invention, needle-shaped γ can be formed as described later by heating to the annealing temperature T described later or holding at a further annealing temperature T. Regarding this, if heated to the single-phase region of γ above Ac3 (°C), the needle-shaped γ merges with adjacent γ, and the morphology of the γ becomes equiaxed. Therefore, in this invention, two-phase annealing is required.

[0200] Furthermore, regarding the annealing temperature T, if (T-Ac1) / (Ac3-Ac1) is less than 0.5, the reverse phase transformation to γ ​​will not occur sufficiently, acicular γ will not form, and only equiaxed γ will form along the recrystallized ferrite grain boundaries. Therefore, the annealing temperature T is set to 0.5≤(T-Ac1) / (Ac3-Ac1)<1.0.

[0201] Furthermore, good chemical conversion properties are not achieved at temperatures above 840℃. Therefore, the temperature T is set below 840℃.

[0202] Furthermore, when the dew point Td is below -50°C, it promotes the formation of Si oxides, which negatively impact chemical conversion treatability, thus resulting in poor chemical conversion treatability. Conversely, when the dew point Td exceeds -30°C, internal oxide layers containing oxides selectively form at grain boundaries within the microstructure, thus negatively affecting corrosion resistance, etc. Therefore, the dew point Td is set to be between -50°C and -30°C. Preferably, the dew point Td is -48°C or higher, more preferably -46°C or higher. Furthermore, preferably, the dew point Td is -32°C or lower, more preferably -34°C or lower.

[0203] There is no particular limitation on the soaking time at the annealing temperature T. From the viewpoint of element distribution in two-phase annealing, it is preferably set to 25 to 350 seconds, and more preferably 50 to 300 seconds.

[0204] It should be noted that Ac1(°C) can be calculated using the following formula based on empirical rules.

[0205] Ac1(℃)=723+22×[Si%]-18×[Mn%]+17×[Cr%]+4.5[Mo%]+16×[V%]

[0206] Ac3 (°C) can be calculated using the following formula based on empirical rules.

[0207] Ac3 (°C) = 910 - 203 × [C%] 1 / 2 +44.7×[Si%]-30×[Mn%]+700×[P%]+400×[sol.Al%]-20×[Cu%]+31.5×[Mo%]+104×[V%]+400×[Ti%]

[0208] It should be noted that [X%] in the above formula is the content (mass%) of element X in the steel plate, and is sometimes set to "0" if it does not contain it.

[0209] The needle-like γ-ray tissue formed by the above-mentioned heat preservation treatment has a number density of 5 per 1000 μm. 2 above

[0210] In this invention, acicular γ-structures are utilized to achieve the desired formability. If a large amount of acicular austenite (acicular γ-structure) is formed, a large amount of residual γ-structure with a high aspect ratio is easily formed. In this invention, to achieve the desired formability, the acicular γ-structure formed by heating to the annealing temperature T and holding it in a homogenous state needs to have a number density of 5 acicular γ-structures per 1000 μm. 2 In conclusion, considering the properties of needle-like γ particles, there is no upper limit, and a higher number of needle-like γ particles is preferable.

[0211] In this invention, for cold-rolled steel sheets having the above-mentioned composition and structure, the average heating rate in the temperature range of 500°C or higher and Ac1 or lower is set to 0.5 to 15°C / second or lower, and the sheets are heated to the annealing temperature T. The sheets are then kept in a furnace atmosphere at the annealing temperature T and kept in a homogenized state. This allows the number density of the acicular γ structure to be adjusted to the desired range.

[0212] Number density of needle-like gamma tissue

[0213] When evaluating microstructures formed at high temperatures, microstructures formed by freezing the microstructure through water cooling are typically evaluated. In this invention, an important point is that the needle-like γ-structures formed during the annealing process through homogenization up to the annealing temperature T contribute to the formation of residual γ-structures with high aspect ratios and high processing stability during subsequent cooling. The number density of these needle-like γ-structures is measured. Observation specimens are cut from the steel plate with a section parallel to the rolling direction as the observation plane. The thick section is etched using a 1% (v / v) nitric acid ethanol solution and magnified to 2000x using a scanning electron microscope (SEM). The section at thickness t / 4 is examined at a depth of 3000 μm. 2 The above areas were photographed. Figure 1 (b) The SEM image shown is a photograph of the microstructure after being held at a temperature T within the range of the present invention during the annealing process and then water-cooled, forming needle-like γ and blocky γ ferrite microstructures. Figure 2The diagram illustrates a method for determining the aspect ratio of acicular γ-rays. Here, austenite with an aspect ratio of 3.0 or greater, surrounded by recrystallized ferrite with the same orientation, is defined as acicular γ-rays. It should be noted that it is acceptable for the tip of the acicular austenite to be in contact with other austenite grains; in this case, it is sufficient to confirm that adjacent ferrite grains are of the same orientation using electron beam backscatter diffraction (EBSD). According to this definition, during the annealing process, the number of acicular γ-rays in a steel sheet that has undergone homogenization up to the annealing temperature T is measured in five fields of view. The number density (number of acicular γ-rays / 1000 μm) is determined by dividing the number of acicular γ-rays by the total area observed. 2 ).

[0214] First Cooling: Set the average cooling rate within the temperature range of 750–550℃ to at least 6.0℃ / second, cooling down to a first cooling stop temperature Tc1 below 550℃ but above 400℃. First Holding: After the first cooling, hold at the first cooling stop temperature Tc1 for at least 25 seconds.

[0215] During the first cooling process, within a temperature range of 750–550°C, ferrite and pearlite phase transformations primarily occur. If excessive ferrite or pearlite phase transformations occur, the acicular γ-rays undergo ferrite phase transformation. Therefore, the average cooling rate within the 750–550°C temperature range is set to 6.0°C / second or higher to suppress ferrite phase transformation. The average cooling rate is preferably 8.0°C / second or higher, and more preferably 10.0°C / second or higher.

[0216] Here, the average cooling rate (°C / sec) is calculated from (750°C (cooling start temperature) - 550°C (cooling stop temperature)) / (cooling time (sec) from cooling start temperature to cooling stop temperature).

[0217] The first cooling stop temperature Tc1 in the first cooling process corresponds to the temperature on the high-temperature side of the two-stage austenitic tempering, and is set within the temperature range where the upper bainitic transformation with minimal carbide precipitation occurs, i.e., below 550°C and above 400°C. When the first cooling stop temperature Tc1 exceeds 550°C, a ferrite or pearlite transformation occurs, without localized carbon distribution, and residual γ with high processing stability is not formed. On the other hand, when the first cooling stop temperature Tc1 is below 400°C, a lower bainitic transformation occurs accompanied by carbide precipitation. Furthermore, depending on the composition and annealing conditions, a portion of the microstructure undergoes a martensitic transformation and transforms into tempered martensite through subsequent holding. Thus, carbon distribution to the untransformed γ is delayed, and residual γ with high processing stability is not formed. Therefore, the first cooling stop temperature Tc1 is set to be below 550°C and above 400°C. The first cooling stop temperature Tc1 is preferably above 420°C, and more preferably above 450°C. In addition, the first cooling stop temperature Tc1 is preferably 530°C or less, and more preferably 510°C or less.

[0218] Furthermore, as the first holding period after the first cooling, the temperature control at the first cooling stop temperature Tc1 is permissible as long as it is within the range of 550°C or below and 400°C or above, and is held for 25 seconds or more. This allows for sufficient phase transformation of the upper bainite. The holding time in the first holding period is preferably 30 seconds or more, more preferably 35 seconds or more. Additionally, the holding time in the first holding period is preferably 60 seconds or less, more preferably 55 seconds or less.

[0219] Second cooling: Cooling to a second cooling stop temperature Tc2 that is below the first cooling stop temperature Tc1 and below 450°C but above 300°C.

[0220] Second holding time: Hold at the second cooling stop temperature Tc2 for 20–3000 seconds.

[0221] In the second cooling, the second cooling stop temperature Tc2 corresponds to the temperature of the low-temperature side of the two-stage austenitic tempering. By holding the steel sheet at the second cooling stop temperature Tc2, the amount of dissolved carbon that will undergo martensitic transformation during final cooling after holding at the first cooling stop temperature Tc1 is small, causing the thermally unstable untransformed γ to undergo bainitic transformation. This allows residual γ to form. When the second cooling stop temperature Tc2 is below the first cooling stop temperature Tc1 but above 450°C, the bainitic transformation stops, and therefore the untransformed γ undergoes excessive martensitic transformation during final cooling. On the other hand, when the second cooling stop temperature Tc2 is below 300°C, the untransformed γ undergoes martensitic transformation, promoting carbide formation by holding it. This hinders the formation of residual γ with high dissolved carbon content and high processing stability. Therefore, the second cooling stop temperature Tc2 is set to be below Tc1 and below 450°C but above 300°C. The second cooling stop temperature Tc2 is preferably 320°C or higher, more preferably 340°C or higher. In addition, the second cooling stop temperature Tc2 is preferably 430°C or less, and more preferably 410°C or less.

[0222] Furthermore, as a second holding period after the second cooling, the temperature control at the second cooling stop temperature Tc2 is permissible as long as it is below the first cooling stop temperature Tc1 and within the range of 450°C to 300°C. A holding time of 20 seconds or more promotes the formation of residual γ with high processing stability. On the other hand, from an operational point of view, the holding time in the second holding period is set to 3000 seconds or less. The holding time in the second holding period is preferably 100 seconds or more, more preferably 200 seconds or more. Additionally, the holding time in the second holding period is preferably 2500 seconds or less, more preferably 2000 seconds or less.

[0223] Third cooling: After the second holding period, cooling is carried out.

[0224] After the second holding period described above, the plate is cooled to room temperature (10-30°C) to obtain the steel plate of the present invention.

[0225] There are no particular limitations after the above annealing process; for example, surface finishing with an elongation of 0.05 to 0.5% can be performed.

[0226] The steel sheet of the present invention, obtained by the manufacturing method of the steel sheet of the present invention, preferably has a thickness of 0.5 mm or more. Furthermore, the thickness is preferably 2.0 mm or less.

[0227] Next, the components of the present invention and their manufacturing method will be described.

[0228] The component of the present invention is formed by performing at least one of forming and joining processes on the steel plate of the present invention. Furthermore, the manufacturing method of the component of the present invention includes a step of forming and joining processes on the steel plate of the present invention to produce the component.

[0229] The steel sheet of the present invention has a tensile strength of 590 MPa or higher, and exhibits high ductility, excellent extension flange formability, and good chemical conversion treatment properties. Therefore, components obtained using the steel sheet of the present invention also possess high strength, exhibiting superior high ductility, excellent extension flange formability, and good chemical conversion treatment properties compared to conventional high-strength components. Furthermore, lightweighting can be achieved by using components of the present invention. Therefore, components of the present invention can be suitably used, for example, in vehicle body frame parts. The components of the present invention also include welded joints.

[0230] Forming processes can utilize general processing methods such as pressure processing without limitation. Furthermore, joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and rivet joining without limitation.

[0231] Example

[0232] The embodiments of the present invention will be described below.

[0233] Hot rolling (billet heating temperature: 1250℃, soaking time: 30 minutes, finishing rolling temperature: Ar3+50℃, coiling temperature: 550℃) and pickling treatment were carried out on the steel billet with a thickness of 250mm having the composition shown in Table 1. The resulting hot-rolled steel sheet was then cold-rolled under the conditions shown in Table 2 to manufacture cold-rolled steel sheet.

[0234] Next, the cold-rolled steel sheet was annealed using a continuous annealing production line under the conditions shown in Table 2, and then surface-rolled with an elongation of 0.2 to 0.4% to produce a steel sheet for evaluation.

[0235] In addition, after a second holding at the second cooling stop temperature Tc2, the temperature is cooled to room temperature (20°C) as a third cooling process.

[0236]

[0237]

[0238] The obtained steel plates were evaluated using the following methods.

[0239] (1) Determination of the area ratio of steel microstructure and determination of the number density of needle-like γ-rays

[0240] The specimen was cut from the steel plate with the cross-section perpendicular to the steel plate surface and parallel to the rolling direction as the observation plane. The thick section was etched and revealed using a 1% (v / v) nitric acid ethanol solution. The specimen was then magnified to 2000x using a scanning electron microscope (SEM) at a depth of 3000 μm in the t / 4 section of the plate. 2 Tissue photographs were taken of the above areas. The following items (i) to (iv) were measured respectively. The results are shown in Table 3. It should be noted that t represents plate thickness and w represents plate width.

[0241] (i) Polygonal ferrite and upper bainite

[0242] Both polygonal ferrite (recrystallized F) and upper bainite (UB) appear gray in SEM images, but can be identified by their shape. An example of a SEM image is shown alongside an SEM image of a microstructure held at temperature T and then water-cooled. Figure 1 middle. Figure 1 The area indicated by the dashed line in (a) represents a needle-like γ-structure formed by homogenization and holding at an annealing temperature T up to the scope of this invention during the annealing process. Upper bainite (UB) forms within this structure, surrounded by high aspect ratio residual γ-structure or fresh martensite (M). The same structure was also observed in the blocky γ-structure formed by homogenization and holding at an annealing temperature T. The area ratios of polygonal ferrite and upper bainite were determined using a point-based method according to ASTM E562-11 (2014). The area ratios of polygonal ferrite and upper bainite were the average values ​​of measurements taken at five locations.

[0243] (ii) Fresh martensite and residual γ

[0244] Both fresh martensite and residual γ appear white in SEM images and are indistinguishable. Therefore, residual γ was measured separately using the method described later. Additionally, the area ratio of fresh martensite and residual γ was determined from SEM images using a point-based method according to ASTM E562-11 (2014). The area ratio of fresh martensite was then determined by subtracting the area ratio of residual γ, as determined by the method described later, from this total area ratio. The total area ratio of fresh martensite and residual γ was determined using a point-based method, and the area ratio of fresh martensite was set as the value obtained by subtracting the volume fraction of residual γ, as determined by the method described later, from the average value of the measurements taken at five locations.

[0245] (iii) Tempered martensite and / or lower bainite

[0246] Tempered martensite and lower bainite are carbide-containing microstructures observed as fine, white particles in SEM images. While they can be distinguished at a more microscopic scale, they are difficult to differentiate in SEM images. Therefore, in this invention, tempered martensite and lower bainite are defined as a single microstructure, and the combined area ratio of tempered martensite and lower bainite is determined using a point-based algorithm according to ASTM E562-11 (2014). The average value of measurements from five locations is taken as the combined area ratio of tempered martensite and lower bainite.

[0247] (iv) Excess Fabric

[0248] The area ratio of the remaining structure is defined as the area ratio of polygonal ferrite, upper bainite, fresh martensite, residual γ, tempered martensite, and lower bainite determined by the above method, which is subtracted from 100%.

[0249] (2) Determination of the volume fraction of residual γ

[0250] After grinding the steel plate to 1 / 4 of its thickness, it is further ground by 0.1 mm using chemical grinding. The resulting surfaces are then subjected to X-ray diffraction using Kα rays of Mo to measure the integrated reflection intensity of the (200), (220), and (311) surfaces of FCC iron (γ) and the (200), (211), and (220) surfaces of BCC iron (ferrite). The volume fraction of residual γ is determined by the intensity ratio of the integrated reflection intensity from each surface of FCC iron (γ) to the integrated reflection intensity from each surface of BCC iron (ferrite). In this invention, the volume fraction of residual γ can be used as the area fraction of residual γ.

[0251] (3) Number density of needle-like γ tissue

[0252] When evaluating microstructures formed at high temperatures, microstructures formed by freezing the microstructure through water cooling are typically evaluated. In this invention, an important point is that the needle-like γ-structures formed during the annealing process through homogenization up to the annealing temperature T contribute to the formation of residual γ-structures with high aspect ratios and high processing stability during subsequent cooling. The number density of these needle-like γ-structures is measured. Observation specimens are cut from the steel plate with a section parallel to the rolling direction as the observation plane. The thick section is etched using a 1% (v / v) nitric acid ethanol solution and magnified to 2000x using a scanning electron microscope (SEM). The section at thickness t / 4 is examined at a depth of 3000 μm. 2 Take tissue photos in the above areas. Figure 1 (b) shows a SEM image of the microstructure after being held at a temperature T within the range of the present invention during the annealing process and then water-cooled, forming needle-like γ and blocky γ ferrite microstructures. Figure 2 The diagram illustrates a method for determining the aspect ratio of acicular γ-rays. Here, austenite with an aspect ratio of 3.0 or greater, surrounded by recrystallized ferrite with the same orientation, is defined as acicular γ-rays. It should be noted that it is acceptable for the tip of the acicular austenite to be in contact with other austenite grains; in this case, it is sufficient to confirm that adjacent ferrite grains are of the same orientation using electron beam backscatter diffraction (EBSD). According to this definition, during the annealing process, the number of acicular γ-rays in a steel sheet that has undergone homogenization up to the annealing temperature T is measured in five fields of view. The number density (number of acicular γ-rays / 1000 μm) is determined by dividing the number of acicular γ-rays by the total area observed. 2 The results are shown in Table 3.

[0253] (4) Equivalent circle diameter and aspect ratio of fresh martensite particles and / or residual γ particles

[0254] The specimen was cut from the steel plate with the cross-section parallel to the rolling direction as the observation plane. The microstructure of the thick section was revealed by corrosion using Lepera etching solution. A laser microscope (LM) was used to magnify the specimen to 1000x at a depth of 10000 μm on the t / 4 section of the plate. 2 Take tissue photos in the above areas.

[0255] Lepera etching is a color etching process that extracts fresh martensite particles and / or residual γ particles by representing fresh martensite and / or residual γ particles with white contrast, performs image analysis, and thereby determines the equivalent circle diameter and aspect ratio of the fresh martensite particles and / or residual γ particles.

[0256] Of all the particles obtained, those with an equivalent circle diameter of less than 1.2 μm were selected as the target particles, and their number was measured to calculate the proportion of the total number of particles.

[0257] In addition, among all the obtained particles, taking particles with an equivalent circle diameter of 1.2 μm or more as objects, the number of particles with an aspect ratio of 2.5 or more is measured, and the proportion of particles having an aspect ratio of 2.5 or more and an equivalent circle diameter of 1.2 μm or more relative to all particles having an equivalent circle diameter of 1.2 μm or more is calculated. The results are shown in Table 3.

[0258] (5) Texture of cold-rolled structure

[0259] A test specimen with a cross-section parallel to the rolling direction as the measurement surface is cut from the cold-rolled steel sheet after the cold rolling process. After mechanical grinding or electrolytic polishing of the measurement surface, an area of 80000 μm 2 or more is measured by the SEM-EBSD method (measurement conditions: WD: 20 mm, acceleration voltage: 20 kV). The area fraction of bcc phase with {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation and {100}<011> orientation for the rolled {ND plane}<RD direction> is quantified, and the ratio to the area fraction of bcc phase with all orientations is calculated, thereby evaluating the texture of the cold-rolled steel sheet.

[0260] (6) Tensile test

[0261] JIS No. 5 tensile test pieces having a tensile direction perpendicular to the rolling direction were produced from the obtained steel sheets. Each test piece is subjected to a tensile test in accordance with the provisions of JIS Z 2241 (2011). The crosshead speed of the tensile test is set to 10 mm / min. It should be noted that the measurement is carried out twice, and the measurement value is obtained by averaging, which is taken as the tensile strength (TS) of each steel sheet.

[0262] (7) Hole expansion test

[0263] A test piece of 100 mm × 100 mm is cut out, and a hole expansion test in accordance with JFST 1001 (Japan Iron and Steel Federation Standard) is carried out three times at each cutting position, and the average value of three tests (total value of three tests (%) / 3) is taken as the hole expansion ratio λ (%).

[0264] (8) Evaluation

[0265] In the present invention, a steel sheet having a tensile strength (TS) of 590 MPa or more is evaluated as high-strength.

[0266] When tensile strength (TS) × total elongation (T.El) ≥ 22000 MPa·%, ductility El is evaluated as excellent, and when the hole expansion ratio λ (%) satisfies the following (A1) or (A2), stretch flange formability λ is evaluated as excellent.

[0267] (A1) The porosity λ is 60% or more when the tensile strength is 590 MPa or more and less than 780 MPa.

[0268] (A2) When the tensile strength is above 780 MPa, the porosity λ is above 35%.

[0269] (9) Chemical conversion treatment

[0270] Annealed steel plates are treated with 20-35 A / dm 2 The surface was degreased and surface-adjusted by sulfuric acid electrolysis for 2 seconds at a current density, followed by chemical conversion treatment with zinc phosphate chemical conversion solution. The degreasing process involved a spray degreasing process at 40°C for 120 seconds; the surface-adjusting process involved a pH of 9.5, a room temperature treatment time of 20 seconds; and the chemical conversion treatment process involved a chemical conversion solution at 35°C for 120 seconds. It should be noted that the degreasing agent FC-E2011, the surface conditioner PL-X, and the chemical conversion solution PB-L3065 manufactured by Parkerizing Co., Ltd. of Japan were used sequentially in the degreasing, surface-adjusting, and chemical conversion treatment processes. The solution was applied at a magnification of 2000 times at a depth of 10000 μm. 2 SEM observations were performed in the above areas to observe the surface chemical transformation structure. Samples with chemical transformation film observed on the entire surface were rated as 0, while samples with no chemical transformation film observed only partially were rated as ×. The results are shown in Table 3.

[0271]

[0272] As shown in Table 3, the steel plate of the present invention has a tensile strength of over 590 MPa, high ductility and excellent elongation flange formability, and excellent chemical conversion treatment properties.

[0273] Furthermore, it is understood that since the steel plate of the present invention has high strength, high ductility, excellent elongation flange formability, and good chemical conversion treatment properties, the components obtained by forming the steel plate of the present invention, the components obtained by joining the steel plate, and the components obtained by forming and joining the steel plate are also high in strength, high in ductility, excellent elongation flange formability, and good chemical conversion treatment properties, just like the steel plate of the present invention.

Claims

1. A steel plate having, by mass percent, the following composition: C: 0.06-0.24%, Si: 0.4% or more and less than 1.60%, Mn: 1.5-3.2%, P: less than 0.02%, S: less than 0.01%, sol.Al: less than 1.0%, N: less than 0.015%, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities. Si / Mn < 0.50… Equation (1) In equation (1), Si and Mn represent the Si content (mass%) and Mn content (mass%), respectively. The steel plate has a microstructure with an area ratio of polygonal ferrite of 20% to 85%, an area ratio of upper bainite of 9% to 45%, a volume ratio of retained austenite of 3% to 15%, an area ratio of fresh martensite of 3% to 15%, a combined area ratio of tempered martensite and lower bainite of 50% or less (including 0%), and an area ratio of the remaining microstructure of 5% or less. The total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of less than 1.2 μm accounts for more than 50% of the total number of fresh martensite particles and retained austenite particles. Furthermore, the total number of fresh martensite particles and retained austenite particles with an aspect ratio of 2.5 or higher and an equivalent circle diameter of 1.2 μm or higher is 40% or higher relative to the total number of fresh martensite particles and retained austenite particles with an equivalent circle diameter of 1.2 μm or higher.

2. The steel plate according to claim 1, wherein, As part of the composition, the material further contains, by mass%, one or more of the following: less than 0.2% Nb, less than 0.2% Ti, less than 0.2% V, less than 0.2% B, less than 0.01% Cu, less than 0.2% Ni, less than 0.2% Cr, less than 0.4% Mo, less than 0.15%.

3. The steel plate according to claim 1 or 2, wherein, As part of the composition, the ingredient further contains, by mass%, one or more of the following: Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.10% or less, Sb: 0.10% or less, and REM: 0.0050% or less.

4. A component made of steel plate according to any one of claims 1 to 3.

5. A method for manufacturing a steel plate, comprising: In the cold rolling process, a steel billet having the composition described in any one of claims 1 to 3 is hot-rolled and pickled, and the resulting hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet; and The annealing process involves annealing the cold-rolled steel sheet to obtain the steel sheet. The cold rolling process includes the following cold rolling treatment: setting the cumulative cold rolling rate to 30-85%, and setting the reduction rate of the first pass to 5% or more and less than 25%, thereby obtaining {111}<0-11> orientation, {111}<11-2> orientation, {211}<0-11> orientation, and {100} <011> The cold-rolled steel sheet in which the total area percentage of the oriented microstructure relative to the total microstructure of the bcc phase is 35% or more and 75% or less. The annealing process includes the annealing treatment as described below: For the cold-rolled steel sheet, the average heating rate within the temperature range of 500°C to Ac1 is set to 0.5–15°C / second, and heating continues until the annealing temperature T below 840°C satisfies 0.5 ≤ (T-Ac1) / (Ac3-Ac1) < 1.

0. After heating, the material is held at the annealing temperature T in a furnace atmosphere with a dew point Td between -50°C and -30°C, thereby obtaining a acicular austenite structure with a number density of 5 acicular atoms / 1000 μm. 2 The above steel plates, Next, a first cooling process is implemented, in which the average cooling rate within the temperature range of 750–550°C is set to 6.0°C / second or higher, and the temperature is cooled to a first cooling stop temperature Tc1 below 550°C but above 400°C. After the first cooling, a first holding period is implemented at the first cooling stop temperature Tc1 for more than 25 seconds. After the first holding period, a second cooling process is implemented, cooling to a second cooling stop temperature Tc2 that is below the first cooling stop temperature Tc1 and below 450°C and above 300°C. A second holding period is implemented, maintaining the temperature at the second cooling stop temperature Tc2 for 20 to 3000 seconds. After the second cooling is completed, a third cooling process is implemented.

6. A method for manufacturing a component, comprising a step of forming a component by performing at least one of forming or joining processes on a steel plate as described in any one of claims 1 to 3.

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