WO2022269742A1

WO2022269742A1 - Hot-rolled steel sheet and method for manufacturing same

Info

Publication number: WO2022269742A1
Application number: PCT/JP2021/023549
Authority: WO
Inventors: 菜緒子加藤; 栄作桜田; 仁之二階堂
Original assignee: 日本製鉄株式会社
Priority date: 2021-06-22
Filing date: 2021-06-22
Publication date: 2022-12-29
Also published as: EP4306664A1; JPWO2022269742A1; MX2023012061A; CN117120647A; KR20230156108A; EP4306664A4; US20240167133A1

Abstract

This hot-rolled steel sheet has a prescribed chemical composition, and the metallographic structure thereof includes, by area ratio, 53.0% to 76.0% of ferrite, 3.0% to 10.0% of martensite, 14.0% to 39.0% of bainite, and 2.6% or less of pearlite. The average diameter of the martensite is 0.26 μm to 0.70 μm. Of the entire martensite interface, the total length of the interface between the martensite and the bainite is 75.0% or more relative to the total length of the entire martensite interface.

Description

Hot-rolled steel sheet and manufacturing method thereof

The present invention relates to a hot-rolled steel sheet and its manufacturing method.

In recent years, from the perspective of protecting the global environment, efforts have been made to reduce the weight of automobile bodies and parts with the aim of improving the fuel efficiency of automobiles. In order to further reduce the weight of automobile bodies and parts, it is necessary to increase the strength of steel sheets applied to the bodies and parts.

　Among automobile parts, underbody parts such as lower arms have complex shapes in order to fulfill their functions. Therefore, from the viewpoint of ensuring both strength and workability, high-strength hot-rolled steel sheets with a thickness of 2.0 to 6.0 mm are sometimes applied to chassis parts. In addition, since hot-rolled steel sheets used for automobile parts are processed into complicated shapes as described above, ductility and hole expansibility are particularly required among workability. In recent years, in particular, high-strength hot-rolled steel sheets having a tensile strength of 780 MPa or more and excellent ductility and hole expansibility are required as hot-rolled steel sheets to be applied to automobile parts.

As a method for improving the strength of hot-rolled steel sheets, there is a method using Ti and Nb. Ti and Nb are elements that precipitate fine alloy carbides in ferrite, and these fine alloy carbides contribute to strength improvement. Si is sometimes added to the hot-rolled steel sheet in order to strengthen the precipitation of ferrite by such Ti and Nb. In particular, since the line length of the section from finish rolling to winding is limited in the hot rolling line, when Si is added when forming ferrite in this section and precipitating carbides of Ti and Nb There are many. On the other hand, if the hot-rolled steel sheet contains Si, there is a risk that scale patterns will form on the surface of the steel sheet, impairing the appearance of the steel sheet and also deteriorating the fatigue properties.

In order to solve such problems, Patent Document 1 discloses a steel sheet mainly composed of ferrite-bainite containing 0.25% or less by mass of Si and Al, and a manufacturing method thereof. .

In addition, Patent Document 2 discloses a high-strength steel sheet that has both elongation (ductility) and hole expansibility and improved fatigue strength by using a ferrite-based structure in the metal structure and reducing the area ratio of martensite. is disclosed.

Japanese Patent Application Laid-Open No. 2004-204326 WO2014/051005

However, in a hot-rolled steel sheet containing a large amount of Ti and Nb as in Patent Document 1, although ductility and hole expandability can be secured, peeling of the sheared edge surface (hereinafter also referred to as fine cracking of the sheared edge surface) occurs during shearing. There is a problem that forming cracks occur in the stretch flange portion starting from fine cracks in the sheared end face. In other words, conventional hot-rolled steel sheets have a problem that sufficient stretch-flange formability cannot be obtained due to the generation of fine cracks on the sheared edge surface during shearing.

In view of the above problems, the object of the present invention is to provide a hot-rolled steel sheet having excellent strength, ductility, hole expansibility and stretch-flangeability, and a method for producing the same.

In view of the above-mentioned problems, the present inventors diligently studied the relationship between the chemical composition and metallographic structure of hot-rolled steel sheets and the above characteristics. As a result, the following findings (a) to (e) were obtained, and the present invention was completed.

(a) In order to obtain excellent strength, it is effective to include a desired amount of martensite in the metal structure.
(b) In order to obtain excellent ductility, it is necessary to include a desired amount of ferrite in the metal structure and to control the amount of bainite in the metal structure within a desired range.
(c) In order to obtain excellent expansibility, it is important to control the amount of pearlite in the metal structure within a desired range.
(d) In order to obtain excellent stretch flangeability, it is important to prevent fine cracks on the sheared edge during shearing. In order to prevent fine cracks on the sheared end face, bainite, which has an intermediate deformability between ferrite and martensite, is arranged so as to cover martensite, that is, arranged adjacent to martensite, and martensite It is useful to control the average diameter of the sites within the desired range. In this way, by appropriately arranging each metal structure and controlling the shape of martensite, it is possible to prevent fine cracking of the sheared end face during shearing, and as a result, excellent stretch flangeability can be obtained. .
(e) It is important to precisely control the cooling conditions after hot rolling in order to obtain the appropriate arrangement of the metal structure and the average grain size of martensite within the desired range as described above. In particular, it is important to generate a large number of uniform bainites by controlling the cooling conditions in the bainite transformation temperature region after hot rolling to a desired range.

The gist of the present invention made based on the above knowledge is as follows.
[1] A hot-rolled steel sheet according to an aspect of the present invention has a chemical composition, in mass%,
C: 0.035% or more and 0.085% or less,
Si: 0.001% or more and 0.15% or less,
Mn: 0.70% or more and 1.80% or less,
P: 0.020% or less,
S: 0.0050% or less,
Ti: 0.075% or more and 0.170% or less,
Nb: 0.003% or more and 0.050% or less,
Al: 0.10% or more and 0.40% or less,
N: 0.0080% or less,
Cr: 0% or more and 0.27% or less,
B: 0% or more and 0.0050% or less,
Ca: 0% or more and 0.0050% or less,
Mo: 0% or more and 0.40% or less,
Ni: 0% or more and 0.50% or less,
Cu: 0% or more and 0.50% or less, and REM: 0% or more and 0.0300% or less,
The balance consists of Fe and impurities,
metal structure
Ferrite has an area ratio of 53.0% or more and 76.0% or less,
Martensite has an area ratio of 3.0% or more and 10.0% or less,
Bainite is more than 14.0% and 39.0% or less in area ratio,
Perlite contains 2.6% or less in area ratio,
The average diameter of martensite is 0.26 μm or more and 0.70 μm or less,
Of all the martensite interfaces, the total length of the interfaces between the martensite and the bainite is 75.0% or more of the total length of all the martensite interfaces.
[2] The hot-rolled steel sheet according to [1] above, wherein the chemical composition is, by mass,
Cr: 0.06% or more and 0.27% or less,
B: 0.0003% or more and 0.0050% or less,
Ca: 0.0003% or more and 0.0050% or less,
Mo: 0.01% or more and 0.40% or less,
Ni: 0.01% or more and 0.50% or less,
Cu: 0.01% or more and 0.50% or less, and REM: 0.0003% or more and 0.0300% or less of the group consisting of 1 or 2 or more may be contained.
[3] A method for manufacturing a hot-rolled steel sheet according to an aspect of the present invention, for a slab having the chemical composition described in [1] or [2] above,
A hot rolling step of rolling under conditions where the final finishing temperature is 880 ° C. or higher and 950 ° C. or lower;
After the hot rolling step, a primary cooling step of cooling to a primary cooling stop temperature of 680° C. or higher and 760° C. or lower at an average cooling rate of 60° C./sec or higher;
After the primary cooling step, a secondary cooling step of cooling for 1.6 seconds or more and 6.3 seconds or less at an average cooling rate of 20° C./second or less;
After the secondary cooling step, a tertiary cooling step of cooling to a tertiary cooling stop temperature of 195° C. or higher and 440° C. or lower at an average cooling rate of 60° C./s or higher and 130° C./s or lower;
After the tertiary cooling step, the quaternary cooling is water-cooled at a water volume density of 2.0 m ³ /min/mm ² or more and 7.2 m ³ /min/mm ² or less for 0.33 seconds or more and 1.50 seconds or less. process and
A fifth cooling step of air-cooling for 3.0 seconds or more and 5.0 seconds or less after the fourth cooling step;
and a winding step of winding at less than 180° C. after the fifth cooling step.

According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent strength, ductility, hole expansibility and stretch-flangeability, and a method for producing the same.

FIG. 1 is a graph showing the relationship between the formed height of a stretch flange formed portion and the fracture limit strain in a side bend test in a conventional hot-rolled steel sheet having a tensile strength of 340 to 780 MPa. FIG. 2 is a schematic diagram of a test piece used in the side bend test. FIG. 3 is a photograph taken with a microscope of a crack generated in a sheared end surface subjected to stretch flanging. FIG. 4A is a cross-sectional photograph of a sheared end face before undergoing stretch flanging. FIG. 4B is an SEM photograph of the vicinity of the crack formed on the sheared end face shown in FIG. 4A. FIG. 5 is a diagram showing the relationship between the rupture limit strain and coverage in this example. FIG. 6 is a diagram showing the relationship between the fracture limit strain and the average diameter dM of martensite in this example. FIG. 7 is a diagram showing the relationship between the coverage rate and the water volume density in the quaternary cooling process in this embodiment. FIG. 8 is a diagram showing the relationship between the average diameter dM of martensite and the cooling time of the quaternary cooling step in this example. FIG. 9 is a diagram showing the relationship between coverage and air cooling time in this embodiment. FIG. 10A is a structure photograph (SEM photograph) of Test No. 21 in this example. FIG. 10B is a micrograph (SEM photograph) in the vicinity of the sheared end face after shearing was applied to Test No. 21 in this example. FIG. 11A is a structure photograph (SEM photograph) of Test No. 17 in this example. FIG. 11B is an enlarged view of area A shown in FIG. 11A.

First, the present inventors will explain the results of examination of the factors that affect stretch-flangeability in hot-rolled steel sheets, and the new knowledge obtained about the relationship between stretch-flangeability and metallographic structure.

For conventional hot-rolled steel sheets, it was known that the appropriate design of the chemical composition, especially the active use of Ti and Nb, was effective in improving ductility and hole expansibility. However, as a result of further intensive research by the present inventors, when a hot-rolled steel sheet with good ductility and hole expansibility is subjected to shearing, fine flaking occurs on the sheared edge surface, and further fine cracking occurs on the sheared edge surface. It was found that when stretch-flanging is applied to a hot-rolled steel sheet, forming cracks may occur at the stretch-flange portion. That is, in conventional hot-rolled steel sheets, even if ductility and hole expansibility are good, the present inventors found that fine cracks on the sheared end surface during shearing and forming at the stretch flange portion due to these fine cracks It has been found that cracking occurs and as a result, sufficient stretch-flange formability may not be obtained. Therefore, the present inventors investigated the factors that cause forming cracks in the stretch-flange portion and the index that indicates the stretch-flangeability.

First, using a slab having the steel composition B in Table 1 of Examples described later, hot-rolled steel sheets were manufactured by variously changing the cooling conditions after the hot rolling process. Samples were cut from the /4 position and examined for each property. As a result, the strength and ductility (elongation at break) in the direction perpendicular to the rolling direction and the hole expansibility were good. However, when hot-rolled steel sheets manufactured under certain cooling conditions were sheared and then stretch-flanged (side bend test), cracks penetrated in the sheet thickness direction and fractured. rice field. FIG. 3 shows the observation result of the fractured surface of the fractured portion.
FIG. 3 is a photograph taken with a microscope of a crack generated in a sheared end surface subjected to stretch flanging. As shown in FIG. 3, among the sheared edges after shearing, cracks parallel to the sheet surface were generated on the sheared edges subjected to stretch flanging in the direction perpendicular to the rolling direction. In other words, the reason why hot-rolled steel sheets manufactured under certain cooling conditions fractured as described above is presumed to be that cracks originating parallel to the sheet surface penetrated in the sheet thickness direction. .

The cracks parallel to the plate surface were investigated in detail. FIG. 4A shows a cross-sectional photograph of the sheared edge before stretch flanging. As shown in FIG. 4A, it was found that cracks parallel to the sheet surface were already formed on the sheared edge surface during the shearing process.
Furthermore, FIG. 4B shows the investigation result of the relationship between the cracks parallel to the plate surface and the metal structure. FIG. 4B is an SEM photograph of the vicinity of the crack formed on the sheared end face shown in FIG. 4A. Note that FIG. 4B is an SEM photograph taken after repeller corrosion, in which white areas indicate martensite, gray areas indicate ferrite, and black areas indicate voids generated by cracking of martensite. As a result of the investigation, as shown in FIG. 4B, in the vicinity of the crack formed on the sheared end face, there are places where martensite is deformed and peeled (arrow (1)) and cracks in martensite itself (arrow (2) ) was observed. From this, it was found that delamination and shape cracking of martensite itself as indicated by arrow (1) in FIG. In this specification, the places where martensite is deformed and peeled off (arrow (1)) and the cracks of martensite itself (arrow (2)) shown in FIG. 4B are collectively referred to as “molding of martensite "Crack".

Based on the above investigation results, next, in order to prevent forming cracks in martensite, we focused on bainite, whose deformability is intermediate between ferrite and martensite, and investigated the morphology of the effective metal structure. . As a result, the present inventors found that the presence of bainite so as to surround martensite, rather than simply specifying the area ratio of bainite, is effective against forming cracks in martensite. That is, the present inventors have found that by increasing the ratio of the interfaces with bainite among all the interfaces of martensite, formation cracking of martensite can be prevented, and as a result, the stretch flangeability is improved.
Although the mechanism by which bainite acts effectively against forming cracks in martensite is not clear, it is thought that it played a role like a cushion.

In addition to forming cracks in martensite, we investigated whether there were any other factors that could cause cracks during shear surface processing, and found that the diameter of martensite also had an effect.
Some of the cracks during shear surface processing are caused by voids, and we investigated how to suppress the formation of voids. As a result, it was found that That is, it is important to control the diameter of martensite within a desired range in order to suppress the formation of voids and prevent the occurrence of cracks.

That is, according to the present inventors, it is important to prevent cracks parallel to the plate surface during shear surface processing. In other words, if the diameter of the martensite is too large, even if the martensite is surrounded by bainite, the difference in hardness between the two causes deformation to concentrate at the interface between the bainite and martensite, resulting in the formation of voids. There is a risk of Therefore, in order to prevent cracks parallel to the plate surface during shear surface processing and improve stretch flangeability, the arrangement of bainite and martensite should be set as desired, and the diameter of martensite should be within the desired range. It is important to.

Next, the stretch flange formability in the present invention will be explained.
Conventionally, a hole expanding test, a notch tensile test, and the like have been generally applied as methods for evaluating stretch flangeability. However, these tests sometimes fail to accurately evaluate the stretch-flangeability of automobile parts, especially in the case of automobile parts where tensile deformation is predominant, such as frame parts and underbody parts.
For example, in the hole expansion test method, the strain distribution changes rapidly in the circumferential direction, whereas in the stretch flanging deformation, it changes with a gentle strain gradient in the circumferential radial direction. In this way, the conventionally used hole expansion test cannot evaluate the stretch flangeability that reflects the influence of the shape of the part when actually forming the part and the properties of the steel sheet that is the material. Therefore, it was difficult to accurately evaluate the stretch flangeability in line with the actual conditions of molding.

Therefore, in the present invention, the fracture limit strain evaluated by the side bend test is used as an index of stretch flangeability. That is, the stretch flange formability referred to in the present invention refers to the fracture limit strain (hereinafter simply referred to as the limit strain) at the time when the open cross section of the steel plate is deformed in the plane and the crack penetrates in the plate thickness direction. . In the present invention, stretch-flange formability is evaluated based on the rupture limit strain obtained by this side bend test.

Next, the chemical composition and metallographic structure of the hot-rolled steel sheet (hereinafter sometimes simply referred to as steel sheet) according to the present embodiment will be described more specifically below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.
The numerical limits described below with "-" in between include the lower limit and the upper limit. Any numerical value indicated as "less than" or "greater than" excludes that value from the numerical range. In the following description, percentages relating to the chemical composition of the steel sheet are percentages by mass unless otherwise specified.

(chemical composition)
The hot-rolled steel sheet according to the present embodiment has a chemical composition of C: 0.035% or more and 0.085% or less, Si: 0.001% or more and 0.15% or less, Mn: 0.001% or more, and Mn: 0.15% or less. 70% or more and 1.80% or less P: 0.020% or less S: 0.005% or less Ti: 0.075% or more and 0.170% or less Nb: 0.003% or more 0.003% or less 050% or less, Al: 0.10% or more and 0.40% or less, N: 0.008% or less, and the balance: Fe and impurities. Each element will be described in detail below.

C: 0.035% or more and 0.085% or less The C (carbon) content is 0.035% or more and 0.085% or less. If the C content is less than 0.035%, it becomes difficult to ensure a sufficient area ratio of martensite. Therefore, the C content should be 0.035% or more. Moreover, it is not preferable to excessively reduce the C content from the viewpoint of steelmaking costs. For these reasons, the C content is preferably 0.037% or more, more preferably 0.040% or more. On the other hand, if the C content exceeds 0.085%, the area ratio of martensite may become excessively large. Therefore, the C content should be 0.085% or less. Also, in order to reduce the incidence of slab cracking in the casting process, it is preferable to suppress the C content. For these reasons, the C content is preferably 0.065% or less.

Si: 0.001% or more and 0.15% or less The lower the Si (silicon) content, the better the appearance of the steel sheet. Moreover, if the Si content exceeds 0.15%, the area ratio of ferrite may become excessively large. Therefore, the Si content should be 0.15% or less. In addition, in order to reduce the cost in the pickling process for removing the scale generated in the hot rolling process, the lower the Si content, the better. For these reasons, the Si content is preferably 0.07% or less. On the other hand, if the Si content is excessively lowered, the manufacturing cost in the steelmaking process will be significantly increased and the aforementioned effects will be saturated, so the Si content is made 0.001% or more. The Si content is preferably 0.003% or more.

Mn: 0.70% or more and 1.80% or less Mn (manganese) has the effect of suppressing ferrite transformation and increasing the strength of the hot-rolled steel sheet. If the Mn content is less than 0.70%, the area ratio of ferrite increases and the desired strength cannot be obtained. Therefore, the Mn content should be 0.70% or more. The Mn content is preferably 0.80% or more, more preferably 0.90% or more. On the other hand, if the Mn content exceeds 1.80%, the area ratio of ferrite is excessively decreased and the area ratio of bainite is increased, resulting in deterioration of ductility. Therefore, the Mn content should be 1.80% or less. The Mn content is preferably 1.75% or less and 1.70% or less. If the Mn content is less than 1.20%, a wrinkled pattern may be formed at the ends of the steel sheet or steel sheet coil in the width direction. Usually, such a wrinkle-shaped portion is trimmed, which leads to a decrease in yield. Therefore, the Mn content is preferably 1.20% or more.

P: 0.020% or less P (phosphorus) is an element generally contained as an impurity, and has the effect of increasing the strength of the hot-rolled steel sheet by solid-solution strengthening. Therefore, although P may be intentionally contained, P is an element that segregates at grain boundaries and also has the effect of causing a decrease in ductility. On the other hand, if the P content exceeds 0.020%, the toughness of the hot-rolled slab is lowered, and there is a risk of cracking during the rolling process, especially at the corners of the slab (corner cracking). Therefore, the P content should be 0.020% or less. The P content is preferably 0.015% or less. There is no need to specify the lower limit of the P content, and the lower the P content, the better. The lower limit of P content can include 0%. However, if the P content is less than 0.001%, the refining cost in the steelmaking process becomes extremely high. Therefore, the P content is preferably 0.001% or more.

S: 0.0050% or less S (sulfur) is an element contained as an impurity, and is an element that forms non-metallic inclusions to reduce the ductility of the hot-rolled steel sheet. On the other hand, when the S content exceeds 0.0050%, the ductility of the hot-rolled steel sheet is remarkably lowered, and flaws and breakage occur in the hot rolling process. Therefore, the S content should be 0.0050% or less. The S content is preferably 0.0040% or less. There is no need to specify the lower limit of the S content, and the lower the S content, the better. The lower limit of S content can include 0%. However, if the S content is less than 0.0001%, the refining cost in the steelmaking process increases. From the viewpoint of refining cost, the S content is preferably 0.0001% or more.

Ti: 0.075% or more and 0.170% or less Ti (titanium) is an element that precipitates fine alloy carbides in ferrite and has the effect of increasing strength. If the Ti content is less than 0.075%, sufficient strength cannot be obtained. Therefore, the Ti content should be 0.075% or more. Ti is also an effective element for improving hole expandability. In order to obtain these effects, the Ti content is preferably 0.090% or more. On the other hand, if the Ti content exceeds 0.170%, the cold slab may crack. Therefore, the Ti content is set to 0.170% or less. The Ti content is preferably 0.150% or less.

Nb: 0.003% to 0.050% Nb (niobium) is an element that precipitates fine alloy carbides. Further, Nb suppresses the grain growth of austenite during hot rolling, thereby suppressing the coarsening of the grain size of ferrite that is transformed and formed thereafter. By exerting these actions, the strength of the steel sheet can be increased. In order to obtain this effect, the Nb content should be 0.003% or more. Nb also has the effect of suppressing coarsening of crystal grains in the heat-affected zone during arc welding and suppressing softening of the heat-affected zone. In order to exhibit these effects, the Nb content is preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.050%, the toughness of the hot-rolled slab is lowered, and as a result, cracks and flaws may occur during the rolling process. Therefore, the Nb content should be 0.050% or less. The Nb content is preferably 0.045% or less.

Al: 0.10% or more and 0.40% or less Al (aluminum) is an element effective in deoxidizing steel and making the steel plate sound. Al is an element that effectively acts to improve the area ratio of ferrite. If the Al content is less than 0.10%, the area ratio of ferrite becomes insufficient. Therefore, the Al content is set to 0.10% or more. In addition, Al also has the effect of lowering the melting point of scale formed on the surface of the steel sheet during the hot rolling process, so that the scale can be easily removed during hot rolling. In order to obtain these effects, the Al content is preferably 0.20% or more. On the other hand, if the Al content exceeds 0.40%, the area ratio of ferrite becomes excessively large, resulting in insufficient strength. Therefore, the Al content should be 0.40% or less. The Al content is preferably 0.35% or less.

N: 0.0080% or less N (nitrogen) is an element that forms a nitride such as Ti, Nb or Al. These nitrides lower the toughness of the hot-rolled slab and cause flaws and cracks in the rolling process, especially corner cracks of the cast slab (corner cracks). Therefore, from the viewpoint of production, the lower the N content, the better, and the N content is set to 0.0080% or less. The lower limit of N content may include 0%. On the other hand, if the N content is less than 0.0005%, the steelmaking cost may become extremely high. Therefore, the N content is preferably 0.0005% or more, more preferably 0.0010% or more.

The rest of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, impurities are elements that are mixed from ore and scrap as raw materials, or elements that are intentionally added in small amounts from the manufacturing environment, etc., and have an adverse effect on the hot-rolled steel sheet according to the present embodiment. It means what is permissible within the range not given.

In addition to the above elements, the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements in order to improve strength, ductility or other properties. That is, instead of part of Fe, one or more of Cr, B, Ca, Mo, Ni, Cu and REM may be contained as arbitrary elements within the range described later. The lower limit of the content when these optional elements are not contained is 0%. Each arbitrary element will be described in detail below.

Cr: 0.06% to 0.27% Cr (chromium) has the effect of increasing the tensile strength of the steel sheet. In order to obtain the effect of this action, it is preferable to set the Cr content to 0.06% or more. Cr content is more preferably 0.10%. On the other hand, if the Cr content exceeds 0.27%, the area ratio of bainite may become excessively large. Therefore, the Cr content is preferably 0.27% or less. Cr content is more preferably 0.25% or less.

B: 0.0003% or more and 0.0050% or less B (boron) has the effect of increasing the tensile strength of the steel sheet. In order to obtain the effect of this action, it is preferable to set the B content to 0.0003% or more. The B content is more preferably 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, it becomes difficult to obtain a sufficient area ratio of ferrite, and in addition, the area ratio of bainite may become excessively large. Therefore, the B content is preferably 0.0050% or less. The B content is more preferably 0.0040% or less.

Ca: 0.0003% or more and 0.0050% or less Ca (calcium) has the effect of spheroidizing non-metallic inclusions to increase ductility. In order to obtain the effect of this action, it is preferable to set the Ca content to 0.0003% or more. Ca content is more preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.0050%, the toughness of the slab is lowered, and cracks and flaws may occur in the slab during the rolling process. Therefore, the Ca content is preferably 0.0050% or less. Ca content is more preferably 0.0040% or less.

Mo: 0.01% to 0.40% Mo has the effect of increasing the tensile strength of the steel sheet. In order to obtain the effect of this action, it is preferable to set the Mo content to 0.01% or more. Mo content is more preferably 0.03% or more. On the other hand, if the Mo content exceeds 0.40%, the area ratio of bainite may become excessively large. Therefore, the Mo content is preferably 0.40% or less. Mo content is more preferably 0.35% or less.

Ni: 0.01% to 0.50% Ni has the effect of increasing the tensile strength of the steel sheet. In order to obtain the effect of this action, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.08% or more. On the other hand, if the Ni content exceeds 0.50%, the area ratio of bainite may become excessively large. Therefore, the Ni content is preferably 0.50% or less. The Ni content is more preferably 0.40% or less.

Cu: 0.01% to 0.50% Cu has the effect of increasing the tensile strength of the steel sheet. In order to obtain the effect of this action, it is preferable to set the Cu content to 0.01% or more. Cu content is more preferably 0.08% or more. On the other hand, if the Cu content exceeds 0.50%, the area ratio of bainite may become excessively large. Therefore, the Cu content is preferably 0.50% or less. Cu content is more preferably 0.40% or less.

REM: 0.0003% or more, 0.0300% or less REM (rare earth metal) is an element that has the effect of reducing the size of inclusions and contributes to the improvement of hole expansibility and ductility (elongation at break). be. If the REM content is less than 0.0003%, these effects cannot be sufficiently obtained. Therefore, it is preferable to set the REM content to 0.0003% or more. The REM content is more preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0300%, castability and hot workability may deteriorate, so the REM content is preferably 0.0300% or less.

Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metals.

The chemical composition of the hot-rolled steel sheet described above may be measured by ICP emission spectroscopic analysis using chips according to JIS G 1201:2014. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

(metal structure)
Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, the metal structure, in terms of area %, contains 53.0% or more and 76.0% or less ferrite, 3.0% or more and 10.0% or less martensite, and 14% bainite. .0% or more and 39.0% or less, including 2.6% or less perlite. Further, in the hot-rolled steel sheet according to the present embodiment, the average diameter of martensite is 0.26 μm or more and 0.70 μm or less, and the total length of the interfaces between martensite and bainite among all interfaces of martensite is It is 75.0% or more of the total length of all the interfaces of the sites.
In addition, in this embodiment, the metal structure is defined at a depth of 1/4 of the plate thickness from the surface of the plate thickness cross section parallel to the rolling direction and at the central position in the plate width direction. The reason is that the metallographic structure at this position shows the typical metallographic structure of the hot-rolled steel sheet.

Ferrite: 53.0% or more and 76.0% or less Since ferrite is a soft structure, it is a metal structure that is mainly responsible for deformation. By using a dual-phase steel sheet containing martensite, such as the hot-rolled steel sheet of the present embodiment, it is possible to obtain an effect of increasing elongation as the area ratio of ferrite increases, that is, an effect of improving ductility. However, if the ferrite area ratio is less than 53.0%, the ductility is lowered. Therefore, the area ratio of ferrite is set to 53.0% or more. The area ratio of ferrite is preferably 57.0% or more, more preferably 60.0% or more. On the other hand, if the ferrite area ratio exceeds 76.0%, the desired strength may not be obtained. Therefore, the area ratio of ferrite is set to 76.0% or less. The area ratio of ferrite is preferably 73.0% or less, more preferably 70.0% or less.

Martensite: 3.0% or more and 10.0% or less Since martensite is a hard structure, it contributes to improvement in the strength of the hot-rolled steel sheet. If the martensite area ratio is less than 3.0%, the desired strength may not be obtained. Therefore, the area ratio of martensite is set to 3.0% or more. The area ratio of martensite is preferably 4.0% or more. On the other hand, if the area ratio of martensite exceeds 10.0%, the hole expansibility may be remarkably deteriorated. Therefore, the area ratio of martensite is set to 10.0% or less. The area ratio of martensite is preferably 9.0% or less, more preferably 8.0% or less, and even more preferably 7.0% or less.

Bainite: 14.0% to 39.0% Bainite is a structure that improves the strength and ductility of hot-rolled steel sheets. In addition, by arranging the metal structure such that martensite is surrounded by bainite, stretch flangeability can be enhanced. If the area ratio of bainite is less than 14.0%, it becomes difficult to arrange the metal structure as described above, and the desired stretch flangeability cannot be obtained. Therefore, the area ratio of bainite is set to 14.0% or more. The area ratio of bainite is preferably 17.0% or more, more preferably 20.0% or more, still more preferably 25.0% or more. On the other hand, when the area ratio of bainite exceeds 39.0%, the ductility (elongation at break) may deteriorate significantly. Therefore, the area ratio of bainite is set to 39.0% or less. The area ratio of bainite is preferably 35.0% or less, more preferably 30.0% or less, and even more preferably 28.0% or less.

Pearlite: 2.6% or less If the area ratio of pearlite exceeds 2.6%, the hole expansibility may deteriorate. Therefore, the area ratio of pearlite is set to 2.6% or less. It is preferably 1.7% or less, more preferably 1.2% or less. The area ratio of pearlite may be 0%.

In addition to the above metallographic structure, it may also contain retained austenite. However, if the area ratio of retained austenite exceeds 4.0%, toughness may decrease. Therefore, when retained austenite is included, the area ratio is preferably 4.0% or less, more preferably 3.0% or less. The area ratio of retained austenite may be 0%.

Next, with regard to the metal structure of the steel sheet of this embodiment, the ratio (area %) of each structure can be measured by the following method.

For the area ratio of the metal structure of the hot-rolled steel sheet, a value measured from metal structure information such as a metal structure photograph taken by a scanning electron microscope in a cross section parallel to the rolling direction of the hot-rolled steel sheet may be used. For metallographic information such as metallographic photographs, the width center position in the direction perpendicular to the rolling direction and the plate thickness direction is cut out parallel to the rolling direction, and the depth of 3/8 of the plate thickness from the surface in the plate thickness direction is the center. should be an observation field of view included in . Three or more observation fields are set, and the average value of the area ratio of the metal structure measured in each field of view can be used as the value of the area ratio of the typical metal structure of the steel sheet. The area ratios of ferrite, pearlite, bainite, martensite, and retained austenite, the average diameter of martensite, and the coverage are measured in the same field of view.

In the present embodiment, the area ratio of ferrite (hereinafter sometimes referred to as Vα) refers to the area ratio of ferrite structure determined by an electron backscatter diffraction (EBSD) method. In order to measure the ferrite area ratio, first, the EBSD method is used to obtain crystal orientation information (crystal orientation mapping data). For the measurement, an apparatus composed of a thermal field emission scanning electron microscope ("JSM-7001F" manufactured by JEOL) and an EBSD detector ("Hikari detector" manufactured by TSL) is used. The crystal orientation mapping data can be obtained using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. During measurement, the degree of vacuum in the apparatus may be 9.6×10 ⁻⁵ Pa or less, and the acceleration voltage may be 20 kv.
The procedure for determining the area ratio of ferrite from this crystal orientation mapping data is divided into the following three steps.

The first step is to define grains from the crystal orientation mapping data. Here, the crystal grain refers to a boundary where the crystal orientation difference between an arbitrary measurement point in the crystal orientation mapping data and a measurement point adjacent thereto is 15° or more, that is, a region surrounded by grain boundaries.

The second step determines whether or not the grains defined in the first step are ferrite grains. A local misorientation average (GAM value) is used for this determination method of ferrite. This GAM value is a value that indicates misorientation of crystal grains. If the GAM value of the crystal grain to be determined is within 0.35°, the crystal grain is determined to be ferrite.

The third step is to perform the determination of the second step on all crystal grains recorded in the crystal orientation mapping data. Then, the ratio of the number of measurement points belonging to the crystal grain determined to be ferrite to all the number of measurement points of the crystal orientation mapping data is calculated. Let this ratio be the area ratio of ferrite.

In order to sufficiently reduce the error due to the measurement position of the ferrite area ratio, the crystal orientation mapping data should include a total of 1000 crystal grains. The measurement magnification for obtaining crystal orientation mapping data by EBSD analysis is set so that the field of view includes 1000 crystal grains. In addition, in the analysis at a low magnification of less than 200 times, the measurement accuracy is lowered due to the distortion of the electron beam. Therefore, the measurement magnification should be 250 times. In this embodiment, an area of 500 μm×500 μm is measured at a magnification of 250 times.

The measurement range for the area ratio of ferrite is a quadrangle consisting of sides in the sheet thickness direction and the rolling direction. The side in the sheet thickness direction should be 500 μm, and the side in the rolling direction should be equal to the side in the sheet thickness direction. The area ratio of ferrite is measured in a range including a position 3/8 of the plate thickness from the surface in the plate thickness direction. The crystal orientation measurement interval within the measurement range is 0.03 μm. If the measurement interval is less than 0.03 μm, the electron beam interference range may overlap. On the other hand, if the measurement interval exceeds 0.03 μm, the number of measurement points for the crystal orientation contained in the crystal grain is insufficient, and measurement errors are likely to occur.

A sample for ferrite measurement should be cut out parallel to the rolling direction at the width center position in the direction perpendicular to the rolling direction and the plate thickness direction, and observed from the direction perpendicular to the rolling direction and the plate thickness direction.

In the present embodiment, the area ratio of martensite (hereinafter sometimes referred to as VM) refers to a value measured from the metal structure revealed by repeller corrosion. Among the metal structures exposed by repeller corrosion, the metal structure observed with white contrast is identified as martensite. The ratio of the area of the metal structure identified as martensite to the area of the entire observation field is the area ratio VM of martensite.

A method for measuring the area ratio VM of martensite will be described below.
First, a scanning electron microscope is used for photographing a field of view used for measuring the area ratio VM of martensite. In order to improve the measurement accuracy, the metal structure should be photographed at a magnification of 5000 times. Also, if the magnification is 5000 times, at least one martensite grain can be photographed within one field of view. Therefore, it is preferable to set the magnification to 5000 times. In this embodiment, the area ratio of martensite is measured by observing a region of 500 μm×500 μm at a magnification of 5000 times.

　When measuring the area ratio VM of martensite, the acceleration voltage when irradiating the electron beam is in the range of 10.0 kV or more and 15.0 kV or less. If the acceleration voltage exceeds 15.0 kV, grain boundaries may become blurred. On the other hand, if the acceleration voltage is less than 10.0 kV, the resolution is lowered, which is not suitable for observation.

A backscattered electron image obtained from these observation samples and observation conditions is used to measure the area ratio VM of martensite. Specifically, in order to reduce errors between observation fields, the area ratio VM of martensite is obtained from a measurement range in which the total number of crystal grains is 600 or more. The total number of crystal grains within the measurement range should be 1000 pieces. The measurement range is a range including a position 3/8 of the plate thickness from the surface in the plate thickness direction. The measurement range is 500 μm in the plate thickness direction and 500 μm in the rolling direction.

The area ratio of bainite (hereinafter sometimes referred to as VB) is obtained by subtracting the sum of the area ratios of ferrite, martensite, retained austenite described later, and pearlite obtained by the above method from 100%, and the remainder is the bainite structure. is the area ratio VB.

The pearlite area ratio (hereinafter sometimes referred to as VP) refers to the value measured from the metal structure revealed by nital corrosion.

A sample for pearlite measurement can be obtained by cutting out the width center position in the direction perpendicular to the rolling direction and the plate thickness direction parallel to the rolling direction and observing from the direction perpendicular to the rolling direction and the plate thickness direction.
A photograph of the metallographic structure is obtained in a measurement range centered on the ⅛ position in the plate thickness direction from the surface of the steel plate in the collected pearlite measurement sample. The sample for pearlite measurement is the same as the sample for measuring the area ratios of ferrite and martensite.

In this embodiment, a scanning electron microscope is used to obtain a metallographic photograph for measuring the area ratio of pearlite. In photographing with a scanning electron microscope, the acceleration voltage at the time of electron beam irradiation is in the range of 10.0 kV to 15.0 kV. If the acceleration voltage exceeds 15.0 kV, grain boundaries may become blurred. On the other hand, if the acceleration voltage is less than 10.0 kV, the resolution is lowered, which is not suitable for observation. In order to sufficiently improve the measurement accuracy, the metal structure should be photographed at a magnification of 2000 times or more. If the magnification is 10000 times or less, one or more perlite grains can be captured in one visual field. Therefore, the magnification should be 10000 times or less. Note that the magnification is preferably 5000 times to reduce the number of fields of view and obtain measurement accuracy. The measurement range is 10 μm or more and 40 μm or less in the thickness direction, and 10 μm or more and 55 μm or less in the rolling direction.

The area ratio of retained austenite (hereinafter sometimes referred to as Vγ) is the number of crystal orientation measurement points where the crystal structure is determined to be fcc among the crystal orientation mapping data used to obtain the above-mentioned ferrite area ratio Vα. is divided by the number of all measurement points of the crystal orientation mapping data. The crystal orientation mapping data used for measuring the area ratio Vγ of retained austenite is the same as that used for measuring the area ratio Vα of ferrite. That is, the measurement range, measurement magnification and field of view may be the same as the method for measuring the area ratio of ferrite.

(morphology of metal structure)
Next, the morphology of the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
In order to obtain excellent stretch-flangeability in the hot-rolled steel sheet of the present embodiment, the metal structure is configured as described above, and the area ratio is within the desired range. It is important to set the interfacial length ratio between martensite and bainite and the average martensite diameter dM within the desired ranges.

The ratio of the interfacial length between martensite and bainite to the total interfacial length of martensite (hereinafter sometimes referred to as coverage) is 75.0% or more. Since bainite is a metal structure having a strength intermediate between that of ferrite and martensite, it is considered to play a role of mitigating the deformation difference between ferrite and martensite, that is, playing a role like a cushion. If the coverage of martensite by bainite is less than 75.0%, the role of this cushion becomes insufficient, and fine cracks occur on the sheared edge surface. As a result, it becomes difficult to obtain excellent stretch flangeability. Moreover, when the coverage is less than 75.0%, the fracture limit strain, which will be described later, is lowered. Therefore, the ratio of the interfacial length between martensite and bainite to the total interfacial length of martensite is preferably as high as possible, preferably 78% or more. On the other hand, the upper limit of the ratio of the interfacial length between martensite and bainite to the total interfacial length of martensite is not particularly defined, and may be 100%.

As explained above, in order to improve stretch flangeability, not only the area ratio of bainite is increased, but also bainite is arranged so as to surround martensite. It is effective to increase the proportion of the interfacial length of .

The average diameter dM of martensite is set to 0.26 μm or more and 0.70 μm or less from the viewpoint of suppressing voids. By setting the average diameter dM within the above range, it is possible to suppress fine cracking of the sheared end surface, and as a result, it is possible to obtain high stretch flangeability. When the average diameter dM of martensite is more than 0.70 μm, the deformation concentrates at the interface between martensite and bainite due to the difference in hardness between the two. As a result, voids may be formed in the vicinity of the interface between martensite and bainite even if the bainite coverage is satisfactory. Further, if voids are formed, fine cracks may occur in the sheared end face, and the fracture limit strain described later may decrease. Therefore, the average diameter dM of martensite is set to 0.70 μm or less. The average diameter dM of martensite is preferably 0.65 μm or less, more preferably 0.60 μm or less. On the other hand, when the average diameter dM of martensite is less than 0.26 μm, martensite may not contribute to the strength. Moreover, when the average diameter dM of martensite is less than 0.26 μm, the coverage may be lowered. Therefore, the average diameter dM of martensite is set to 0.26 μm or more. Preferably, the average diameter dM of martensite is 0.30 μm or more.

Here, the ratio of the interfacial length of martensite and bainite to the total interfacial length of martensite is the total boundary length (interface length) between martensite and other metal structures adjacent to it, It represents the ratio of the total boundary length (interface length) between martensite and bainite. A method for obtaining the ratio will be described below.

First, the total interfacial length of martensite, i.e., the sum of the boundary lengths between martensite and other adjacent metal structures, in the martensite identified by the method described above, is The total value of the measured boundary lengths (interface lengths). The total interfacial length of martensite can be obtained by using a metal structure photograph taken by the same method as the method for measuring the average diameter dM of martensite, which will be described later. Specifically, 300 martensite grains are selected from the photographed metallographic structure, and the interfacial length of these grains is determined. For each martensite, measure the length of the boundary between martensite and other metal structures adjacent to it, and add up all the measured values to obtain the total length of the boundary between martensite and other metal structures adjacent to it. is the total interfacial length of martensite.

Next, the sum of boundary lengths between martensite and bainite is obtained. The total boundary length between martensite and bainite is the total value of the measured lengths of the boundaries between martensite and bainite identified by the method described above. This value is a value measured using the same martensite as the object of measurement when measuring the total interfacial length of martensite, and the number of measurements is also the same. That is, the "boundary between martensite and bainite" means the boundary between martensite and bainite among the boundaries between martensite and other metal structures adjacent thereto obtained by the above-described method. The total length is the "boundary length between martensite and bainite".
The value obtained by dividing the sum of the boundary lengths between martensite and bainite obtained by the above method by the sum of the boundary lengths between martensite and other metal structures adjacent to it is the coverage ratio of martensite with bainite. is the ratio of the interfacial length of martensite and bainite to the total interfacial length of martensite.

Next, how to obtain the average diameter dM of martensite will be described.
The martensite of the hot-rolled steel sheet of this embodiment has a plate-like form. Therefore, the grains of martensite are approximated to an ellipsoid, the major axis and the minor axis are measured, and the average value is taken as the average diameter of the measured martensite. Then, the average value of all measured diameters of martensite is calculated, and the average value of these values is defined as the average diameter dM of martensite of the hot-rolled steel sheet.
The number of measured average diameters dM of martensite is 300 pieces. Note that most of the martensite to be measured is fine (having a diameter of several μm or less). Therefore, it is preferable to perform the measurement using a metal structure photograph taken at a magnification of 5000 times. The field of view of the metallographic photograph and the measurement sample used when measuring the average diameter dM are the same as the field of view used when measuring the above-mentioned area ratio of martensite.

(Characteristic)
Next, the characteristics of the hot-rolled steel sheet of this embodiment will be described.
The hot-rolled steel sheet according to the present embodiment has a tensile strength of 780 MPa or more, a ductility (elongation at break) of 15.0% or more, and a hole expandability (hole expansion ratio) of 60% or more. good. Moreover, the fracture limit strain in a side bend test, which will be described later, may be 0.5 or more. By satisfying these properties in the hot-rolled steel sheet, it is possible to obtain a hot-rolled steel sheet that is suitable not only for automobile bodies, but also for automobile parts that are processed into complicated shapes (particularly chassis parts).

<Tensile strength>
The hot-rolled steel sheet according to this embodiment may have a tensile strength of 780 MPa or more. By setting the tensile strength to 780 MPa or more, it is possible to contribute to weight reduction of the vehicle body and parts. Although the upper limit is not particularly limited, it may be 950 MPa or less.

<Ductility (elongation at break)>
The hot-rolled steel sheet according to this embodiment may have an elongation at break of 15.0% or more.

<Hole expansibility>
The hot-rolled steel sheet according to the present embodiment may have a hole expansibility (hole expansibility) of 60% or more.

Tensile strength and elongation at break are measured according to JIS Z 2241:2011 using JIS Z 2241:2011 No. 5 test piece. A tensile test piece is taken in a direction perpendicular to the rolling direction and the plate thickness direction (plate width direction) so as to include a 1/4 part from the edge of the steel plate. At this time, the tensile test piece is taken with the direction perpendicular to the rolling direction as the longitudinal direction. The crosshead speed in the tensile test may be carried out under conditions such that the strain rate is constant at 0.005 s ⁻¹ .

The hole expandability (hole expansion ratio) is evaluated by the hole expansion ratio (λ) specified in JIS Z 2256:2010. Specifically, using a punch of φ10 mm, a die diameter is selected so that the clearance becomes 12.5%, and holes are punched out. After that, using a conical die with a tip angle of 60°, a hole expansion test is performed at a stroke speed of 10 mm/min with the burr on the outside. When the crack formed around the hole penetrates through the plate thickness, the test is stopped and the hole diameters before and after the hole expansion test are compared.

<Stretch flangeability (breaking limit strain)>
In the hot-rolled steel sheet of the present embodiment, the fracture limit strain evaluated by the side bend test described below is used as an index of stretch flangeability. The breaking limit strain of the hot-rolled steel sheet of this embodiment may be 0.5 or more.

Here, when molding an automobile part, it is desirable to mold so that the vertical wall height (molding height) of the stretch flange portion can be sufficiently secured for the purpose of ensuring the rigidity of the part. That is, as a material steel sheet for parts, a material steel sheet that can withstand this increase in forming height is desired. Generally, it is desired to use a material steel plate that can secure a forming height of 18 mm or more, for example. Therefore, the present inventors used conventional hot-rolled steel sheets with a tensile strength of 340 to 780 MPa to investigate the relationship between the formed height of the stretch flange formed part and the fracture limit strain by a side bend test. The results are shown in FIG. Note that, in the symbols in the graph shown in FIG. 1, the white symbols represent cases where molding was possible, and the black symbols represent cases where cracking occurred. Also, in the graph, for example, "780 material" means a 780 MPa material.

From the graph shown in FIG. 1, it can be seen that all the 340 MPa, 440 MPa and 590 MPa materials with a limit strain of 0.5 or more can be molded to a molding height of 18 mm or more without breakage or cracking. However, the 780 MPa material with a critical strain of 0.35 can be formed up to a forming height of about 15 mm, but cracks occur when the forming height exceeds that. That is, it was found that by setting the rupture limit strain of the steel plate as a material to 0.5 or more, it is possible to withstand forming in which the formed height of the stretch flange formed part is 18 mm or more.
From the above, the hot-rolled steel sheet of the present embodiment may have a rupture limit strain of 0.5 or more, preferably 0.6 or more, obtained by the side bend test described below.

(Side bend test method)
The rupture limit strain, which is an index of stretch flanging formability, is the value measured in the following side bend test. The side bend test in this embodiment employs the method described in "Shin Nippon Steel Technical Report No. 393, (2012) pp. 18-24" and "Japanese Patent Application Laid-Open No. 2009-145138".

The shape of the test piece for the side bend test shall be the shape shown in Fig. 2. The semicircular portion of the test piece may be made by shearing. Specifically, first, a plate of 35 mm×100 mm is cut out from a steel plate. After that, a punch of φ30 mm is used to punch a semicircular hole in the plate under the condition that the plate thickness clearance (the value obtained by dividing the gap between the punch and the die by the plate thickness) is 12.5%. These steps produce the test piece shown in FIG. Also, the radius of the semicircular portion of the test piece is 15 mm. Before conducting the side bend test, it is preferable to draw a grid pattern of 2 mm on the surface of the test piece in order to measure the rupture limit strain.

Using the above-mentioned side bend test piece, it is carried out by the apparatus and method described in "Shin Nippon Steel Technical Report No. 393, (2012) pp. 18-24" and "Japanese Patent Laid-Open No. 2009-145138". Specifically, the test piece is deformed at a stroke speed of 10 mm/min, and the formation of a crack at the edge of the hole is defined as "fracture". After breaking, the fracture limit strain with a gauge length of 6.0 mm is measured using a total of three elements, the judged element and the element adjacent thereto.
From the hot-rolled steel sheet to be evaluated, three or more side-bend test pieces are produced, and the rupture limit strain described above is measured for each test piece. Then, the average value of those rupture limit strains is defined as the rupture limit strain of the hot-rolled steel sheet in the side bend test.

(Thickness)
The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.6 to 8.0 mm. In particular, when considering application to the chassis of automobiles, the plate thickness is often 1.6 mm or more. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.6 mm or more. It is preferably 1.8 mm or more and 2.0 mm or more. Further, by setting the plate thickness to 8.0 mm or less, the metal structure can be easily refined, and the metal structure described above can be easily secured. Therefore, the plate thickness may be 8.0 mm or less. Preferably, it is 7.0 mm or less.

(Plating layer)
The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and metallographic structure may be provided with a plating layer on the surface thereof for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy. Examples of hot-dip coating layers include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn--Al alloy plating, hot-dip Zn--Al--Mg alloy plating, and hot-dip Zn--Al--Mg--Si alloy plating. be. The amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.

(manufacturing conditions)
Next, a method for manufacturing a hot-rolled steel sheet according to this embodiment will be described. The temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate. In this embodiment, the temperature of the hot-rolled steel sheet is measured with a contact or non-contact thermometer if it is the extreme end portion in the width direction of the steel sheet. If it is other than the extreme end portion in the width direction of the hot-rolled steel sheet, it is measured by a thermocouple or calculated by heat transfer analysis.

The method for manufacturing a hot-rolled steel sheet according to the present embodiment includes a hot rolling step of rolling a slab having the above-described chemical composition under conditions where the final finishing temperature is 880 ° C. or higher and 950 ° C. or lower; After the process, a primary cooling step of cooling to a primary cooling stop temperature of 680° C. or higher and 760° C. or lower at an average cooling rate of 60° C./second or higher, and after the primary cooling step, an average cooling rate of 20° C./second or lower: 1. A secondary cooling step in which cooling is performed for 6 seconds or more and 6.3 seconds or less; A tertiary cooling step of cooling to the tertiary cooling stop temperature, and after the tertiary cooling step, at a water volume density of 2.0 m ³ /min/mm ² or more and 7.2 m ³ /min/mm ² or less, 0.33 seconds or more, 1 A fourth cooling step of water cooling for 50 seconds or less, a fifth cooling step of air cooling for 3.0 seconds or more and 5.0 seconds or less after the fourth cooling step, and less than 180 ° C after the fifth cooling step and a winding step of winding with.

(Hot rolling process)
First, after rough rolling the hot slab having the chemical composition described above, finish rolling is performed under the condition that the final rolling delivery side temperature (final finishing temperature) is 880° C. or higher and 950° C. or lower. By performing finish rolling under such conditions, the area ratio of ferrite can be adjusted to an appropriate range. If the final finishing temperature is less than 880°C, the ferrite area ratio becomes excessively large. Also, if the final finishing temperature exceeds 950° C., it becomes difficult to secure a sufficient area ratio of ferrite. Therefore, the final finishing temperature should be 880° C. or higher and 950° C. or lower, preferably 890° C. or higher and 940° C. or lower.

(Primary cooling process)
After the hot rolling step, the steel sheet is cooled to a primary cooling stop temperature of 680°C or higher and 760°C or lower at an average cooling rate of 60°C/sec or higher (primary cooling step). In this primary cooling step, if the average cooling rate is less than 60° C./sec, pearlite is excessively formed, making it difficult to improve the hole expansibility. Therefore, it is preferable that the average cooling rate in the primary cooling step is 65° C./second or more. Although the upper limit of the average cooling rate in the primary cooling step is not specified, it may be 150° C./second or less, or 110° C./second or less. The cooling stop temperature (primary cooling stop temperature) in the primary cooling step may be 680° C. or higher and 760° C. or lower. If the primary cooling stop temperature is less than 680°C, the area ratio of ferrite may be insufficient. Also, even if the primary cooling stop temperature exceeds 760° C., the area ratio of ferrite is insufficient, and the area ratio of bainite may increase.

(Secondary cooling process)
After the primary cooling step, cooling is performed at an average cooling rate of 20° C./second or less for 1.6 seconds or more and 6.3 seconds or less (secondary cooling step). If the average cooling rate in the secondary cooling step exceeds 20° C./sec, the area ratio of ferrite may become insufficient. Therefore, the average cooling rate in the secondary cooling step should be 20° C./second or less, preferably 18° C./second or less. Further, if the cooling time in the secondary cooling step is less than 1.6 seconds, the area ratio of ferrite may become insufficient, and the area ratio of bainite may increase. On the other hand, if the cooling time in the secondary cooling step exceeds 6.3 seconds, the area ratio of ferrite may excessively increase, making it difficult to improve the strength. Moreover, if the cooling time in the secondary cooling step is too long, the area ratio of bainite may be insufficient. Therefore, the cooling time in the secondary cooling step is preferably 1.8 seconds or more and 6.1 seconds or less.

(Tertiary cooling process)
After the secondary cooling step, it is cooled to a tertiary cooling stop temperature of 195° C. or higher and 440° C. or lower at an average cooling rate of 60° C./second or higher and 130° C./second or lower. In this embodiment, a desired metallographic structure is obtained by precisely controlling the quaternary cooling process and the tertiary cooling process, which will be described later. Therefore, the tertiary cooling step and the quaternary cooling step are important steps from the viewpoint of ensuring stretch flangeability. That is, by increasing the average cooling rate in the tertiary cooling process, a large number of bainite is formed from the interface between ferrite and austenite generated in the primary cooling process and the secondary cooling process, or from the austenite/austenite grain boundary. can increase the bainite coverage of the remaining austenite. After that, in the quaternary cooling step, the remaining austenite is transformed into martensite, so that the bainite coverage of the present embodiment can be increased.

In the tertiary cooling process, the desired amount of bainite can be secured by setting the average cooling rate to 60°C/second or more and 130°C/second or less. If the average cooling rate in the tertiary cooling step is less than 60° C./sec, a sufficient degree of supercooling cannot be ensured, and a large amount of bainite is formed only from specific grain boundaries. As a result, it becomes difficult to obtain a sufficient coverage of martensite with bainite after the quaternary cooling step. Therefore, in the tertiary cooling step, the average cooling rate is set to 60° C./second or more, preferably 65° C./second or more, more preferably 70° C./second or more. On the other hand, if the average cooling rate in the tertiary cooling process exceeds 130° C./sec, the formation of bainite does not proceed sufficiently, making it difficult to obtain a sufficient coverage of martensite with bainite after the quaternary cooling process. Therefore, in the tertiary cooling step, the average cooling rate is set to 130° C./second or less, preferably 125° C./second or less, more preferably 120° C./second or less.

The temperature at which the tertiary cooling process ends (tertiary cooling stop temperature) should be 195°C or higher and 440°C or lower. If the tertiary cooling stop temperature is less than 195°C, the area ratio of bainite will be insufficient. Therefore, the tertiary cooling stop temperature is preferably 220° C. or higher, more preferably 250° C. or higher. On the other hand, if the tertiary cooling stop temperature exceeds 440°C, the area ratio of bainite increases, making it difficult to obtain good elongation at break. Therefore, the tertiary cooling stop temperature is preferably 420° C. or lower, more preferably 400° C. or lower.

(Quaternary cooling process)
After the tertiary cooling step, water cooling is performed at a water volume density of 2.0 m ³ /min/mm ² or more and 7.2 m ³ /min/mm ² or less for 0.33 seconds or more and 1.50 seconds or less.

Like the tertiary cooling process, this quaternary cooling process is an important process for controlling the coverage of martensite with bainite, the average diameter of martensite, and the area ratio of martensite.
In the quaternary cooling step, if the water density is less than 2.0 m ³ /min/mm ² , there is a possibility that the coverage of martensite with bainite cannot be ensured. The investigation by the inventors revealed that the coverage of martensite with bainite increases as the water density in the quaternary cooling process increases. Although the detailed mechanism is unknown, it is thought that the decrease in water density leads to a decrease in the driving force for bainite transformation, and as a result, the bainite transformation around martensite is delayed. In this embodiment, by setting the water volume density in the quaternary cooling process to 2.0 m ³ /min/mm ² or more, the coverage of martensite with bainite can be increased. From the viewpoint of the coverage of martensite with bainite, the upper limit of the water density is not particularly specified, but if it is 7.2 m ³ /min/mm ² or more, plate deformation due to water pressure may occur. Therefore, the water density is less than 7.2 m ³ /min/mm ² , preferably 7.0 m ³ /min/mm ² or less, more preferably 6.8 m ³ /min/mm ² or less.

In the quaternary cooling process, the water volume density is set within the above range, and the cooling time is set to 0.33 seconds or more and 1.50 seconds or less. Investigations by the present inventors have revealed that the average diameter dM of martensite changes depending on the cooling time of the quaternary cooling step. Specifically, when the cooling time is less than 0.33 seconds, the average diameter dM of martensite becomes excessively small. On the other hand, if the cooling time exceeds 1.50 seconds, the average diameter dM of martensite becomes excessively large. Therefore, the cooling time in the quaternary cooling step is set to 0.33 seconds or more and 1.50 seconds or less, preferably 0.40 seconds or more and 1.40 seconds or less.

(Fifth cooling process)
After the fourth cooling step, air cooling is performed for 3.0 seconds or more and 5.0 seconds or less to cool to less than 180°C (fifth cooling step).
In the fifth cooling step, after the fourth cooling step, air cooling is performed without water cooling for a period of 3.0 seconds or more and 5.0 seconds or less. The time without water cooling, that is, the air cooling time affects the coverage of martensite with bainite. Although the detailed mechanism is unknown, it is presumed that the martensite coverage was improved by the formation of bainite during air cooling in the fifth cooling step. If the air cooling time is less than 3.0 seconds, the coverage may be insufficient. On the other hand, if the air cooling time exceeds 5.0 seconds, the area ratio of bainite may excessively increase.

As a result of investigations by the present inventors, the coverage of martensite with bainite was less than 75.0% when the air cooling time was less than 3.0 seconds or more than 5.0 seconds. In order to obtain a more sufficient effect, the air cooling time should be 4.0 seconds or more and 4.8 seconds.

In the fifth cooling step, the steel sheet is coiled after air-cooling to a coiling temperature of less than 180°C.
When the coiling temperature is 180° C. or higher, the area ratio of martensite becomes insufficient, making it difficult to obtain excellent strength. Therefore, the winding temperature is preferably less than 180°C.

The hot-rolled steel sheet according to the present embodiment can be manufactured by the method described above.

The threading speed of the steel plate in the quaternary cooling process may be 360 to 790 mpm (meter per minute).

Also, after winding the hot-rolled steel sheet into a hot-rolled coil, the hot-rolled coil may be unwound and pickled for the purpose of removing the oxide film. Further, skin pass rolling may be performed within a range in which ductility is not deteriorated.

Also, in the present embodiment, equipment for performing each of the above cooling steps is not limited. Industrially, it is preferable to use a water spray device capable of precisely controlling the water volume density. For example, a water spray device may be placed between the transport rollers that transport the steel plate, and the steel plate may be cooled by spraying a predetermined amount of water from above and below. At this time, the heat history of the supercooling process as described above can be achieved by controlling the density of the water to be injected or by changing the opening/closing position of the valve.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one example of conditions. It is not limited. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

Steel sheet coils (hot-rolled steel sheets) with a width of 800 mm to 1080 mm were manufactured under the conditions shown in Tables 2A to 2C using the cast slabs having the chemical compositions shown in Tables 1A and 1B. In the quaternary cooling process, the plate threading speed was set in the range of 360 to 780 mpm (meter per minute). Further, in each cooling process, a predetermined heat history (cooling rate) was obtained by changing the open/close position of the valve on the run-out table (ROT). Further, the plate thickness of the hot-rolled steel plate was within the range of 2.0 mm to 6.0 mm. In addition, "FT" in Table 1 means the final finish temperature of finish rolling in the hot rolling process.

Regarding the microstructure of the manufactured hot-rolled steel sheet, the area ratio of each structure, the average grain size of martensite, and the coating ratio of martensite with bainite were measured by the above-described measurement methods. Each property of the hot-rolled steel sheet was evaluated by the following methods. Evaluation results are shown in Tables 3A to 3C.

"Tensile strength (TS)"
The tensile strength (TS) of the hot-rolled steel sheet was determined according to the test method described in JIS Z 2241:2011 using No. 5 test pieces of JIS Z 2241:2011. A tensile test piece was taken in a direction (sheet width direction) perpendicular to the rolling direction and the sheet thickness direction so as to include a 1/4 part from the edge of the steel sheet. At this time, the tensile test piece was sampled with the direction perpendicular to the rolling direction as the longitudinal direction. The crosshead speed in the tensile test was performed under the condition that the strain rate was constant at 0.005 s ^-1 . A tensile strength of 780 MPa or more was determined to be acceptable, and a tensile strength of less than 780 MPa was determined to be unacceptable.

"Breaking elongation"
The elongation at break, which is an index of ductility, was determined according to the test method described in JIS Z 2241:2011 in the same manner as the evaluation method for tensile strength (TS). When the elongation at break (%) was 15.0% or more, it was determined to be acceptable, and when it was less than 15.0%, it was determined to be unacceptable.

"Hole expansibility"
The hole expansibility was evaluated by the hole expansibility λ (%) measured according to JIS Z 2256:2010. Specifically, using a punch of φ10 mm, a die diameter was selected so that the clearance was 12.5%, and holes were punched out. After that, using a conical die with a tip angle of 60°, a hole expansion test was performed at a stroke speed of 10 mm/min with the burr on the outside. The test was stopped when the cracks formed around the hole penetrated through the plate thickness, and the hole diameters before and after the hole expanding test were compared to calculate the hole expanding ratio (%). A hole expansion ratio (%) of 60% or more was determined to be acceptable, and a case of less than 60% was determined to be unacceptable.

"Minor cracks on the sheared edge"
The hot-rolled steel sheet was sheared and the appearance of fine cracks on the sheared edge was visually observed. Specifically, shear processing is performed by observing the end of the punched semicircular part of the side bend test piece below with a microscope at a magnification of 50 times, and detecting cracks that do not penetrate the plate thickness existing only at the punched end. defined as micro-cracks. In this test, no crack penetrating through the sheet thickness occurred. The punching clearance at this time was set to 12.5%.

"Side bend test"
As an index of stretch flangeability, the limit strain at break evaluated by a side bend test was used. For the side bend test, the method described in "Nippon Steel Technical Report No. 393, (2012) pp. 18-24" and "Japanese Patent Laid-Open No. 2009-145138" was adopted.
Specifically, first, a plate of 35 mm×100 mm was cut out from a steel plate. After that, using a punch of φ30 mm, a semicircular hole was punched in the plate under the condition that the plate thickness clearance (the value obtained by dividing the gap between the punch and the die by the plate thickness) was 12.5%. . Through these steps, the test piece shown in FIG. 2 was produced.
The specimen was then deformed at a stroke speed of 10 mm/min, and the formation of a crack at the edge of the hole was defined as "fracture". After breaking, the fracture limit strain with a gauge length of 6.0 mm was measured using a total of three elements, the judged element and the adjacent element. In addition, in this example, three side bend test pieces were produced, the above-described rupture limit strain was measured for each test piece, and the average value of those rupture limit strains was evaluated. When the breaking limit strain was 0.5 or more, it was determined to be acceptable, and when it was less than 0.5, it was determined to be unacceptable.

According to Tables 3A to 3C, test numbers 11 to 25, 37 to 41 and test numbers 56 to 68, which are invention examples, have high strength and are excellent in ductility, hole expansibility and stretch flangeability.

Fig. 5 shows the relationship between the breaking limit strain and coverage in test numbers 2, 4, 5, 8, 9, 11-25. In all of Test Nos. 11 to 25, the average diameter of martensite and the area ratio of bainite are within the scope of the present invention. As shown in FIG. 5, a hot-rolled steel sheet in which the average diameter of martensite and the area ratio of bainite are within the scope of the present invention, and the coverage of martensite with bainite is 75.0% or more , it was found that the rupture limit strain due to side bending can be 0.5 or more. In other words, even if the average diameter of martensite and the area ratio of bainite are within the range of the present invention, if the coverage of martensite with bainite is less than 75.0%, it is difficult to increase the rupture limit strain. It turned out to be

Also, FIG. 6 shows the relationship between the fracture limit strain and the average martensite diameter dM in test numbers 6, 7, and 11 to 25. In all of Test Nos. 11 to 25, the area ratio of bainite and the coverage of martensite with bainite are within the scope of the present invention. As shown in FIG. 6, by using a hot-rolled steel sheet in which the area ratio and coverage of bainite are within the range of the present invention and the average diameter dM of martensite is within the range of the present invention, It was found that the breaking limit strain can be set to 0.5 or more. In other words, even if the area ratio and coverage of bainite are within the range of the present invention, it is difficult to increase the rupture limit strain if the average diameter dM of martensite is outside the range.

Also, Fig. 7 shows the relationship between the coverage ratio and the water volume density in the quaternary cooling process in test numbers 4, 5, and 11 to 25. In all of Test Nos. 11 to 25, the production conditions other than the water volume density in the quaternary cooling process are within the scope of the present invention. As shown in FIG. 7, the coverage of the hot-rolled steel sheet is sufficiently improved by manufacturing the hot-rolled steel sheet under the conditions in which all the manufacturing conditions, including the water density in the quaternary cooling process, are within the scope of the present invention. I found it possible. In other words, even if the production conditions other than the water density in the quaternary cooling process are within the scope of the present invention, if the water density in the quaternary cooling process is outside the range, it is difficult to increase the coverage rate. I found out.

Also, FIG. 8 shows the relationship between the average martensite diameter dM and the cooling time of the quaternary cooling process in test numbers 6, 7, and 11 to 25. In all of Test Nos. 11 to 25, manufacturing conditions other than the cooling time of the quaternary cooling process are within the scope of the present invention. As shown in FIG. 8, by manufacturing a hot-rolled steel sheet under conditions in which all manufacturing conditions, including the cooling time of the quaternary cooling process, are within the scope of the present invention, the average martensite diameter dM is sufficiently improved. I found it possible. In other words, even if the manufacturing conditions other than the cooling time of the quaternary cooling step are within the scope of the present invention, if the cooling time of the quaternary cooling step is outside the range, the average diameter dM of martensite can be increased. proved difficult.

Fig. 9 shows the relationship between the coverage of martensite with bainite and the air cooling time in test numbers 8, 9, and 11 to 25. In all of Test Nos. 11 to 25, the manufacturing conditions other than the air cooling time are within the scope of the present invention. As shown in FIG. 9, it was found that the coverage can be sufficiently improved by manufacturing the hot-rolled steel sheet under conditions in which all the manufacturing conditions, including the air cooling time, are within the scope of the present invention. In other words, even if the manufacturing conditions other than the air-cooling time are within the range of the present invention, if the air-cooling time is out of the range, it is difficult to increase the coverage of martensite with bainite.

As described above, it can be seen that Test Nos. 11 to 25, 37 to 41 and Test Nos. 56 to 68, which are invention examples within the scope of the present invention, are excellent in all properties.
For example, when the microstructure of Test No. 21, which is an invention example, was observed, the microstructure shown in FIG. 10A was obtained. As is clear from FIG. 10A, in Test No. 21, which is an invention example, martensite is sufficiently covered with bainite (dotted line area in the figure).

In addition, in order to subject Test No. 21 to the side bend test, shearing was performed on Test No. 21, and the structure near the sheared end face was observed. As a result, a micrograph as shown in FIG. 10B was obtained. According to FIG. 10B, no cracks were observed in the deformed martensite itself or at the interface between the martensite and other structures, and it can be seen that voids are generated in the ferrite with large deformation. In other words, bainite is arranged so as to cover the periphery of martensite, and this bainite plays a role like a cushion that mitigates the difference in deformation amount between ferrite and martensite, thereby preventing martensite from cracking. could have been suppressed.

As described above, it can be seen that the comparative examples of Test Nos. 1 to 10, 26 to 36 and Test Nos. 42 to 55 are inferior in one or more properties.
For example, in Test Nos. 4 and 5, which are comparative examples, the water volume density in the quaternary cooling process was less than 2.0 m ³ /min/mm ² , so the coverage of martensite with bainite was less than 75.0%. rice field. In addition, microcracks were observed on the sheared end face, and the breaking limit strain in the side bend test could not reach the target.

In test numbers 26 to 36, which are comparative examples, although the coverage of martensite with bainite and the average diameter dM of martensite were within the scope of the invention, other requirements for the metal structure were not satisfied. characteristics were not satisfied. For example, in Test No. 27 using steel type G having an appropriate chemical composition, the final finishing temperature of finish rolling exceeded 950° C., so the area ratio of ferrite was less than 53.0%. As a result, the ductility decreased.

In addition, Test No. 50, which is a comparative example, was manufactured under manufacturing conditions within the scope of the present invention, but the Al content in the chemical composition was high, so the area ratio of ferrite increased. As a result, the tensile strength was less than 780 MPa. In the case of Test No. 50, in which the conditions of the quaternary cooling process are appropriate and the area ratio of bainite exceeds 14.0%, the rupture limit strain of the side bend reaches the target. From this, it is extremely important to control the conditions of the quaternary cooling process within the scope of the present invention in order to suppress fine cracks on the sheared end face and thereby improve the rupture limit strain of the side bend. Do you get it.

Also, FIG. 11A shows a structure photograph (SEM photograph) of Test No. 6, which is a comparative example with a retention time shorter than 0.33 seconds. Moreover, FIG. 11B is an enlarged view of the area A shown in FIG. 11A. In the portion indicated by the arrow in FIG. 11B, linear contrast different from the grain boundary is observed. The contrast is considered to be the interface with austenite after the bainite transformation by tertiary cooling is completed. It can be seen that the martensite transformation progresses at the austenite-ferrite interface and the portion close to it, and as a result, the coverage of martensite with bainite is reduced.

Claims

The chemical composition, in mass %,
C: 0.035% or more and 0.085% or less,
Si: 0.001% or more and 0.15% or less,
Mn: 0.70% or more and 1.80% or less,
P: 0.020% or less,
S: 0.0050% or less,
Ti: 0.075% or more and 0.170% or less,
Nb: 0.003% or more and 0.050% or less,
Al: 0.10% or more and 0.40% or less,
N: 0.0080% or less,
Cr: 0% or more and 0.27% or less,
B: 0% or more and 0.0050% or less,
Ca: 0% or more and 0.0050% or less,
Mo: 0% or more and 0.40% or less,
Ni: 0% or more and 0.50% or less,
Cu: 0% or more and 0.50% or less, and REM: 0% or more and 0.0300% or less,
The balance consists of Fe and impurities,
metal structure
Ferrite has an area ratio of 53.0% or more and 76.0% or less,
Martensite has an area ratio of 3.0% or more and 10.0% or less,
Bainite is 14.0% or more and 39.0% or less in area ratio,
Perlite contains 2.6% or less in area ratio,
The average diameter of martensite is 0.26 μm or more and 0.70 μm or less,
The hot rolling characterized in that the total length of the interfaces between the martensite and the bainite among all the interfaces of the martensite is 75.0% or more of the total length of all the interfaces of the martensite. steel plate.
the chemical composition, by mass,
Cr: 0.06% or more and 0.27% or less,
B: 0.0003% or more and 0.0050% or less,
Ca: 0.0003% or more and 0.0050% or less,
Mo: 0.01% or more and 0.40% or less,
Ni: 0.01% or more and 0.50% or less,
Cu: 0.01% or more and 0.50% or less, and REM: 0.0003% or more and 0.0300% or less containing one or two or more of the group consisting of
The hot-rolled steel sheet according to claim 1, characterized in that:
For a slab having the chemical composition according to claim 1 or 2,
A hot rolling step of rolling under conditions where the final finishing temperature is 880 ° C. or higher and 950 ° C. or lower;
After the hot rolling step, a primary cooling step of cooling to a primary cooling stop temperature of 680° C. or higher and 760° C. or lower at an average cooling rate of 60° C./sec or higher;
After the primary cooling step, a secondary cooling step of cooling for 1.6 seconds or more and 6.3 seconds or less at an average cooling rate of 20° C./second or less;
After the secondary cooling step, a tertiary cooling step of cooling to a tertiary cooling stop temperature of 195° C. or higher and 440° C. or lower at an average cooling rate of 60° C./s or higher and 130° C./s or lower;
After the tertiary cooling step, the quaternary cooling is water-cooled at a water volume density of 2.0 m 3 /min/mm 2 or more and 7.2 m 3 /min/mm 2 or less for 0.33 seconds or more and 1.50 seconds or less. process and
A fifth cooling step of air-cooling for 3.0 seconds or more and 5.0 seconds or less after the fourth cooling step;
A method for producing a hot-rolled steel sheet, comprising: a winding step of winding at a temperature of less than 180°C after the fifth cooling step.