Hot-rolled steel sheet and method for producing same

Abstract

Provided are: a hot-rolled steel sheet having a tensile strength (TS) of 780 MPa or more and excellent workability; and a method for producing the hot-rolled steel sheet. Provided is a hot-rolled steel sheet which has an arbitrarily selected component composition of C, Si, Mn, P, S, Al, N, and Ti, and which has a ferrite content of 95% or more in terms of the area ratio of the metallographic structure, a carbide array containing Ti present in a curved shape, a particle dispersion strengthening amount of 290 MPa or more, a stress increase rate at 80% strain of a uniform elongation of 1200 MPa or more, and a tensile strength of 780 MPa or more. Furthermore, the method for manufacturing the hot-rolled steel sheet comprises a rough rolling step in which a steel slab having the above-mentioned component composition is heated or not heated, a finish rolling step in which the starting temperature exceeds 1000 DEG C and the reduction rates of the first pass and the second pass are 35% or more, respectively, a cooling step in which the cooling step is carried out, and a winding step in which the cooling step is carried out in the first pass and the second pass. The total reduction rate from the third pass to the end of rolling is 85% or less.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent workability, and a method for manufacturing the same. The hot-rolled steel sheet of the present invention is suitable as a blank for automotive parts. Background Art

[0002] In recent years, from the viewpoint of protecting the global environment, in order to limit CO2 emissions, the entire automotive industry has been oriented towards improving the fuel consumption of automobiles. For improving the fuel consumption of automobiles, weight reduction of automobiles due to thinning of used parts is the most effective. Therefore, in recent years, the usage amount of high-strength steel sheets as blanks for automotive parts has been gradually increasing.

[0003] Generally, as the steel sheet becomes higher in strength, the formability tends to deteriorate. Therefore, in order to further expand the popularization of high-strength steel sheets, the formability must be improved. In particular, hot-rolled steel sheets are mostly used for forming suspension arm parts having complex shapes, and thus are required to have excellent workability.

[0004] Therefore, in order to solve these problems, various techniques for increasing the strength and workability of steel sheets have been proposed so far.

[0005] For example, Patent Document 1 discloses a hot-rolled steel sheet having a ferrite phase with an area ratio of more than 95% as a main phase of a matrix, and finely precipitated Ti carbides having an average particle diameter of less than 10 nm in ferrite grains. Thereby, a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent workability can be obtained.

[0006] In addition, Patent Document 2 discloses a method for manufacturing a hot-rolled steel sheet, in which hot rolling including rough rolling with a rolling start temperature of 1200 °C or more and finish rolling with a rolling end temperature of 900 °C or more, and coiling treatment at 580 °C or more are performed. In the hot-rolled steel sheet, fine TiN having an average particle diameter of 20 nm or more and fine carbide of Ti having an average particle diameter of less than 6 nm are dispersed in a ferrite phase and a metal structure with an area ratio of 95% or more. As a result, a high-tensile hot-rolled steel sheet having a tensile strength of 590 MPa to 750 MPa and excellent punching property and flange stretching property can be obtained.

[0007] Patent Document 3 discloses a hot-rolled steel sheet in which, in order to improve hardenability, after adding 1.0% or more of Mn, upper bainite with an area ratio of 75.0% or more and less than 97.0% is used as a main phase, and the number density of second-phase particles of 0.5 μm or more is 150000 pieces / mm 2 or less. As a result, a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more can be obtained.

[0008] Prior Art Documents

[0009] Patent Document

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-95996

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2013-133525

[0012] Patent Document 3: WO 2018 / 150955 Summary of the Invention

[0013] However, the prior art disclosed in the above patent documents has the following problems.

[0014] In the technologies proposed in Patent Document 1 and Patent Document 2, fine carbides that contribute to enhanced particle dispersion are arranged linearly. In this case, it is difficult to achieve a high stress increase rate.

[0015] In addition, in Patent Document 3, an upper bainite structure having Fe-based carbides and / or retained austenite (including a structure without Fe-based carbides and retained austenite) is the main phase between lath-shaped bainitic ferrite. In such a steel structure that actively utilizes bainite, martensite, and retained austenite, a large amount of alloying elements are required, and the material cost becomes high.

[0016] The present invention has been developed in view of the above problems of the prior art, and an object thereof is to provide a hot-rolled steel sheet having a tensile strength (TS) of 780 MPa or more and excellent workability, and a method for manufacturing the same.

[0017] In order to solve the above problems, the inventors have conducted in-depth research on factors that combine high strength and workability in hot-rolled steel sheets. The thickness of the hot-rolled steel sheet targeted in the present application is 1.0 mm to 35.0 mm. In order to suppress the addition of alloying elements as much as possible and obtain high strength, fine carbides are dispersed in the ferrite phase, which is a structure rich in workability, thereby strengthening the metal structure.

[0018] As the strength increases, the stretch formability and bend workability deteriorate. Therefore, it is necessary to balance high strength and workability. The formability that affects the tensile characteristics was studied, and it was found that the workability is improved if the generation of local necking is suppressed. Specifically, if the stress increase rate at 80% strain of the uniform elongation rate is 1200 MPa or more, strict forming conditions can be tolerated.

[0019] Furthermore, the limit of the strengthening increase of the metal structure brought about by the particle dispersion strengthening mechanism strengthened by the fine carbide was investigated. As a result, it was found that when strengthening with curved carbide rows, the stress increase rate and the particle dispersion strengthening amount reached high levels compared to the linear carbide rows of the prior art. It was found that the curved carbide rows only appear when the interface velocity between austenite and ferrite is above a certain level during the austenite-to-ferrite phase transformation. In addition, it was also known that the curved carbide rows can be obtained by combining specific steel components and hot rolling conditions. However, with the refinement of the carbide particle size, the particle dispersion strengthening amount showed an increasing trend, but it was difficult to accurately capture nanosized carbides using a transmission electron microscope.

[0020] Therefore, the particle dispersion strengthening amount of the curved carbide rows was compared with the prior art. By dispersing carbides containing nanosized Ti in a larger amount than before, an increase in the stress increase rate considered to be caused by dislocations generated around the carbides could be observed. It was found that in components requiring strict workability, due to the increase in this stress increase rate being conducive to stress dispersion in the high strain region, good workability could be obtained.

[0021] The hot rolled steel sheet of the present invention developed based on the above insights is configured as follows.

[0022] [1] A hot rolled steel sheet having the following composition:

[0023] By mass%, it contains 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%, N: 0.0060% or less, Ti: 0.12% to 0.28%,

[0024] 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%, and optionally further contains one or both of the following components in Group A and Group B,

[0025] Group A: B: 0.0002% to 0.0050%,

[0026] Group B: Any one or more of Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, Ta, Pb, Cs, Te, Bi, and Se: 1% or less in total,

[0027] The balance is composed of Fe and unavoidable impurities;

[0028] The hot-rolled steel sheet has, in terms of the area ratio of the metallographic structure, ferrite of 95% or more, and the curvature of the Ti-containing carbide columns present in a bent shape is 1.8×10 -3 nm -1 or more, the amount of particle dispersion strengthening is 290 MPa or more, the stress increase rate at 80% strain of the uniform elongation rate is 1200 MPa or more, and the tensile strength is 780 MPa or more.

[0029] [2] In the hot-rolled steel sheet of the above [1], a plating layer is provided on the surface of the hot-rolled steel sheet.

[0030] The manufacturing method of the hot-rolled steel sheet of the present invention developed based on the above insights is configured as follows:

[0031] [3] A manufacturing method of a hot-rolled steel sheet, including the following steps: a rough rolling step of heating a steel billet having the composition described in the above [1] to a heating temperature of 1200°C or more or performing rough rolling without heating after casting to form a thin steel sheet; a finish rolling step of finish rolling the thin steel sheet in such a manner that the starting temperature of rolling exceeds 1000°C, the reduction ratios of the first pass and the second pass are 35% or more respectively, and the total reduction ratio from the third pass to the end of rolling is 85% or less to form a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 600°C to 700°C at an average cooling rate of 40°C / s or more; and a coiling step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 600°C to 700°C.

[0032] [4] In the manufacturing method of the hot-rolled steel sheet described in the above [3], a casting step is included before the above rough rolling step or the above finish rolling step: casting a steel billet having the composition described in the above [1] and having a thickness of 35 mm to 200 mm; and forming a thin steel sheet with or without applying the above rough rolling step.

[0033] [5] A manufacturing method of a hot-rolled steel sheet, including the following steps: a rough rolling step of heating a steel billet having the composition described in the above [1] to a heating temperature of 1200°C or more and performing rough rolling to form a thin steel sheet; a joining step of joining the rough-rolled thin steel sheet with a previous thin steel sheet at 1010°C or more; a finish rolling step of finish rolling the joined thin steel sheet in such a manner that the starting temperature of rolling exceeds 1000°C, the reduction ratios of the first pass and the second pass are 35% or more respectively, and the total reduction ratio from the third pass to the end of rolling is 85% or less to form a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 600°C to 700°C at an average cooling rate of 40°C / s or more; and a coiling step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 600°C to 700°C.

[0034] [6] In the method for manufacturing a hot-rolled steel sheet according to any one of [3] to [5] above, the following steps are further included: a hot-rolled sheet annealing step of annealing the above hot-rolled steel sheet at an annealing temperature of 720°C or lower; and a plating step of performing a plating treatment on the annealed above hot-rolled steel sheet.

[0035] [7] In the method for manufacturing a hot-rolled steel sheet according to [6] above, an alloying step is further included: performing an alloying treatment on the plated above hot-rolled steel sheet at 400°C to 500°C.

[0036] According to the present invention, it is possible to manufacture a hot-rolled steel sheet having a high strength with a tensile strength (TS) of 780 MPa or more and excellent workability. If the hot-rolled steel sheet of the present invention is applied to automotive parts, the weight reduction of automotive parts can be further achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a photograph of an example of the metal structure and precipitates in the hot-rolled steel sheet of the present embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] Hereinafter, the hot-rolled steel sheet of the present embodiment will be described.

[0039] <Chemical composition of the hot-rolled steel sheet>

[0040] In the composition of the hot-rolled steel sheet, by mass%, it contains 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%, N: 0.0060% or less, Ti: 0.12% to 0.28%, 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%. Each component will be described below. In the following description, "%" indicating the content of the component means "mass%".

[0041] C: 0.030% to 0.080%

[0042] C contributes to the high strength of the steel sheet by combining with Ti. In order to obtain a steel sheet with a tensile strength of 780 MPa or more, the C content is set to 0.030% or more. On the other hand, during the transformation of austenite to ferrite, C accumulates at the interface between austenite and ferrite, and the interface migration rate decreases. Due to this decrease in the interface migration rate, curved carbide rows cannot be obtained in the metal structure. Therefore, the C content is set to 0.080% or less. Preferably, it is 0.035% to 0.070%.

[0043] Si: 1.5% or less

[0044] Si increases the driving force for the transformation from austenite to ferrite and increases the interface velocity between austenite and ferrite, and it is easy to obtain a row of curved carbides. To exhibit such an effect, the Si content is preferably 0.18% or more. On the other hand, if Si is higher than 1.5%, the driving force for the transformation from austenite to ferrite is too high, and the transformation from austenite to ferrite starts at a high temperature during the cooling process after hot rolling, and the carbides coarsen, so that a particle dispersion strengthening amount of 290 MPa or more cannot be obtained. Therefore, the Si content is set to 1.5% or less. The Si content is preferably 0.26% to 1.1%.

[0045] Mn: 1.6% or less

[0046] Mn reduces the driving force for the transformation from austenite to ferrite and reduces the interface migration velocity between austenite and ferrite. Therefore, the Mn content is set to 1.6% or less. The content of Mn is preferably 1.5% or less. Although 0.05% is inevitably mixed in during manufacturing, even if it is 0.00%, the effects of the present invention will not be impaired.

[0047] It should be noted that in order to obtain a row of curved carbides and to control the driving force for the transformation from austenite to ferrite, it is preferable to satisfy the following formula (1).

[0048] 2.8[%Si]-12([%C]-12 / 48[%Ti*])-[%Mn]≥0···(1)

[0049] Wherein, [%Ti*]=[%Ti]-48[%N] / 14, and [%M](M = C, Si, Mn, N, Ti) refers to the content of each element in mass%.

[0050] P: 0.05% or less

[0051] P is a harmful element that segregates at the grain boundaries and reduces the workability, so it is preferably reduced as much as possible. In the present embodiment, the P content is allowed up to 0.05%. The P content is preferably 0.04% or less, but in order to be used under more stringent working conditions, it is more preferably 0.02% or less. On the other hand, 0.002% of P is sometimes inevitably mixed in during manufacturing.

[0052] S: 0.010% or less

[0053] S forms coarse sulfides in steel, which extend during hot rolling to become wedge-shaped inclusions, thus having an adverse effect on the stretch formability. Therefore, S is also a harmful element and it is preferably reduced, and it can be allowed up to 0.010%. The S content is preferably 0.003% or less, but in order to be used under more stringent processing conditions, it is more preferably 0.001% or less. Sometimes 0.0001% of S may inevitably be mixed in during manufacturing.

[0054] Al: 0.005% - 0.080%

[0055] When Al is added as a deoxidizer during the steelmaking stage, the Al content is 0.005% or more. The workability is reduced due to the formation of oxides by Al. Therefore, the Al content is set to 0.080% or less. The Al content is preferably 0.010% - 0.070%.

[0056] N: 0.0060% or less

[0057] Since N combines with Ti to form coarse TiN, it is a harmful element that reduces the strength and workability. Therefore, the N content is preferably reduced as much as possible, and it can be allowed up to 0.0060%. The N content is preferably 0.0050% or less. Sometimes 0.0005% of N may inevitably be mixed in during manufacturing.

[0058] Ti: 0.12% - 0.28%

[0059] Ti combines with C to contribute to the high strength of the steel plate. 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 is higher than 0.28%, coarse Ti-containing carbides cannot be dissolved in the heating process before hot rolling, not only the effect of high strength saturation, but also it has an adverse effect on the workability. Therefore, the Ti content is set to 0.12% - 0.28%. The Ti content is preferably 0.13% - 0.25%.

[0060] C that is not utilized as Ti-containing carbide accumulates at the interface between austenite and ferrite, thereby reducing the interface migration rate between austenite and ferrite. In order to suppress such an adverse effect, it is preferable to satisfy the following formula (2).

[0061] ([%C] / 12) / ([%Ti*] / 48) < 1.3 ··· (2)

[0062] Here, [%Ti*] = [%Ti] - 48[%N] / 14, and [%M] (M = C, N, Ti) refers to the content of each element in mass%.

[0063] 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%

[0064] V, Mo, Nb, Hf, W, and Zr are elements that combine with C to form carbides. However, when added in combination with Ti, during the austenite-to-ferrite phase transformation, the utilization of C becomes unstable, increasing the risk of not obtaining the desired curved carbide rows. Therefore, it is set as 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%.

[0065] The above is the basic composition of the chemical composition of the hot-rolled steel sheet according to the embodiment, and it may optionally further contain one or both of the following components in Group A and Group B.

[0066] Group A: B: 0.0002% to 0.0050%

[0067] Group B: Any one or more of Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, Ta, Pb, Cs, Te, Bi, and Se: 1% or less in total

[0068] B: 0.0002% to 0.0050%

[0069] B is an effective element for improving hardenability and has the effect of suppressing the austenite-to-ferrite phase transformation and promoting the stable formation of curved carbide rows during the cooling process after hot rolling. Therefore, by setting the B content to 0.0002% or more, it helps to stably obtain the desired structure. On the other hand, if the B content exceeds 0.0050%, the effect on the hardenability of the steel saturates, so it is set to 0.0050% or less. The B content is preferably 0.0002% to 0.0050%. The B content is more preferably 0.0004% to 0.0030%.

[0070] Any one or more of Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, Ta, Pb, Cs, Te, Bi, and Se: 1% or less in total

[0071] If any one or more are contained in the range of 1% or less in total, the influence on the properties of the hot-rolled steel sheet according to this embodiment is very small, so it is allowed. On the other hand, it is preferable to limit the content of each element to 0.03% or less.

[0072] The chemical composition of the hot-rolled steel sheet according to this embodiment contains the above elements, and the remaining part is Fe and inevitable impurities.

[0073] <Metallographic Structure and Properties of Hot-Rolled Steel Sheet>

[0074] Next, the metallographic structure and properties of the hot-rolled steel sheet will be described.

[0075] The metallographic 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 Ti-containing carbide columns existing in a curved shape is 1.8×10 -3 nm -1 or more, the amount of particle dispersion strengthening is 290 MPa or more, the stress increase rate at 80% strain of the uniform elongation is 1200 MPa or more, and the tensile strength of the hot-rolled steel sheet is 780 MPa or more.

[0076] In the following description, "%" indicating the metallographic structure refers to "area ratio".

[0077] The area ratio of ferrite is 95% or more

[0078] Ferrite is a steel structure that is more workable than bainite, martensite, and tempered martensite. If the structure other than ferrite such as bainite, martensite, tempered martensite, and retained austenite exceeds 5%, the amount of particle dispersion strengthening of 290 MPa or more and the stress increase rate of 1200 MPa or more cannot be obtained. From this point of view, ferrite is made 95% or more. Ferrite is preferably 97% or more.

[0079] The curvature of the Ti-containing carbide columns existing in a curved shape is 1.8×10 -3 nm -1 or more

[0080] The presence of curved carbide columns is a major feature of the hot-rolled steel sheet of this embodiment. Through the curved carbide, a high stress increase rate can be obtained. When observed with a transmission electron microscope (TEM) with (001)α incidence, curved Ti-containing carbides can be observed inside the ferrite grains. Since it may not always be observed as a carbide depending on the incident direction of the electron beam, the confirmation of the curved carbide columns is performed by the ferrite grains with (001)α incidence.

[0081] A micrograph of the representative carbide structure existing in a curved shape is shown in Figure 1 . Figure 1 It is a bright-field image taken with (001)α incidence. The granular black contrast is the curved carbide columns. In the measurement of the curvature of the carbides, more than 10 carbide columns are photographed, and for the top 30% of the carbide columns with the largest curvature among them, the curvature is measured at the position with the largest curvature of the carbide columns. The average value of this measured value is used as the curvature of the carbide columns specified in the present invention.

[0082] On the other hand, the amount of particle dispersion strengthening depends on the grain size of the carbide. According to Figure 1 it is known that the carbide is very fine, so it is very difficult to accurately determine the grain size of the carbide. Therefore, the feature of the technology of the hot-rolled steel sheet according to the present embodiment is not defined by the grain size of the carbide but by the amount of particle dispersion strengthening obtained by the carbide containing Ti.

[0083] The amount of particle dispersion strengthening is 290 MPa or more

[0084] In order to obtain a tensile strength of 780 MPa or more, at least an amount of particle dispersion strengthening of 290 MPa or more is required. In order to stably satisfy a tensile strength of 780 MPa, the amount of particle dispersion strengthening is preferably 300 MPa or more. The amount of particle dispersion strengthening can be obtained by the following formula (3).

[0085] [Amount of particle dispersion strengthening (MPa)] = [Yield strength] - 32 [%Mn] - 83 [%Si] - 17.4d -0.5 -54 ··· (3)

[0086] Here, d is the ferrite grain size (unit: mm), and [%M] (M = Mn, Si) is the content of Mn and Si in mass%. It should be noted that for a structure with a ferrite area ratio of less than 95%, formula (3) is not applicable and is excluded from the range obtained in the present invention.

[0087] The stress increase rate at 80% strain of the uniform elongation is 1200 MPa or more

[0088] Under strict forming conditions, the state of stress dispersion caused by work hardening at the stress concentration part is an important factor for forming. The hot-rolled steel sheet of the present embodiment has high work hardening by dispersing curved carbide rows. Since the curved carbide rows are finer and more dispersed than the straight carbide rows, it is presumed that a large amount of dislocations caused by the mismatch between the matrix and the carbide are generated around the carbide during the deformation of the matrix, which is the cause of work hardening. Therefore, the stress increase rate at 80% strain of the uniform elongation is 1200 MPa or more.

[0089] This stress increase rate is calculated as the differential value of the true stress with respect to the true strain at 80% of the uniform elongation in the relationship with the true strain and the true stress. There are cases where the differential value is unstable due to the responsiveness of the testing machine, etc. Therefore, in the present embodiment, the stress increase rate is the average value of 20 adjacent points obtained with the true strain at 80% of the uniform elongation as the center. It should be noted that in order not to cause defects such as necking during forming, a high uniform elongation is preferred, and the product of TS and U-El obtained from the hot-rolled steel sheet of the present embodiment is 7800 MPa·%. This U-El is calculated based on the nominal strain.

[0090] The hot-rolled steel sheet according to this embodiment preferably has a coating on its surface. Even if a coating is formed, the functions of the hot-rolled steel sheet are not impaired. The composition of the coating is preferably selected from one or more of Zn, Si, Al, Ni, and Mg.

[0091] It should be noted that the coated steel sheet in this embodiment can be any one of a steel sheet (GI) obtained by hot-dip galvanizing treatment, a steel sheet (GA) obtained by further performing an alloying treatment after hot-dip galvanizing treatment, and a steel sheet (EG) obtained by electro-galvanizing treatment.

[0092] <Manufacturing method of hot-rolled steel sheet>

[0093] Next, a first method for manufacturing the hot-rolled steel sheet according to this embodiment will be described.

[0094] Generally, in the manufacture of hot-rolled steel sheets, after casting, a slab (billet) cooled to a temperature below 1000 °C is charged into a heating furnace, heated for a short time, and then reduced to a specified thickness on a hot-rolling production line and coiled into a steel coil. Or after casting, a slab (billet) temporarily cooled to room temperature is heated in a heating furnace for a long time, and then reduced to a specified thickness on a hot-rolling production line and coiled into a steel coil. In addition, there is also the following manufacturing method: a cast slab (billet) is directly sent to a hot-rolling production line without being heated in a heating furnace, reduced to a specified thickness, and coiled into a steel coil.

[0095] In the manufacturing method of the hot-rolled steel sheet of this embodiment, it is applicable not only to the process of heating the billet after casting, but also to the process of directly sending the billet to the hot-rolling production line without heating the billet after casting.

[0096] <Billet for the first method>

[0097] The melting method for manufacturing the billet in this embodiment is not particularly limited, and known melting methods such as a converter and an electric furnace can be used. In addition, secondary refining can be performed in a vacuum degassing furnace. Then, considering productivity and quality, it is preferable to make the molten steel adjusted to the above composition into a slab (billet) by a continuous casting method. In addition, a slab can be formed by an ingot-cogging rolling method or other known casting methods.

[0098] <Rough rolling process of the first method>

[0099] In this embodiment, the billet is heated to a heating temperature of 1200 °C or higher, or the billet is rough-rolled without heating after casting to form a thin steel sheet.

[0100] <Finish rolling process of the first method>

[0101] Next, hot rolling is performed with the start temperature of finish rolling being above 1000°C, the reduction ratios of the first and second passes being above 35% each, and the total reduction ratio from the third pass to the end of rolling being below 85% to produce a hot-rolled steel sheet.

[0102] <Cooling step of the first embodiment>

[0103] Next, the hot-rolled hot-rolled steel sheet is cooled to a cooling stop temperature of 600° C. to 700° C. at an average cooling rate of 40° C. / s or more.

[0104] <Coiling step of the first method>

[0105] Then, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C to 700°C.

[0106] Heating of steel billet: heating to above 1200℃ or not heating

[0107] The coarse carbides containing Ti precipitated in the slab (steel billet) are dissolved in the heating process before hot rolling, so fine carbides containing Ti are precipitated after hot rolling. Therefore, in order to obtain a particle dispersion strengthening amount of 290 MPa or more, the heating temperature is set to 1200°C or more. The heating temperature is preferably 1220°C or more, and when the Ti content is 0.13% or more, it is more preferable to heat the slab (steel billet) to 1240°C or more. There is no particular upper limit, but 1300°C is a manufacturing limit to avoid heat damage in the heating furnace.

[0108] When the steel billet maintained at 1200° C. or higher after casting is directly sent to a hot rolling line, the steel billet after casting is not heated.

[0109] Finish rolling starting temperature: above 1000℃

[0110] Since the hot-rolled steel sheet according to the present embodiment has a steel composition in which the driving force of the phase transformation from austenite to ferrite is enhanced, under the hot rolling conditions of the conventional method, the phase transformation from austenite to ferrite begins in the high temperature region of the cooling process after hot rolling, and no curved carbide array is obtained. If carbides are precipitated in the high temperature region of the cooling process after hot rolling, the carbides are coarsened, and not only the desired particle dispersion strengthening amount cannot be obtained, but also the desired stress increase rate cannot be obtained due to the generation of linear carbides.

[0111] Therefore, in order to increase the hot rolling temperature and avoid rolling in the non-recrystallized region of austenite as much as possible, the start temperature of the finish rolling is set to be above 1000°C. The start temperature of the finish rolling is preferably above 1010°C. From the perspective of the properties of the steel, there is no particular upper limit, but unless a heating device is provided on the hot rolling line, the slab heating temperature is the actual upper limit temperature, which is mostly below 1200°C.

[0112] The reduction ratios in the first pass and the second pass are each 35% or more.

[0113] In finish rolling, by increasing the rolling ratio in the high-temperature region where austenite recrystallizes, the degree of work of austenite at the end of finish rolling can be reduced. As a result, the nucleation of ferrite is suppressed, and during the cooling process after hot rolling, the phase transformation of austenite to ferrite in the high-temperature region can be avoided. Therefore, the reduction ratios in the first pass and the second pass are each 35% or more.

[0114] The reduction ratios in the first pass and the second pass are each preferably 38% or more.

[0115] The reduction ratios in the first pass and the second pass can be calculated by the following formulas (4) and (5), respectively.

[0116] Reduction ratio in the first pass = (t0 - t1) / t0 ··· (4)

[0117] Reduction ratio in the second pass = (t1 - t2) / t1 ··· (5)

[0118] Here, t0, t1, and t2 are the plate thickness before finish rolling, the plate thickness after the first pass, and the plate thickness after the second pass, respectively.

[0119] Total reduction ratio from the third pass to the end of rolling: 85% or less

[0120] Until the end of finish rolling, it is necessary to control the degree of work of austenite and reduce the density of ferrite nucleation sites. Therefore, the total reduction ratio from the third pass to the end of rolling is set to 85% or less. The total reduction ratio from the third pass to the end of rolling is preferably 80% or less. The total reduction ratio from the third pass to the end of rolling can be calculated by the following formula (6).

[0121] Total reduction ratio from the third pass to the end of rolling = (t2 - t f ) / t2 ··· (6)

[0122] Here, t f is the plate thickness after finish rolling.

[0123] Until the cooling stop temperature after finish rolling reaches 600°C to 700°C, the average cooling rate is 40°C / s or more

[0124] After finish rolling, in order to suppress the formation of ferrite, it is necessary to rapidly cool the hot-rolled steel sheet. If the cooling stop temperature exceeds 700°C, ferrite is formed during the cooling process. Therefore, 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.

[0125] On the other hand, if the cooling stop temperature is less than 600°C, the precipitation amount of the carbide containing Ti is insufficient, and a particle dispersion strengthening amount of 290 MPa or more cannot be obtained. Therefore, the cooling stop temperature is 600°C or higher.

[0126] Therefore, the range of the cooling stop temperature is set to 600°C to 700°C. The range of the cooling stop temperature is preferably 610°C to 690°C.

[0127] Here, the average cooling rate can be calculated by {(cooling start temperature) - (cooling end temperature)} / (forced cooling time other than air cooling) in the forced cooling other than air cooling after hot rolling. As a method of forced cooling, for example, water cooling can be cited.

[0128] Coiling temperature: 600°C to 700°C

[0129] For the same reason as the cooling stop temperature, the coiling temperature is set to 600°C to 700°C. It is preferably 610°C to 690°C. If coiling is performed in this temperature range, the formation of bainite, martensite, and retained austenite can be extremely suppressed.

[0130] Next, a second method of the manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described. In the present embodiment, the differences from the first method will be described.

[0131] <Casting process of the second method>

[0132] The hot-rolled steel sheet according to the present embodiment can also be manufactured by the thin slab continuous casting method. In the case of manufacturing by the thin slab continuous casting method, a steel slab material with a casting thickness of 35 mm to 200 mm is cast.

[0133] <Rough rolling process of the second method>

[0134] The above-mentioned cast steel slab material is heated to a heating temperature of 1200°C or higher, or is not heated after casting and is rough rolled as needed to form a thin steel sheet.

[0135] After the finish rolling process, it is the same as the first method.

[0136] Here, the thickness of the slab (steel slab material) unique to the thin slab continuous casting method will be described.

[0137] Slab (steel slab material) thickness: thickness 35 mm to 200 mm

[0138] The thin slab continuous casting method is different from the continuous casting method. Since the slab before hot rolling is thin, the working degree of austenite during hot rolling is low. If the slab thickness is less than 35 mm, the desired total reduction ratio from the first pass to the fifth pass cannot be obtained. On the other hand, if the slab thickness is greater than 200 mm, the casting speed becomes slow, and compared with the continuous casting method, the productivity advantage in the thin slab continuous casting method is lost. From the above viewpoints, the slab thickness in the thin slab continuous casting method is set to 35 mm to 200 mm.

[0139] Next, a third method of the manufacturing method of the hot-rolled steel sheet according to the present embodiment will be described. In the present embodiment, the differences from the first method and the second method will be described. The third method can adopt hot continuous rolling technology.

[0140] <Bonding process of the third method>

[0141] The thin steel sheet obtained in the first method or the second method is bonded to the previous thin steel sheet at 1010 °C or higher before finish rolling. If it is lower than 1010 °C, it is difficult to perform rolling at a finish rolling end temperature of 880 °C or higher. The heating temperature of the thin steel sheet during bonding is preferably 1070 °C or higher. After the finish rolling process, it is the same as the first method.

[0142] In the manufacturing method of the hot-rolled steel sheet according to the present embodiment, an annealing process of annealing on a continuous annealing production line at an annealing temperature of 720 °C or lower and a plating process of plating on a continuous plating production line can be applied. In addition, an alloying process of heating the hot-rolled steel sheet obtained by the plating treatment to 400 °C to 500 °C and performing an alloying treatment can be further provided. Even if this annealing treatment or this plating treatment is performed, it will not affect the material of the hot-rolled steel sheet according to the present embodiment. Therefore, by further performing a plating treatment on the surface of the hot-rolled steel sheet, a coating can be provided on the surface of the steel sheet.

[0143] In addition, as described above, since the plating treatment and the composition of the plating bath do not affect the material of the hot-rolled steel sheet according to the present embodiment, any one of hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, and electrogalvanizing treatment can be applied as the plating treatment. The composition of the plating bath can contain one or more of Zn, Al, Mg, Si, and Ni. That is, in the plating treatment, the composition of the coating formed on the surface of the hot-rolled steel sheet can contain one or more of Zn, Si, Al, Ni, and Mg.

[0144] Examples

[0145] The embodiments of the present invention will be further described by examples. It should be noted that the present invention is not limited to the manufacturing conditions and product performances shown in the following examples. The embodiment is a mode in which the desired performance can be achieved within the scope of the present invention.

[0146] <The First Method Based on the Continuous Casting Method>

[0147] A steel billet with a thickness of 250 mm having the component compositions shown in Tables 1-1 to 1-2 is hot-rolled under the rough rolling and finish rolling conditions shown in Table 2. Then, after performing temper rolling with an elongation rate of 0.1 to 0.5% and pickling, a steel plate for evaluation is manufactured.

[0148] <The Second Method Based on the Thin Slab Continuous Casting Method>

[0149] Steel having the component composition shown in Table 1-1 is hot-rolled under the conditions shown in Table 3. After performing temper rolling with an elongation rate of 0.1 to 0.5% and pickling, a steel plate for evaluation is manufactured.

[0150] <The Third Method Based on the Hot Continuous Rolling Method>

[0151] Steel having the component composition shown in Table 1-1 is joined into thin steel sheets under the conditions shown in Table 4. The joined thin steel sheets are hot-rolled. After performing temper rolling with an elongation rate of 0.1 to 0.5% and pickling, it is used for manufacturing a steel plate for evaluation.

[0152] <Manufacturing Method of Applying a Coating to a Hot-Rolled Steel Plate>

[0153] The hot-rolled steel coils (No2, 3) manufactured under the conditions of Table 2 are pickled. Next, Zn plating is applied to the hot-rolled steel plate on a continuous hot-dip coating production line (CGL) according to the conditions shown in Table 5. Thus, continuous hot-dip galvanized steel sheets (GI) and alloyed hot-dip galvanized steel sheets (GA) are manufactured.

[0154]

[0155]

[0156]

[0157] [Table 3]

[0158]

[0159] [Table 4]

[0160]

[0161] [Table 5]

[0162]

[0163] From the viewpoints of the metal structure, tensile properties, bendability, and toughness, the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 were evaluated by the following methods. The results are shown in Tables 6-1 to 6-2.

[0164] (i) Area ratio of the metal structure

[0165] A test piece was cut from the hot-rolled steel sheet with the cross-section parallel to the rolling direction as the observation surface. The center part of the plate thickness was etched with 1% nitric acid ethanol to reveal the structure. It was photographed at 2000 times magnification with a scanning electron microscope (SEM) at an accelerating voltage of 15 kV, and 10 fields of view at the (1 / 4)t part of the plate thickness were taken.

[0166] No corrosion marks were observed within the ferrite grains, and grains with a gray contrast were observed in the SEM photographs. The area ratio of ferrite was determined using image analysis software (Photoshop elements and Image J).

[0167] The ferrite grain size was determined by drawing 10 lines with an actual length of 35 μm in the longitudinal and transverse directions on the above SEM photograph, and then using the cutting method.

[0168] (ii) Observation of carbides

[0169] An observation film was taken from a part corresponding to 1 / 4 of the plate thickness of the hot-rolled steel sheet. The incident direction of the electron beam was adjusted to (001)α incidence using TEM. For more than 20 ferrite grains, the presence or absence of bent carbides was confirmed. For more than 10 carbide rows, the curvature of the carbide rows with the top 30% curvature of the carbide rows was measured, and the average value of the curvature was determined. It should be noted that for the case where the carbide row is linear and the curvature cannot be measured, it is recorded as "unmeasurable" in Tables 6-1 to 6-2, and the level where no carbide row can be observed is recorded as "no precipitation".

[0170] (iii) Tensile test

[0171] Tensile test pieces of JIS No. 5 were made from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 in the direction perpendicular to the rolling direction, and five tensile tests were carried out in accordance with the provisions of JIS Z 2241 (2011) to obtain the average yield strength (YS), tensile strength (TS), uniform elongation (U-El), and stress increase rate (dσ / dε). For the test pieces where the yield point was observed, the lower yield point was taken as the yield strength. For the test pieces where the yield point was not observed, the 0.2% yield stress was taken as the yield strength, and the stress increase rate was obtained by the above method. In Tables 6-1 to 6-2, the tensile strength of 780 MPa or more and the stress increase rate of 1200 MPa or more are taken as the invention examples.

[0172] (iv) Bending test

[0173] In the stamping process of automotive steel sheets, repeated bending processes are usually carried out. To simulate this process, test pieces with a width of 35 mm and a length of 100 mm, whose end faces have been ground, are taken from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5. Bending with an R / t of 1.5 to 2.5 is carried out using the V-block method described in JIS Z 2248, and then a bending test with an R / t of 0.5 or less is carried out under the same conditions as the first time in the direction of the V-shaped punch. A total of 5 repeated bending tests are carried out. In Tables 6-1 to 6-2, the test pieces with an R / t of 0.5 or less in the second bending test have the bending characteristics required by the present invention and are marked as "〇", and under the condition of an R / t of 0.5 or less, the test pieces with cracks observed on the surface of one or more test pieces do not have the bending characteristics required by the present invention and are marked as "×".

[0174] The tensile strength (TS) of any one of the examples of the present invention is 780 MPa or more, and a high amount of particle dispersion strengthening and stress increase rate can be obtained. On the other hand, in the comparative examples outside the scope of the present invention, the tensile strength does not reach 780 MPa or the required amount of particle dispersion strengthening or stress increase rate of the present invention is not obtained.

[0175]

[0176]

Claims

1. A hot-rolled steel sheet having the following composition: By mass %, it contains C: 0.030% - 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% - 0.080%, N: 0.0060% or less, Ti: 0.12% - 0.28%, V: 0% - 0.01%, Mo: 0% - 0.01%, Nb: 0% - 0.01%, Hf: 0% - 0.01%, W: 0% - 0.01%, Zr: 0% - 0.01%, and optionally further contains one or both of the following components in Group A and Group B. Group A: B: 0.0002% - 0.0050%, Group B: Any one or more of Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, Ta, Pb, Cs, Te, Bi, and Se: 1% or less in total, The balance consists of Fe and inevitable impurities; In the hot-rolled steel sheet, the ferrite is 95% or more in terms of the area ratio of the metallographic structure. The curvature of the carbide column containing Ti present in a curved shape is 1.8×10 -3 nm -1 or more, The particle dispersion strengthening amount is 290 MPa or more, the stress increase rate at 80% strain of the uniform elongation rate is 1200 MPa or more, and the tensile strength is 780 MPa or more.

2. The hot-rolled steel sheet according to claim 1, characterized in that The surface of the hot-rolled steel sheet has a coating layer.

3. A method for manufacturing a hot-rolled steel sheet, characterized in that It includes the following processes: Rough rolling process: heating a steel billet with the composition described in claim 1 to a heating temperature of over 1200 °C or performing rough rolling without heating after casting to produce a thin steel sheet. Finish rolling process: finish rolling the thin steel sheet in such a way that the starting temperature of rolling exceeds 1000 °C, the reduction ratios of the first and second passes are respectively 35% or more, and the total reduction ratio from the third pass to the end of rolling is 85% or less to produce a hot-rolled steel sheet. Cooling process: cooling the hot-rolled steel sheet to a cooling stop temperature of 600 °C - 700 °C at an average cooling rate of 40 °C / s or more. And Coiling process: coiling the cooled hot-rolled steel sheet at a coiling temperature of 600 °C - 700 °C.

4. The method for manufacturing a hot-rolled steel sheet according to claim 3, characterized in that Before the rough rolling process or the finish rolling process, it includes a casting process: casting a steel billet with the composition described in claim 1 and a thickness of 35 mm - 200 mm. And, a thin steel sheet is produced with or without applying the rough rolling process.

5. A method for manufacturing a hot-rolled steel sheet, characterized in that It includes the following processes: Rough rolling process: heating a steel billet with the composition described in claim 1 to a heating temperature of over 1200 °C and performing rough rolling to produce a thin steel sheet. Bonding process: bonding the rough-rolled thin steel sheet with a previous thin steel sheet at a temperature of over 1010 °C. Finish rolling process: finish rolling the bonded thin steel sheet in such a way that the starting temperature of rolling exceeds 1000 °C, the reduction ratios of the first and second passes are respectively 35% or more, and the total reduction ratio from the third pass to the end of rolling is 85% or less to produce a hot-rolled steel sheet. Cooling process: cooling the hot-rolled steel sheet to a cooling stop temperature of 600 °C - 700 °C at an average cooling rate of 40 °C / s or more. And Coiling process: coiling the cooled hot-rolled steel sheet at a coiling temperature of 600 °C - 700 °C.

6. The manufacturing method of the hot-rolled steel sheet according to any one of claims 3 to 5, characterized in that, It further includes the following processes: Hot-rolled sheet annealing process: annealing the hot-rolled steel sheet at an annealing temperature of 720 °C or less; and Plating process: performing a plating treatment on the annealed hot-rolled steel sheet.

7. The manufacturing method of the hot-rolled steel sheet according to claim 6, characterized in that, It further includes an alloying process: performing an alloying treatment on the plated hot-rolled steel sheet at 400 °C - 500 °C.

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