Hot-rolled steel sheet and method of manufacturing the same

Abstract

This invention provides a hot-rolled steel sheet with a yield strength of 500 MPa or higher, excellent ductility and shear properties, and a method for manufacturing the same. The hot-rolled steel sheet comprises C, Si, Mn, P, S, Al, N, B, Ti, and optional components. In its microstructure, the ratio of area fraction with a KAM value of 1.0 or higher and 4.0 or lower to area fraction with a KAM value less than 1.0 is 0.05 or higher, the coefficient of variation of the grain size is 0.55 or higher, and the average grain size of the Ti-containing carbides is 10 nm or lower. It also has a yield strength of 500 MPa or higher and a uniform elongation of 10% or higher. In addition, a method for manufacturing hot-rolled steel sheet includes: a rough rolling process in which steel raw material having the above-mentioned composition is heated or cast and then rough rolled without heating, wherein the rough rolling completion temperature is above 1000°C and below 1100°C; a finishing rolling process in which the rolling start temperature is above 950°C and the total reduction rate of the first and second passes is below 70%; a cooling process; a coiling process; and a coil cooling process after coiling.

Description

Technical Field

[0001] This invention relates to hot-rolled steel sheets with a yield strength of 500 MPa or higher, excellent ductility and shear properties, and a method for manufacturing the same. The hot-rolled steel sheets of this invention are suitable as raw materials for automotive parts applications and a wide range of other applications. Background Technology

[0002] In recent years, from the perspective of protecting the Earth's environment, the entire industry has been moving towards increasing the strength of hot-rolled steel sheets to reduce CO2 emissions. In the process of increasing the strength of hot-rolled steel sheets, formability has become a significant issue. For example, in automotive running gear components, bulging forming requiring ductility and extended flange processing for sheared end faces are common applications. Therefore, it is desirable for hot-rolled steel sheets to possess both ductility and shearability. Typically, as the strength of steel sheets increases, formability and shearability tend to deteriorate. Therefore, to further expand the adoption of high-strength hot-rolled steel sheets, it is essential to improve both formability and shearability. Furthermore, from the perspective of extending the lifespan of components and improving their aesthetic appearance, hot-rolled steel sheets with good plating properties are also desired.

[0003] Therefore, in order to solve these problems, various technologies have been proposed to date to improve the formability of hot-rolled steel sheets.

[0004] For example, in Patent Document 1, the total volume fraction of ferrite and bainite phases with small hardness difference is 95% or more, the volume fraction of ferrite phase is 50-90%, and precipitates containing Ti with a size of less than 20 nm are precipitated in a specified amount. As a result, a hot-rolled steel sheet with excellent elongation flange properties and a tensile strength of 780 MPa or more can be obtained.

[0005] Furthermore, in Patent Document 2, Ti carbides with an average particle size of less than 6 nm and TiS with an average particle size of less than 0.5 μm are dispersed and precipitated in a metallic microstructure composed of ferrite grains at a surface area ratio of 95% or more. This yields a hot-rolled steel sheet with excellent bending workability and a tensile strength of 780 MPa or more and 900 MPa or less.

[0006] In Patent Document 3, a chemical composition is specified, with a Ti / C mass ratio of 0.625 to 3.000 and a dislocation density of 1 × 10⁻⁶. 14 ~1×10 16 m -2 Furthermore, TiC precipitates with an average diameter of less than 2.0 nm precipitate within the grains at a specified density or higher. This results in hot-rolled steel sheets with minimal damage to the punched end faces and a tensile strength exceeding 780 MPa.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2011-068945

[0010] Patent Document 2: International Publication No. 2013 / 099196

[0011] Patent Document 3: Japanese Patent Application Publication No. 2017-179539 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] However, the prior art regarding hot-rolled steel plates disclosed in the aforementioned patent documents has the following problems.

[0014] In the technology proposed in Patent Document 1, due to the use of a bainitic structure obtained through low-temperature coiling, it is impossible to obtain sufficiently uniform elongation. Furthermore, the steels invented in Patent Document 1 are mostly Si-added steels that reduce plating properties, thus there is a problem of not being able to obtain hot-rolled steel sheets with good plating properties. Even with steels containing low Si content, due to inappropriate composition and manufacturing methods, it is impossible to obtain a metallic structure and properties that combine yield strength, excellent ductility, and shear strength.

[0015] In the technology proposed in Patent Document 2, the ferrite area ratio is high, and in such a metal structure, the distribution of crystal grain size is narrow, resulting in poor shear properties.

[0016] In the technology proposed in Patent Document 3, the desired yield strength and uniform elongation cannot be obtained due to the presence of a large number of dislocations.

[0017] In addition, the manufacturing methods described in Patent Documents 1 and 3 require complex control on the discharge roller conveyor before the hot-rolled steel sheet is coiled, which results in poor manufacturing stability.

[0018] The present invention was developed in view of the above-mentioned problems existing in the prior art, and aims to provide a high-strength hot-rolled steel sheet with a yield strength of 500 MPa or more, excellent ductility and shear properties, and a method for manufacturing the same.

[0019] Methods for solving problems

[0020] To address the aforementioned problems, the inventors conducted in-depth research on the requirements for combining the ductility and shear strength of hot-rolled steel sheets. The hot-rolled steel sheets targeted in this invention have a thickness of 1.0 mm or more and 3.6 mm or less. High uniform elongation is effective in achieving good ductility in hot-rolled steel sheets. The characteristic of high uniform elongation in hot-rolled steel sheets is easily obtained in metal structures obtained by winding at temperatures above 600°C, where dislocations are easily recovered.

[0021] Previously, the microstructure obtained by hot rolling at temperatures above 600°C was predominantly ferrite. In a ferrite-dominant microstructure, the grains are narrowly distributed and uniform, and the crystallization strain permeates the entire steel sheet. Consequently, crack propagation becomes unstable during shearing, making it impossible to achieve the desired shear properties. Furthermore, many existing technologies utilize silicon (Si) to harden the otherwise soft ferrite. However, the presence of Si in the steel sheet reduces its plating properties.

[0022] Therefore, the phase transformation of the metal structure of steel sheets under coiling conditions above 600°C was studied in a composition system with minimal Si content. As a result, by setting the coiling temperature in hot rolling to above 600°C and below 700°C, and controlling the roughing, finishing, and cooling conditions, a microstructure with previously unseen grain size and strain distribution was obtained.

[0023] High KAM value grains, by dispersing a large number of grains that stabilize crack propagation during shearing, improve shear strength. Furthermore, compared to bainite and martensite with lath-like structures, high KAM value grains exhibit superior ductility. Therefore, it is possible to provide steel sheets that balance excellent shear strength and ductility. The KAM value is an indicator of the plastic strain gradient in minute regions.

[0024] To broaden the grain size distribution, partial recrystallization of austenite through hot rough rolling is effective. Therefore, it is necessary to control the Ti content and rough rolling temperature to alter the recrystallization behavior of austenite. It is known that to obtain a microstructure with a high KAM value, a large amount of fine TiC needs to precipitate at the austenite-ferrite interface during the austenite-ferrite phase transformation, causing the austenite-ferrite interface to shift and change. Therefore, based on a high Ti content, it is necessary to suppress the TiC content precipitated within the austenite grains during finish rolling. That is, it has been found that by controlling the Ti content and finish rolling temperature, the desired steel sheet microstructure can be obtained.

[0025] It was found that in the microstructure with this grain size and strain distribution, the propagation of stable cracks is promoted when shearing steel sheets. It was determined that hot-rolled steel sheets with this new microstructure possess excellent shear properties with a yield strength of ≥500 MPa and a uniform elongation of ≥10%.

[0026] The hot-rolled steel sheet of the present invention, developed based on the above insights, is configured as follows.

[0027] [1] A hot-rolled steel sheet comprising, by mass%, C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: greater than 0.7% and 2.5% or less, P: less than 0.05%, S: less than 0.010%, Al: 0.005% or more and 0.080% or less, N: less than 0.0080% and Ti: 0.06% or more and 0.15% or less, optionally containing at least one of groups A to D below, and the balance being Fe and unavoidable impurities. Group A: Selected from at least one of the following: V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: Selected from at least one of Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less; Group C: Selected from at least one of the following: Ca: ≥0% and ≤0.01%, Mg: ≥0% and ≤0.01%, REM: ≥0% and ≤1.0%, and Co: ≥0% and ≤0.01%; and Group D: Selected from at least one of the following: Sb: 0% or more and 0.01% or less; Sn: 0% or more and 0.01% or less; As: 0% or more and 0.01% or less; Ta: 0% or more and 0.01% or less; Pb: 0% or more and 0.01% or less; Cs: 0% or more and 0.01% or less; Te: 0% or more and 0.01% or less; Bi: 0% or more and 0.01% or less; Zn: 0% or more and 0.01% or less; Ge: 0% or more and 0.01% or less; and Sr: 0% or more and 0.01% or less. In the hot-rolled steel plate, the ratio of the area ratio of KAM values ​​of 1.0 or higher and 4.0 or lower to the area ratio of KAM values ​​less than 1.0 is 0.05 or higher, the coefficient of variation of the crystal grain size is 0.55 or higher, the average grain size of Ti-containing carbides is 10 nm or lower, the yield strength of the hot-rolled steel plate is 500 MPa or higher, and the uniform elongation is 10% or higher.

[0028] [2] The hot-rolled steel sheet described in [1] above has a coating on its surface.

[0029] The method for manufacturing hot-rolled steel sheet of the present invention, developed based on the above insights, is configured as follows.

[0030] [3] A method for manufacturing a hot-rolled steel plate, comprising: A rough rolling process for producing thin slabs from steel raw materials having the following composition by mass percentage: C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: greater than 0.7% and 2.5% or less, P: less than 0.05%, S: less than 0.010%, Al: 0.005% or more and 0.080% or less, N: less than 0.0080%, and Ti: 0.06% or more and 0.15% or less, optionally containing at least one of the following groups A to D, with the balance being Fe and unavoidable impurities. Group A: Selected from at least one of the following: V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: Selected from at least one of Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less; Group C: Selected from at least one of the following: Ca: ≥0% and ≤0.01%, Mg: ≥0% and ≤0.01%, REM: ≥0% and ≤1.0%, and Co: ≥0% and ≤0.01%; and Group D: Selected from at least one of the following: Sb: 0% or more and 0.01% or less; Sn: 0% or more and 0.01% or less; As: 0% or more and 0.01% or less; Ta: 0% or more and 0.01% or less; Pb: 0% or more and 0.01% or less; Cs: 0% or more and 0.01% or less; Te: 0% or more and 0.01% or less; Bi: 0% or more and 0.01% or less; Zn: 0% or more and 0.01% or less; Ge: 0% or more and 0.01% or less; and Sr: 0% or more and 0.01% or less. The finishing rolling process of producing hot-rolled steel plates by precision rolling the above-mentioned thin slabs; The cooling process used to cool the hot-rolled steel sheet; Next, the hot-rolled steel sheet is coiled to form a coiled material; and The cooling process for cooling the wound material. In the aforementioned rough rolling process, the steel raw material is heated to above 1150°C, or after casting, the steel raw material is maintained at above 1150°C for rough rolling, so that the temperature of the thin slab at the end of the rough rolling is in the range of above 1000°C and below 1100°C. In the aforementioned finishing rolling process, the temperature of the material being rolled at the start of finishing rolling is set to above 950°C, the total reduction rate of the first and second passes is set to below 70%, the temperature of the material being rolled at the finishing roll exit is set to above 850°C, and the rolling speed at the finishing roll exit is set to above 500 m / min. In the aforementioned cooling process, the average cooling rate of the hot-rolled steel sheet up to the cooling stop temperature, which is in the range of 600°C or higher and 700°C or lower, is set to 40°C / s or higher. In the above-mentioned coiling process, the coiling temperature of the hot-rolled steel sheet is set to a range of 600°C or higher and 700°C or lower. In the above-mentioned coil cooling process, the average cooling rate of the coiled material up to 500°C is set to 50°C / hour or more.

[0031] [4] The hot-rolled steel plate manufacturing method according to [3] above includes: a joining process in which the rough-rolled thin slab and the preceding thin slab are joined at 1000°C or above between the rough rolling process and the finishing rolling process, and the joined thin slab is finished rolled in the finishing rolling process.

[0032] [5] The manufacturing method of hot-rolled steel plate according to [3] or [4] above further includes: a hot-rolled plate annealing process of annealing the hot-rolled steel plate at an annealing temperature of 720°C or below to produce a hot-rolled annealed plate; and a plating process of plating the hot-rolled annealed plate.

[0033] [6] The manufacturing method of hot-rolled steel plate according to [5] above further includes an alloying process of performing alloying treatment on the hot-rolled steel plate after plating at a temperature of 460°C or higher and 600°C or lower.

[0034] Invention Effects

[0035] According to the present invention, it is possible to manufacture high-strength hot-rolled steel sheets with a yield strength (YS) of 500 MPa or more and a uniform elongation of 10% or more, exhibiting excellent ductility and good shear properties. Using the hot-rolled steel sheet of the present invention allows for thinner wall thicknesses compared to existing materials, which helps reduce CO2 emissions. Detailed Implementation

[0036] Hereinafter, the hot-rolled steel sheet according to an embodiment of the present invention will be described.

[0037] <Chemical Composition of Hot-Rolled Steel Plate>

[0038] The composition of hot-rolled steel plates, expressed as a percentage by mass, contains: C: ≥0.02% and ≤0.12%, Si: ≤0.15%, Mn: ≥0.7% and ≤2.5%, P: ≤0.05%, S: ≤0.010%, Al: ≥0.005% and ≤0.080%, N: ≤0.0080%, and Ti: ≥0.06% and ≤0.15%. The reasons for these limitations are explained below. Unless otherwise specified, the percentage (%) indicating the content of chemical components refers to "percentage by mass".

[0039] C: Above 0.02% and below 0.12%

[0040] Carbon (C) is an element that contributes to the high strength of steel sheets and delays phase transformation during coiling by combining with titanium (Ti). To obtain the desired steel sheet microstructure with a yield strength of 500 MPa or higher, the C content needs to be 0.02% or higher. On the other hand, when the C content exceeds 0.12%, the amount of cementite formed increases excessively, making it impossible to obtain the desired ductility. Therefore, the C content is set in the range of 0.02% or higher and 0.12% or lower. Preferably, the C content is set in the range of 0.035% or higher and 0.10% or lower.

[0041] Si: less than 0.15%

[0042] Si is a detrimental element that reduces plating performance. Furthermore, Si increases the driving force for the austenite-to-ferrite phase transformation and impairs the delaying effect of the austenite-to-ferrite phase transformation during winding; therefore, it is an element that needs to be minimized in this embodiment. Thus, the Si content is set to less than 0.15%. Preferably, the Si content is set to less than 0.10%. It should be noted that even if the Si content is 0%, the effectiveness of the invention will not be compromised.

[0043] Mn: greater than 0.7% and less than 2.5%

[0044] Mn is an element that helps delay the phase transformation during coiling. To obtain the desired steel sheet microstructure, the Mn content is set to be greater than 0.7%. On the other hand, when the Mn content exceeds 2.5%, the phase transformation from austenite to ferrite does not occur during coiling, and the desired microstructure and properties cannot be obtained. Therefore, the Mn content is set in the range of greater than 0.7% and less than 2.5%, preferably in the range of more than 0.8% and less than 2.0%.

[0045] P: below 0.05%

[0046] Phosphorus (P) is a harmful element that segregates at grain boundaries and reduces shear strength, so it is preferable to minimize it as much as possible. In this embodiment, the P content can be allowed down to 0.05%. The P content is preferably set to 0.04% or less. For use in environments requiring more aesthetically pleasing shear strength, the P content is more preferably suppressed to 0.03% or less. On the other hand, in manufacturing, P content is sometimes inevitably mixed in, with a lower limit of 0.002%.

[0047] S: below 0.010%

[0048] Sulfur (S) forms coarse sulfides in steel, which stretch during hot rolling to become wedge-shaped inclusions, thus adversely affecting shear properties. Therefore, S is also a harmful element and should be minimized as much as possible. In this embodiment, S can be allowed down to 0.010%, so the upper limit of S content is set to 0.010%. The S content is preferably 0.003% or less. For use in environments requiring more stringent toughness, the S content is more preferably suppressed to 0.002% or less. In manufacturing, S content is sometimes unavoidably mixed in, with a lower limit of 0.0001%.

[0049] Al: Above 0.005% and below 0.080%

[0050] When Al is added as a deoxidizer during the steelmaking stage, the Al content is 0.005% or more. Al reduces ductility and shear strength by forming oxides. Therefore, the Al content is set to 0.080% or less. Preferably, the Al content is set in the range of 0.010% or more and 0.070% or less.

[0051] N: below 0.0080%

[0052] Nitrogen (N) is a harmful element that reduces strength and shear strength by combining with Ti to form coarse TiN. Therefore, N is preferably minimized. The N content can be as low as 0.0080%. The N content is more preferably set to 0.0060% or less. In manufacturing, N content is sometimes inevitably mixed in, with a lower limit of 0.0005%.

[0053] Ti: ≥0.06% and ≤0.15%

[0054] Ti combines with C to form fine Ti-containing carbides, thereby contributing to the high strength of the steel sheet. Furthermore, Ti is an element that, during the austenite-to-ferrite phase transformation, precipitates as TiC at the austenite-ferrite interface, delaying the phase transformation by pinning the interface. To obtain the desired yield strength and microstructure, the Ti content is set to 0.06% or more. On the other hand, when the Ti content exceeds 0.15%, excessive hardening occurs, making it impossible to obtain a uniform elongation of 10% or more. Therefore, the Ti content is set in the range of 0.06% or more and 0.15% or less. Preferably, the Ti content is set in the range of 0.07% or more and 0.14% or less.

[0055] In order to effectively delay the phase transformation from austenite to ferrite during coiling, it is preferable to satisfy the following equation (1). Equation (1) is a formula that takes into account the element enrichment in austenite near the interface that hinders the movement of the interface from austenite to ferrite, the driving force of the phase transformation from austenite to ferrite, and the pinning effect brought about by Ti-containing carbides.

[0056] 6.2[%C +0.5[%Mn]+3.2[%Ti ]≥1.0 …(1)

[0057] in, [%Ti ]=[%Ti]-48[%N] / 14, [%C = [%C]-12[%Ti] / 48.

[0058] Here, [%C], [%Mn], [%N] and [%Ti] represent the C content, Mn content, N content and Ti content, respectively, expressed as mass%.

[0059] The above describes the basic composition of the hot-rolled steel sheet according to this embodiment. It may also optionally contain at least one of the components from groups A to D below.

[0060] Group A: Selected from at least one of the following: V: ≥0% and ≤0.2%, Nb: ≥0% and ≤0.07%, Mo: ≥0% and ≤0.15%, Zr: ≥0% and ≤0.1%, Hf: ≥0% and ≤0.1%, and W: ≥0% and ≤0.1%.

[0061] V, Nb, Mo, Zr, Hf, and W are elements that contribute to the strengthening of steel sheets by forming precipitates. Therefore, the content of one or more elements selected from V, Nb, Mo, Zr, Hf, and W is preferably set to 0% or more. On the other hand, when each element is present above the upper limit, TiC cannot be dissolved during the reheating of the billet, and fine TiC cannot be sufficiently obtained during hot rolling, thus failing to obtain the desired metallic structure.

[0062] Group B: Selected from at least one of the following: Cu: 0% or more and 1.0% or less; Ni: 0% or more and 1.0% or less; Cr: 0% or more and 1.0% or less; and B: 0% or more and 0.010% or less.

[0063] Cu, Ni, Cr, and B are the main elements that alter the phase transformation behavior from austenite to ferrite. Therefore, the content of one or more elements selected from Cu, Ni, Cr, and B is preferably set to 0% or more. On the other hand, when each element is present above the upper limit, the interface migration rate between austenite and ferrite decreases during the austenite-ferrite phase transformation, making it impossible to obtain a metallic microstructure with a high KAM value.

[0064] Group C: Selected from at least one of the following: Ca: 0% or more and 0.01% or less; Mg: 0% or more and 0.01% or less; REM: 0% or more and 1.0% or less; and Co: 0% or more and 0.01% or less.

[0065] Ca, Mg, REM, and Co are elements that can alter the morphology of inclusions and are expected to improve shearability. Therefore, the content of one or more elements selected from Ca, Mg, REM, and Co is preferably set to 0% or more. On the other hand, considering that even if they are present in large quantities, the effect may saturate and various properties such as weldability may be degraded, the upper limit of the content of Ca, Mg, REM, and Co is specified as described above. REM is a collective term for 17 elements including Sc, Y, and the lanthanides, and is expressed as the total content of each element.

[0066] Group D: Selected from at least one of the following: Sb: 0% or more and 0.01% or less; Sn: 0% or more and 0.01% or less; As: 0% or more and 0.01% or less; Ta: 0% or more and 0.01% or less; Pb: 0% or more and 0.01% or less; Cs: 0% or more and 0.01% or less; Te: 0% or more and 0.01% or less; Bi: 0% or more and 0.01% or less; Zn: 0% or more and 0.01% or less; Ge: 0% or more and 0.01% or less; and Sr: 0% or more and 0.01% or less.

[0067] Sb, Sn, As, Ta, Pb, Cs, Te, Bi, Zn, Ge, and Sr are elements that are mixed in as impurities. The upper limit of the content of each element is specified as above, so as not to affect the properties of the hot-rolled steel sheet of this embodiment.

[0068] The hot-rolled steel sheet of this embodiment has the above-mentioned elements in its chemical composition, with the balance being Fe and unavoidable impurities.

[0069] <Metallic Structure of Hot-Rolled Steel Sheets>

[0070] Next, the metal structure of hot-rolled steel sheets will be described.

[0071] In the microstructure of the hot-rolled steel sheet of this embodiment, the ratio of (area fraction with KAM value of 1.0 or higher and 4.0 or lower) to (area fraction with KAM value less than 1.0) is 0.05 or higher, the coefficient of variation of the grain size is 0.55 or higher, and the average grain size of the Ti-containing carbides is 10 nm or lower. The amount of dissolved Ti in the steel is preferably 0.03% or lower.

[0072] (Area ratio with a KAM value of 1.0 or higher but below 4.0) / (Area ratio with a KAM value less than 1.0): 0.05 or higher

[0073] The KAM value of the steel sheet obtained by electron backscatter diffraction (EBSD) analysis indicates the strain distribution of the grains. A key feature of this embodiment is that it has a microstructure in which crystals with large KAM values, which were previously unattainable at coiling temperatures above 600°C, coexist with crystals with small KAM values ​​that are highly ductile. To obtain properties that combine ductility and shear strength, the ratio of (area fraction with KAM values ​​of 1.0 or higher and 4.0 or lower) to (area fraction with KAM values ​​less than 1.0) (hereinafter also referred to as the "KAM ratio") is 0.05 or higher. Preferably, the KAM ratio is 0.07 or higher. Although no upper limit is specified, from the viewpoint of ensuring sufficient ductility, the KAM ratio is preferably 0.20 or lower.

[0074] Coefficient of variation of crystal grain size: ≥0.55

[0075] The grain size in the steel plate can be obtained by EBSD analysis. By mixing grains with different grain sizes, the shear properties become good. The desired shear properties are obtained by having a coefficient of variation of the grain size of 0.55 or more, as given by equation (2) below. Preferably, the coefficient of variation of the grain size is 0.65 or more. It should be noted that there is no upper limit, but from the viewpoint of material stability, the coefficient of variation of the grain size is preferably 0.75 or less.

[0076] (Coefficient of variation of particle size) = (Standard deviation of particle size) / (Average particle size) ≥ 0.55 …(2)

[0077] Average particle size of Ti-containing carbides: less than 10 nm

[0078] In this embodiment, the steel sheet is strengthened using Ti-containing carbides. To obtain a high-strength hot-rolled steel sheet with a yield strength of 500 MPa or more, the average particle size of the Ti-containing carbides dispersed in the steel is set to 10 nm or less. To stably obtain a yield strength of 500 MPa or more, the average particle size of the Ti-containing carbides is preferably set to 5 nm or less. While there is no limitation on the lower limit, from the viewpoint of strength stability, the average particle size of the Ti-containing carbides is preferably 1 nm or more.

[0079] The amount of dissolved Ti in the steel is less than 0.03%.

[0080] The strengthening effect of Ti-containing carbides depends not only on the average particle size mentioned above, but also on the amount of precipitation. The amount of precipitation is greatly affected by the winding temperature; when the winding temperature is below 600°C, Ti remains in a solid solution state without precipitation, which may prevent the attainment of the desired yield strength. Therefore, to obtain a yield strength of 500 MPa or higher, the amount of dissolved Ti is preferably limited to 0.03% or less, more preferably 0.02% or less. The amount of dissolved Ti can also be zero.

[0081] The hot-rolled steel sheet of this embodiment preferably has a coating on its surface. Even with the formation of the coating, the function of the hot-rolled steel sheet is not impaired. The composition of the coating is preferably selected from at least one of Zn, Si, Al, Ni, and Mg. It should be noted that the coated steel sheet of this embodiment refers to any one of the following: steel sheet that has undergone hot-dip galvanizing treatment (hereinafter also referred to as GI), steel sheet that has undergone further alloying treatment after hot-dip galvanizing treatment (hereinafter also referred to as GA), and steel sheet that has undergone electro-galvanizing treatment (hereinafter also referred to as EG).

[0082] Next, the manufacturing method of the hot-rolled steel sheet according to the embodiment of the present invention will be described.

[0083] <Manufacturing Method of Hot-Rolled Steel Plate>

[0084] The method for manufacturing hot-rolled steel sheet according to this embodiment includes: a roughing process in which steel raw material having a specific composition as described above is rough-rolled to form a thin slab; a finishing process in which the thin slab is finish-rolled to form a hot-rolled steel sheet; a cooling process in which the hot-rolled steel sheet is cooled; a coiling process in which the hot-rolled steel sheet is then coiled to form a coiled material; and a coil cooling process in which the coiled material is cooled. In the roughing process, the steel raw material is heated to 1150°C or higher, or the steel raw material is maintained at 1150°C or higher after casting, and rough-rolled, so that the temperature of the thin slab at the end of roughing is in the range of 1000°C or higher and 1100°C or lower. In the finishing process, the temperature of the material to be rolled at the beginning of finishing rolling is set to 950°C or higher, the total reduction rate of the first and second passes is set to 70% or lower, the temperature of the material to be rolled at the finishing mill exit is set to 850°C or higher, and the rolling speed at the finishing mill exit is set to 500 m / min or higher. In the cooling process, the average cooling rate of the hot-rolled steel sheet up to a cooling stop temperature ranging from 600°C to 700°C is set to 40°C / s or more. In the coiling process, the coiling temperature of the hot-rolled steel sheet is set to a range of 600°C to 700°C. In the coil cooling process, the average cooling rate of the coiled coil up to 500°C is set to 50°C / hour or more.

[0085] In the typical manufacturing process of hot-rolled steel sheets, after casting the steel billet (raw material), the billet (raw material) is cooled to below 1000°C and placed in a heating furnace. After a short period of heating, it is reduced to a specified thickness on a hot rolling production line and then coiled. Alternatively, after casting the steel billet (raw material), the billet (raw material) is temporarily cooled to room temperature and then heated in a heating furnace for a long period of time before being reduced to a specified thickness on a hot rolling production line and coiled. Another manufacturing method involves directly feeding the cast steel billet (raw material) to the hot rolling production line without heating it in a heating furnace, reducing it to a specified thickness, and then coiling it. The hot-rolled steel sheet manufacturing method of this embodiment can be applied not only to processes where the steel raw material is heated after casting, but also to processes where the steel raw material is directly fed to the hot rolling production line without heating it after casting.

[0086] Steel raw material temperature: Heated to above 1150℃, or maintained above 1150℃ after casting.

[0087] During finishing rolling, the presence of coarse Ti-containing carbides in the rolled material reduces ductility and shear strength. If the temperature of the cast billet drops below 1150°C, Ti-containing carbides precipitate and grow into coarse grains. Therefore, when the billet temperature is below 1150°C, it needs to be heated to above 1150°C to dissolve the Ti-containing carbides. A preferred heating temperature is 1180°C or higher. While no specific upper limit is set, a maximum heating temperature of 1300°C is a manufacturing constraint to avoid thermal damage to the furnace. By maintaining the cast billet temperature above 1150°C, hot rolling can be performed without heating the billet. A preferred billet temperature is maintained above 1180°C. Due to manufacturing constraints during billet casting, a maximum billet temperature of approximately 1300°C is preferred.

[0088] Rough rolling finishing temperature: above 1000℃ and below 1100℃

[0089] To obtain the high grain size distribution microstructure required in this embodiment, partial recrystallization of austenite is necessary during rough rolling. If the temperature of the rolled material (slab) at the completion of rough rolling, i.e., the rough rolling completion temperature, is below 1000°C, the austenite becomes an unrecrystallized microstructure. On the other hand, if the rough rolling completion temperature exceeds 1100°C, it becomes a fully recrystallized microstructure, in which case a high grain size distribution microstructure cannot be obtained. Therefore, the rough rolling completion temperature is set in the range of 1000°C or higher and 1100°C or lower. Preferably, the rough rolling completion temperature is in the range of 1010°C or higher and 1080°C or lower.

[0090] Finishing rolling start temperature: above 950℃; total reduction rate of the first and second passes: below 70%.

[0091] In this embodiment, the high KAM value characteristic microstructure is obtained by suppressing the precipitation of coarse Ti-containing carbides in austenite due to the processing effects of fine rolling, and by allowing fine Ti-containing carbides to precipitate at the austenite-ferrite interface during the austenite-ferrite phase transformation. Coarse Ti-containing carbides precipitated in austenite are easily formed under conditions of low temperature and low rolling speed. In this embodiment, the coarse Ti precipitates in austenite are suppressed, and fine TiC precipitates abundantly at the austenite-ferrite interface. Therefore, grain boundary movement, different from usual, can be expected based on the pinning effect of TiC, resulting in grains with a high KAM value.

[0092] To avoid the adverse effects of finishing rolling, the temperature of the thin slab, which serves as the starting temperature for finishing rolling, is set to 950°C or higher. The total reduction rate of the first and second passes, where the rolling speed is slower than that at the finishing roll exit, is limited to 70% or less. Preferably, the total reduction rate of the first and second passes is in the range of 50% to 67%. It should be noted that the starting temperature for finishing rolling is set lower than the finishing temperature for roughing rolling.

[0093] Temperature at the finish mill exit side: above 850℃; Rolling speed at the finish mill exit side: above 500m / min.

[0094] To avoid the adverse effects of finishing rolling, it is necessary to control not only the total reduction rate of the first and second passes, but also the temperature and rolling speed (pass speed) of the hot-rolled steel sheet at the finish mill exit. The precipitation of Ti-containing carbides during rolling varies depending on the processing temperature, the amount of material processed, and the post-processing holding time. To suppress the precipitation of Ti-containing carbides in austenite through processing, increasing the processing temperature to promote the recovery of processed austenite and shortening the post-processing holding time are effective. Therefore, the temperature and rolling speed at the finish mill exit should be set to 850°C or higher and 500 m / min or higher, respectively. Preferably, the temperature and rolling speed at the finish mill exit should be 900°C or higher and 550 m / min or higher, respectively. The thickness of the finished steel sheet should be in the range of 1.0 mm or higher and 3.6 mm or lower. From the viewpoint of suppressing TiC coarsening, the temperature at the finish mill exit is preferably set to an upper limit of approximately 950°C. If we consider that the thickness of the hot-rolled steel sheet to be used in this embodiment is between 1.0 mm and 3.6 mm, then the rolling speed at the finishing mill exit side is preferably at an upper limit of about 800 m / min.

[0095] Cooling process of hot-rolled steel sheet after finishing: The average cooling rate is above 40℃ / s until the cooling stop temperature is between 600℃ and 700℃.

[0096] If the cooling stop temperature of the finished hot-rolled steel sheet exceeds 700°C or the average cooling rate is less than 40°C / s, the austenite-to-ferrite phase transformation begins at higher temperatures, and the particle size of Ti-containing carbides precipitated at the austenite-ferrite interface increases. Consequently, the interfacial pinning effect cannot be fully achieved, the desired microstructure cannot be obtained, and the strengthening effect gained through Ti-containing carbide precipitation is reduced, failing to achieve a yield strength of 500 MPa. Therefore, the cooling stop temperature and average cooling rate of the finished hot-rolled steel sheet are set below 700°C and above 40°C / s, respectively.

[0097] The average cooling rate depends on forced cooling with a cooling rate higher than that of air cooling based on water cooling, etc., and therefore it is preferable to start forced cooling within 3 seconds after finishing rolling. Therefore, the average cooling rate can be calculated as {(finishing rolling completion temperature) - (cooling stop temperature)} / (cooling time based on forced cooling). If the cooling stop temperature is below 600°C, the amount of dissolved Ti and dislocation density increase, making it impossible to obtain a yield strength of 500 MPa or higher or a uniform elongation of 10% or higher. Therefore, the cooling stop temperature of the hot-rolled steel sheet after finishing rolling is set to 600°C or higher. A preferred cooling stop temperature is in the range of 610°C or higher and 690°C or lower, and a preferred average cooling rate is 50°C / s or higher. There is no upper limit, but considering equipment constraints, an average cooling rate of 200°C / s is preferably set as an upper limit.

[0098] Winding process: Winding temperature is between 600℃ and 700℃.

[0099] When the coiling temperature of hot-rolled steel sheet exceeds 700°C, the phase transformation from austenite to ferrite becomes increasingly high-temperature, resulting in larger grain sizes of Ti-containing carbides precipitated at the austenite-ferrite interface. Consequently, the interfacial pinning effect cannot be sufficiently achieved, leading to undesirable microstructure. Furthermore, the strengthening effect obtained through Ti-containing carbide precipitation is reduced, and the yield strength does not reach 500 MPa. On the other hand, when the coiling temperature is below 600°C, the amount of dissolved Ti and the dislocation density increase, making it impossible to obtain a yield strength of 500 MPa or higher, or a uniform elongation of 10% or higher. Based on the above, the coiling temperature of hot-rolled steel sheet is set in the range of 600°C or higher and 700°C or lower. Preferably, the coiling temperature of hot-rolled steel sheet is set in the range of 610°C or higher and 680°C or lower.

[0100] Coil cooling process: After winding, the average cooling rate of the coil up to 500°C is above 50°C / hour.

[0101] When the cooling rate of the wound roll is slow, the KAM ratio and the coefficient of variation of particle size decrease during the cooling process after winding, making it impossible to obtain the microstructure required in this embodiment. To avoid this adverse effect, the roll is cooled to 500°C at an average cooling rate of 50°C / hour or higher after winding. Preferably, the average cooling rate is 75°C / hour or higher. In the temperature range below 500°C, the microstructure changes are small, and conventional air cooling can be used for cooling. There is no upper limit, but if temperature fluctuations within the wound roll are taken into account, the average cooling rate of the roll up to 500°C after winding is preferably set to 150°C / hour or lower.

[0102] Thin slab joining process

[0103] Between the roughing and finishing rolling processes, the rough-rolled slab is joined with the preceding slab at a temperature of 1000°C or higher. If the joining temperature of the slabs is below 1000°C, it becomes difficult to perform finishing rolling at a starting temperature of 950°C or higher in the subsequent finishing rolling process. Preferably, the joining temperature of the slabs at this stage is 1100°C or higher. From the viewpoint of optimizing the metal microstructure, the joining temperature of the slabs at this stage is preferably at an upper limit of 1200°C.

[0104] Plating process and alloying process

[0105] In the hot-rolled steel sheet manufacturing method of this embodiment, an annealing process in which the hot-rolled steel sheet is annealed in a continuous annealing production line with an annealing temperature of 720°C or lower can be applied, as well as a plating process in which the hot-rolled steel sheet is plating using a continuous plating production line. Furthermore, an alloying process can be included in which the plating-treated hot-rolled steel sheet is heated to a temperature range of 460°C or higher and 600°C or lower to perform alloying treatment. Even if this annealing treatment or this plating treatment is performed, it will not affect the material of the hot-rolled steel sheet of this embodiment. Therefore, a plating treatment can be performed on the surface of the hot-rolled steel sheet to have a coating on the surface of the steel sheet.

[0106] Furthermore, as described above, the composition of the plating treatment and the plating bath does not affect the material of the hot-rolled steel sheet of this embodiment. Therefore, as the plating treatment, any one of hot-dip galvanizing, alloyed hot-dip galvanizing, and electro-galvanizing can be applied. The composition of the plating bath may include at least one selected from Zn, Al, Mg, Si, and Ni. That is, the composition of the coating formed on the surface of the hot-rolled steel sheet during the plating treatment may include at least one selected from Zn, Si, Al, Ni, and Mg.

[0107] Example

[0108] The embodiments of the present invention are further illustrated by way of examples. It should be noted that the present invention is not limited to the manufacturing conditions and product performance shown in the following embodiments. When the embodiments are within the scope of the present invention, the desired performance can be achieved.

[0109] <Manufacturing Method Based on Continuous Casting>

[0110] Steel stock with a thickness of 250 mm and the composition shown in Table 1 is hot-rolled under the roughing and finishing conditions shown in Table 2, followed by leveling rolling with an elongation of 0.1–0.5%, pickling, and then a steel sheet for evaluation is produced. The thickness of the finished steel sheet is in the range of 1.0 mm or more and 3.6 mm or less.

[0111]

[0112] <Manufacturing method for applying a coating to hot-rolled steel sheet>

[0113] Hot-rolled coils manufactured under the roughing and finishing conditions shown in Table 3 are pickled. Then, using a continuous hot-dip galvanizing (CGL) production line under the coating conditions shown in Table 3, the hot-rolled steel sheets are subjected to Zn plating. This produces hot-dip galvanized steel sheets (GI) and alloyed hot-dip galvanized steel sheets (GA).

[0114]

[0115] <Manufacturing Method Based on Hot Continuous Rolling>

[0116] Steels with the composition shown in Table 1 are joined together as slabs under the conditions shown in Table 4. The joined slabs are then hot-rolled, followed by leveling rolling with an elongation of 0.1 to 0.5%. After pickling, steel plates for evaluation are manufactured.

[0117]

[0118] From the perspectives of microstructure, tensile properties, and shear strength, the hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4 were evaluated using the following methods. The results are shown in Table 5.

[0119] (i) Analytical methods for metal structure

[0120] The coefficients of variation of KAM values ​​and crystal grain size were determined by the EBSD method. The field of view area of ​​the analyte was set to 2500 μm. 2 Data were obtained with a step size of 0.5 μm. The obtained image data were analyzed using OIM Analysis software (TSL). The KAM value analysis was performed under the condition of the first nearest neighbor. The KAM (Kernel Average Misorientation) value is the average value of the orientation difference between the measurement point of interest and the adjacent parts, and is an index representing the plastic strain gradient of a small region. The crystal grain size distribution was obtained in the "grain size (diameter)" model. Based on its average grain size and standard deviation, the coefficient of variation of the crystal grain size was calculated by the above equation (2).

[0121] (ii) Average particle size of Ti-containing carbides

[0122] A thin film for observation was cut from the surface of the self-heating rolled steel sheet at a position equivalent to 25% of the sheet thickness. More than 300 Ti-containing carbides were photographed using a transmission electron microscope at a magnification of over 600,000x. The equivalent circle diameter of each Ti-containing carbide was calculated, and its average value was taken as the average particle size. The presence of Ti-containing carbides can be determined simply by using EDX analysis accompanying the TEM to confirm the presence of peaks originating from Ti.

[0123] (iii) Analysis of the precipitation of Ti-containing carbides

[0124] The test piece was ground to 25% of its thickness on both sides, and then dissolved in a 10% acetylacetone-1% tetramethylammonium chloride-methanol (10% AA) electrolyte solution. The solution was filtered through a 0.2 μm mesh filter, and the Ti concentration in the filtered electrolyte solution was analyzed using ICP-MS. The amount of Ti in the electrolyte solution was considered the dissolved Ti amount, and the dissolved Ti amount, expressed as a percentage by mass, was obtained from the ratio of the dissolved matrix amount to the dissolved Ti amount.

[0125] (iv) Tensile test

[0126] Hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4 were used to prepare JIS No. 5 tensile test specimens in a direction perpendicular to the rolling direction. Five tensile tests were performed according to JIS Z 2241 (2011) to determine the average yield strength (YS) and tensile strength (TS). The crosshead speed for the tensile tests was set to 10 mm / min. In Table 5, hot-rolled steel sheets with a yield strength of 500 MPa or higher and a uniform elongation of 10% or higher were evaluated as having excellent mechanical properties.

[0127] (v) Evaluation of punching (shearing) properties

[0128] For hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4, three punching processes with a diameter of 10 mm were performed, each with a spacing of 5% to a gap of 5% or more but less than 30%. Then, the length of abnormal portions such as roughness and cracks generated on the punched end face was investigated. The shearability evaluation in Table 5 was performed as follows: If the sum of the lengths of abnormal portions in the three punching processes is less than 3% of the sum of the lengths of the punched end face (=10π×3mm), it is marked as "Good" in the shearability evaluation column. On the other hand, if the sum of the lengths of abnormal portions exceeds 3% of the sum of the lengths of the punched end face, it is marked as "No" in the shearability evaluation column.

[0129] The inventive examples all exhibited good ductility and shear properties with a yield strength (YS) of 500 MPa or more and a uniform elongation of 10% or more. On the other hand, comparative examples outside the scope of this invention failed to obtain tensile properties or had poor shear property evaluations.

[0130]

Claims

1. A hot-rolled steel sheet comprising, by mass%, C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: greater than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0080% or less, and Ti: 0.06% or more and 0.15% or less, optionally containing at least one of groups A to D below, and the balance being Fe and unavoidable impurities. Group A: Selected from at least one of the following: V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: Selected from at least one of Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less; Group C: Selected from at least one of the following: Ca: ≥0% and ≤0.01%, Mg: ≥0% and ≤0.01%, REM: ≥0% and ≤1.0%, and Co: ≥0% and ≤0.01%; and Group D: Selected from at least one of the following: Sb: 0% or more and 0.01% or less; Sn: 0% or more and 0.01% or less; As: 0% or more and 0.01% or less; Ta: 0% or more and 0.01% or less; Pb: 0% or more and 0.01% or less; Cs: 0% or more and 0.01% or less; Te: 0% or more and 0.01% or less; Bi: 0% or more and 0.01% or less; Zn: 0% or more and 0.01% or less; Ge: 0% or more and 0.01% or less; and Sr: 0% or more and 0.01% or less. In the hot-rolled steel plate In metallic microstructures, the ratio of the area fraction with a KAM value of 1.0 or higher but less than 4.0 to the area fraction with a KAM value less than 1.0 is 0.05 or higher. The coefficient of variation for crystal grain size is above 0.

55. The average particle size of Ti-containing carbides is less than 10 nm. The hot-rolled steel plate has a yield strength of 500 MPa or more and a uniform elongation of 10% or more.

2. The hot-rolled steel sheet according to claim 1, wherein it has a coating on its surface.

3. A method for manufacturing a hot-rolled steel plate, comprising: A rough rolling process for producing thin slabs from steel raw materials having the following composition by mass percentage: C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: greater than 0.7% and 2.5% or less, P: less than 0.05%, S: less than 0.010%, Al: 0.005% or more and 0.080% or less, N: less than 0.0080%, and Ti: 0.06% or more and 0.15% or less, optionally containing at least one of the following groups A to D, with the balance being Fe and unavoidable impurities. Group A: Selected from at least one of the following: V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: Selected from at least one of Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less; Group C: Selected from at least one of the following: Ca: ≥0% and ≤0.01%, Mg: ≥0% and ≤0.01%, REM: ≥0% and ≤1.0%, and Co: ≥0% and ≤0.01%; and Group D: Selected from at least one of the following: Sb: 0% or more and 0.01% or less; Sn: 0% or more and 0.01% or less; As: 0% or more and 0.01% or less; Ta: 0% or more and 0.01% or less; Pb: 0% or more and 0.01% or less; Cs: 0% or more and 0.01% or less; Te: 0% or more and 0.01% or less; Bi: 0% or more and 0.01% or less; Zn: 0% or more and 0.01% or less; Ge: 0% or more and 0.01% or less; and Sr: 0% or more and 0.01% or less. The finishing rolling process of producing hot-rolled steel plates by precision rolling the thin slab; The cooling process used to cool the hot-rolled steel sheet; Next, the hot-rolled steel sheet is coiled to form a coiled material; and The cooling process for cooling the wound roll. In the rough rolling process, the steel raw material is heated to above 1150°C, or after casting, the steel raw material is maintained at above 1150°C for rough rolling, so that the temperature of the thin slab at the end of the rough rolling is in the range of above 1000°C and below 1100°C. In the finishing rolling process, the temperature of the material being rolled at the start of finishing rolling is set to above 950°C, the total reduction rate of the first and second passes is set to below 70%, the temperature of the material being rolled at the finishing roll exit is set to above 850°C, and the rolling speed at the finishing roll exit is set to above 500 m / min. In the cooling process, the average cooling rate of the hot-rolled steel sheet up to the cooling stop temperature, which is in the range of 600°C to 700°C, is set to 40°C / s or higher. In the coiling process, the coiling temperature of the hot-rolled steel sheet is set to a range of 600°C or higher and 700°C or lower. In the coil cooling process, the average cooling rate of the coiled material up to 500°C is set to 50°C / hour or higher.

4. The method for manufacturing hot-rolled steel plate according to claim 3, wherein, include: A joining process between the roughing rolling process and the finishing rolling process, in which the rough-rolled slab is joined to the preceding slab at a temperature above 1000°C. In the finishing rolling process, the joined thin slab is finished rolled.

5. The method for manufacturing hot-rolled steel sheet according to claim 3 or 4, wherein, Also includes: The hot-rolled steel sheet is annealed at an annealing temperature below 720°C to produce a hot-rolled annealed sheet; as well as The plating process for applying a plating treatment to the hot-rolled annealed sheet.

6. The method for manufacturing hot-rolled steel plate according to claim 5, wherein, It also includes an alloying process in which the hot-rolled steel sheet after plating is subjected to alloying treatment at a temperature between 460°C and 600°C.

Images