WO2024106000A1 - Tôle d'acier laminée à chaud et son procédé de production - Google Patents

Tôle d'acier laminée à chaud et son procédé de production Download PDF

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WO2024106000A1
WO2024106000A1 PCT/JP2023/033821 JP2023033821W WO2024106000A1 WO 2024106000 A1 WO2024106000 A1 WO 2024106000A1 JP 2023033821 W JP2023033821 W JP 2023033821W WO 2024106000 A1 WO2024106000 A1 WO 2024106000A1
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hot
less
steel sheet
rolled steel
rolling
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PCT/JP2023/033821
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English (en)
Japanese (ja)
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典晃 ▲高▼坂
広志 松田
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Ｊｆｅスチール株式会社
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Priority to JP2024504864A priority Critical patent/JP7541639B1/ja
Publication of WO2024106000A1 publication Critical patent/WO2024106000A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent workability, and a manufacturing method thereof.
  • the hot-rolled steel sheet of the present invention is suitable as a material for automotive components.
  • Patent Document 1 discloses a hot-rolled steel sheet in which the main phase of the matrix is a ferrite phase with an area ratio of more than 95%, and in which Ti carbides with an average particle size of less than 10 nm are finely precipitated within the ferrite crystal grains. This is said to result in a high-strength hot-rolled steel sheet with excellent workability and a tensile strength of 780 MPa or more.
  • Patent Document 2 discloses a method for manufacturing a hot-rolled steel sheet, which includes hot rolling consisting of rough rolling at a rolling start temperature of 1200°C or higher and finish rolling at a rolling end temperature of 900°C or higher, and coiling at 580°C or higher.
  • the hot-rolled steel sheet has a ferrite phase with an area ratio of 95% or higher and fine carbides containing TiN with an average particle size of 20 nm or more and Ti with an average particle size of less than 6 nm dispersed in the metal structure.
  • a high-tensile hot-rolled steel sheet with a tensile strength of 590 MPa to 750 MPa and excellent punchability and stretch flangeability can be obtained.
  • Patent Document 3 discloses a hot-rolled steel sheet in which 1.0% or more of Mn is added for the purpose of improving hardenability, upper bainite having an area ratio of 75.0% or more and less than 97.0% as the main phase, and the number density of second phase particles of 0.5 ⁇ m or more is 150,000 particles/ mm2 or less. As a result, it is said that a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more can be obtained.
  • the main phase is an upper bainite structure (including structures that do not have Fe-based carbides or retained austenite) that has Fe-based carbides and/or retained austenite between bainitic ferrite with a lath-like morphology.
  • Such structure steels that actively utilize bainite, martensite, and retained austenite require large amounts of alloying elements, resulting in high material costs.
  • the present invention was developed in consideration of the above-mentioned problems with conventional technology, and aims to provide a hot-rolled steel sheet with a tensile strength (TS) of 780 MPa or more and excellent workability, as well as a manufacturing method thereof.
  • TS tensile strength
  • the thickness of the hot-rolled steel sheet to be studied in this case is 1.0 mm or more and 35.0 mm or less.
  • stretch formability and bending workability deteriorate, so it is necessary to achieve both high strength and workability.
  • workability can be improved by suppressing the occurrence of local necking. Specifically, it was found that if the stress increase rate at 80% strain of uniform elongation is 1200 MPa or more, it can withstand severe forming conditions.
  • the hot-rolled steel sheet according to the present invention which has been developed based on the above findings, has the following configuration.
  • C 0.030% to 0.080%, Si: 1.5% or less, Mn: 1.6% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% to 0.080% or less, N: 0.0060% or less, Ti: 0.12% to 0.28% or less, V: 0% or more and 0.01% or less, Mo: 0% or more and 0.01% or less, Nb: 0% or more and 0.01% or less, Hf: 0% or more and 0.01% or less, W: 0% or more and 0.01% or less, Zr: 0% or more and 0.01% or less, and optionally, further containing one or both of the following components from group A and group B, Group A; Group B: 0.0002% or more and 0.0050% or less; Group B: Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr
  • the manufacturing method of the hot-rolled steel sheet according to the present invention which was developed based on the above findings, is configured as follows: [3] A rough rolling process in which a steel material having the component composition described in the above [1] is heated to a heating temperature of 1200 ° C.
  • a finish rolling process in which the sheet bar is finish-rolled to form a hot-rolled steel sheet with a rolling start temperature exceeding 1000 ° C., a rolling reduction rate of 35% or more in the first pass and the second pass, and a total rolling reduction rate of 85% or less from the third pass to the end of rolling; a cooling process in which the hot-rolled steel sheet is cooled at an average cooling rate of 40 ° C./s or more to a cooling stop temperature of 600 ° C. or more and 700 ° C. or less; and a coiling process in which the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600 ° C. or more and 700 ° C.
  • a method for producing a hot-rolled steel sheet comprising: a rough rolling step in which a steel material having the composition described in [1] above is heated to a heating temperature of 1200°C or higher and roughly rolled to form a sheet bar; a joining step in which the roughly rolled sheet bar and a preceding sheet bar are joined at 1010°C or higher; a finish rolling step in which the joined sheet bar is finish-rolled to form a hot-rolled steel sheet, with the rolling start temperature exceeding 1000°C, the rolling reductions in the first and second passes being 35% or more, and the total rolling reduction from the third pass to the end of rolling being 85% or less; a cooling step in which the hot-rolled steel sheet is cooled at an average cooling rate of 40°C/s or more to a cooling stop temperature of 600°C or more and 700°C or less; and a coiling step in which the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C or more and 700°C or less.
  • the method for producing a hot-rolled steel sheet further includes a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or less, and a plating step of plating the annealed hot-rolled steel sheet.
  • a hot-rolled steel sheet with high strength, with a tensile strength (TS) of 780 MPa or more, and excellent workability. If the hot-rolled steel sheet according to the present invention is applied to automobile parts, further weight reduction of the automobile parts can be achieved.
  • TS tensile strength
  • FIG. 3 is a photograph showing an example of a metal structure and precipitates in a hot-rolled steel sheet according to the present embodiment.
  • the composition of the hot-rolled steel sheet is, in mass%, C: 0.030% to 0.080%, Si: 1.5% or less, Mn: 1.6% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% to 0.080% or less, N: 0.0060% or less, Ti: 0.12% to 0.28% or less, V: 0% to 0.01% or less, Mo: 0% to 0.01% or less, Nb: 0% to 0.01% or less, Hf: 0% to 0.01% or less, W: 0% to 0.01% or less, and Zr: 0% to 0.01% or less.
  • % representing the content of a component means “mass%”.
  • C 0.030% or more and 0.080% or less C contributes to increasing the strength of the steel plate by combining with Ti.
  • the C content is set to 0.030% or more.
  • C accumulates at the interface between austenite and ferrite during the transformation from austenite to ferrite, and the interface migration speed decreases. Due to this decrease in the interface migration speed, curved carbide rows cannot be obtained in the metal structure. Therefore, the C content is set to 0.080% or less. It is preferably 0.035% or more and 0.070% or less.
  • Si 1.5% or less Si increases the driving force for the transformation from austenite to ferrite, increases the austenite-ferrite interface speed, and makes it easier to obtain curved carbide rows.
  • the Si content is preferably 0.18% or more.
  • the Si content exceeds 1.5%, the driving force for the transformation from austenite to ferrite increases excessively, and the transformation from austenite to ferrite begins at high temperatures during the cooling process after hot rolling, causing the carbides to coarsen, making it impossible to obtain a particle dispersion strengthening amount of 290 MPa or more. Therefore, the Si content is set to 1.5% or less.
  • the Si content is 0.26% or more and 1.1% or less.
  • Mn 1.6% or less Mn reduces the driving force for the transformation from austenite to ferrite and reduces the interface movement speed between austenite and ferrite. Therefore, the Mn content is set to 1.6% or less. Preferably, the Mn content is 1.5% or less. In manufacturing, 0.05% is inevitably mixed in, but even if it is 0.00%, the effect of the present invention is not impaired. In order to control the driving force for the transformation from austenite to ferrite in order to obtain a curved carbide row, it is preferable to satisfy the following formula (1).
  • P 0.05% or less
  • P is a harmful element that segregates at grain boundaries and reduces workability, so it is preferable to reduce it as much as possible.
  • the P content can be tolerated up to 0.05%.
  • the P content is 0.04% or less, but for use under more severe processing conditions, it is more preferable to make it 0.02% or less.
  • 0.002% P may be inevitably mixed in during production.
  • S 0.010% or less S forms coarse sulfides in steel, which expand during hot rolling to become wedge-shaped inclusions, adversely affecting elongation workability. Therefore, since S is also a harmful element, it is preferable to reduce it, and up to 0.010% is acceptable. Preferably, the S content is 0.003% or less, but for use under more severe processing conditions, it is more preferable to make it 0.001% or less. In manufacturing, 0.0001% S may be inevitably mixed in.
  • Al 0.005% to 0.080%
  • the Al content is 0.005% or more. Al forms oxides, which reduces workability. Therefore, the Al content is set to 0.080% or less.
  • the Al content is 0.010% to 0.070%.
  • N 0.0060% or less N is a harmful element that combines with Ti to form coarse TiN, thereby reducing strength and workability. Therefore, it is preferable to reduce the N content as much as possible, and up to 0.0060% is acceptable. Preferably, the N content is 0.0050% or less. In manufacturing, 0.0005% N may be inevitably mixed in.
  • Ti 0.12% or more and 0.28% or less Ti combines with C and contributes to increasing the strength of the steel sheet. In order to obtain a tensile strength of 780 MPa or more, the Ti content is 0.12% or more. On the other hand, if the Ti content exceeds 0.28%, it becomes impossible to dissolve the coarse Ti-containing carbides in the heating process before hot rolling, and not only does the effect of increasing the strength become saturated, but it also has an adverse effect on the workability. Therefore, the Ti content is 0.12% or more and 0.28% or less. Preferably, the Ti content is 0.13% or more and 0.25% or less.
  • V 0% to 0.01%
  • Mo 0% to 0.01%
  • Nb 0% to 0.01%
  • Hf 0% to 0.01%
  • W 0% to 0.01%
  • Zr 0% to 0.01%
  • V, Mo, Nb, Hf, W, and Zr are elements that combine with C to form carbides, but by adding them in combination with Ti, the utilization of C becomes unstable when transforming from austenite to ferrite, increasing the risk that the desired curved carbide row cannot be obtained. For this reason, V: 0% to 0.01%, Mo: 0% to 0.01%, Nb: 0% to 0.01%, Hf: 0% to 0.01%, W: 0% to 0.01%, Zr: 0% to 0.01%.
  • Group A B: 0.0002% or more and 0.0050% or less;
  • Group B Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, Ta, Pb, Cs, Te, Bi, and Se, any one or more of which is 1% or less in total.
  • B 0.0002% or more and 0.0050% or less B is an element effective for improving hardenability, and has the effect of suppressing the transformation from austenite to ferrite during the cooling process after hot rolling and stably promoting the formation of curved carbide rows. Therefore, by setting the B content to 0.0002% or more, it contributes to stably obtaining a desired structure. On the other hand, if the B content exceeds 0.0050%, the effect on the hardenability of the steel is saturated, so it is set to 0.0050% or less.
  • the B content is set to 0.0002% or more and 0.0050% or less. More preferably, the B content is set to 0.0004% or more and 0.0030% or less.
  • any one or more of Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, Ta, Pb, Cs, Te, Bi, and Se is contained in a total amount of 1% or less. Any one or more of these elements is contained in a total amount of 1% or less, and thus is permissible since it has little effect on the properties of the hot-rolled steel sheet according to the present embodiment. On the other hand, the content of each element is preferably limited to 0.03% or less.
  • the chemical composition of the hot-rolled steel sheet according to this embodiment contains the above elements, with the remainder being Fe and unavoidable impurities.
  • the metal structure and mechanical properties of the hot-rolled steel sheet of this embodiment are such that the area ratio of ferrite is 95% or more, the curvature of the curved Ti-containing carbide row is 1.8 x 10-3 nm -1 or more, the amount of particle dispersion strengthening is 290 MPa or more, the stress increase rate at 80% strain of uniform elongation is 1200 MPa or more, and the tensile strength is 780 MPa or more.
  • the "%" representing the metal structure means the "area ratio”.
  • Ferrite area ratio is 95% or more Ferrite is a steel structure with better workability than bainite, martensite, and tempered martensite. If structures other than ferrite, such as bainite, martensite, tempered martensite, and retained austenite, exceed 5%, particle dispersion strengthening of 290 MPa or more and stress increase rate of 1200 MPa or more cannot be obtained. From this viewpoint, ferrite is set to 95% or more. Preferably, ferrite is 97% or more.
  • the curvature of the curved Ti-containing carbide row is 1.8 ⁇ 10-3 nm -1 or more
  • the presence of the curved carbide row is one of the major features of the hot-rolled steel sheet according to this embodiment, and the curved carbides provide a high stress increase rate.
  • the curved Ti-containing carbides can be observed in ferrite grains when observed with a transmission electron microscope (TEM) at (001) ⁇ incidence. Depending on the incidence direction of the electron beam, they may not necessarily be observed as carbides, so the curved carbide row is confirmed in the (001) ⁇ incidence ferrite grains.
  • TEM transmission electron microscope
  • Figure 1 is a bright field image taken with (001) alpha incidence. The granular black contrast is the curved carbide row.
  • To measure the carbide curvature 10 or more rows of carbide rows are photographed, and the curvature is measured at the position with the greatest curvature for the top 30% of the carbide rows with the greatest curvature. The average of these measurements is defined as the curvature of the carbide row as defined in this invention.
  • the amount of particle dispersion strengthening depends on the particle size of the carbides, but as can be seen from Figure 1, the carbide particles are very fine, making it very difficult to accurately determine the particle size of the carbides. Therefore, the technical features of the hot-rolled steel sheet according to this embodiment are defined not by the particle size of the carbides, but by the amount of particle dispersion strengthening obtained by the Ti-containing carbides.
  • Particle dispersion strengthening amount is 290 MPa or more In order to obtain a tensile strength of 780 MPa or more, the particle dispersion strengthening amount must be at least 290 MPa or more. In order to stably achieve a tensile strength of 780 MPa, the particle dispersion strengthening amount is preferably 300 MPa or more.
  • d is the ferrite grain size (unit: mm)
  • Stress increase rate at 80% strain of uniform elongation is 1200 MPa or more Under severe forming conditions, the state of stress dispersion due to work hardening of the stress concentration area is an important factor in whether or not the steel can be formed.
  • the hot-rolled steel sheet of this embodiment has high work hardening due to the dispersion of curved carbide rows. Since curved carbide rows are finer and more abundantly dispersed than linear carbide rows, it is presumed that the cause of work hardening is the large amount of dislocations caused by the incompatibility between the matrix and carbides that occurs around the carbides during deformation of the matrix. Therefore, the stress increase rate at 80% strain of uniform elongation is set to 1200 MPa or more.
  • This stress increase rate is calculated as the differential value of true stress of true strain at 80% of uniform elongation in relation to true strain and true stress. There are cases where the differential value is unstable due to the responsiveness of the testing machine, etc. For this reason, in this embodiment, the stress increase rate is calculated as the average value of 20 adjacent points centered on the true strain at 80% of uniform elongation. Note that in order to prevent defects such as necking during forming, it is desirable to have a high uniform elongation, and the product of TS and U-El obtained with the hot-rolled steel sheet of this embodiment is 7800 MPa ⁇ %. This U-El was calculated using the nominal strain.
  • the hot-rolled steel sheet according to the present embodiment preferably has a plating layer on the surface. Even if the plating layer is formed, the function of the hot-rolled steel sheet is not impaired.
  • the composition of the plating layer is preferably one or more selected from Zn, Si, Al, Ni, and Mg.
  • the plated steel sheet includes any of those that have been subjected to a hot-dip galvanizing treatment (GI), those that have been subjected to an alloying treatment after hot-dip galvanizing treatment (GA), and those that have been subjected to an electrolytic galvanizing treatment (EG).
  • GI hot-dip galvanizing treatment
  • GA alloying treatment after hot-dip galvanizing treatment
  • EG electrolytic galvanizing treatment
  • hot-rolled steel sheets are manufactured by loading a slab (steel material) that has been cooled to 1000°C or less after casting into a heating furnace, heating it for a short time, and then reducing it to a predetermined thickness in a hot rolling line and winding it into a coil.
  • a slab (steel material) that has been cooled to room temperature after casting is heated for a long time in a heating furnace, and then reducing it to a predetermined thickness in a hot rolling line and winding it into a coil.
  • a manufacturing method in which a cast slab (steel material) is directly sent to a hot rolling line without being heated in a heating furnace, and then reduced to a predetermined thickness and wound into a coil.
  • the manufacturing method of the hot-rolled steel sheet according to this embodiment can be applied not only to a process in which the steel material is heated after casting, but also to a process in which the steel material is directly sent to a hot rolling line without being heated after casting.
  • the smelting method for producing the steel material of this embodiment is not particularly limited, and known smelting methods such as converters and electric furnaces can be adopted. Secondary refining may also be performed in a vacuum degassing furnace.
  • the molten steel thus adjusted to the above-mentioned composition is then preferably made into a slab (steel material) by a continuous casting method, taking into consideration productivity and quality.
  • the slab may be made into a slab by an ingot casting-blooming rolling method or other known casting methods.
  • the steel material is heated to a heating temperature of 1200° C. or higher, or is not heated after casting, and is roughly rolled to form a sheet bar.
  • the steel sheet is subjected to hot rolling in which the starting temperature of the finish rolling is 1000°C or higher, the rolling reductions of the first and second passes are each 35% or higher, and the total rolling reduction from the third pass to the end of rolling is 85% or less, to produce a hot-rolled steel sheet.
  • the hot-rolled steel sheet is cooled to a cooling stop temperature of 600° C. or higher and 700° C. or lower at an average cooling rate of 40° C./s or higher.
  • the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C or higher and 700°C or lower.
  • Heating of steel material heating to 1200°C or higher, or not heating.
  • Coarse carbides containing Ti precipitated in the slab (steel material) are dissolved in a heating process before hot rolling, so that fine carbides containing Ti precipitate after hot rolling. Therefore, in order to obtain a particle dispersion strengthening amount of 290 MPa or more, the heating temperature is 1200°C or higher.
  • the heating temperature is 1220°C or higher, and when the Ti content is 0.13% or more, it is more preferable to heat the slab (steel material) to 1240°C or higher.
  • Finish rolling start temperature 1000°C or higher
  • the hot-rolled steel sheet according to the present embodiment has a steel composition with an increased driving force for the transformation from austenite to ferrite
  • the transformation from austenite to ferrite starts in the high temperature region during the cooling process after hot rolling under normal hot rolling conditions, and curved carbide rows are not obtained.
  • carbides precipitate in the high temperature region during the cooling process after hot rolling the carbides become coarse, and not only is it not possible to obtain the desired amount of particle dispersion strengthening, but also because linear carbides are generated, the desired stress increase rate is not obtained. Therefore, in order to increase the hot rolling temperature and avoid rolling in the unrecrystallized region of austenite as much as possible, the finish rolling start temperature is 1000° C. or higher.
  • the finish rolling start temperature is 1010° C. or higher.
  • the slab heating temperature is the substantial upper limit temperature unless a heating device is provided in the hot rolling line, and it is often 1200° C. or lower.
  • the reduction ratios of the first and second passes are each 35% or more.
  • the reduction ratio is increased in the high temperature range where austenite recrystallizes, thereby reducing the degree of austenite at the end of the finish rolling.
  • the reduction ratios of the first and second passes are each 35% or more.
  • the rolling reduction in the first pass and the second pass is 38% or more.
  • the rolling reductions in the first and second passes can be calculated by the following formulas (4) and (5), respectively.
  • t 0 , t 1 , and t 2 are the sheet thickness before finish rolling, the sheet thickness after one pass, and the sheet thickness after two passes, respectively.
  • Total reduction ratio from the third pass to the completion of rolling 85% or less It is necessary to control the degree of austenite processing and reduce the density of ferrite nucleation sites until the completion of finish rolling. For this reason, the total reduction ratio from the third pass to the completion of rolling is set to 85% or less. Preferably, the total reduction ratio from the third pass to the completion of rolling is set to 80% or less.
  • the total reduction ratio from the third pass to the completion of rolling can be calculated by the following formula (6).
  • Total reduction rate from the third pass to the end of rolling (t 2 ⁇ t f )/t 2 (6)
  • tf is the plate thickness after the completion of finish rolling.
  • Cooling stop temperature after finish rolling is 600°C to 700°C at an average cooling rate of 40°C/s or more. After finish rolling, the hot-rolled steel sheet needs to be quenched in order to suppress the formation of ferrite. If the cooling stop temperature exceeds 700°C, ferrite will form during the cooling process, so the hot-rolled steel sheet is cooled from the finish rolling temperature to 700°C at an average cooling rate of 40°C/s or more. On the other hand, if the cooling stop temperature is less than 600° C., the amount of precipitation of Ti-containing carbides becomes insufficient, and the amount of particle dispersion strengthening of 290 MPa or more cannot be obtained. Therefore, the cooling stop temperature is set to a range of 600° C. to 700° C.
  • the cooling stop temperature is set to a range of 610° C. to 690° C.
  • the average cooling rate may be calculated by the following formula: ⁇ (cooling start temperature)-(cooling completion temperature) ⁇ /(forced cooling time other than natural cooling) after hot rolling.
  • An example of the forced cooling method is water cooling.
  • Coiling temperature 600° C. or higher and 700° C. or lower
  • the coiling temperature is set to 600° C. or higher and 700° C. or lower.
  • the coiling temperature is preferably 610° C. or higher and 690° C. or lower. If coiling is performed in this temperature range, the generation of bainite, martensite, and retained austenite can be suppressed as much as possible.
  • the hot rolled steel sheet according to the present embodiment can also be produced by a thin slab continuous casting method.
  • a steel material having a thickness of 35 mm to 200 mm is cast.
  • ⁇ Second type rough rolling step> The cast steel material is heated to a heating temperature of 1200° C. or higher, or is not heated after casting, and is roughly rolled as necessary to form a sheet bar.
  • the process after the finish rolling step is the same as that of the first embodiment.
  • Slab (steel material) thickness 35 mm to 200 mm
  • the thin slab before hot rolling is thin in the thin slab continuous casting method, so the degree of austenite processing in hot rolling is low. If the slab thickness is less than 35 mm, the desired total reduction rate from the first pass to the fifth pass cannot be obtained. On the other hand, if the slab thickness exceeds 200 mm, the casting speed becomes slow, and the productivity advantage of the thin slab continuous casting method is lost compared to the continuous casting method. From the above viewpoints, the slab thickness in the thin slab continuous casting method is set to 35 mm to 200 mm.
  • a third embodiment of the method for producing a hot-rolled steel sheet according to the present embodiment will be described.
  • the difference from the first and second embodiments will be described.
  • a continuous hot rolling technique can be applied.
  • ⁇ Joining process of the third embodiment The sheet bar obtained in the first or second embodiment is joined to the preceding sheet bar at 1010° C. or higher before finish rolling. If the temperature is lower than 1010° C., it becomes difficult to roll at the finish rolling completion temperature of 880° C. or higher.
  • the preferred heating temperature of the sheet bar during joining is 1070° C. or higher.
  • the process after the finish rolling step is the same as in the first embodiment.
  • the manufacturing method for the hot-rolled steel sheet according to this embodiment can apply an annealing process in which the sheet is annealed in a continuous annealing line at an annealing temperature of 720°C or less, and a plating process in which the sheet is plated in a continuous plating line.
  • the method may have an alloying process in which the plated hot-rolled steel sheet is heated to 400°C or more and 500°C or less and alloyed. This annealing process or this plating process does not affect the material properties of the hot-rolled steel sheet according to this embodiment. Therefore, it is possible to further plate the surface of the hot-rolled steel sheet to provide a plating layer on the surface of the steel sheet.
  • the plating process and the composition of the plating bath do not affect the material of the hot-rolled steel sheet according to this embodiment, and therefore any of hot-dip galvanizing, alloyed hot-dip galvanizing, and electrolytic galvanizing processes can be applied as the plating process.
  • the composition of the plating bath can include one or more of Zn, Al, Mg, Si, and Ni.
  • the composition of the plating layer formed on the surface of the hot-rolled steel sheet in the plating process can include one or more of Zn, Si, Al, Ni, and Mg.
  • the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 were evaluated in terms of metal structure, tensile properties, bending workability, and toughness using the following methods. The results are shown in Tables 6-1 to 6-2.
  • test pieces with R/t of 0.5 or less in the second bending test were marked with " ⁇ " as the bending characteristics required in the present invention, and test pieces with cracks on the surface of the test pieces at least once under the condition of R/t of 0.5 or less were marked with "X” as not having the bending characteristics required in the present invention.
  • All of the examples of the present invention had a tensile strength (TS) of 780 MPa or more, and a high degree of particle dispersion strengthening and stress increase rate were obtained.
  • the comparative examples outside the range of the present invention either did not reach a tensile strength of 780 MPa or did not achieve the degree of particle dispersion strengthening or stress increase rate required by the present invention.

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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

L'invention concerne : une tôle d'acier laminée à chaud ayant une résistance à la traction (TS) de 780 MPa ou plus et ayant une excellente aptitude au façonnage ; et son procédé de production. La présente tôle d'acier laminée à chaud a une composition de composants arbitrairement sélectionnée de C, Si, Mn, P, S, Al, N et Ti, dans laquelle : la ferrite est de 95 % ou plus en termes de rapport de surface de la structure métallographique ; une ligne de carbure comprenant du Ti et ayant une forme incurvée existe ; la quantité renforcée de dispersion de particules est de 290 MPa ou plus ; le rapport de hausse de la contrainte à une déformation de 80 % d'allongement constant est de 1 200 MPa ou plus ; et la résistance à la traction est de 780 MPa ou plus. Le présent procédé de production d'une tôle d'acier laminée à chaud implique la réalisation, pour un matériau d'acier ayant la composition de composants décrite ci-dessus dans un état chauffé ou non chauffé, de : une étape de laminage grossier ; une étape de laminage de finition dans laquelle la température de départ dépasse 1 000 °C, la réduction au laminage dans chacun d'un premier passage et d'un deuxième passage est de 35 % ou plus, et la réduction au laminage totale à partir d'un troisième passage jusqu'à l'achèvement du laminage est de 85 % ou moins ; une étape de refroidissement ; et une étape d'enroulement.
PCT/JP2023/033821 2022-11-16 2023-09-19 Tôle d'acier laminée à chaud et son procédé de production WO2024106000A1 (fr)

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2013053330A (ja) * 2011-09-02 2013-03-21 Jfe Steel Corp 溶接部耐食性に優れた高強度熱延鋼板およびその製造方法
JP2016183413A (ja) * 2015-03-26 2016-10-20 Ｊｆｅスチール株式会社 連続鋳造スラブ及びその製造方法並びに加工性に優れた高張力鋼板の製造方法

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2013053330A (ja) * 2011-09-02 2013-03-21 Jfe Steel Corp 溶接部耐食性に優れた高強度熱延鋼板およびその製造方法
JP2016183413A (ja) * 2015-03-26 2016-10-20 Ｊｆｅスチール株式会社 連続鋳造スラブ及びその製造方法並びに加工性に優れた高張力鋼板の製造方法

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