JP7688287B2 - HOT-ROLLED STEEL SHEET AND ITS MANUFACTURING METHOD - Google Patents

Description

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

In recent years, from the viewpoint of protecting the global environment, efforts are being made to reduce the weight of automobile bodies and parts in order to improve 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 the steel sheets used in the automobile bodies and parts.

Among automobile parts, suspension parts such as lower arms have complex shapes to fulfill their part 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 may be applied to suspension parts. In addition, since the hot-rolled steel sheets used for automobile parts are processed into complex shapes as described above, ductility and hole expandability are particularly required among the workability. In particular, in recent years, high-strength hot-rolled steel sheets with a tensile strength of 780 MPa or more and excellent ductility and hole expandability are required as hot-rolled steel sheets 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 improving the strength. In order to strengthen the precipitation of ferrite by Ti and Nb, Si may be added to the hot-rolled steel sheets. In particular, since the line length of the section from finish rolling to coiling is limited in the hot-rolling line, Si is often added when ferrite is formed in this section and carbides of Ti and Nb are precipitated. On the other hand, when Si is contained in the hot-rolled steel sheet, a scale pattern is generated on the surface of the steel sheet, which impairs the appearance, and the fatigue properties may be deteriorated.

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

Furthermore, Patent Document 2 discloses a high-strength steel sheet having both elongation (ductility) and hole expandability and improved fatigue strength by using a metal structure mainly composed of ferrite and reducing the area ratio of martensite.

Japanese Patent Application Publication No. 2004-204326 International Publication No. 2014/051005

However, in the 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 end surface (hereinafter also referred to as microcracks on the sheared end surface) occurs during shear processing, and forming cracks occur in the stretch flange portion starting from the microcracks on the sheared end surface. That is, in the conventional hot-rolled steel sheet, there was a problem that sufficient stretch flangeability could not be obtained due to the occurrence of microcracks on the sheared end surface during shear processing.

In view of the above problems, the present invention aims to provide a hot-rolled steel sheet having excellent strength, ductility, hole expandability and stretch flangeability, and a manufacturing method thereof.

In view of the above-mentioned problems, the present inventors have conducted extensive research into the relationship between the chemical composition and metal structure of a hot-rolled steel sheet and the above-mentioned characteristics, and have obtained the following findings (a) to (e) and completed the present invention.

(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 hole expandability, 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 microcracks on the sheared end surface during shear processing. In order to prevent microcracks on the sheared end surface, it is effective to arrange bainite, which has an intermediate deformability between ferrite and martensite, so that it is covered by martensite, that is, arranged adjacent to martensite, and to control the average diameter of martensite to a desired range. In this way, by appropriately arranging each metal structure and controlling the shape of martensite, it is possible to prevent microcracks on the sheared end surface during shear processing, and as a result, it is possible to obtain excellent stretch flangeability.
(e) In order to achieve the above-mentioned appropriate arrangement of the metal structure and the desired average grain size of martensite, it is important to precisely control the cooling conditions after hot rolling. In particular, it is important to generate a large amount of uniform bainite by controlling the cooling conditions in the bainite transformation temperature range after hot rolling to be within the desired range.

The gist of the present invention, which has been made based on the above findings, is as follows.
[1] A hot-rolled steel sheet according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.035% or more, 0.085% or less,
Si: 0.001% or more, 0.15% or less,
Mn: 0.70% or more, 1.80% or less,
P: 0.020% or less,
S: 0.0050% or less,
Ti: 0.075% or more, 0.170% or less,
Nb: 0.003% or more, 0.050% or less,
Al: 0.10% or more, 0.40% or less,
N: 0.0080% or less,
Cr: 0% or more, 0.27% or less,
B: 0% or more, 0.0050% or less,
Ca: 0% or more, 0.0050% or less,
Mo: 0% or more, 0.40% or less,
Ni: 0% or more, 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 is Fe and impurities,
The metal structure at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction is
Ferrite is 53.0% or more and 76.0% or less in area ratio,
The area ratio of martensite is 3.0% or more and 10.0% or less,
Bainite is more than 14.0% and not more than 39.0% in terms of area ratio;
The area ratio of pearlite is 2.6% or less , and
The retained austenite is 4.0% or less ,
The average diameter of martensite is 0.26 μm or more and 0.70 μm or less,
Of all the interfaces of the martensite, the total length of interfaces between the martensite and the bainite is 75.0% or more of the total length of all the interfaces of the martensite.
[2] The hot-rolled steel sheet according to the above [1], wherein the chemical composition is, by mass,
Cr: 0.06% or more, 0.27% or less,
B: 0.0003% or more, 0.0050% or less,
Ca: 0.0003% or more, 0.0050% or less,
Mo: 0.01% or more, 0.40% or less,
Ni: 0.01% or more, 0.50% or less,
One or more selected from the group consisting of Cu: 0.01% or more and 0.50% or less, and REM: 0.0003% or more and 0.0300% or less may be contained.
[3] A method for producing a hot-rolled steel sheet according to one aspect of the present invention is a method for producing a hot-rolled steel sheet according to the above [1] or [2], wherein the slab having the chemical composition according to the above [1] or [2] is
A hot rolling process in which rolling is performed under conditions in which the final finishing temperature is 880°C or more and 950°C or less;
After the hot rolling step, a primary cooling step of cooling the steel sheet to a primary cooling stop temperature of 680° C. or more and 760° C. or less at an average cooling rate of 60° C./sec or more;
A secondary cooling step in which cooling is performed at an average cooling rate of 20° C./sec or less for 1.6 seconds or more and 6.3 seconds or less after the primary cooling step;
After the secondary cooling step, a tertiary cooling step is performed at an average cooling rate of 60° C./sec or more and 130° C./sec or less to a tertiary cooling stop temperature of 195° C. or more and 440° C. or less;
After the tertiary cooling step, a fourth cooling step is performed in which the mixture is water-cooled for 0.33 seconds or more and 1.50 seconds or less at a water flow rate of ^{2.0 m3} /min/ ^mm2 or more and 7.2 ^m3 /min/mm2 or less.
A fifth cooling step of cooling the mixture in air for 3.0 seconds or more and 5.0 seconds or less after the fourth cooling step;
After the quinary cooling step, a winding step is performed at a temperature lower than 180°C.

According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having excellent strength, ductility, hole expandability and stretch flangeability, and a manufacturing method thereof.

FIG. 1 is a graph showing the relationship between the forming height of the stretch flange formed portion and the fracture limit strain in a side bend test for conventional hot rolled steel sheets having a tensile strength of 340 to 780 MPa. FIG. 2 is a schematic diagram of a test piece used in a side bend test. FIG. 3 is a photograph taken with a microscope of cracks that occurred on the sheared edge surface that had been subjected to stretch flange forming. FIG. 4A is a cross-sectional photograph of the sheared edge prior to stretch flange forming. FIG. 4B is an SEM photograph of the vicinity of the crack formed on the sheared end surface shown in FIG. 4A. FIG. 5 is a diagram showing the relationship between the fracture limit strain and the coverage in this embodiment. 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 flow rate in the quaternary cooling step 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 embodiment. FIG. 9 is a diagram showing the relationship between the coverage rate and the air-cooling time in this embodiment. FIG. 10A is a structural photograph (SEM photograph) of Test No. 21 in this example. FIG. 10B is a structural photograph (SEM photograph) of the vicinity of the sheared end surface after the shearing process was performed on Test No. 21 in this example. FIG. 11A is a structural 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 describe the results of their investigation into factors that affect the stretch flangeability of hot-rolled steel sheets, as well as their new findings regarding the relationship between the stretch flangeability and metallographic structure.

In conventional hot-rolled steel sheets, it was known that appropriate design of the chemical composition, especially active use of Ti and Nb, is effective in improving ductility and hole expandability. However, as a result of further intensive research by the present inventors, it was found that when a hot-rolled steel sheet with good ductility and hole expandability is sheared, fine peeling occurs on the sheared end surface, and when the hot-rolled steel sheet with microcracks on the sheared end surface is stretch-flanged, forming cracks may occur in the stretch flange portion. That is, the present inventors found that in conventional hot-rolled steel sheets, even if the ductility and hole expandability are good, microcracks occur on the sheared end surface during shearing, and forming cracks occur in the stretch flange portion due to these microcracks, and as a result, stretch flangeability may not be sufficiently obtained. Therefore, the present inventors have studied factors that cause forming cracks in the stretch flange portion and indicators that indicate stretch flangeability.

First, a hot-rolled steel sheet was manufactured by changing various cooling conditions after the hot rolling process using a slab having the steel composition B in Table 1 of the examples described later, and samples were cut out from a 1/4 position in the width direction of the manufactured hot-rolled steel sheet to examine each characteristic. As a result, the strength, ductility (fracture elongation), and hole expandability in the direction perpendicular to the rolling direction were good. However, when the hot-rolled steel sheet manufactured under some cooling conditions was subjected to shear processing and further to stretch flange forming (side bend test), a crack penetrated in the plate thickness direction and broke. The observation result of the fracture surface of the broken part is shown in FIG. 3.
Fig. 3 is a photograph taken with a microscope of cracks occurring on the sheared end surface that has been subjected to stretch flange forming. As shown in Fig. 3, among the sheared end surfaces after shear processing, cracks parallel to the sheet surface have occurred on the sheared end surface that has been subjected to stretch flange forming in a direction perpendicular to the rolling direction. In other words, it is presumed that the reason why the hot-rolled steel sheet manufactured under certain cooling conditions broke as described above is that the cracks parallel to the sheet surface started as the starting point and penetrated in the sheet thickness direction.

The cracks parallel to the plate surface were investigated in detail. Figure 4A shows a cross-sectional photograph of the sheared edge before stretch flange forming. As shown in Figure 4A, it was found that the cracks parallel to the plate surface on the sheared edge were already formed at the shear processing stage.
Furthermore, the results of the investigation of the relationship between the cracks parallel to the plate surface and the metal structure are shown in FIG. 4B. FIG. 4B is an SEM photograph of the vicinity of the crack formed on the sheared end surface shown in FIG. 4A. Note that FIG. 4B is an SEM photograph taken after Lepera corrosion, in which the white parts indicate martensite, the gray parts indicate ferrite, and the black parts indicate voids generated by cracks in the martensite. As a result of the investigation, as shown in FIG. 4B, in the vicinity of the crack formed on the sheared end surface, a part where the martensite deformed while peeling (arrow (1)) and a crack in the martensite itself (arrow (2)) were observed. From this, it was found that the peeling as shown by the arrow (1) in FIG. 4B and the crack in the martensite itself are the main factors that cause cracks parallel to the plate surface during the processing of the sheared surface. Note that in this specification, the part where the martensite deformed while peeling (arrow (1)) and the crack in the martensite itself (arrow (2)) shown in FIG. 4B are collectively referred to as "molding cracks in martensite".

Based on the above investigation results, next, in order to prevent cracks in martensite during forming, the inventors investigated the morphology of the metal structure that is effective for preventing cracks in martensite during forming, focusing on bainite, which has a deformability between ferrite and martensite. As a result, the inventors found that, rather than simply specifying the area ratio of bainite, having bainite surround martensite is effective for preventing cracks in martensite during forming. In other words, they found that by increasing the ratio of interfaces with bainite among all interfaces of martensite, it is possible to prevent cracks in martensite during forming, and as a result, stretch flangeability is improved.
The mechanism by which bainite effectively prevents cracks in martensite during forming is not clear, but it is thought to act as a cushion.

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

That is, according to the inventors, in order to prevent cracks parallel to the sheet surface during shear surface processing, it is important to: In other words, if the diameter of martensite is too large, even if martensite is surrounded by bainite, the hardness of the two is different, so that deformation is concentrated at the interface between bainite and martensite, and as a result, voids may be formed. Therefore, in order to prevent cracks parallel to the sheet surface during shear surface processing and improve stretch flangeability, it is important to arrange bainite and martensite in a desired arrangement and to set the diameter of martensite in a desired range.

Next, the stretch flangeability in the present invention will be described.
Conventionally, hole expansion tests, notch tensile tests, etc. have been commonly used as methods for evaluating stretch flangeability. However, these tests sometimes fail to accurately evaluate stretch flangeability, particularly in the case of automotive parts, such as frame parts and suspension parts, where tensile deformation is dominant.
For example, in the hole expansion test, the strain distribution changes rapidly in the circumferential direction, whereas in the stretch flange deformation, the strain changes with a gentle gradient in the circumferential and radial directions. Thus, the hole expansion test used conventionally does not allow evaluation of the stretch flangeability that reflects the shape of the part when actually forming the part and the influence of the properties of the steel sheet as the material, and it has been difficult to accurately evaluate the stretch flangeability that corresponds to the actual forming conditions.

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 flangeability in the present invention refers to the fracture limit strain (hereinafter also simply referred to as the limit strain) at the time when the open cross section of the steel sheet is deformed in-plane and a crack penetrates in the sheet thickness direction. In the present invention, the stretch flangeability is evaluated by the fracture limit strain obtained by the side bend test.

Next, the chemical composition and metal structure of the hot-rolled steel sheet according to this embodiment (hereinafter, may be simply referred to as steel sheet) will be described in more detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention.
In the following, the numerical ranges described with "to" include the lower and upper limits. Numerical values indicated as "less than" or "more than" are not included in the numerical range. In the following description, percentages related to the chemical composition of the steel sheet are mass% unless otherwise specified.

(chemical composition)
The chemical composition of the hot-rolled steel sheet according to this embodiment includes, 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: 0.005% 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, and N: 0.008% or less, and the balance: Fe and impurities. Each element will be described in detail below.

C: 0.035% or more, 0.085% or less The C (carbon) content is 0.035% or more, 0.085% or less. If the C content is less than 0.035%, it is difficult to sufficiently secure the area ratio of martensite. Therefore, the C content is 0.035% or more. In addition, from the viewpoint of steelmaking costs, it is not preferable to reduce the C content excessively. 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 is 0.085% or less. In addition, it is preferable to suppress the C content in order to reduce the occurrence rate of cracks in the slab during the casting process. For these reasons, the C content is preferably 0.065% or less.

Si: 0.001% or more, 0.15% or less The content of Si (silicon) is preferably low in terms of the appearance of the steel sheet. If the Si content exceeds 0.15%, the area ratio of ferrite may become excessively large. Therefore, the Si content is set to 0.15% or less. In order to reduce the cost of the pickling process for removing the scale generated in the hot rolling process, the lower the Si content, the more preferable it is. For these reasons, the Si content is preferably 0.07% or less. On the other hand, if the Si content is excessively reduced, the manufacturing cost in the steelmaking process increases significantly and the above-mentioned effects become saturated, so the Si content is set to 0.001% or more. The Si content is preferably 0.003% or more.

Mn: 0.70% or more, 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 is set to 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 decreases excessively and the area ratio of bainite increases, resulting in deterioration of ductility. Therefore, the Mn content is set to 1.80% or less. The Mn content is preferably 1.75% or less, 1.70% or less. Note that if the Mn content is less than 1.20%, a selvedge pattern may be formed at the widthwise end of the steel sheet or steel sheet coil. Usually, such a selvedge pattern 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, but has the effect of increasing the strength of hot-rolled steel sheets by solid solution strengthening. Therefore, P may be intentionally contained, but P is an element that segregates at grain boundaries and also has the effect of causing a decrease in ductility. In addition, if the P content exceeds 0.020%, the toughness of the hot-rolled slab is reduced, and cracks may occur during the rolling process, especially cracks at the corners of the slab (corner cracks). Therefore, the P content is set to 0.020% or less. The P content is preferably 0.015% or less. There is no need to specify a lower limit for the P content, and the lower the P content, the more preferable it is. The lower limit of the P content may 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 set to 0.001% or more.

S: 0.0050% or less S (sulfur) is an element contained as an impurity, and forms nonmetallic inclusions to reduce the ductility of the hot-rolled steel sheet. In addition, if the S content exceeds 0.0050%, the ductility of the hot-rolled steel sheet is significantly reduced, and defects or breakage may occur during the hot rolling process. Therefore, the S content is set to 0.0050% or less. The S content is preferably 0.0040% or less. There is no need to specify a lower limit for the S content, and the lower the S content, the more preferable it is. The lower limit of the S content may include 0%. However, if the S content is less than 0.0001%, the refining cost in the steelmaking process will be high. From the viewpoint of refining cost, the S content is preferably set to 0.0001% or more.

Ti: 0.075% or more, 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 is set to 0.075% or more. Ti is also an effective element for improving hole expandability. In order to obtain these effects more effectively, the Ti content is preferably set to 0.090% or more. On the other hand, if the Ti content exceeds 0.170%, the cold-cut 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% or more, 0.050% or less Nb (niobium) is an element that precipitates fine alloy carbides. In addition, Nb has the effect of suppressing the grain growth of austenite during hot rolling, thereby suppressing the coarsening of the grain size of ferrite that is transformed and generated thereafter. By exerting these effects, the strength of the steel plate can be increased. In order to obtain this effect, the Nb content is set to 0.003% or more. In addition, Nb also has the effect of suppressing the coarsening of grains in the heat affected zone during arc welding and suppressing the softening of the heat affected zone. In order to express these effects, the Nb content is preferably set to 0.010% or more. On the other hand, if the Nb content exceeds 0.050%, the toughness of the hot rolled slab decreases, and as a result, cracks and scratches may occur in the rolling process. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.045% or less.

Al: 0.10% or more, 0.40% or less Al (aluminum) is an element that is effective in deoxidizing steel and improving the soundness of the steel sheet. In addition, 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 the scale formed on the steel sheet surface in the hot rolling process, making it possible to easily remove the scale in the hot rolling. In order to obtain these effects more effectively, the Al content is preferably set to 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 is set to 0.40% or less. The Al content is preferably set to 0.35% or less.

N: 0.0080% or less N (nitrogen) is an element that forms nitrides such as Ti, Nb, or Al. These nitrides reduce the toughness of hot-rolled slabs and cause flaws and cracks during the rolling process, especially cracks at the corners of the slab (corner cracks). Therefore, in terms of production, the lower the N content, the better, and the N content is set to 0.0080% or less. The lower limit of the 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, and more preferably 0.0010% or more.

The balance of the chemical composition of the heat-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities refer to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, or elements that are intentionally added in small amounts, and are allowed within a range that does not adversely affect the heat-rolled steel sheet according to the present embodiment.

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 a part of Fe, one or more of Cr, B, Ca, Mo, Ni, Cu, and REM may be contained as optional elements in the ranges described below. When these optional elements are not contained, the lower limit of the content is 0%. Each optional element will be described in detail below.

Cr: 0.06% or more, 0.27% or less Cr (chromium) has the effect of increasing the tensile strength of the steel plate. In order to obtain the effect of this action, the Cr content is preferably 0.06% or more. The 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. The Cr content is more preferably 0.25% or less.

B: 0.0003% or more, 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, the B content is preferably 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 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, 0.0050% or less Ca (calcium) has the effect of spheroidizing nonmetallic inclusions to increase ductility. In order to obtain the effect of this action, the Ca content is preferably 0.0003% or more. The Ca content is more preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, the toughness of the slab decreases, and cracks and scratches may occur in the slab during the rolling process. Therefore, the Ca content is preferably 0.0050% or less. The Ca content is more preferably 0.0040% or less.

Mo: 0.01% or more, 0.40% or less Mo has the effect of increasing the tensile strength of the steel sheet. In order to obtain the effect of this action, the Mo content is preferably 0.01% or more. The 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. The Mo content is more preferably 0.35% or less.

Ni: 0.01% or more, 0.50% or less 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% or more, 0.50% or less Cu has the effect of increasing the tensile strength of the steel sheet. In order to obtain the effect of this action, the Cu content is preferably 0.01% or more. The 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. The 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 improving hole expandability and ductility (fracture elongation). If the REM content is less than 0.0003%, the effects of these actions cannot be fully obtained. Therefore, it is preferable that the REM content is 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 it is preferable that the REM content is 0.0300% or less.

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

The chemical composition of the above-mentioned hot-rolled steel sheet may be measured by ICP emission spectrometry using chips in accordance with 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 a combustion-infrared absorption method, and N may be measured using an 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 heat-rolled steel sheet according to the present embodiment, the metal structure contains, in area percentages, 53.0% to 76.0% ferrite, 3.0% to 10.0% martensite, 14.0% to 39.0% bainite, and 2.6% or less pearlite. In addition, in the heat-rolled steel sheet according to the present embodiment, the average diameter of the martensite is 0.26 μm to 0.70 μm, and 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.
In this embodiment, the metal structure at a quarter depth of the sheet thickness from the surface and at the center position in the sheet width direction in the sheet thickness cross section parallel to the rolling direction is specified because the metal structure at this position shows a typical metal structure of a hot-rolled steel sheet.

Ferrite: 53.0% or more, 76.0% or less Ferrite is a soft structure, and is therefore a metal structure that is mainly responsible for deformation. By making a multi-phase steel sheet containing martensite, such as the hot-rolled steel sheet of this embodiment, it is possible to obtain an effect of increasing elongation with an increase in the area ratio of ferrite, that is, an effect of improving ductility. However, if the ferrite area ratio is less than 53.0%, the ductility decreases. 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, 10.0% or less Martensite is a hard structure, so it contributes to improving 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 expandability may be significantly 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% or more, 39.0% or less Bainite is a structure that improves the strength and ductility of a hot-rolled steel sheet. In addition, stretch flangeability can be improved by arranging the metal structure so that bainite surrounds martensite. 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, and even more preferably 25.0% or more. On the other hand, if the area ratio of bainite exceeds 39.0%, the ductility (elongation at break) may be significantly deteriorated. 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 expandability 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 metal structure, the steel may contain retained austenite. However, if the area ratio of the retained austenite exceeds 4.0%, the toughness may decrease. Therefore, if the steel contains retained austenite, the area ratio is preferably 4.0% or less, and more preferably 3.0% or less. The area ratio of the retained austenite may be 0%.

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

The area ratio of the metal structure of the hot-rolled steel sheet may be 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. The metal structure information such as a metal structure photograph may be obtained by cutting out the width center position in the direction perpendicular to the rolling direction and the sheet thickness direction parallel to the rolling direction, and observing the field of view including the center at a depth of 3/8 of the sheet thickness from the surface in the sheet thickness direction. The observation field is three or more locations, 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 representative 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 this embodiment, the area ratio of ferrite (hereinafter, sometimes referred to as Vα) refers to the area ratio of the ferrite structure determined by the electron backscatter diffraction (EBSD) method. To measure the ferrite area ratio, first, measurement is performed by the EBSD method to obtain crystal orientation information (crystal orientation mapping data). For the measurement, an apparatus consisting 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 the measurement, the degree of vacuum in the device 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 a crystal grain from the crystal orientation mapping data, where a crystal grain refers to a boundary in the crystal orientation mapping data where the difference in crystal orientation between any measurement point and its adjacent measurement point is 15° or more, i.e., an area surrounded by a grain boundary.

The second step is to determine whether the grains defined in the first step are ferrite grains. The method for determining whether the grains are ferrite uses the local misorientation average (GAM value). The GAM value is a value that indicates the misorientation of the grains. If the GAM value of the grain to be determined is within 0.35°, the grain is determined to be ferrite.

In the third step, the determination in the second step is performed for all crystal grains recorded in the crystal orientation mapping data. Then, the ratio of the number of measurement points belonging to the crystal grains determined to be ferrite to the number of all measurement points in the crystal orientation mapping data is calculated. This ratio is defined as the area ratio of ferrite.

In order to sufficiently reduce the error of the ferrite area ratio due to the measurement position, the crystal orientation mapping data should contain a total of 1000 crystal grains. The measurement magnification when obtaining the crystal orientation mapping data by EBSD analysis is set so that the field of view contains 1000 crystal grains. Furthermore, in an analysis at a low magnification of less than 200 times, the measurement accuracy decreases due to the influence of distortion of the electron beam, etc. 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 of the ferrite area ratio is a rectangle consisting of sides in the sheet thickness direction and the rolling direction. The side in the sheet thickness direction is 500 μm, and the side in the rolling direction is equal to the side in the sheet thickness direction. The measurement of the ferrite area ratio is performed in a range including a position of 3/8 of the sheet thickness from the surface in the sheet thickness direction. In addition, the measurement interval of the crystal orientation within the measurement range is 0.03 μm. If the measurement interval is less than 0.03 μm, the interference range of the electron beam may overlap. On the other hand, if the measurement interval exceeds 0.03 μm, the number of measurement points of the crystal orientation contained in the crystal grain is insufficient, and measurement errors are likely to occur.

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

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

A method for measuring the area fraction VM of martensite will be described below.
First, a scanning electron microscope is used to photograph the visual field used in measuring the martensite area ratio VM. To improve the measurement accuracy, the metal structure may be photographed at 5000x magnification. Furthermore, if the magnification is 5000x, at least one martensite grain can be photographed within one visual field. Therefore, the magnification is preferably 5000x. In this embodiment, the martensite area ratio is measured by observing an area of 500 μm×500 μm at a magnification of 5000x.

When measuring the area ratio VM of martensite, the acceleration voltage for irradiating the electron beam is set to be in the range of 10.0 kV or more and 15.0 kV or less. If the acceleration voltage is more than 15.0 kV, the grain boundary may become unclear. On the other hand, if the acceleration voltage is less than 10.0 kV, the resolution decreases and it is not suitable for observation.

The backscattered electron images obtained by these observation samples and observation conditions are used to measure the area ratio VM of martensite. Specifically, in order to reduce the error between the 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 in the measurement range may be 1000. 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 fraction of bainite (hereinafter sometimes referred to as VB) is the remainder obtained by subtracting the total of the area fractions of ferrite, martensite, retained austenite (described later), and pearlite obtained by the above-mentioned method from 100%, and this is defined as the area fraction VB of the bainite structure.

The area ratio of pearlite (hereinafter, sometimes referred to as VP) refers to a value measured from a metal structure revealed by nital etching.

A sample for measuring pearlite may 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.
A metallographic photograph is taken of the sample for measuring pearlite in a measurement range centered at 3/8 position from the surface of the steel sheet in the sheet thickness direction. The sample for measuring pearlite is the same as the sample for measuring the area ratio of ferrite and martensite.

In this embodiment, a metal structure photograph for measuring the area ratio of pearlite is obtained by a scanning electron microscope. In photographing with a scanning electron microscope, the acceleration voltage when irradiating the electron beam is in the range of 10.0 kV or more and 15.0 kV. If the acceleration voltage is more than 15.0 kV, the grain boundary may become unclear. On the other hand, if the acceleration voltage is less than 10.0 kV, the resolution decreases and it is not suitable for observation. And, in order to sufficiently improve the measurement accuracy, the metal structure may be photographed at a magnification of 2000 times or more. If the magnification is 10000 times or less, one or more pearlite grains can be photographed in one field of view. Therefore, the magnification may be 10000 times or less. In order to reduce the number of fields of view and obtain measurement accuracy, the magnification is preferably 5000 times. In addition, the measurement range is 10 μm or more and 40 μm or less in the plate thickness direction, and 10 μm or more and 55 μm or less in the rolling direction.

The area fraction of retained austenite (hereinafter sometimes referred to as Vγ) is the value obtained by dividing the number of measurement points of the crystal orientations in which the crystal structure is determined to be fcc, among the crystal orientation mapping data used to determine the above-mentioned ferrite area fraction Vα, by the total number of measurement points in the crystal orientation mapping data. The crystal orientation mapping data used to measure the area fraction of retained austenite Vγ is the same data as that used to measure the ferrite area fraction Vα. In other words, the measurement range, measurement magnification, and field of view may also be the same as those used in the measurement method for the ferrite area fraction.

(Metal structure morphology)
Next, the form 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 this embodiment, it is important to have the above-mentioned metal structure and an area ratio within a desired range, and to set the ratio of the interface length between martensite and bainite to the total interface length of martensite and the average diameter dM of martensite within a desired range.

The ratio of the interface length between martensite and bainite to the total interface length of martensite (hereinafter, sometimes referred to as coverage) is 75.0% or more. Bainite is a metal structure with intermediate strength between ferrite and martensite, so it is considered to play a role of mitigating the deformation difference between ferrite and martensite, that is, a role like a cushion. If the coverage rate of martensite by bainite is less than 75.0%, this role as a cushion becomes insufficient, and microcracks occur on the shear end surface. As a result, it becomes difficult to obtain excellent stretch flangeability. In addition, if the coverage rate is less than 75.0%, the fracture limit strain described later decreases. Therefore, the higher the ratio of the interface length between martensite and bainite to the total interface length of martensite, the more preferable, and it is preferably 78% or more. On the other hand, the upper limit of the ratio of the interface length between martensite and bainite to the total interface length of martensite is not particularly specified, and it may be 100%.

As described above, in order to improve stretch flangeability, it is effective not only to increase the area fraction of bainite but also to arrange bainite so as to surround martensite, that is, to increase the ratio of the interface length between martensite and bainite to the total interface length of martensite.

From the viewpoint of suppressing voids, the average diameter dM of martensite is set to 0.26 μm or more and 0.70 μm or less. By setting the average diameter dM in this range, it is possible to suppress microcracks on the sheared end surface, and as a result, high stretch flangeability can be obtained. If the average diameter dM of martensite is more than 0.70 μm, the hardness of martensite and bainite is different, so that deformation is concentrated at the interface between the two. As a result, even if the coverage rate of bainite is satisfied, voids may be formed near the interface between martensite and bainite. In addition, if voids are formed, microcracks may occur on the sheared end surface, 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, if the average diameter dM of martensite is less than 0.26 μm, martensite may not contribute to strength. Furthermore, if the average diameter dM of martensite is less than 0.26 μm, the coverage may decrease. 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 interface length between martensite and bainite to the total interface length of martensite refers to the ratio of the total interface length between martensite and bainite to the total interface length between martensite and other metallic structures adjacent thereto. A method for determining this ratio will be described below.

First, the total interface length of martensite, i.e., the total boundary length between martensite and other metal structures adjacent thereto, is the total value obtained by measuring the boundary length (interface length) between martensite identified by the above-mentioned method and other metal structures adjacent thereto. This total interface length of martensite may be obtained by using a metal structure photograph taken by the same method as the measurement method of the average diameter dM of martensite described later. Specifically, 300 martensite grains are selected from the metal structure photograph taken, and the interface length of these grains is obtained. For each martensite, the boundary length between martensite and other metal structures adjacent thereto is measured, and the total value of all the measured values is the total boundary length between martensite and other metal structures adjacent thereto, i.e., the total interface length of martensite.

Next, the total boundary length between martensite and bainite is calculated. The total boundary length between martensite and bainite is the total of the measured lengths of the boundaries where the martensite and bainite contact, identified by the above-mentioned method. This value is measured using the same martensite as that measured when measuring the total interface length of the martensite, and the number of measurements is also the same. In other words, the "boundary between martensite and bainite" means the boundary between martensite and bainite among the boundaries between the martensite and other metallic structures adjacent thereto, calculated by the above-mentioned method, and the total length of the boundaries is the "boundary length between martensite and bainite."
The total boundary length between martensite and bainite obtained by the above method is divided by the total boundary length between martensite and other metallic structures adjacent thereto, to obtain the coverage rate of martensite with bainite, i.e., the ratio of the interface length between martensite and bainite to the total interface length of martensite.

Next, a method for determining the average diameter dM of martensite will be described.
The martensite of the hot-rolled steel sheet of this embodiment has a plate-like shape. Therefore, the crystal grains of martensite are approximated as ellipses, and the major and minor axes are measured and the average value is taken as the average diameter of the measured martensite. Then, the average value of all the measured diameters of the martensite is calculated, and the average value of these values is taken as the average diameter dM of the martensite of the hot-rolled steel sheet.
The number of pieces measured for the average diameter dM of martensite is 300. Most of the martensite to be measured is fine (diameter is several μm or less). Therefore, it is preferable to perform the measurement using a metallographic 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 those used when measuring the area ratio of martensite described above.

(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 may satisfy the following characteristics: tensile strength of 780 MPa or more, ductility (elongation at break) of 15.0% or more, and hole expansion property (hole expansion ratio) of 60% or more. The breaking limit strain by the side bend test described later may be 0.5 or more. By satisfying these characteristics in the hot-rolled steel sheet, it is possible to obtain a hot-rolled steel sheet suitable for use as a material not only for automobile bodies but also for automobile parts (especially suspension parts) that are processed into complex shapes.

<Tensile strength>
The hot-rolled steel sheet according to the present embodiment may have a tensile strength of 780 MPa or more. By making the tensile strength 780 MPa or more, it is possible to contribute to weight reduction of the vehicle body and parts. There is no need to particularly limit the upper limit, but it may be 950 MPa or less.

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

<Hole expandability>
The hot-rolled steel sheet according to this embodiment may have a hole expandability (hole expansion ratio) of 60% or more.

The tensile strength and elongation at break are measured in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The tensile test piece is taken from the end of the steel plate in a direction perpendicular to the rolling direction and the plate thickness direction (plate width direction) so as to include a 1/4 portion. 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 performed 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, a φ10 mm punch is used to select a die diameter so that the clearance is 12.5%, and a hole is punched out. Then, a conical die with a tip angle of 60° is used so that the burr is on the outside, and a hole expansion test is performed at a stroke speed of 10 mm/min. The test is stopped when the crack formed around the hole penetrates the plate thickness, and the hole diameter before and after the hole expansion test is compared.

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

Here, when forming an automobile part, it is desirable to form the part so that the vertical wall height (forming height) of the stretch flange portion can be sufficiently secured in order to ensure 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. In general, for example, a material steel sheet that can secure a forming height of 18 mm or more is desired. Therefore, the present inventors investigated the relationship between the forming height of the stretch flange forming portion and the fracture limit strain by the side bend test using a conventional hot-rolled steel sheet having a tensile strength of 340 to 780 MPa. The results are shown in FIG. 1. In addition, in the symbols in the graph shown in FIG. 1, white symbols represent the case where forming was possible, and black symbols represent the case where cracks occurred. In addition, in the graph, for example, "780 material" means a 780 MPa material.

From the graph shown in Figure 1, it can be seen that the 340 MPa, 440 MPa, and 590 MPa materials, which have a limit strain of 0.5 or more, can all be formed to a forming height of 18 mm or more without breaking or cracking. However, it can be seen that the 780 MPa material, which has a limit strain of 0.35, can be formed to a forming height of about 15 mm, but cracks will occur if the forming height is greater than that. In other words, it was found that by setting the breaking limit strain of the steel sheet, which is the raw material, to 0.5 or more, it can withstand forming to a forming height of 18 mm or more of the stretch flange forming part.
In view of the above, the heat-rolled steel sheet of this embodiment may have a fracture limit strain obtained by the side bend test described below of 0.5 or more, and preferably 0.6 or more.

(Side Bend Test Method)
The fracture limit strain, which is an index of stretch flangeability, is a value measured by the following side bend test. Note that the side bend test in this embodiment employs the method described in "Nippon Steel Technical Review No. 393, (2012) pp. 18-24" and "JP Patent Publication No. 2009-145138".

The shape of the test piece for the side bend test is the shape shown in FIG. 2. The semicircular portion of the test piece may be formed by shear processing. Specifically, first, a plate of 35 mm x 100 mm is cut out from a steel plate. Then, a semicircular hole is punched into the plate using a φ30 mm punch under the condition of a plate thickness clearance (a value obtained by dividing the gap between the punch and the die by the plate thickness) of 12.5%. Through these steps, the test piece shown in FIG. 2 is created. The radius of the semicircular portion of the test piece is 15 mm. Before performing the side bend test, a 2 mm grid pattern may be drawn on the surface of the test piece to measure the fracture limit strain.

Using the above side bend test piece, the test is carried out with the apparatus and method described in "Nippon Steel Technical Report No. 393, (2012) pp. 18-24" and "JP Patent Publication No. 2009-145138". Specifically, the test piece is deformed at a stroke speed of 10 mm/min, and the formation of a crack on the edge of the hole is defined as "fracture". After fracture, the fracture limit strain is measured at a gauge length of 6.0 mm using a total of three elements, including the determined element and the adjacent elements.
Three or more side bend test pieces are prepared from the hot-rolled steel sheet to be evaluated, and the fracture limit strain is measured for each test piece. The average value of the fracture limit strains is defined as the fracture 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 undercarriage of an automobile, the 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, 2.0 mm or more. Moreover, by making the thickness 8.0 mm or less, it becomes easy to refine the metal structure, and the above-mentioned metal structure can be easily secured. Therefore, the thickness may be 8.0 mm or less. It is preferably 7.0 mm or less.

(Plating layer)
The hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be provided with a plating layer on the surface 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-dip plating layer. Examples of the electroplating layer include electrogalvanizing and electrogalvanizing Zn-Ni alloy plating. Examples of the hot-dip plating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, 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. The coating weight is not particularly limited and may be the same as in the past. In addition, it is also possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application of a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.

(manufacturing conditions)
Next, a manufacturing method of a hot-rolled steel sheet according to the present embodiment will be described. In this embodiment, the temperature of the slab and the temperature of the steel sheet refer to the surface temperature of the slab and the surface temperature of the steel sheet. In this embodiment, the temperature of the hot-rolled steel sheet is measured by a contact or non-contact thermometer at the end in the width direction of the sheet. If the temperature is other than the end 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 producing a hot-rolled steel sheet according to this embodiment includes a hot rolling process in which a slab having the above-mentioned chemical composition is rolled under conditions in which the final finishing temperature is 880°C or more and 950°C or less, a primary cooling process in which, after the hot rolling process, cooling is performed at an average cooling rate of 60°C/sec or more to a primary cooling stop temperature of 680°C or more and 760°C or less, a secondary cooling process in which, after the primary cooling process, cooling is performed at an average cooling rate of 20°C/sec or less for 1.6 seconds or more and 6.3 seconds or less, a tertiary cooling process in which, after the secondary cooling process, cooling is performed at an average cooling rate of 60°C/sec or more and 130°C/sec or less to a tertiary cooling stop temperature of 195°C or more and 440°C or less, and a cooling rate of 2.0 ^m3 /min/mm2 ^or more and 7.2 ^m3 /min/mm2 or more after the tertiary cooling process. The method includes a quaternary cooling step of water-cooling the sheet at a water flow rate of ² or less for a period of 0.33 seconds or more and 1.50 seconds or less, a quinary cooling step of air-cooling the sheet at a period of 3.0 seconds or more and 5.0 seconds or less after the quaternary cooling step, and a winding step of winding the sheet at a temperature less than 180° C. after the quinary cooling step.

(Hot rolling process)
First, a hot slab having the above-mentioned chemical composition is rough-rolled, and then finish-rolled under conditions where the final rolling delivery temperature (final finishing temperature) is 880°C or more and 950°C or less. By performing finish rolling under such conditions, the area ratio of ferrite can be set to an appropriate range. If the final finishing temperature is less than 880°C, the area ratio of ferrite becomes excessively large. Also, if the final finishing temperature exceeds 950°C, it becomes difficult to sufficiently secure the area ratio of ferrite. Therefore, the final finishing temperature is set to 880°C or more and 950°C or less, and preferably, the final finishing temperature is set to 890°C or more and 940°C or less.

(Primary cooling process)
After the hot rolling process, the steel is cooled to a primary cooling stop temperature of 680°C or more and 760°C or less at an average cooling rate of 60°C/s or more (primary cooling process). In this primary cooling process, if the average cooling rate is less than 60°C/s, pearlite is excessively generated, making it difficult to improve the hole expandability. Therefore, it is preferable that the average cooling rate in the primary cooling process is 65°C/s or more. The upper limit of the average cooling rate in the primary cooling process is not particularly specified, but it may be 150°C/s or less, or 110°C/s or less. The cooling stop temperature in the primary cooling process (primary cooling stop temperature) may be 680°C or more and 760°C or less. If the primary cooling stop temperature is less than 680°C, the area ratio of ferrite may be insufficient. In addition, even if the primary cooling stop temperature is more than 760°C, the area ratio of ferrite may be insufficient and the area ratio of bainite may increase.

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

(Tertiary cooling process)
After the secondary cooling step, the steel is cooled to a tertiary cooling stop temperature of 195°C to 440°C at an average cooling rate of 60°C/s to 130°C/s. In this embodiment, the desired metal structure is obtained by precisely controlling the tertiary cooling step and the quaternary cooling step 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 step, a large amount of bainite can be formed from the interface between ferrite and austenite generated in the primary cooling step and the secondary cooling step, or the austenite/austenite grain boundary, and the bainite coverage of the remaining austenite can be increased. Then, in the quaternary cooling step, the remaining austenite is transformed into martensite, thereby increasing the bainite coverage of this embodiment.

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

The temperature at which the tertiary cooling step is ended (tertiary cooling stop temperature) may be 195°C or higher and 440°C or lower. If the tertiary cooling stop temperature is lower than 195°C, the area ratio of bainite becomes 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 fracture elongation. 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, the slab is water-cooled at a water flow rate of 2.0 m ³ /min/mm ² to 7.2 m ³ /min/mm ² for 0.33 seconds to 1.50 seconds.

This quaternary cooling step, like the tertiary cooling step, is an important step in controlling the coverage rate of martensite with bainite, the average diameter of martensite, and the area ratio of martensite.
In the fourth cooling step, if the water flow density is less than 2.0 ^{m 3} /min/mm ² , the coverage of martensite with bainite may not be ensured. The inventors' investigations have revealed that the coverage of martensite with bainite increases as the water flow density in the fourth cooling step increases. Although the detailed mechanism is unclear, it is believed that a decrease in the water flow density leads to a decrease in the driving force of bainite transformation, and as a result, the bainite transformation around martensite is delayed. In this embodiment, the coverage of martensite with bainite can be increased by setting the water flow density in the fourth cooling step to 2.0 m ³ /min/mm ² or more. Note that, from the viewpoint of the coverage of martensite with bainite, the upper limit of the water flow 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 flow rate is less than 7.2 m ³ /min/mm ² , preferably 7.0 m ³ /min/mm ² or less, and more preferably 6.8 m ³ /min/mm ² or less.

In the fourth cooling step, the water flow rate is set within the above range, and the cooling time is set to 0.33 seconds or more and 1.50 seconds or less. In the investigation by the present inventors, it was found that the average diameter dM of martensite changes depending on the cooling time of the fourth cooling step. Specifically, if 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 of the fourth cooling step is set to 0.33 seconds or more and 1.50 seconds or less, and preferably 0.40 seconds or more and 1.40 seconds or less.

(Fiveth cooling process)
After the fourth cooling step, the mixture is air-cooled for 3.0 seconds or more and 5.0 seconds or less to a temperature lower than 180° C. (fifth cooling step).
In the fifth cooling step, after the fourth cooling step, air cooling is performed for 3.0 seconds or more and 5.0 seconds or less without water cooling. This time without water cooling, i.e., the air cooling time, affects the coverage of martensite with bainite. Although the detailed mechanism is unknown, it is presumed that the coverage of martensite is improved by forming bainite even in the air cooling of 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 is more than 5.0 seconds, the area ratio of bainite may increase excessively.

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

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

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

The speed at which the steel plate is passed through the fourth cooling process is 360 to 790 mpm (meter per
minute).

In addition, after the hot-rolled steel sheet is wound into a hot-rolled coil, the hot-rolled coil may be unwound and subjected to pickling for the purpose of removing an oxide film. In addition, skin pass rolling may be performed within a range in which ductility is not deteriorated.

In this embodiment, the equipment for performing each of the above cooling steps is not limited. Industrially, it is preferable to use a water spray device that can precisely control the water density. For example, a water spray device may be disposed between the transport rollers that transport the steel sheet, and a predetermined amount of water may be sprayed from above and below the steel sheet to cool it. In addition, by controlling the density of the water sprayed or changing the opening and closing positions of the valves, the thermal history of the supercooling step as described above can be achieved.

Next, an embodiment of the present invention will be described, but the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Using the cast slabs having the chemical compositions shown in Tables 1A and 1B, steel sheet coils (hot-rolled steel sheets) having widths of 800 mm to 1080 mm were produced under the conditions shown in Tables 2A to 2C. In the fourth cooling step, the sheet threading speed was set to a range of 360 to 780 mpm (meter per minute). In each cooling step, a predetermined thermal history (cooling rate) was achieved by changing the opening and closing positions of the valves on the run-out table (ROT). The thickness of the hot-rolled steel sheets was set to a range of 2.0 mm to 6.0 mm. In Table 1, "FT" means the final finishing temperature of the finish rolling in the hot rolling step.

Regarding the microstructure of the produced hot-rolled steel sheets, the area ratio of each structure, the average grain size of martensite, and the coverage of martensite with bainite were measured by the above-mentioned measurement methods. In addition, each property of the hot-rolled steel sheets was evaluated by the following methods. The evaluation results are shown in Tables 3A to 3C.

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

"Elongation at break"
The breaking elongation, which is an index of ductility, was determined in the same manner as the evaluation method of the tensile strength (TS) described above, according to the test method described in JIS Z 2241: 2011. A breaking elongation (%) of 15.0% or more was judged as pass, and a breaking elongation (%) of less than 15.0% was judged as fail.

"Hole expansion ability"
The hole expandability was evaluated by the hole expansion ratio λ (%) measured in accordance with JIS Z 2256:2010. Specifically, a φ10 mm punch was used to select a die diameter so that the clearance was 12.5%, and a hole was punched out. Then, a hole expansion test was performed at a stroke speed of 10 mm/min using a conical die with a tip angle of 60° so that the burrs were on the outside. The test was stopped when the crack formed around the hole penetrated the plate thickness, and the hole diameters before and after the hole expansion test were compared to calculate the hole expansion ratio (%). A hole expansion ratio (%) of 60% or more was judged to be pass, and a hole expansion ratio (%) of less than 60% was judged to be fail.

"Microcracks on sheared edge"
The hot-rolled steel sheet was sheared, and the occurrence of microcracks on the sheared end surface was visually observed. Specifically, the end of the punched semicircular portion of the side bend test piece described below was observed with a microscope at a magnification of 50 times, and a crack that does not penetrate the plate thickness and exists only at the punched end was defined as a microcrack. In this test, no cracks penetrating the plate thickness occurred. The punching clearance at this time was 12.5%.

"Side Bend Test"
The fracture limit strain evaluated by a side bend test was used as an index of stretch flangeability. The side bend test was performed according to the method described in "Nippon Steel Technical Review No. 393, (2012) pp. 18-24" and "JP Patent Publication No. 2009-145138".
Specifically, a 35 mm x 100 mm plate was cut out from the steel plate. A semicircular hole was then punched into the plate using a φ30 mm punch under the condition of a plate thickness clearance (a value obtained by dividing the gap between the punch and the die by the plate thickness) of 12.5%. Through these steps, the test piece shown in FIG. 2 was produced.
Next, the test piece was 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 fracture, the fracture limit strain was measured at a gauge length of 6.0 mm using a total of three elements, including the judged element and the adjacent element. In this example, three side bend test pieces were prepared, and the fracture limit strain was measured for each test piece, and the average value of the fracture limit strains was used for evaluation. A fracture limit strain of 0.5 or more was judged as pass, and a fracture limit strain of less than 0.5 was judged as fail.

According to Tables 3A to 3C, it can be seen that Test Nos. 11 to 25, 37 to 41 and Test Nos. 56 to 68, which are examples of the invention, have high strength and are excellent in ductility, hole expandability and stretch flangeability.

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

FIG. 6 shows the relationship between the fracture limit strain and the average diameter dM of martensite in test numbers 6, 7, and 11 to 25. Note that the area ratio of bainite and the coverage of martensite by bainite are all within the range of the present invention in test numbers 11 to 25. As shown in FIG. 6, it was found that the fracture limit strain due to side bending can be made 0.5 or more 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. In other words, it was found that even if the area ratio and coverage of bainite are within the range of the present invention, it is difficult to increase the fracture limit strain if the average diameter dM of martensite is outside the range.

FIG. 7 shows the relationship between the coverage rate and the water density in the quaternary cooling process in test numbers 4, 5, and 11 to 25. In addition, in all of test numbers 11 to 25, the manufacturing conditions other than the water density in the quaternary cooling process are within the range of the present invention. As shown in FIG. 7, it was found that the coverage rate of the hot-rolled steel sheet can be sufficiently improved by manufacturing the hot-rolled steel sheet under conditions in which all manufacturing conditions, including the water density in the quaternary cooling process, are within the range of the present invention. In other words, it was found that even if the manufacturing conditions other than the water density in the quaternary cooling process are within the range of the present invention, it is difficult to increase the coverage rate if the water density in the quaternary cooling process is outside the range.

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

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

As described above, it is clear that all of the test numbers 11 to 25, 37 to 41 and test numbers 56 to 68, which are invention examples within the scope of the present invention, are excellent in characteristics.
For example, when the microstructure of the inventive example, Test No. 21, was observed, the microstructure shown in Fig. 10A was obtained. As is clear from Fig. 10A, in the inventive example, Test No. 21, martensite is sufficiently covered with bainite (the dotted line region in the figure).

In addition, in order to subject Test No. 21 to a side bend test, shear processing was performed on Test No. 21, and the structure near the sheared end surface was observed. As a result, the structure photograph shown in FIG. 10B was obtained. According to FIG. 10B, no cracks were observed in the deformed martensite itself or in the interface between the martensite and other structures, and voids were generated in the highly deformed ferrite. In other words, it is considered that the bainite was arranged to cover the martensite, and this bainite played a role like a cushion that mitigated the difference in the amount of deformation between the ferrite and martensite, thereby suppressing the cracking of the martensite.

As described above, it is clear that the comparative examples, that is, test numbers 1 to 10, 26 to 36 and test numbers 42 to 55, are inferior in one or more characteristics.
For example, in the comparative examples of Test Nos. 4 and 5, the water flow rate in the quaternary cooling step was less than 2.0 ^m3 /min/ ^mm2 , so the coverage of martensite with bainite was less than 75.0%. In addition, microcracks were observed on the sheared edge, and the fracture limit strain in the side bend test did not reach the target.

In the comparative examples, Test Nos. 26 to 36, the coverage of martensite with bainite and the average diameter dM of martensite were within the range of the invention, but the requirements for the other metal structures were not met, and therefore one or more of the characteristics were not satisfied. For example, in Test No. 27, which used steel type G with a proper chemical composition, the final finishing temperature of the finish rolling exceeded 950°C, and therefore the area ratio of ferrite was less than 53.0%. As a result, the ductility was reduced.

In addition, the comparative example, test number 50, was manufactured under manufacturing conditions within the range 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. Note that in test number 50 and other samples in which the conditions of the quaternary cooling step were appropriate and the area ratio of bainite was more than 14.0%, the fracture limit strain of the side bend reached the target. This shows that it is extremely important to control the conditions of the quaternary cooling step within the range of the present invention in order to suppress microcracks on the sheared end surface and thereby improve the fracture limit strain of the side bend.

Fig. 11A shows a structural photograph (SEM photograph) of Test No. 6, which is a comparative example in which the holding time is shorter than 0.33 seconds. Fig. 11B is an enlarged view of Region A shown in Fig. 11A. In the portion indicated by the arrow in Fig. 11B, a linear contrast different from the grain boundary is observed. This contrast is considered to be the interface with austenite after the bainite transformation by tertiary cooling is completed. It can be seen that martensitic transformation progresses at the austenite-ferrite interface and in the vicinity thereof, and as a result, the coverage rate of martensite by bainite is reduced.

Claims (3)

The chemical composition, in mass%, is
C: 0.035% or more, 0.085% or less,
Si: 0.001% or more, 0.15% or less,
Mn: 0.70% or more, 1.80% or less,
P: 0.020% or less,
S: 0.0050% or less,
Ti: 0.075% or more, 0.170% or less,
Nb: 0.003% or more, 0.050% or less,
Al: 0.10% or more, 0.40% or less,
N: 0.0080% or less,
Cr: 0% or more, 0.27% or less,
B: 0% or more, 0.0050% or less,
Ca: 0% or more, 0.0050% or less,
Mo: 0% or more, 0.40% or less,
Ni: 0% or more, 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 is Fe and impurities,
The metal structure at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction is
Ferrite is 53.0% or more and 76.0% or less in area ratio,
The area ratio of martensite is 3.0% or more and 10.0% or less,
Bainite is 14.0% or more and 39.0% or less in area ratio,
The area ratio of pearlite is 2.6% or less , and
Retained austenite is 4.0% or less
It consists of :
The average diameter of martensite is 0.26 μm or more and 0.70 μm or less,
A hot-rolled steel sheet, characterized in that, among all interfaces of the martensite, a total length of interfaces between the martensite and the bainite is 75.0% or more of a total length of all interfaces of the martensite. The chemical composition, by mass, is
Cr: 0.06% or more, 0.27% or less,
B: 0.0003% or more, 0.0050% or less,
Ca: 0.0003% or more, 0.0050% or less,
Mo: 0.01% or more, 0.40% or less,
Ni: 0.01% or more, 0.50% or less,
Contains one or more of the group consisting of Cu: 0.01% or more and 0.50% or less, and REM: 0.0003% or more and 0.0300% or less;
The hot rolled steel sheet according to claim 1 . The method for producing a hot-rolled steel sheet according to claim 1 or 2,
For a slab having the chemical composition according to claim 1 or 2,
A hot rolling process in which rolling is performed under conditions in which the final finishing temperature is 880°C or more and 950°C or less;
After the hot rolling step, a primary cooling step of cooling the steel sheet to a primary cooling stop temperature of 680° C. or more and 760° C. or less at an average cooling rate of 60° C./sec or more;
A secondary cooling step in which cooling is performed at an average cooling rate of 20° C./sec or less for 1.6 seconds or more and 6.3 seconds or less after the primary cooling step;
After the secondary cooling step, a tertiary cooling step is performed at an average cooling rate of 60° C./sec or more and 130° C./sec or less to a tertiary cooling stop temperature of 195° C. or more and 440° C. or less;
After the tertiary cooling step, a fourth cooling step is performed in which the mixture is water-cooled for 0.33 seconds or more and 1.50 seconds or less at a water flow rate of ^{2.0 m3} /min/ ^mm2 or more and 7.2 ^m3 /min/mm2 or less.
A fifth cooling step of cooling the mixture in air for 3.0 seconds or more and 5.0 seconds or less after the fourth cooling step;
A coiling step of coiling the steel sheet at a temperature lower than 180°C after the quinary cooling step.

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