HOT-ROLLED STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Abstract

A hot-rolled steel sheet has a certain chemical composition, the area ratio of ferrite in a metallographic structure is 95% or greater, curved carbide strings containing Ti are present, the particle-dispersion strengthening amount is 290 MPa or greater, the stress increase rate of the steel sheet subjected to uniform elongation is 1200 MPa or greater at a point where the steel sheet has a strain of 80%, and the tensile strength is 780 MPa or greater. The method for producing the hot-rolled steel sheet includes a finishing-rolling step of performing finishing rolling by setting the start temperature to greater than 1000° C., setting the rolling reduction for each of first and second passes to 35% or greater, and setting the total rolling reduction for passes from a third pass to the completion of the rolling to 85% or less.

Description

TECHNICAL FIELD

The present invention relates to a hot-rolled steel sheet having a tensile strength of 780 MPa or greater and excellent workability, and a method for producing the same. Such a hot-rolled steel sheet of the present invention is suitable for materials of members of automobiles.

BACKGROUND ART

In recent years, from the perspective of global environmental protection, attempts have been made throughout the automobile industry to improve fuel efficiency of automobiles for the purpose of regulating CO₂ emissions. To improve the fuel efficiency of an automobile, it is most effective to reduce the weight of the automobile by thinning parts used therefor. Thus, in recent years, there has been an increase in the volume of high-strength steel sheets that are used as the materials of automobile parts.

Typically, as the strength of a steel sheet increases, its formability tends to decrease. Therefore, to further promote the widespread use of high-strength steel sheets, it is essential to improve their formability. In particular, hot-rolled steel sheets are often used for suspension arm parts that are formed into complex shapes. Thus, such hot-rolled steel sheets are required to have excellent workability.

To solve the foregoing problems, various technologies for increasing the strength as well as the workability of steel sheets have been proposed so far.

For example, Patent Literature 1 discloses a hot-rolled steel sheet having a microstructure in which a matrix includes, as a primary phase, more than 95% of ferritic phase on an area ratio basis, and fine Ti carbides having an average grain size of less than 10 nm are precipitated in the crystal grains of the ferritic phase. Accordingly, it is presumably possible to obtain a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or greater and having high workability.

Patent Literature 2 discloses a method for producing a hot-rolled steel sheet that includes hot rolling, which includes rough rolling performed at a rolling start temperature of 1200° C. or higher, and finishing rolling performed at a rolling end temperature of 900° C. or higher, and a winding process performed at a temperature of 580° C. or higher. The hot-rolled steel sheet includes 95% or more of ferritic phase on an area ratio basis, and contains, dispersed in its metallographic structure, TiN with an average grain size of 20 nm or greater and Ti-containing fine carbides with an average grain size of less than 6 nm. Consequently, it is presumably possible to obtain a high-tensile hot-rolled steel sheet that has a tensile strength of 590 MPa to 750 MPa, and thus is excellent in punchability and stretch-flangeability.

Patent Literature 3 discloses a hot-rolled steel sheet containing 1.0% or more of Mn added thereto to improve hardenability, and having a microstructure including 75.0% or greater but less than 97.0% by area ratio of a primary phase composed of an upper bainite phase, in which the number density of grains of a secondary phase of 0.5 μm or greater is 150,000 grains/mm² or less. Accordingly, it is presumably possible to obtain a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or greater.

CITATION LIST

Patent Literature

• Patent Literature 1: JP-A-2013-95996
• Patent Literature 2: JP-A-2013-133525
• Patent Literature 3: International Publication No. WO 2018/150955

SUMMARY OF INVENTION

Technical Problem

However, the conventional technologies disclosed in the foregoing patent literatures have the following problems.

According to the technologies proposed in Patent Literatures 1 and 2, fine carbides, which contribute to particle-dispersion strengthening, are arranged linearly. In such a case, it will be difficult to attain a high stress increase rate.

Meanwhile, according to Patent Literature 3, the primary phase is composed of an upper bainite phase with a structure of lath-like bainitic ferrite, the structure containing an Fe-based carbide and/or a residual austenite phase between grains of the bainitic ferrite (which also includes a structure in which an Fe-based carbide and a residual austenite phase are not present between the grains of the bainitic ferrite). Such a steel structure in which bainite, martensite, or residual austenite is actively used will require a large amount of alloying element, which results in an increased cost of materials.

The present invention has been developed in view of the foregoing problems of the conventional technologies, and it is an object of the present invention to provide a hot-rolled steel sheet having a tensile strength (TS) of 780 MPa or greater and excellent workability, and a method for producing the same.

Solution to Problem

To solve the foregoing problems, the inventors have conducted concentrated studies about the requirements for the production of a hot-rolled steel sheet having both high strength and high workability. The thickness of the target hot-rolled steel sheet herein is 1.0 mm or more but 35.0 mm or less. To minimize the addition of alloying elements and obtain high strength, the inventors have attempted to strengthen the metallographic structure of the steel sheet by dispersing fine carbides in a ferrite phase that is a microstructure with high workability.

The stretch formability and the bendability of a hot-rolled steel sheet will decrease with an increase in strength. Thus, it is necessary to achieve both high strength and high workability at the same time. The inventors have studied the formability related to tensile properties, and has found that suppressing the generation of local necking can increase workability. Specifically, it has been found that a hot-rolled steel sheet can tolerate severe forming conditions if the stress increase rate of the steel sheet subjected to uniform elongation is 1200 MPa or greater at a point where the steel sheet has a strain of 80%.

Further, the inventors have pursued the limit of an increase in the strengthening of the metallographic structure based on a particle-dispersion strengthening mechanism for strengthening the metallographic structure using fine carbides. Then, the inventors have found that it is possible to achieve a higher stress increase rate as well as a greater particle-dispersion strengthening amount when the carbides are arranged as curved strings rather than when the carbides are arranged as linear strings as in the conventional technologies. The inventors have also found that such curved carbide strings appear for the first time when, during the phase transformation from austenite to ferrite, the speed of the interface between the austenite and the ferrite has reached a constant level or higher. The inventors have also found that such curved carbide strings can be obtained by combining specific steel components with specific hot-rolling conditions. However, although the particle-dispersion strengthening amount tends to increase with a decrease in the grain size of carbides, it has been difficult to accurately capture the nanosized carbides using a transmission electron microscope.

In response, the inventors have compared the particle-dispersion strengthening amount based on the curved carbide strings with those of the conventional technologies. Then, it has been recognized that when nanosized carbides containing Ti are dispersed in larger quantities than in the conventional technologies, the stress increase rate will increase presumably due to dislocation that occurs around the carbides. It has been also found that when such an increase in the stress increase rate is achieved for a member that should satisfy sever workability requirements, stress is advantageously dispersed in a high strain range, and thus, excellent workability can be obtained.

A hot-rolled steel sheet according to the present invention developed based on the foregoing findings has the following features.

[1] A hot-rolled steel sheet having a chemical composition including, in mass %: C: 0.030% or more but 0.080% or less, Si: 1.5% or less, Mn: 1.6% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more but 0.080% or less, N: 0.0060% or less, Ti: 0.12% or more but 0.28% or less, V: 0% or more but 0.01% or less, Mo: 0% or more but 0.01% or less, Nb: 0% or more but 0.01% or less, Hf: 0% or more but 0.01% or less, W: 0% or more but 0.01% or less, and Zr: 0% or more but 0.01% or less, and further optionally including one or both of Groups A and B of components below:

• • Group A: B: 0.0002% or more but 0.0050% or less; and
  • Group B: 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,
  • with a balance being Fe and unavoidable impurities,
  • characterized in that an area ratio of ferrite in a metallographic structure is 95% or greater, a curvature of a curved carbide string containing Ti is 1.8×10⁻³ nm⁻¹ or greater, a particle-dispersion strengthening amount is 290 MPa or greater, a stress increase rate of the steel sheet subjected to uniform elongation is 1200 MPa or greater at a point where the steel sheet has a strain of 80%, and a tensile strength is 780 MPa or greater.

[2] The hot-rolled steel sheet according to [1] above, further including a plating layer on a surface of the hot-rolled steel sheet.

A method for producing a hot-rolled steel sheet according to the present invention developed based on the foregoing findings has the following features.

[3] A method for producing a hot-rolled steel sheet, including a rough-rolling step of rough-rolling a steel material having the chemical composition of [1] above after heating the steel material to a temperature of 1200° C. or higher, or without heating the steel material that has been cast, thereby obtaining a sheet bar; a finishing-rolling step of finishing-rolling the sheet bar by setting a rolling start temperature to higher than 1000° C., setting a rolling reduction for each of a first pass and a second pass to 35% or greater, and setting a total rolling reduction for passes from a third pass to completion of the rolling to 85% or less, thereby obtaining a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet down to a cooling stop temperature of 600° C. or higher but 700° C. or lower at an average cooling rate of 40° C./s or greater; and a winding step of winding the cooled hot-rolled steel sheet at a winding temperature of 600° C. or higher but 700° C. or lower.

[4] The method for producing a hot-rolled steel sheet according to [3] above, further including a casting step of casting the steel material having the chemical composition of [1] above and having a thickness of 35 mm or more but 200 mm or less before the rough-rolling step or the finishing-rolling step, in which the sheet bar is obtained by applying or not applying the rough-rolling step.

[5] A method for producing a hot-rolled steel sheet, including a rough-rolling step of rough-rolling a steel material having the chemical composition of [1] above after heating the steel material to a temperature of 1200° C. or higher, thereby obtaining a sheet bar; a joining step of joining the sheet bar obtained through the rough rolling to a preceding sheet bar at a temperature of 1010° C. or higher; a finishing-rolling step of finishing-rolling the joined sheet bars by setting a rolling start temperature to higher than 1000° C., setting a rolling reduction for each of a first pass and a second pass to 35% or greater, and setting a total rolling reduction for passes from a third pass to completion of the rolling to 85% or less, thereby obtaining a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet down to a cooling stop temperature of 600° C. or higher but 700° C. or lower at an average cooling rate of 40° C./s or greater; and a winding step of winding the cooled hot-rolled steel sheet at a winding temperature of 600° C. or higher but 700° C. or lower.

[6] The method for producing a hot-rolled steel sheet according to any one of [3] to [5] above, further including a hot-band annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720° C. or lower; and a plating step of plating the annealed hot-rolled steel sheet.

[7] The method for producing a hot-rolled steel sheet according to [6] above, further including an alloying step of alloying the plated hot-rolled steel sheet at a temperature of 400° C. or higher but 500° C. or lower.

Advantageous Effects of Invention

According to the present invention, it is possible to produce a hot-rolled steel sheet having high strength, specifically, a tensile strength (TS) of 780 MPa or greater, and excellent workability. When the hot-rolled steel sheet according to the present invention is applied to an automobile part, the weight of the automobile part can be further reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph showing an example of the metallographic structure of a hot-rolled steel sheet according to the present embodiment, and precipitates therein.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a hot-rolled steel sheet according to the present embodiment will be described.

<Chemical Composition of Hot-Rolled Steel Sheet>

The hot-rolled steel sheet has a chemical composition including, in mass %, C: 0.030% or more but 0.080% or less, Si: 1.5% or less, Mn: 1.6% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more but 0.080% or less, N: 0.0060% or less, Ti: 0.12% or more but 0.28% or less, V: 0% or more but 0.01% or less, Mo: 0% or more but 0.01% or less, Nb: 0% or more but 0.01% or less, Hf: 0% or more but 0.01% or less, W: 0% or more but 0.01% or less, and Zr: 0% or more but 0.01% or less. Each component will be described below. In the following description, “%” representing the content of each component means “mass %.”

C: 0.030% or more but 0.080% or less

C contributes to increasing the strength of the steel sheet by binding to Ti. The C content is set to 0.030% or greater to obtain a steel sheet with a tensile strength of 780 MPa or greater. Meanwhile, during the phase transformation from austenite to ferrite, C deposits at the interface between the austenite and the ferrite, which results in a decreased movement speed of the interface. With such decrease in the movement speed of the interface, it is not possible to obtain curved carbide strings in the metallographic structure. Therefore, the C content is set to 0.080% or less. Preferably, the C content is set to 0.035% or more but 0.070% or less.

Si: 1.5% or less

Si increases the driving force for the phase transformation from austenite to ferrite, and thus increases the movement speed of the interface between the austenite and the ferrite, which makes it easier to obtain curved carbide strings. To achieve such an effect, the Si content is preferably set to 0.18% or greater. Meanwhile, if the Si content exceeds 1.5%, the driving force for the phase transformation from austenite to ferrite becomes excessively high, so that the phase transformation from austenite to ferrite starts at a high temperature during a cooling process following hot rolling. In such a case, the resulting carbides become coarse, which makes it difficult to achieve a particle-dispersion strengthening amount of 290 MPa or greater. Therefore, the Si content is set to 1.5% or less. Preferably, the Si content is set to 0.26% or more but 1.1% or less.

Mn: 1.6% or less

Mn reduces the driving force for the phase transformation from austenite to ferrite, and thus reduces the movement speed of the interface between the austenite and the ferrite. Thus, the Mn content is set to 1.6% or less. Preferably, the Mn content is set to 1.5% or less. At least 0.05% of Mn is unavoidably mixed during production, but even if the Mn content is 0.00%, the advantageous effects of the present invention are not impaired.

To obtain curved carbide strings, Expression (1) below should be satisfied to control the driving force for the phase transformation from austenite to ferrite.

$\begin{array}{cc}2.8\left[\%\mathrm{Si}\right]-12\left(\left[\%C\right]-12/48\left[\%{\mathrm{Ti}}^{*}\right]\right)-\left[\%\mathrm{Mn}\right]\ge 0,& \left(1\right)\end{array}$

where [% Ti*]=[% Ti]−48 [% N]/14, and [% M] (M=C, Si, Mn, N, or Ti) represents the content of each element in mass %.

P: 0.05% or less

P is a detrimental element that decreases workability by being segregated at grain boundaries. Thus, the P content is preferably minimized. In the present embodiment, the acceptable upper limit of the P content is 0.05%. The P content should be 0.04% or less. Preferably, the P content should be 0.02% or less when the hot-rolled steel sheet is used under more severe working conditions. Meanwhile, 0.002% P may be unavoidably mixed during production.

S: 0.010% or less

S forms coarse sulfides in steel, and such sulfides are elongated during hot rolling so as to become wedge-shaped inclusions, thus adversely affecting elongation workability. Therefore, the content of S, which is also a detrimental element, is preferably reduced. The acceptable upper limit of the S content is 0.010%. The S content should be 0.003% or less. Preferably, the S content should be 0.001% or less when the hot-rolled steel sheet is used under more severe working conditions. Meanwhile, 0.0001% S may be unavoidably mixed during production.

Al: 0.005% or more but 0.080% or less

When Al is added as a deoxidizing agent during a steelmaking process, the Al content is set to 0.005% or greater. Al reduces workability by forming oxide. Thus, the Al content should be 0.080% or less. Preferably, the Al content should be 0.010% or more but 0.070% or less.

N: 0.0060% or less

N is a detrimental element that reduces strength and workability by forming coarse TiN as a result of binding to Ti. Therefore, the N content should be minimized. The acceptable upper limit of the N content is 0.0060%. Preferably, the N content should be 0.0050% or less. 0.0005% N may be unavoidably mixed during production.

Ti: 0.12% or more but 0.28% or less

Ti contributes to increasing the strength of the steel sheet by binding to C. To achieve a tensile strength of 780 MPa or greater, the Ti content is set to 0.12% or greater. Meanwhile, when the Ti content exceeds 0.28%, it is difficult to melt coarse Ti-containing carbides in a heating step performed before hot rolling, thereby not only causing the effect of increasing the strength to saturate, but also adversely affecting workability. Therefore, the Ti content should be 0.12% or more but 0.28% or less. Preferably, the Ti content should be 0.13% or more but 0.25% or less.

C that is not used as a Ti-containing carbide will deposit at the interface between austenite and ferrite, thus decreasing the movement speed of the interface between the austenite and the ferrite. To suppress such adverse effects, Expression (2) below is preferably satisfied.

$\begin{array}{cc}\left(\left[\%C\right]/12\right)/\left(\left[\%{\mathrm{Ti}}^{*}\right]/48\right)<1.3,& \left(2\right)\end{array}$

where [% Ti*]=[% Ti]−48 [% N]/14, and [% M] (M=C, N, or Ti) represents the content of each element in mass %.

V: 0% or more but 0.01% or less, Mo: 0% or more but 0.01% or less, Nb: 0% or more but 0.01% or less, Hf: 0% or more but 0.01% or less, W: 0% or more but 0.01% or less, and Zr: 0% or more but 0.01% or less

Each of V, Mo, Nb, Hf, W, and Zr is an element that forms a carbide by binding to C. However, if such an element is added in combination with Ti, the utilization of C will be difficult during the phase transformation from austenite to ferrite, which will increase the risk that the desired curved carbide strings may not be obtained. Therefore, the content of each element is set as follows: V: 0% or more but 0.01% or less, Mo: 0% or more but 0.01% or less, Nb: 0% or more but 0.01% or less, Hf: 0% or more but 0.01% or less, W: 0% or more but 0.01% or less, and Zr: 0% or more but 0.01% or less.

The hot-rolled steel sheet according to the embodiment has the foregoing basic chemical composition, and may further optionally contain one or both of Groups A and B of components described below.

• • Group A: B: 0.0002% or more but 0.0050% or less
  • Group B: 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
  • B: 0.0002% or more but 0.0050% or less

B is an element effective in increasing hardenability and has the effect of suppressing the phase transformation from austenite to ferrite during a cooling process following hot rolling, and thus stably promoting the formation of curved carbide strings. Thus, setting the B content to 0.0002% or greater can contribute to stably obtaining a desired microstructure. Meanwhile, if the B content exceeds 0.0050%, the effect of increasing the hardenability of steel will be saturated. Thus, the B content is set to 0.0050% or less. Preferably, the B content is set to 0.0002% or more but 0.0050% or less. More preferably, the B content is set to 0.0004% or more but 0.0030% or less.

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 acceptable range of the total content of one or more of such elements is 1% or less as such a range has little influence on the properties of the hot-rolled steel sheet according to the present embodiment. Meanwhile, the content of each element is preferably limited to 0.03% or less.

The hot-rolled steel sheet according to the present embodiment has a chemical composition including the foregoing elements, with the balance being Fe and unavoidable impurities.

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

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

The metallographic structure and the mechanical properties of the hot-rolled steel sheet of the present embodiment are as follows: the area ratio of ferrite is 95% or greater, the curvature of curved carbide strings containing Ti is 1.8×10⁻³ nm⁻¹ or greater, the particle-dispersion strengthening amount is 290 MPa or greater, the stress increase rate of the steel sheet subjected to uniform elongation is 1200 MPa or greater at a point where the steel sheet has a strain of 80%, and the tensile strength is 780 MPa or greater.

In the following description, “%” used to represent the metallographic structure refers to the “area ratio.”

Area ratio of ferrite: 95% or greater

Ferrite is a steel microstructure having higher workability than bainite, martensite, and tempered martensite. When the proportion of microstructures other than ferrite, such as bainite, martensite, tempered martensite, and residual austenite, is over 5%, it is difficult to achieve a particle-dispersion strengthening amount of 290 MPa or greater, and a stress increase rate of 1200 MPa or greater. From such a perspective, the area ratio of ferrite is set to 95% or greater. Preferably, the area ratio of ferrite is set to 97% or greater.

Curvature of curved carbide strings containing Ti: 1.8×10⁻³ nm⁻¹ or greater

The presence of curved carbide strings is one of the major features of the hot-rolled steel sheet according to the present embodiment. With such curved carbide strings, a high stress increase rate can be achieved. Such curved carbide strings containing Ti can be observed within ferrite grains when the observation is performed with a transmission electron microscope (TEM) based on (001)α incidence. Depending on the electron beam incident direction, such curved carbide strings may not always be observed as carbides. Thus, such curved carbide strings are checked within ferrite grains based on (001)α incidence.

FIG. 1 shows a photograph of representative microstructures of curved carbides. FIG. 1 is a bright-field image taken at (001)α incidence. Granular black lines as contrasts correspond to curved carbide strings. To measure the curvature of the carbides, 10 or more carbide strings are imaged, and the top 30% of carbide strings with larger curvatures are selected from among them, and then, the curvature is measured for each of the selected carbide strings at a position with the largest curvature. The mean value of the measured values is determined as the curvature of the carbide strings as defined in the present invention.

Meanwhile, the particle-dispersion strengthening amount depends on the grain size of the carbides, but as is clear from FIG. 1, it is quite difficult to determine the accurate grain size of the carbides as they are extremely fine. Thus, a technical feature of the hot-rolled steel sheet according to the present embodiment is defined based not on the grain size of carbides, but on the particle-dispersion strengthening amount achieved with carbides containing Ti.

Particle-dispersion strengthening amount: 290 MPa or greater

To achieve a tensile strength of 780 MPa or greater, the particle-dispersion strengthening amount needs to be at least 290 MPa or greater. To stably obtain a tensile strength of 780 MPa or greater, the particle-dispersion strengthening amount is preferably 300 MPa or greater. The particle-dispersion strengthening amount can be determined from Expression (3) below.

$\begin{array}{cc}\left[\mathrm{Particle}-\mathrm{dispersion}\mathrm{strengthening}\mathrm{amount}\left(\mathrm{MPa}\right)\right]=\left[\mathrm{yield}\mathrm{strength}\right]-32\left[\%\mathrm{Mn}\right]-83\left[\%\mathrm{Si}\right]-17.4d^{-0.5}-54,& \left(3\right)\end{array}$

where d represents the ferrite grain size (unit: mm), and [% M] (M=Mn or Si) represents the content of Mn or Si in mass %. Note that Expression (3) is not applied to a microstructure in which the area ratio of ferrite is less than 95%, and such a microstructure is thus excluded from the range determined in the present invention.

Stress increase rate of hot-rolled steel sheet subjected to uniform elongation at point where it has strain of 80%: 1200 MPa or greater

The state in which stress of a stress concentration portion is dispersed through work-hardening serves as an important factor determining if forming is possible under severe forming conditions. The hot-rolled steel sheet of the present embodiment has a high work-hardening degree as it has curved carbide strings dispersed therein. Since such curved carbide strings are dispersed more finely and in larger quantities than linear carbide strings, it is estimated that work-hardening is attributed to the fact that dislocation, which results from misalignment between the matrix and the carbides, occurs in large quantities around the carbides during the matrix deformation. Thus, the stress increase rate of the hot-rolled steel sheet subjected to uniform elongation at a point where it has a strain of 80% is set to 1200 MPa or greater.

Such a stress increase rate is calculated as, in terms of the relationship between true strain and true stress, the differential value of the true stress with respect to a true strain of 80% during uniform elongation. There may be a case where the differential value is unstable due to the responsiveness of a testing machine, for example. Therefore, in the present embodiment, the stress increase rate is determined by calculating the mean value for 20 adjacent points around a true strain of 80% during uniform elongation as the center. To avoid failures, such as necking, during forming, the hot-rolled steel sheet is desirably uniformly elongated to a high degree. The product of TS and U-El obtained with the hot-rolled steel sheet of the present embodiment is 7800 MPa·%. U-El herein is calculated based on the nominal strain.

The hot-rolled steel sheet according to the present embodiment preferably has a plating layer formed on its surface. Even when a plating layer is formed, the functions of the hot-rolled steel sheet are not impaired. As the composition of the plating layer, one or more of Zn, Si, Al, Ni, and Mg are preferably selected.

Note that the plated steel sheet of the present embodiment may be any one of a steel sheet that has been subjected to a hot-dip galvanizing process (GI), a steel sheet that has been subjected to a hot-dip galvanizing process that is further followed by an alloying process (GA), and a steel sheet that has been subjected to an electrogalvanizing process (EG).

<Method for Producing Hot-Rolled Steel Sheet>

Next, a first aspect of a method for producing a hot-rolled steel sheet according to the present embodiment will be described.

Typically, a hot-rolled steel sheet is produced by putting a slab (i.e., steel material), which has been cast and cooled down to a temperature of 1000° C. or lower, into a heating furnace to heat the slab for a short time, and then reducing the thickness of the slab down to a predetermined thickness through a hot-rolling line, and further winding the slab into a coil form. Alternatively, a hot-rolled steel sheet is produced by heating a slab (i.e., steel material), which has been cast and once cooled down to room temperature, in a heating furnace for a long time, and then reducing the thickness of the slab down to a predetermined thickness through a hot-rolling line, and further winding the slab into a coil form. There is also a production method that includes directly transferring a slab (i.e., a steel material), which has been cast, to a hot-rolling line without heating it in a heating furnace, and then reducing the thickness of the slab down to a predetermined thickness, and further winding the slab into a coil form.

The method for producing the hot-rolled steel sheet according to the present embodiment is applicable to not only a process of heating a steel material, which has been cast, but also a process of directly transferring a steel material, which has been cast, to a hot-rolling line without heating it.

<Steel Material of First Aspect>

A method for melting to produce a steel material of the present embodiment is not limited to a particular method. For example, a known melting method that involves the use of a converter, an electric furnace, and the like may be adopted. Further, secondary refining may be performed in a vacuum degassing furnace. When taking productivity and quality into consideration, it is preferred that a molten steel adjusted to have the foregoing chemical composition in such a manner be formed into a slab (i.e., steel material) through a continuous casting process. Alternatively, a slab may be formed using an ingot making-blooming process or any other known casting processes.

<Rough-Rolling Step of First Aspect>

In the present embodiment, the steel material is heated to a temperature of 1200° C. or higher, or not heated after being cast. Then, the steel material is rough-rolled to obtain a sheet bar.

<Finishing-Rolling Step of First Aspect>

Next, hot rolling is performed as finishing rolling in such a manner that the start temperature of the finishing rolling is set to 1000° C. or higher, the rolling reduction for each of a first pass and a second pass is set to 35% or greater, and the total rolling reduction for passes from a third pass to the completion of the rolling is set to 85% or less, whereby a hot-rolled steel sheet is obtained.

<Cooling Step of First Aspect>

Next, the hot-rolled steel sheet that has been obtained through hot rolling is cooled to a cooling stop temperature of 600° C. or higher but 700° C. or lower at an average cooling rate of 40° C./s or greater.

<Winding Step of First Aspect>

The cooled hot-rolled steel sheet is then wound at a winding temperature of 600° C. or higher but 700° C. or lower.

Heating of steel material: heated to 1200° C. or higher, or not heated

Coarse carbides containing Ti that have been precipitated in the slab (i.e., steel material), are melted in a heating step performed before the hot rolling, so that fine carbides containing Ti will be precipitated after the hot rolling. To achieve a particle-dispersion strengthening amount of 290 MPa or greater, the heating temperature should be 1200° C. or higher. The heating temperature should be 1220° C. or higher. If the Ti content is 0.13% or greater, the slab (i.e., steel material) should be preferably heated to 1240° C. or higher. While there is no specific upper limit to the heating temperature, it should not be over 1300° C. in terms of production restrictions for avoiding thermal damage to the heating furnace.

In the case where the steel material held at 1200° C. or higher after casting is transferred directly to a hot-rolling line, the cast steel material is not heated.

Start temperature of finishing rolling: 1000° C. or higher

The hot-rolled steel sheet according to the present embodiment contains the steel components with an increased driving force for the phase transformation from austenite to ferrite. Thus, under the hot-rolling conditions of a common process, phase transformation from austenite to ferrite will start in a high temperature range during a cooling process following the hot rolling, which makes it impossible to obtain curved carbide strings. If carbides are precipitated in a high temperature range during a cooling process following the hot rolling, these carbides become coarse, whereby not only it is impossible to obtain the desired particle-dispersion strengthening amount, but it is also impossible to obtain the desired stress increase rate as linear carbides are generated.

Thus, to increase the hot-rolling temperature and thus minimize rolling in an unrecrystallized region of austenite, the start temperature of the finishing rolling is set to 1000° C. or higher. Preferably, the start temperature of the finishing rolling is set to 1010° C. or higher. The upper limit of the start temperature is not particularly provided in view of the properties of steel. However, unless there is a heating apparatus in the hot-rolling line, the heating temperature for the slab is substantially the upper limit temperature, which is often 1200° C. or lower.

Rolling reduction for each of first pass and second pass: 35% or greater

During the finishing rolling, increasing the rolling reduction in a high temperature range where austenite is recrystallized can lower the work degree of austenite at the completion of the finishing rolling. Consequently, it is possible to suppress the nucleation of ferrite, and thus avoid the phase transformation from austenite to ferrite in a high temperature range during a cooling process following the hot rolling. Therefore, the rolling reduction for each of the first pass and the second pass is set to 35% or greater.

Preferably, the rolling reduction for each of the first pass and the second pass is set to 38% or greater.

The rolling reductions for the first pass and the second pass can be respectively calculated using Expressions (4) and (5) below.

$\begin{array}{cc}\mathrm{The}\mathrm{rolling}\mathrm{reduction}\mathrm{for}\mathrm{the}\mathrm{first}\mathrm{pass}=\left(t_0-t_1\right)/t_0& \left(4\right)\end{array}$ $\begin{array}{cc}\mathrm{The}\mathrm{rolling}\mathrm{reduction}\mathrm{for}\mathrm{the}\mathrm{second}\mathrm{pass}=\left(t_1-t_2\right)/t_1,& \left(5\right)\end{array}$

where t₀, t₁, and t₂ respectively represent the sheet thickness before the finishing rolling, the sheet thickness after the first pass, and the sheet thickness after the second pass.

Total rolling reduction for passes from third pass to completion of rolling: 85% or less

It is necessary to control the work degree of austenite, and thus decrease the density of the ferrite nucleation sites until the completion of the finishing rolling. Therefore, the total rolling reduction for passes from the third pass to the completion of the rolling is set to 85% or less. Preferably, the total rolling reduction for passes from the third pass to the completion of the rolling is set to 80% or less. The total rolling reduction for passes from the third pass to the completion of the rolling can be calculated using Expression (6) below.

$\begin{array}{cc}\mathrm{The}\mathrm{total}\mathrm{rolling}\mathrm{reduction}\mathrm{for}\mathrm{passes}\mathrm{from}\mathrm{the}\mathrm{third}\mathrm{pass}\mathrm{to}\mathrm{the}\mathrm{completion}\mathrm{of}\mathrm{the}\mathrm{rolling}=\left(t_2-t_f\right)/t_2,& \left(6\right)\end{array}$

where t_f represents the sheet thickness after the completion of the finishing rolling.

Average cooling rate for hot-rolled steel sheet, which has been subjected to finishing rolling, down to cooling stop temperature to 600° C. or higher but 700° C. or lower: 40° C./s or greater

After the finishing rolling, it is necessary to quench the hot-rolled steel sheet to suppress the generation of ferrite. If the cooling stop temperature is over 700° C., ferrite will be generated during the cooling process. Thus, the hot-rolled steel sheet is cooled at an average cooling rate of 40° C./s or greater in the temperature range of the finishing rolling temperature to 700° C.

Meanwhile, if the cooling stop temperature is lower than 600° C., there will be an insufficient amount of the Ti-containing carbides precipitated. In such a case, it will be difficult to achieve a particle-dispersion strengthening amount of 290 MPa or greater. Therefore, the cooling stop temperature is set to 600° C. or higher. Thus, the cooling stop temperature should be 600° C. or higher but 700° C. or lower. Preferably, the cooling stop temperature should be 610° C. or higher but 690° C. or lower.

Herein, the average cooling rate may be calculated as, through forced cooling other than natural cooling performed after the hot rolling, {(cooling start temperature)−(cooling completion temperature)}/(time required for the forced cooling other than natural cooling). Examples of the forced cooling include water-cooling.

Winding temperature: 600° C. or higher but 700° C. or lower

The winding temperature should be 600° C. or higher but 700° C. or lower for the same reason as the cooling stop temperature. Preferably, the winding temperature should be 610° C. or higher but 690° C. or lower. When winding is performed in such a temperature range, the generation of bainite, martensite, and residual austenite can be minimized.

Next, a second aspect of a method for producing a hot-rolled steel sheet according to the present embodiment will be described. In this embodiment, the differences from the first aspect are described.

<Casting Step of Second Aspect>

The hot-rolled steel sheet according to the present embodiment can also be formed using a thin-slab continuous casting process. When a thin-slab continuous casting process is used, a steel material with a thickness of 35 mm or more but 200 mm or less is cast.

<Rough-Rolling Step of Second Aspect>

The cast steel material is heated to 1200° C. or higher, or is not heated after being cast, and is then subjected to rough rolling as appropriate so that a sheet bar is obtained.

The finishing-rolling step and the following steps are similar to those of the first aspect.

Hereinafter, the thickness of the slab (i.e., steel material) that is specific to the thin-slab continuous casting process will be described.

Thickness of slab (i.e., steel material): thickness of 35 mm or more but 200 mm or less

When the thin-slab continuous casting process is used, the slab before being subjected to hot rolling is thin unlike the continuous casting process. Thus, the work degree of austenite during hot rolling is low. If the thickness of the slab is less than 35 mm, it will be difficult to achieve the desired total rolling reduction for passes from a first pass to a fifth pass. Meanwhile, if the thickness of the slab is over 200 mm, the casting speed becomes slow. In such a case, the advantage of the thin-slab continuous casting process over the continuous casting process in terms of productivity will be lost. From such perspectives, the thickness of the slab in the thin-slab continuous casting process should be 35 mm or more but 200 mm or less.

Next, a third aspect of a method for producing a hot-rolled steel sheet according to the present embodiment will be described. In this embodiment, the differences from the first aspect and the second aspect will be described. The third aspect may employ a hot continuous rolling technology.

<Joining Step of Third Aspect>

The sheet bar obtained in the first aspect or the second aspect is joined to a preceding sheet bar at a temperature of 1010° C. or higher before performing finishing rolling. If the temperature is lower than 1010° C., it will be difficult to perform finishing rolling at a finishing-rolling completion temperature of 880° C. or higher. The preferred heating temperature for the sheet bars when joining them is 1070° C. or higher. The finishing-rolling step and the following steps are similar to those of the first aspect.

The method for producing the hot-rolled steel sheet according to the present embodiment may include an annealing step of performing annealing in a continuous annealing line at an annealing temperature of 720° C. or lower, and a plating step of performing plating in a continuous plating line. Further, the method may also include an alloying step of performing an alloying process by heating the plated hot-rolled steel sheet to 400° C. or higher but 500° C. or lower. Even when such an annealing process or a plating process is performed, there will be no impact on the quality of the materials of the hot-rolled steel sheet according to the present embodiment. Therefore, the surface of the hot-rolled steel sheet may be further subjected to a plating process to form a plating layer on the surface of the steel sheet.

Further, as described above, neither the plating process nor the composition of the plating bath has impact on the quality of the materials of the hot-rolled steel sheet according to the present embodiment. Thus, any one of a hot-dip galvanizing process, an alloying hot-dip galvanizing process, and an electrogalvanizing process is applicable as the plating process. The composition of the plating bath may include one or more of Zn, Al, Mg, Si, and Ni. That is, the composition of the plating layer formed on the surface of the hot-rolled steel sheet during the plating process may include one or more of Zn, Si, Al, Ni, and Mg.

EXAMPLE

The embodiment of the present invention will be described in further detail by way of Example. Note that the present invention is not limited to the production conditions or the product performance described below as Example. The desired performance can be achieved as long as the embodiment is within the range of the present invention.

<First Aspect Involving Continuous Casting Process>

Steel materials each having a thickness of 250 mm and having compositions of components shown in Tables 1-1 to 1-2 were hot-rolled under the conditions of rough rolling and finishing rolling shown in Table 2. Then, each steel material was subjected to temper rolling with an elongation ratio of 0.1 to 0.5% and then to pickling, thus producing a steel sheet to be evaluated.

<Second Aspect Involving Thin-Slab Continuous Casting Process>

Thin slabs of steel with the chemical composition shown in Table 1-1 were each hot-rolled under the conditions shown in Table 3. The hot-rolled thin slabs were then subjected to temper rolling with an elongation ratio of 0.1 to 0.5% and then to pickling, thus producing a steel sheet to be evaluated.

<Third Aspect Involving Hot Continuous Rolling Process>

Sheet bars of steel with the chemical composition shown in Table 1-1 were joined under the conditions shown in Table 4, and then, the joined sheet bars were hot-rolled, followed by being subjected to temper rolling with an elongation ratio of 0.1 to 0.5% and then to pickling, thus producing a steel sheet to be evaluated.

<Production Method Involving Forming Plating Layer on Hot-Rolled Steel Sheet>

Hot-rolled coils produced under the conditions of Table 2 (Nos. 2 and 3) were pickled. Then, the resulting hot-rolled steel sheets were galvanized in a continuous galvanizing line (CGL) under the conditions shown in Table 5. Accordingly, a continuous galvanized steel sheet (GI) and an alloyed galvanized steel sheet (GA) were produced.

TABLE 1-1
Steel	Chemical composition (mass %)	Expression	Expression
No.	C	Si	Mn	P	S	Al	N	Ti	Others	(1)	(2)	Remarks
A	0.038	1.05	1.30	0.02	0.0030	0.04	0.0032	0.154	—	1.61	1.06	Invention
												Example
B	0.041	0.73	0.85	0.02	0.0016	0.04	0.0026	0.161	—	1.16	1.08	Invention
												Example
C	0.062	0.57	0.70	0.01	0.0028	0.03	0.0020	0.238	B: 0.0012	0.85	1.07	Invention
												Example
D	0.038	0.46	0.98	0.02	0.0010	0.05	0.0049	0.163	B: 0.0012	0.29	1.04	Invention
												Example
E	0.043	0.84	0.43	0.03	0.0028	0.05	0.0020	0.163	B: 0.0015	1.87	1.10	Invention
												Example
F	0.042	0.69	0.35	0.01	0.0027	0.04	0.0024	0.160	B: 0.0013	1.53	1.11	Invention
												Example
G	0.042	0.31	0.26	0.03	0.0019	0.03	0.0033	0.157	B: 0.0018	0.54	1.15	Invention
												Example
H	0.038	0.80	0.94	0.01	0.0034	0.04	0.0043	0.161	REM: 0.002	1.28	1.04	Invention
									Ni: 0.01			Example
									Cu: 0.01
									Cr: 0.01
I	0.040	0.94	0.32	0.03	0.0026	0.04	0.0026	0.161	Mg: 0.002	2.29	1.05	Invention
									Sn: 0.003			Example
									Co: 0.002
									Ta: 0.002
									Pb: 0.001
J	0.038	1.00	0.94	0.01	0.0026	0.03	0.0035	0.159	Cs: 0.001	1.85	1.03	Invention
									As: 0.001			Example
									Mg: 0.002
									Ca: 0.002
									Sb: 0.001
Expression (1): 2.8[% Si] − 12([% C] − 12/48 [% Ti*]) − [% Mn] [% Ti*] = [% Ti] − 48 [% N]/14
Expression (2): ([% C]/12)/([% Ti*]/48)

TABLE 1-2
Steel	Chemical composition (mass %)	Expression	Expression
No.	C	Si	Mn	P	S	A1	N	Ti	Others	(1)	(2)	Remarks
K	0.025	0.29	0.50	0.03	0.0016	0.05	0.0036	0.156	—	0.44	0.70	Comparative
												Example
L	0.041	0.09	0.88	0.02	0.0012	0.04	0.0044	0.161	—	−0.68	1.12	Comparative
												Example
M	0.045	0.85	1.76	0.01	0.0011	0.03	0.0037	0.162	—	0.53	1.21	Comparative
												Example
N	0.045	0.08	0.78	0.01	0.0011	0.03	0.0037	0.166	—	−0.64	1.17	Comparative
												Example
O	0.039	0.38	0.34	0.01	0.0030	0.06	0.0031	0.110	—	0.55	1.57	Comparative
												Example
P	0.043	0.67	0.76	0.02	0.0032	0.03	0.0037	0.165	Nb: 0.05	1.06	1.13	Comparative
												Example
Q	0.039	0.58	0.83	0.02	0.0033	0.05	0.0022	0.165	V: 0.1	0.80	0.99	Comparative
												Example
R	0.039	0.93	0.84	0.03	0.0020	0.06	0.0049	0.159	Mo: 0.1	1.72	1.10	Comparative
									Hf: 0.05			Example
S	0.040	0.31	0.44	0.02	0.0028	0.06	0.0049	0.164	W: 0.08	0.39	1.09	Comparative
									Zr: 0.12			Example
Expression (1): 2.8[% Si] − 12([% C] − 12/48 [% Ti*]) − [% Mn] [% Ti*] = [% Ti] − 48 [% N]/14
Expression (2): ([% C]/12)/([% Ti*]/48)

	TABLE 2
	Finishing
	rolling conditions
	Total	Conditions after
	rolling	finishing rolling
			Rolling	Rolling	Rolling	reduction		Cooling
		Heating	start	reduction	reduction	(%) for	Average	stop	Winding
		temper-	temper-	(%) for	(%) for	third pass	cooling	temper-	temper-
Steel	Steel	ature	ature	first	second	and subaequent	rate	ature	ature
sheet No.	No.	(° C.)	(° C.)	pass	pass	passes	(° C./s)	(° C.)	(° C.)	Remarks
1	A	1240	1038	39	43	69	51	631	625	Invention
										Example
2		1250	1024	47	44	80	66	675	667	Invention
										Example
3		1240	1037	43	45	70	66	627	623	Invention
										Example
4		1220	985	48	40	70	77	642	635	Comparative
										Example
5		1240	1049	32	38	75	50	666	658	Comparative
										Example
6		1240	1024	40	34	73	61	667	662	Comparative
										Example
7		1250	1021	48	38	86	59	627	622	Comparative
										Example
8		1250	1047	46	48	79	23	648	639	Comparative
										Example
9		1240	1029	48	48	80	50	712	708	Comparative
										Example
10		1250	1049	56	46	80	52	489	482	Comparative
										Example
11	C	1260	1024	42	48	80	77	612	609	Invention
										Example
12	D	1260	1030	47	44	80	51	637	628	Invention
										Example
13	E	1240	1035	42	50	77	78	634	627	Invention
										Example
14	F	1260	1022	61	55	73	60	627	621	Invention
										Example
15	G	1250	1020	40	43	73	79	649	646	Invention
										Example
16	H	1240	1039	53	42	78	63	630	620	Invention
										Example
17	I	1250	1024	38	39	79	69	666	657	Invention
										Example
18	J	1240	1033	40	38	77	77	687	680	Invention
										Example
19	K	1250	1021	41	38	78	74	613	607	Comparative
										Example
20	L	1250	1031	40	39	75	76	615	610	Comparative
										Example
21	M	1250	1031	43	45	70	80	627	619	Comparative
										Example
22	N	1240	1022	39	44	70	81	630	621	Comparative
										Example
23	O	1250	1043	47	41	68	58	634	630	Comparative
										Example
24	P	1250	1047	40	41	77	73	680	674	Comparative
										Example
25	Q	1250	1043	53	42	78	56	666	661	Comparative
										Example
26	R	1250	1045	41	38	78	78	674	671	Comparative
										Example
27	S	1230	1025	43	45	70	70	670	662	Comparative
										Example

	TABLE 3
	Finishing rolling conditions
						Total
						rolling
			Rolling	Rolling	Rolling	reduction		Cooling
		Slab	start	reduction	reduction	(%) for	Average	stop	Winding
		thick-	temper-	(%) for	(%) for	third pass	cooling	temper-	temper-
Steel	Steel	ness	ature	first	second	and subsequent	rate	ature	ature
sheet No.	No.	(mm)	(° C.)	pass	pass	passes	(° C./s)	(° C.)	(° C.)	Remarks
28	B	85	1024	42	48	80	77	612	609	Invention
										Example

	TABLE 4
	Finishing rolling conditions
							Total
							rolling
				Rolling	Rolling	Rolling	reduction		Cooling
			Temperature	start	reduction	reduction	(%) for	Average	stop	Winding
		Heating	(° C.) for	temper-	(%) for	(%) for	third pass	cooling	temper-	temper-
Steel	Steel	temperature	joining	ature	first	second	and subsequent	rate	ature	ature
sheet No.	No.	(° C.)	slabs	(° C.)	pass	pass	passes	(° C./s)	(° C.)	(° C.)	Remarks
29	A	1250	1130	1011	51	50	74	71	628	625	Invention
											Example

	TABLE 5
	CGL Threading Conditions
			Heating	Alloying
Steel	Plated steel		temperature	temperature
sheet No.	sheet No.	Surface	(° C.)	(° C.)
30	2	GA	685	577
31	3	GI	683	—

The hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 were evaluated in terms of the metallographic structure, tensile properties, bendability, and toughness, using the following methods. The results thereof are shown in Tables 6-1 to 6-2.

(i) Area Ratio of Metallographic Structure

A specimen was cut out of each hot-rolled steel sheet so that a cross-section parallel with the rolling direction was observed. Then, the central portion of the specimen in the thickness direction was corroded with a 1% nital solution so as to allow the microstructure to appear. Then, the microstructure was magnified 2000× with a scanning electron microscope (SEM), and a portion of ¼t of the specimen in the thickness direction was imaged for 10 visual fields at an accelerating voltage of 15 kV.

Ferrite is a crystal grain that is observed to have no corrosion marks therein and is observed to appear gray in contrast to others in an SEM photograph. The area ratio of ferrite was determined using image analysis software (Photoshop elements and Image J).

The grain size of ferrite was determined with an intercept method by drawing 10 lines with an actual length of 35 μm in each of the longitudinal and lateral directions on the SEM photograph.

(ii) Observation of Carbides

A thin film to be observed was collected from a site corresponding to ¼ of the hot-rolled steel sheet in the thickness direction, and the electron beam incident direction was adjusted to be (001)α incidence using a TEM, whereby the presence or absence of curved carbides was checked for 20 or more ferrite grains. From among 10 or more carbide strings, the top 30% of carbide strings with larger curvatures were selected, and then, the curvatures of the selected carbide strings were measured. Then, the mean value of the measured curvatures was determined. Regarding linear carbide strings for which the measurement of a curvature was impossible, the relevant column in each of Tables 6-1 to 6-2 is indicated as “Unmeasurable,” and regarding a thin film in which no carbide strings were observed, the relevant column in each of Tables 6-1 to 6-2 is indicated as “No Precipitation.”

(iii) Tensile Test

From each of the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, a tensile specimen of JIS No. 5 was prepared in the direction perpendicular to the rolling direction. Then, a tensile test was conducted five times in compliance with the provision of JIS Z 2241 (2011) so that the average yield strength (YS), tensile strength (TS), uniform elongation (U-El), and stress increase rate (dσ/dε) were determined. Regarding a specimen for which a yield point was recognized, the lower yield point was determined as the yield strength, and regarding a specimen for which a yield point was not recognized, a proof stress of 0.2% was determined as the yield strength. The stress increase rate was determined with the method described above. In Tables 6-1 to 6-2, specimens having a tensile strength of 780 MPa or greater and a stress increase rate of 1200 MPa or greater correspond to examples of the invention.

(iv) Bending Test

A press working process for an automobile steel sheet often includes repeated bending steps. To simulate such steps, a specimen with a width of 35 mm and a length of 100 mm and having ground end faces was collected from each of the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5. A flex test was performed five times in total using the V-block method described in JIS Z 2248, which specifically includes bending with R/t of 1.5 to 2.5, and a bending test with R/t of 0.5 or less performed under the condition that the direction of a V-shaped punch is set the same as that at the first time. In Tables 6-1 to 6-2, a specimen for which R/t was 0.5 or less in a second bending test was evaluated as “∘” as having the bending property required in the present invention, while a specimen for which surface cracking was found once or more under the condition that R/t was 0.5 or less was evaluated as “x” as not having the bending property required in the present invention.

Each of the examples of the present invention was found to have a tensile strength (TS) of 780 MPa or greater, and also have a high particle-dispersion strengthening amount as well as a high stress increase rate. Meanwhile, each of the comparative examples that are outside the range of the present invention was found to have a tensile strength of less than 780 MPa, or have neither the particle-dispersion strengthening amount nor the stress increase rate required in the present invention.

	TABLE 6-1
	Analysis of Microstructure	Mechanical properties
		Ferrite	Curvature				Stress	Particle-
	Area ratio	grain	(×10−3 nm−1) of	Yield	Tensile		increase	dispersion
Steel	(%) of	size	carbide	strength	strength	U-El	rate	strengthening	Bending
sheet No.	ferrite	(μm)	strings	(MPa)	(MPa)	(%)	(MPa)	amount (MPa)	property	Remarks
1	100	3	7.6	776	808	11.2	1382	326	∘	Invention
										Example
2	99	3	8.8	761	818	10.8	1270	311	∘	Invention
										Example
3	99	6	10.8	744	809	10.7	1376	388	∘	Invention
										Example
4	99	6	Unmeasurable	802	844	9.3	1158	446	x	Comparative
										Example
5	99	5	Unmeasurable	757	823	9.2	1143	380	x	Comparative
										Example
6	99	6	Unmeasurable	795	837	9.3	1161	439	x	Comparative
										Example
7	100	5	Unmeasurable	758	806	9	1121	380	x	Comparative
										Example
8	99	5	Unmeasurable	659	755	9.6	1151	281	x	Comparative
										Example
9	100	5	Unmeasurable	682	742	9.9	1152	304	x	Comparative
										Example
10	35	3	No precipitation	767	834	6.7	1125	—	x	Comparative
										Example
11	99	4	5.1	923	982	9.1	1490	524	∘	Invention
										Example
12	100	4	7.0	773	831	11.1	1337	374	∘	Invention
										Example
13	99	5	2.5	780	821	9.8	1384	396	∘	Invention
										Example
14	100	4	5.0	739	803	11.2	1381	341	∘	Invention
										Example
15	100	4	2.7	780	813	10.7	1337	406	∘	Invention
										Example
16	99	6	9.3	780	813	9.7	1307	405	∘	Invention
										Example
17	99	5	10.6	771	838	10.6	1278	382	∘	Invention
										Example
18	99	6	12.5	778	837	11.4	1287	386	∘	Invention
										Example

	TABLE 6-2
	Analysis of microstructure	Mechanical properties
		Ferrite	Curvature				Stress	Particle-
	Area ratio	grain	(×10−3 nm−1) of	Yield	Tensile		increase	dispersion
Steel	(%) of	size	carbide	strength	strength	U-El	rate	strengthening	Bending
sheet No.	ferrite	(μm)	strings	(MPa)	(MPa)	(%)	(MPa)	amount (MPa)	property	Remarks
19	100	6	Unmeasurable	531	618	10.9	1103	288	∘	Comparative
										Example
20	100	6	Unmeasurable	751	819	8.9	1103	288	x	Comparative
										Example
21	99	6	Unmeasurable	754	820	9.1	1130	348	x	Comparative
										Example
22	99	3	Unmeasurable	756	821	12.3	1056	289	x	Comparative
										Example
23	99	6	6.2	633	711	9.5	1262	323	∘	Comparative
										Example
24	99	5	Unmeasurable	807	849	9.3	1181	426	x	Comparative
										Example
25	100	6	Unmeasurable	778	828	9.1	1172	424	x	Comparative
										Example
26	99	3	Unmeasurable	783	824	8.9	1122	306	x	Comparative
										Example
27	100	6	Unmeasurable	784	843	8.8	1108	465	x	Comparative
										Example
28	100	6	12.1	735	799	9.5	1276	387	∘	Invention
										Example
29	100	6	7.5	769	836	11.4	1391	413	∘	Invention
										Example
30	100	5	7.3	773	805	11.3	1317	395	∘	Invention
										Example
31	99	4	4.8	776	843	10.2	1320	369	∘	Invention
										Example

Claims

1. A hot-rolled steel sheet comprising a chemical composition including, in mass %:

C: 0.030% or more but 0.080% or less,

Si: 1.5% or less,

Mn: 1.6% or less,

P: 0.05% or less,

S: 0.010% or less,

Al: 0.005% or more but 0.080% or less,

N: 0.0060% or less,

Ti: 0.12% or more but 0.28% or less,

V: 0% or more but 0.01% or less,

Mo: 0% or more but 0.01% or less,

Nb: 0% or more but 0.01% or less,

Hf: 0% or more but 0.01% or less,

W: 0% or more but 0.01% or less, and

Zr: 0% or more but 0.01% or less, and further optionally including one or both of Groups A and B of components below:

Group A:

B: 0.0002% or more but 0.0050% or less; and

Group B:

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,

with a balance being Fe and unavoidable impurities,

characterized in that:

an area ratio of ferrite in a metallographic structure is 95% or greater,

a curvature of a curved carbide string containing Ti is 1.8×10−3 nm−1 or greater,

a particle-dispersion strengthening amount is 290 MPa or greater,

a stress increase rate of the steel sheet subjected to uniform elongation is 1200 MPa or greater at a point where the steel sheet has a strain of 80%, and

a tensile strength is 780 MPa or greater.

2. The hot-rolled steel sheet according to claim 1, further comprising a plating layer on a surface of the hot-rolled steel sheet.

3. A method for producing a hot-rolled steel sheet, comprising:

a rough-rolling step of rough-rolling a steel material having the chemical composition of claim 1 after heating the steel material to a temperature of 1200° C. or higher, or without heating the steel material that has been cast, thereby obtaining a sheet bar;

a finishing-rolling step of finishing-rolling the sheet bar by setting a rolling start temperature to higher than 1000° C., setting a rolling reduction for each of a first pass and a second pass to 35% or greater, and setting a total rolling reduction for passes from a third pass to completion of the rolling to 85% or less, thereby obtaining a hot-rolled steel sheet;

a cooling step of cooling the hot-rolled steel sheet down to a cooling stop temperature of 600° C. or higher but 700° C. or lower at an average cooling rate of 40° C./s or greater; and

a winding step of winding the cooled hot-rolled steel sheet at a winding temperature of 600° C. or higher but 700° C. or lower.

4. The method for producing a hot-rolled steel sheet according to claim 3, further comprising:

a casting step of casting the steel material having a thickness of 35 mm or more but 200 mm or less before the rough-rolling step or the finishing-rolling step,

wherein:

the sheet bar is obtained by applying or not applying the rough-rolling step.

5. A method for producing a hot-rolled steel sheet, comprising:

a rough-rolling step of rough-rolling a steel material having the chemical composition of claim 1 after heating the steel material to a temperature of 1200° C. or higher, thereby obtaining a sheet bar;

a joining step of joining the sheet bar obtained through the rough rolling to a preceding sheet bar at a temperature of 1010° C. or higher;

a finishing-rolling step of finishing-rolling the joined sheet bars by setting a rolling start temperature to greater than 1000° C., setting a rolling reduction for each of a first pass and a second pass to 35% or greater, and setting a total rolling reduction for passes from a third pass to completion of the rolling to 85% or less, thereby obtaining a hot-rolled steel sheet;

a cooling step of cooling the hot-rolled steel sheet down to a cooling stop temperature of 600° C. or higher but 700° C. or lower at an average cooling rate of 40° C./s or greater; and

a winding step of winding the cooled hot-rolled steel sheet at a winding temperature of 600° C. or higher but 700° C. or lower.

6. The method for producing a hot-rolled steel sheet according to claim 3, further comprising:

a hot-band annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720° C. or lower; and

a plating step of plating the annealed hot-rolled steel sheet.

7. The method for producing a hot-rolled steel sheet according to claim 6, further comprising an alloying step of alloying the plated hot-rolled steel sheet at a temperature of 400° C. or higher but 500° C. or lower.

8. The method for producing a hot-rolled steel sheet according to claim 4, further comprising:

a hot-band annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720° C. or lower; and

a plating step of plating the annealed hot-rolled steel sheet.

9. The method for producing a hot-rolled steel sheet according to claim 5, further comprising:

a hot-band annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720° C. or lower; and

a plating step of plating the annealed hot-rolled steel sheet.

10. The method for producing a hot-rolled steel sheet according to claim 8, further comprising an alloying step of alloying the plated hot-rolled steel sheet at a temperature of 400° C. or higher but 500° C. or lower.

11. The method for producing a hot-rolled steel sheet according to claim 9, further comprising an alloying step of alloying the plated hot-rolled steel sheet at a temperature of 400° C. or higher but 500° C. or lower.