WO2024058096A1 - High-strength hot-rolled steel sheet - Google Patents

Abstract

Provided is a high-strength hot-rolled steel sheet having a predetermined chemical composition, wherein at all of the positions of 1/10W, 3/10W, 5/10W, 7/10W, and 9/10W from the end in the width direction (W is the total width in the direction perpendicular to the rolling direction and the sheet thickness direction), the metal structure at 1/4 position of the sheet thickness contains, in area%, at least 95% of tempered martensite, 5% or less of fresh martensite, and 5% of less in total of at least one among ferrite, upper bainite, and pearlite, and the difference between the maximum and minimum tensile strengths at all of the positions in the width direction is 30 MPa or less.

Description

High strength hot rolled steel plate

The present invention relates to high-strength hot-rolled steel sheets.

In recent years, the automobile industry has been increasing the strength of steel plates from the perspective of reducing environmental impact and ensuring passenger safety. As the strength of steel sheets increases, the main structure constituting the metallographic structure of steel sheets is becoming martensitic.

For example, in Patent Document 1, in mass %, C: 0.08% or more and less than 0.16%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.0%, P: 0. 025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, Nb: 0.001 to 0.05%, Ti: 0.001 ~0.05%, Cr: 0.01~1.0%, B: 0.0005~0.0050%, with the balance consisting of Fe and inevitable impurities, and a martensite phase or tempered martensite. phase is the main phase, the main phase has a volume percentage of 90% or more of the entire structure, and the average grain size of prior austenite grains is 20 μm or less in a cross section parallel to the rolling direction and 15 μm or less in a cross section perpendicular to the rolling direction. A high-strength hot-rolled steel sheet with excellent low-temperature toughness is described, which has a structure in which the aspect ratio of prior austenite grains in a cross section parallel to the rolling direction is 18 or less. Further, in Patent Document 1, according to the above structure, without containing expensive Mo, it has high strength of yield strength YS: 960 MPa or more, high toughness of vE _-40 of 40 J or more, and A hot-rolled steel sheet with excellent bending workability and delayed fracture resistance, as well as a surface hardness of 360HB or more on Brinell hardness, and excellent wear resistance, making it suitable for structural members of construction machinery and industrial machinery. It is described that it can be easily produced and has great industrial effects.

In Patent Document 2, in mass %, C: 0.05 to 0.14%, Si: 0.01 to 1.0%, Mn: 0.50 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05 %, Cr: 0.01 to 1.0%, B: 0.0005 to 0.0050%, with the balance consisting of Fe and unavoidable impurities, and the steel structure has a martensitic phase and a sintered phase. It has a main phase consisting of at least one of the tempered martensitic phases and has an area ratio of 95% or more to the entire steel structure, and the average grain size in the laths of the martensitic phase and/or tempered martensitic phase is 0.5 μm or less A high-strength hot-rolled steel sheet is described, which contains cementite and has a cementite content of 0.01 to 0.08% by mass. Further, in Patent Document 2, according to the above structure, a high-strength hot-rolled steel sheet that has high toughness and excellent punchability and punching bending fatigue strength characteristics without containing Mo, which is an expensive alloying element, is specifically described. It has high tensile strength TS: 980 MPa or more, high toughness with absorbed energy vE _-40 of 40 J or more in the Charpy impact test at a test temperature of -40°C, and also has excellent punching properties and punching bending fatigue strength properties. It is stated that it is possible to provide a high-strength hot-rolled steel sheet that has excellent properties.

In Patent Document 3, in mass %, C: 0.10 to 0.25%, Si: 0.10% or less, Mn: 1.0 to 2.0%, P: 0.025% or less, S: 0 .005% or less, Al: 0.005 to 0.10%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.05 to 1.0%, B :0.0005 to 0.0050%, the balance is Fe and unavoidable impurities, the tempered martensite phase accounts for 95% or more by volume of the entire structure, and the prior austenite grains In the width direction, the average grain size of A high-strength hot-rolled steel sheet with excellent strength uniformity is described. Further, in Patent Document 3, the structure of the steel sheet is such that the main phase is tempered martensite over the entire width direction, the average grain size of prior austenite (γ) grains in a cross section parallel to the rolling direction is 20 μm or less, and It is taught that by creating a structure in which the average grain size of prior austenite grains in a cross section perpendicular to has been done.

JP 2016-211073 Publication Japanese Patent Application Publication No. 2018-188675 Japanese Patent Application Publication No. 2016-183414

In hot-rolled steel sheets as described in Patent Documents 1 to 3, when the metal structure is made into a martensite single phase or a structure closer to a martensite single phase in order to increase the strength, uneven cooling during cooling or during transformation may occur. The shape of the steel sheet may collapse due to transformation plasticity, etc., and in such a case, it becomes difficult to maintain sufficient flatness in the obtained hot rolled steel sheet. For example, in Patent Document 3, a high-strength hot-rolled steel plate with a uniform yield strength YS in the width direction is studied, but there is no specific method for improving the flatness of the high-strength hot-rolled steel plate. No consideration has been given.

Therefore, an object of the present invention is to provide a high-strength hot-rolled steel sheet with improved flatness using a novel configuration.

In order to achieve the above object, the present inventors conducted a study focusing particularly on the metal structure in the width direction of a hot rolled steel sheet. As a result, the present inventors succeeded in ensuring high strength by changing the metal structure of a hot-rolled steel sheet having a predetermined chemical composition into a structure with tempered martensite as the main phase, while also making the structure uniform in the width direction. It has been found that by distributing the strength in the width direction, it is possible to reduce the strength variation in the width direction. In this way, by reducing the strength variation in the width direction, it is possible to provide a hot rolled steel sheet in which the flatness in the width direction of the hot rolled steel sheet is significantly improved.

The present invention that achieves the above object is as follows.
(1) In mass%,
C: 0.050-0.100%,
Si: 0.010-0.200%,
Mn: 1.00-2.50%,
Ti: 0.001 to 0.120%,
Al: 0.001-0.050%,
B: 0.0005-0.0050%,
P: 0.100% or less,
S: 0.050% or less,
N: 0.0050% or less,
O: 0 to 0.0050%,
Cu: 0 to 0.20%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%,
Cr: 0 to 0.40%,
Mo: 0-0.20%,
Nb: 0 to 0.05%,
V: 0-0.10%,
As: 0 to 0.100%,
Zr: 0 to 0.100%,
Ca: 0-0.0050%,
Mg: 0-0.100%,
Bi: 0 to 0.020%,
Co: 0 to 0.20%,
W: 0-0.20%,
Zn: 0-0.20%,
It has a chemical composition consisting of REM: 0 to 0.1000%, and the balance: Fe and impurities,
When the total width in the direction perpendicular to the rolling direction and the plate thickness direction is W, 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the end in the width direction At all positions, the metal structure at the 1/4 position of the plate thickness is expressed as area%,
Tempered martensite: 95% or more,
Fresh martensite: 5% or less, and at least one of ferrite, upper bainite, and pearlite: 5% or less in total,
A high-strength hot-rolled steel sheet, characterized in that the difference between the maximum and minimum tensile strengths at all positions in the width direction is 30 MPa or less.
(2) the chemical composition is in mass%;
O: 0.0001 to 0.0050%,
Cu: 0.001 to 0.20%,
Ni: 0.001 to 0.20%,
Sn: 0.001 to 0.10%,
Cr: 0.001-0.40%,
Mo: 0.001 to 0.20%,
Nb: 0.001-0.05%,
V: 0.001 to 0.10%,
As: 0.001 to 0.100%,
Zr: 0.0001 to 0.100%,
Ca: 0.0001-0.0050%,
Mg: 0.0001-0.100%,
Bi: 0.0001-0.020%,
Co: 0.001 to 0.20%,
W: 0.001-0.20%,
Zn: 0.001~0.20%, and REM: 0.0001~0.1000%
The high-strength hot-rolled steel sheet described in (1) above, characterized by containing at least one of the following.
(3) The high-strength hot rolled steel sheet according to (1) or (2) above, wherein the prior austenite grain size in the metal structure is 40 μm or less.

According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet with improved flatness.

<High-strength hot-rolled steel plate>
The high-strength hot-rolled steel sheet according to the embodiment of the present invention has, in mass%,
C: 0.050-0.100%,
Si: 0.010-0.200%,
Mn: 1.00-2.50%,
Ti: 0.001 to 0.120%,
Al: 0.001-0.050%,
B: 0.0005-0.0050%,
P: 0.100% or less,
S: 0.050% or less,
N: 0.0050% or less,
O: 0 to 0.0050%,
Cu: 0 to 0.20%,
Ni: 0 to 0.20%,
Sn: 0 to 0.10%,
Cr: 0-0.40%,
Mo: 0-0.20%,
Nb: 0 to 0.05%,
V: 0-0.10%,
As: 0 to 0.100%,
Zr: 0 to 0.100%,
Ca: 0-0.0050%,
Mg: 0-0.100%,
Bi: 0 to 0.020%,
Co: 0 to 0.20%,
W: 0-0.20%,
Zn: 0-0.20%,
It has a chemical composition consisting of REM: 0 to 0.1000%, and the balance: Fe and impurities,
When the total width in the direction perpendicular to the rolling direction and the plate thickness direction is W, 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the end in the width direction At all positions, the metal structure at the 1/4 position of the plate thickness is expressed as area%,
Tempered martensite: 95% or more,
Fresh martensite: 5% or less, and at least one of ferrite, upper bainite, and pearlite: 5% or less in total,
It is characterized in that the difference between the maximum and minimum tensile strengths at all positions in the width direction is 30 MPa or less.

As mentioned earlier, when the metal structure of a hot rolled steel sheet is made into a martensite single phase or a structure closer to a martensite single phase in order to increase the strength, the metal structure is affected by uneven cooling during cooling and transformation plasticity during transformation. In such a case, it becomes difficult to maintain sufficient flatness in the resulting hot rolled steel sheet. Regarding deformation of the steel plate shape, specifically, warpage may occur in the width direction of the steel plate (direction perpendicular to the rolling direction and the plate thickness direction). If a steel plate is warped in the width direction, it may cause shape defects or cracks during forming when such a steel plate is used to form a longitudinal member. An existing method for improving such warpage is correction (flattening treatment) using a leveler or the like. However, if the strength of the steel plate increases, it may not always be possible to perform sufficient correction using a leveler or the like. Furthermore, if pre-processing is performed by straightening with a leveler or the like, such pre-processing consumes a part of the steel sheet's inherent ductility, resulting in a decrease in residual ductility. When the residual ductility decreases, forming defects are likely to occur during pressing of the steel plate, and as a result, productivity also decreases. Therefore, it is required that the flatness of the hot-rolled steel sheet is good even without correction using a leveler or the like. The main reason for the deformation of the steel sheet is that the cooling rate after finish rolling is fast. If the cooling rate after finish rolling is high, the controllability of the amount of water used for cooling deteriorates, and uneven cooling becomes noticeable as some parts are locally overcooled. As a result, thermal stress is generated due to temperature unevenness in the width direction of the steel plate, and warpage occurs in the width direction of the steel plate. Therefore, from the viewpoint of ensuring flatness, it is not necessarily appropriate to increase the cooling rate after finish rolling excessively. On the other hand, in order to obtain a martensite single-phase structure or a structure closer to a martensite single-phase structure from the viewpoint of increasing strength, it is necessary to cool at a cooling rate higher than the critical cooling rate.

Therefore, in order to improve the flatness of the steel sheet while achieving high strength, the present inventors selected an appropriate steel composition and conducted studies focusing on the metallographic structure in the width direction of the hot-rolled steel sheet. Ta. First, the present inventors achieved high strength, more specifically, high strength of 980 MPa or more, by changing the metal structure of a hot rolled steel sheet having a predetermined chemical composition to a structure with tempered martensite as the main phase. I found out that it can be done. Next, the present inventors thought that it would be effective to uniformly distribute the structure including such tempered martensite as the main phase even in the width direction, and conducted further studies. First, in order to make the metal structure uniform in the width direction, it is necessary to cool properly after finishing rolling.Also, in order to form a structure with tempered martensite as the main phase, automatic cooling during cooling is necessary. You need to make good use of temper. Therefore, the present inventors have developed a method for manufacturing hot-rolled steel sheets while promoting auto-tempering during cooling by limiting the content of Si, which can delay or suppress auto-tempering, to 0.200% by mass or less in steel sheets. As will be explained in detail later, the amount of cooling water injected into the steel sheet during cooling is uniformly distributed between the top and bottom surfaces of the steel sheet from after finish rolling until the temperature corresponds to the end temperature of martensitic transformation. In other words, by uniformly cooling the upper and lower surfaces of the steel plate to approximately 200°C with cooling water after finish rolling, a structure in which tempered martensite accounts for 95% or more by area over the entire width of the hot-rolled steel plate is created. It was found that it is possible to distribute the More specifically, the present inventors have particularly determined that the Si content of the hot rolled steel sheet is 0.200% by mass or less, and that the cooling after finish rolling is appropriately controlled as described above. When the total width in the direction perpendicular to the rolling direction and plate thickness direction is W, 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W from the end in the width direction. It has been found that at all positions, the metal structure at the 1/4 position of the plate thickness can be a structure of tempered martensite: 95% or more in area%. As a result, the present inventors have determined that the strength variation in the width direction of the hot rolled steel sheet can be sufficiently reduced to a level where the difference between the maximum and minimum tensile strengths at all positions in the width direction is 30 MPa or less. Despite the high strength, the flatness of the hot rolled steel sheet can be significantly improved in relation to the uniformity of the metallographic structure in the entire width direction and the reduction of such strength variations. I discovered that it can be done.

Conventionally, in cooling after finish rolling, the amount of water sprayed onto the top surface of the steel sheet is generally greater than the amount of water sprayed onto the bottom surface of the steel sheet. Therefore, by uniformly cooling the upper and lower surfaces of the steel plate after finish rolling to a temperature that corresponds to the end temperature of martensitic transformation, uniformity of the metal structure and reduction of strength variations in the entire width direction can be achieved as described above. However, the fact that it is possible to significantly improve the flatness of a hot rolled steel sheet despite its high strength was revealed for the first time by the present inventors. Furthermore, since the high-strength hot-rolled steel sheet according to the embodiment of the present invention has sufficient flatness in the hot-rolled state, there is no need for pre-processing using a leveler or the like. Such pre-processing does not consume part of the inherent ductility of the steel sheet. In this regard, it is possible to reduce the risk of forming defects during pressing of the high-strength hot-rolled steel sheet, and it is also possible to significantly improve productivity. Therefore, it goes without saying that the high-strength hot-rolled steel sheet according to the embodiment of the present invention is particularly useful in the automobile field, but can also be used very effectively in other fields.

Hereinafter, the high-strength hot-rolled steel sheet according to the embodiment of the present invention will be explained in more detail. In the following description, "%", which is the unit of content of each element, means "% by mass" unless otherwise specified. In addition, in this specification, "~" indicating a numerical range is used to include the numerical values written before and after it as a lower limit and an upper limit, unless otherwise specified.

[C:0.050-0.100%]
C is an element effective in increasing the strength of steel sheets. In order to sufficiently obtain such effects, the C content is set to 0.050% or more. The C content may be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if too much C is contained, it becomes difficult to control the variation in strength in the width direction within a predetermined range due to excessively high strength. Therefore, the C content is set to 0.100% or less. The C content may be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.

[Si:0.010-0.200%]
Si is an element effective for increasing strength as a solid solution strengthening element. In order to sufficiently obtain such effects, the Si content is set to 0.010% or more. The Si content may be 0.020% or more, 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more. On the other hand, if Si is contained excessively, auto-tempering during cooling of the steel sheet may be delayed or suppressed, and in such a case, it becomes impossible to obtain a hot-rolled steel sheet having a desired metal structure. Therefore, the Si content is set to 0.200% or less. The Si content may be 0.180% or less, 0.160% or less, 0.140% or less, or 0.120% or less.

[Mn: 1.00-2.50%]
Mn is an element effective in increasing hardenability and strength as a solid solution strengthening element. If the Mn content is low, hardenability is insufficient, and a relatively large amount of soft phases such as ferrite are generated during cooling, making it impossible to uniformly distribute the structure with tempered martensite as the main phase in the width direction. . Further, due to expansion of the steel plate due to such transformation, warpage may occur in the width direction, and the shape of the steel plate may collapse. Therefore, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.40% or more, 1.60% or more, or 1.80% or more. On the other hand, if Mn is contained excessively, martensite may not be sufficiently tempered even by auto-tempering during cooling of the steel sheet due to improved hardenability, and fresh martensite may not be sufficiently reduced in the final metal structure. be. Therefore, the Mn content is set to 2.50% or less. The Mn content may be 2.40% or less, 2.20% or less, 2.00% or less, or 1.90% or less.

[Ti: 0.001 to 0.120%]
Ti is an element that contributes to improving strength through precipitation strengthening and the like. In addition, Ti consumes solid solution N in steel by combining with N to form titanium nitride (TiN), which has the effect of suppressing the decrease in the amount of solid solution B caused by the formation of BN. . In order to fully obtain these effects, the Ti content is set to 0.001% or more. The Ti content may be 0.010% or more, 0.020% or more, 0.040% or more, or 0.060% or more. On the other hand, since Ti is also an element that suppresses the recrystallization of austenite, if Ti is contained excessively, the driving force for ferrite transformation etc. during cooling will be reduced due to the presence of unrecrystallized austenite containing relatively many dislocations. In some cases, a soft phase such as ferrite is likely to be generated from the unrecrystallized austenite, making it impossible to obtain a desired metal structure. Therefore, the Ti content is set to 0.120% or less. The Ti content may be 0.110% or less, 0.100% or less, 0.090% or less, or 0.080% or less.

[Al: 0.001-0.050%]
Al is an element that acts as a deoxidizing agent. In order to sufficiently obtain such effects, the Al content is set to 0.001% or more. The Al content may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, when Al is contained excessively, coarse oxides are formed, which may reduce toughness. Therefore, the Al content is set to 0.050% or less. The Al content may be 0.045% or less or 0.040% or less.

[B:0.0005-0.0050%]
B is an element that enhances the hardenability of steel and contributes to improving its strength. In order to sufficiently obtain such effects, the B content is set to 0.0005% or more. The B content may be 0.0008% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if B is contained excessively, toughness and/or weldability may deteriorate. Therefore, the B content is set to 0.0050% or less. The B content may be 0.0045% or less, 0.0040% or less, 0.0030% or less, or 0.0025% or less.

[P: 0.100% or less]
Excessive P content may adversely affect weldability. Therefore, the P content is set to 0.100% or less. The P content may be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in cost. Therefore, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[S: 0.050% or less]
If S is contained in an excessive amount, a large amount of MnS may be generated and the toughness may be reduced. Therefore, the Si content is set to 0.050% or less. The S content may be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in cost. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[N: 0.0050% or less]
If N is contained excessively, it may form coarse nitrides and reduce toughness. Further, N combines with B in the steel to form boron nitride (BN), thereby reducing the amount of solid solution B, which may reduce the hardenability improvement effect of B addition. Therefore, the lower the N content, the better, and it is set to 0.0050% or less. The N content may be 0.0045% or less, 0.0040% or less, or 0.0035% or more. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in cost. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

The basic chemical composition of the high-strength hot-rolled steel sheet according to the embodiment of the present invention is as described above. Further, the high-strength hot-rolled steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary. For example, high-strength hot-rolled steel sheets include O: 0 to 0.0050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Sn: 0 to 0.10%, and Cr: 0 to 0. .40%, Mo: 0-0.20%, Nb: 0-0.05%, V: 0-0.10%, As: 0-0.100%, Zr: 0-0.100%, Ca : 0-0.0050%, Mg: 0-0.100%, Bi: 0-0.020%, Co: 0-0.20%, W: 0-0.20%, Zn: 0-0. 20%, and REM: 0 to 0.1000%. These optional elements will be explained in detail below.

[O: 0 to 0.0050%]
O is an element mixed in during the manufacturing process. The O content may be 0%. However, reducing the O content to less than 0.0001% requires time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if O is contained excessively, coarse inclusions may be formed and the toughness of the steel sheet may be reduced. Therefore, the O content is preferably 0.0050% or less. The O content may be 0.0040% or less, 0.0035% or less, or 0.0030% or less.

[Cu: 0-0.20%]
Cu is an element that contributes to improving strength and/or corrosion resistance. Although the Cu content may be 0%, in order to obtain these effects, the Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Cu content may lead to deterioration of toughness and weldability. Therefore, the Cu content is preferably 0.20% or less. The Cu content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[Ni: 0-0.20%]
Ni is an element that improves the hardenability of steel and contributes to improving its strength and/or corrosion resistance. Although the Ni content may be 0%, in order to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if Ni is contained excessively, the effect is saturated, leading to an increase in manufacturing costs. Therefore, the Ni content is preferably 0.20% or less. The Ni content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.

[Sn: 0 to 0.10%]
Sn is an element effective in improving corrosion resistance. The Sn content may be 0%, but in order to obtain such effects, the Sn content is preferably 0.001% or more, 0.005% or more, and 0.01% or more. Or it may be 0.02% or more. On the other hand, excessively containing Sn may lead to a decrease in toughness. Therefore, the Sn content is preferably 0.10% or less. The Sn content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[Cr: 0-0.40%]
Cr is an element that improves the hardenability of steel and contributes to improving its strength and/or corrosion resistance. Although the Cr content may be 0%, in order to obtain these effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, even if Cr is contained excessively, the effect is saturated, leading to an increase in manufacturing costs. Therefore, the Cr content is preferably 0.40% or less. The Cr content may be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.

[Mo: 0-0.20%]
Mo is an element that enhances the hardenability of steel and contributes to improving its strength, and also contributes to improving its corrosion resistance. Although the Mo content may be 0%, in order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if Mo is contained excessively, deformation resistance during hot working may increase, and equipment load may increase. Therefore, the Mo content is preferably 0.20% or less. The Mo content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[Nb: 0 to 0.05%]
Nb is an element that forms carbides, nitrides, and/or carbonitrides in steel and contributes to refinement of the structure through a pinning effect, and thus to higher strength of the steel sheet. Although the Nb content may be 0%, in order to obtain such an effect, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more or 0.01% or more. On the other hand, when Nb is contained excessively, coarse carbides and the like are generated in the steel, which may reduce the toughness of the steel sheet. Therefore, the Nb content is set to 0.05% or less. The Nb content may be 0.04% or less, 0.03% or less or 0.02%.

[V: 0-0.10%]
V is an element that contributes to improving strength through precipitation strengthening and the like. Although the V content may be 0%, in order to obtain such an effect, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if too much V is contained, a large amount of precipitates may be generated and the toughness may be reduced. Therefore, the V content is preferably 0.10% or less. The V content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[As: 0 to 0.100%]
As is an element effective in improving corrosion resistance. The As content may be 0%, but in order to obtain such effects, the As content is preferably 0.001% or more, 0.005% or more, and 0.008% or more. Or it may be 0.010% or more. On the other hand, even if As is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the As content is preferably 0.100% or less. The As content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[Zr: 0 to 0.100%]
Zr is an element that can control the morphology of sulfides. Although the Zr content may be 0%, in order to obtain such an effect, the Zr content is preferably 0.0001% or more. The Zr content may be 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, even if Zr is contained excessively, the effect is saturated, leading to an increase in manufacturing costs. Therefore, the Zr content is preferably 0.100% or less. The Zr content may be 0.050% or less, 0.030% or less, or 0.020% or less.

[Ca: 0-0.0050%]
Ca is an element that can control the morphology of sulfides. Although the Ca content may be 0%, in order to obtain such an effect, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if Ca is contained excessively, the effect is saturated, leading to an increase in manufacturing costs. Therefore, the Ca content is preferably 0.0050% or less. The Ca content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[Mg: 0 to 0.100%]
Mg is an element that can control the morphology of sulfides. The Mg content may be 0%, but in order to obtain such an effect, the Mg content is preferably 0.0001% or more, 0.001% or more, 0.005% or more, or It may be 0.008% or more. On the other hand, even if Mg is contained excessively, the effect is saturated, leading to an increase in manufacturing costs. Therefore, it is preferable that the Mg content is 0.100% or less. The Mg content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[Bi: 0-0.020%]
Bi is an element effective in improving corrosion resistance. Although the Bi content may be 0%, in order to obtain such an effect, the Bi content is preferably 0.0001% or more. The Bi content may be 0.0005% or more, 0.001% or more, or 0.003% or more. On the other hand, even if Bi is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Bi content is preferably 0.020% or less. The Bi content may be 0.010% or less, 0.008% or less, or 0.005% or less.

[Co: 0-0.20%]
Co is an element that contributes to improving hardenability and/or heat resistance. Although the Co content may be 0%, in order to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if Co is contained excessively, hot workability may decrease, leading to an increase in raw material cost. Therefore, the Co content is preferably 0.20% or less. The Co content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[W: 0-0.20%]
W is an element that enhances the hardenability of steel and contributes to improving its strength. Although the W content may be 0%, in order to obtain such an effect, the W content is preferably 0.001% or more. The W content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if W is contained excessively, weldability may deteriorate. Therefore, the W content is preferably 0.20% or less. The W content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[Zn: 0-0.20%]
Zn is an element effective in controlling the shape of inclusions. In order to obtain such effects, the Zn content is preferably 0.001% or more. The Zn content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if Zn is contained excessively, the effect is saturated, leading to an increase in manufacturing costs. Therefore, the Zn content is preferably 0.20% or less. The Zn content may be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.

[REM: 0 to 0.1000%]
REM (rare earth metal) is an element that can control the morphology of sulfides. Although the REM content may be 0%, in order to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if REM is contained excessively, the effect is saturated, leading to an increase in manufacturing costs. Therefore, the REM content is preferably 0.1000% or less. The REM content may be 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids such as lanthanum (La) with atomic number 57 to lutetium (Lu with atomic number 71). ), and the REM content is the total content of these elements.

In the high-strength hot rolled steel sheet according to the embodiment of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of high-strength hot rolled steel sheets due to various factors in the production process, including raw materials such as ores and scraps.

The chemical composition of the high-strength hot-rolled steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the high-strength hot rolled steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N using an inert gas melting-thermal conductivity method, and O using an inert gas melting-non-dispersive infrared absorption method.

[Metal structure]
In the high-strength hot-rolled steel sheet according to the embodiment of the present invention, when the total width of the high-strength hot-rolled steel sheet in the direction perpendicular to the rolling direction and the plate thickness direction is W, the position is 1/10W from the end in the width direction. , 3/10W position, 5/10W position, 7/10W position, and 9/10W position, the metal structure at the 1/4 plate thickness position is tempered martensite (tM) in area%: 95% or more, fresh martensite (fM): 5% or less, and at least one of ferrite (α), upper bainite (B), and pearlite (P): 5% or less in total. In the present invention, "total width" refers to the length of a high-strength hot-rolled steel plate (for example, a coiled high-strength hot-rolled steel plate) in a direction perpendicular to the rolling direction and the plate thickness direction.

Here, if the rolling direction of the hot rolled steel sheet is not clear, the rolling direction of the hot rolled steel sheet can be specified by the following method. After finishing the thickness section of the hot rolled steel plate by mirror polishing, the S concentration is measured using an electron probe micro analyzer (EPMA). The measurement conditions are an accelerating voltage of 15 kV, a measurement pitch of 1 μm, and a distribution image in a 500 μm square range at the center of the plate thickness. At this time, the stretched region with a high S concentration is determined to be an inclusion such as MnS. When observing, you may observe from multiple fields of view. Next, using the plate thickness cross section first observed using the above method as a reference, rotate the plate thickness direction in 5° increments in the range of 0° to 180° as an axis, and then take a cross section using the above method. Observe. The average value of the lengths of the long axes of the plurality of inclusions in each of the obtained cross sections is calculated for each cross section, and the cross section in which the average value of the lengths of the long axes of the inclusions is maximum is specified. The direction parallel to the long axis direction of the inclusion in the cross section is specified as the rolling direction of the hot rolled steel sheet.
Each organization will be explained in more detail below.

[Tempered martensite: 95% or more]
At all positions of 1/10W, 3/10W, 5/10W, 7/10W, and 9/10W from the edge in the width direction, the metal structure at the 1/4 plate thickness position is By setting the area percentage of returned martensite to 95% or more, it is possible to achieve high strength due to the structure with martensite as the main phase, while also reducing the maximum and minimum tensile strengths at all positions in the width direction. It is possible to reliably control the difference in value to 30 MPa or less, and therefore it is possible to significantly reduce strength variations in the width direction. At all positions in the width direction, the area ratio of tempered martensite may be 96% or more, 97% or more, or 98% or more. The upper limit of the area ratio of tempered martensite is not particularly limited and may be 100%.

[Fresh martensite: 5% or less]
In the high-strength hot-rolled steel sheet according to the embodiment of the present invention, it is necessary to control the area ratio of fresh martensite to 5% or less at all positions in the width direction. If the area ratio of fresh martensite exceeds 5% at any one position, the strength at that position becomes too high, and strength variations in the width direction may not be sufficiently reduced. Therefore, from the viewpoint of reducing strength variations, the area ratio of fresh martensite is preferably as low as possible at all positions in the width direction, for example, 4% or less, 3% or less, 2% or less, or 1% or less. Good too. The lower limit of the area ratio of fresh martensite is not particularly limited and may be 0%.

[At least one of ferrite, upper bainite, and pearlite: 5% or less in total]
In the high-strength hot-rolled steel sheet according to the embodiment of the present invention, the remaining structure other than tempered martensite and fresh martensite is composed of at least one of ferrite, upper bainite, and pearlite. Similarly, at least one of ferrite, upper bainite, and pearlite must be controlled to a total of 5% or less at all positions in the width direction. If the area ratio of at least one of ferrite, upper bainite, and pearlite exceeds 5% in total at any one position, the strength at that position will become too low, and strength variations in the width direction will be sufficiently reduced. You may not be able to do so. Therefore, from the viewpoint of reducing strength variations, it is preferable that the area ratio of at least one of ferrite, upper bainite, and pearlite be as low as possible at all positions in the width direction, for example, 4% or less, 3% or less, 2% or less in total. % or less or 1% or less. The lower limit of the area ratio of at least one of ferrite, upper bainite, and pearlite is not particularly limited and may be 0% in total. For example, the total area percentage of fresh martensite, ferrite, upper bainite, and pearlite may be 5% or less, 4% or less, 3% or less, 2% or less, or 1% or less. Similarly, the total area ratio of fresh martensite, ferrite, upper bainite, and pearlite may be 0%.

[Identification of metallographic structure and calculation of area ratio]
Identification of metal structure and calculation of area ratio are performed by FE-SEM (field emission scanning electron microscope) and optical microscope after corrosion using nital reagent or Repeller liquid, and X-ray diffraction method. Structure observation using FE-SEM and an optical microscope is performed at a magnification of 1,000 to 50,000 times on a 100 μm×100 μm area in a steel plate cross section parallel to the rolling direction and perpendicular to the plate surface. For any metal structure, 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the edge in the width direction (direction perpendicular to the rolling direction and the plate thickness direction) At each position, there are three measurement points, and the area ratio at each position is determined by calculating the average value of the three measurement values.

The area ratio of ferrite is 100 μm within the range of 1/8 to 3/8 of the plate thickness centered at the 1/4 position of the plate thickness in an electron channeling contrast image by FE-SEM (field emission scanning electron microscope). It is determined by observing an area of ×100 μm. More specifically, within the above region, a portion that appears with uniform contrast can be identified as ferrite, and its area ratio can be calculated using image analysis software Image J.

The area ratio of fresh martensite (as-quenched martensite) is determined by the following procedure. First, the observation surface of the sample is etched with repeller liquid, and then an area of 100 μm x 100 μm within the range of 1/8 to 3/8 of the plate thickness centered at 1/4 of the plate thickness is observed using FE-SEM. In repeller corrosion, fresh martensite and retained austenite are not corroded, so they appear as flat areas with brighter contrast than other parts on the SEM image. The area percentage of uncorroded areas corresponds to the total area percentage of fresh martensite and retained austenite, if present. The area ratio of fresh martensite is calculated by subtracting the area ratio of retained austenite measured by an X-ray diffraction method, which will be described later, from the area ratio of this uncorroded region.

The area ratio of retained austenite is calculated by X-ray diffraction method. First, the sample is removed by mechanical polishing and chemical polishing from the surface of the sample to a depth of 1/4 in the thickness direction. Next, the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220) and (311) of the fcc phase obtained using MoKα rays at a position of 1/4 of the plate thickness. From this, the tissue fraction of retained austenite is calculated. A general 5-peak method is used as this calculation method. The calculated microstructure fraction of retained austenite is determined as the area fraction of retained austenite.

Identification of upper bainite and tempered martensite and calculation of area ratio are performed as follows. First, the observation surface of the sample is corroded with a nital reagent, and then an area of 100 μm x 100 μm within the range of 1/8 to 3/8 of the plate thickness, centered on 1/4 of the plate thickness, is observed using FE-SEM. Upper bainite and tempered martensite are identified in the following manner from the position and arrangement of cementite contained within the structure in this observation region. In upper bainite, cementite or retained austenite exists at the interface of lath-like bainitic ferrite. Based on such feature points, upper bainite is identified, and the area ratio of upper bainite is calculated by dividing the area of the identified bainite by the area of the observation field. On the other hand, in tempered martensite, cementite exists inside the martensite lath, but since there are two or more types of crystal orientations of martensite lath and cementite, and cementite has multiple variants, it is difficult to identify tempered martensite. Can be done. The area of tempered martensite thus identified is divided by the area of the observation field, and the value is calculated as the area ratio of tempered martensite.

Identification of pearlite and calculation of area ratio are performed in the following steps. First, the observation surface of the sample is corroded with a nital reagent, and then a range of 1/8 to 3/8 of the plate thickness, centered on 1/4 of the plate thickness, is observed using an optical microscope. A region where carbide and ferrite exist in a layered manner in an image observed with an optical microscope is identified as pearlite, and the value obtained by dividing this region by the area of the observation field is calculated as the area ratio of pearlite.

[The difference between the maximum and minimum tensile strengths at all positions 1/10W, 3/10W, 5/10W, 7/10W, and 9/10W from the end in the width direction is 30 MPa below]
As described above, in the high-strength hot-rolled steel sheet according to the embodiment of the present invention, from the end in the width direction, the positions are 1/10W, 3/10W, 5/10W, 7/10W, and 9/10W. By setting the tempered martensite in the metal structure at 1/4 of the plate thickness to 95% or more in area% at all positions, high strength can be achieved due to the structure with martensite as the main phase. While achieving this, it is possible to reliably control the difference between the maximum and minimum tensile strengths at all positions in the width direction to 30 MPa or less, and therefore it is possible to significantly reduce strength variations in the width direction. Become. Further, in relation to the uniformity of the metal structure in the entire width direction and the reduction of such strength variations, the flatness of the hot rolled steel sheet can be significantly improved despite its high strength. From the viewpoint of improving the flatness of the hot rolled steel sheet, the smaller the difference between the maximum value and the minimum value of the tensile strength, the better, for example, 28 MPa or less, 25 MPa or less, 22 MPa or less, 20 MPa or less, 17 MPa or less, or 15 MPa or less. Good too. Although the lower limit is not particularly limited, for example, the difference between the maximum and minimum tensile strengths is acceptable if it is 5 MPa or more, 8 MPa or more, or 10 MPa or more.

The difference between the maximum and minimum tensile strengths is determined as follows. First, a test was conducted in the direction parallel to the rolling direction at each of the 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the widthwise end of the hot rolled steel plate. A No. 5 tensile test piece of JIS Z2241:2011 with the direction is taken. Next, five tensile strength values are obtained by conducting a tensile test based on JIS Z2241:2011 using these tensile test pieces, and finally, the difference between the maximum value and the minimum value is calculated.

[Tensile strength]
The minimum value among the above five tensile strength values is determined as the tensile strength of the high strength hot rolled steel sheet according to the embodiment of the present invention. The high-strength hot-rolled steel sheet according to the embodiment of the present invention has the above-described chemical composition and metal structure, thereby achieving high tensile strength, specifically, a tensile strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more, 1050 MPa or more, or 1100 MPa or more. According to the high-strength hot-rolled steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, in relation to the uniformity of the metal structure and the reduction of strength variations in the entire width direction, Very good flatness can be achieved. Although the upper limit of the tensile strength is not particularly limited, for example, the tensile strength of the high-strength hot rolled steel sheet may be 1300 MPa or less, 1250 MPa or less, 1200 MPa or less, or 1180 MPa or less.

[Prior austenite grain size in metal structure: 40 μm or less]
According to a preferred embodiment of the present invention, the prior austenite grain size in the metallographic structure is 40 μm or less. As mentioned above, the high-strength hot-rolled steel sheet according to the embodiment of the present invention can achieve extremely excellent flatness in relation to the uniformity of the metal structure and the reduction of strength variations in the entire width direction. However, in addition to this, controlling the prior austenite grain size within such a fine range makes it possible to further improve additional properties such as toughness. From the viewpoint of improving toughness, the smaller the prior austenite grain size is, the more preferable it is, and may be, for example, 37 μm or less, 35 μm or less, 32 μm or less, 30 μm or less, 27 μm or less, or 25 μm or less. Although the lower limit is not particularly limited, for example, the prior austenite grain size may be 10 μm or more, 12 μm or more, 15 μm or more, 18 μm or more, or 20 μm or more.

The prior austenite grain size in the metal structure is determined as follows. First, a 200 μm x 200 μm area in the L cross section of a steel piece sampled from the surface of a hot rolled steel sheet at a position 1/4 of the sheet thickness is analyzed by SEM/EBSD (scanning electron microscope/backscattered electron diffraction). More specifically, a predetermined crystal orientation transformation is applied to the crystal orientation data obtained by SEM/EBSD ("Study for improving the accuracy of the method for reconstructing the austenite structure of steel", Kengo Hata, Masayuki Wakita, Tomoya Fujiwara). , Kaori Kono, Nippon Steel & Sumikin Technical Report No. 404 (2016), p. 24-30) to obtain an image in which the prior austenite grains are reconstructed. The diameter of a circle having the same area, that is, the equivalent circle diameter, is determined from the prior austenite grains in the image. This operation is performed for a total of 10 prior austenite grains, and the obtained 10 circle equivalent diameters are averaged to determine the prior austenite grain size.

[Full width W]
The high-strength hot-rolled steel sheet according to the embodiment of the present invention can have any overall width W. Although not particularly limited, for example, in the case of the full width W of a hot rolled steel plate (coil) in a wound state, the full width W may be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more. The upper limit is not particularly limited, but from the viewpoint of ensuring improvement in flatness, the total width is preferably 2500 mm or less, such as 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm. It may be the following.

[Plate thickness]
The high-strength hot-rolled steel plate according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although it is not particularly limited. For example, the plate thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.

<Production method of high strength hot rolled steel plate>
Next, a preferred method for manufacturing a high-strength hot-rolled steel sheet according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a high-strength hot-rolled steel sheet according to an embodiment of the present invention, and the high-strength hot-rolled steel sheet is manufactured as described below. It is not intended to be limited to those manufactured by the method.

The method for manufacturing a high-strength hot-rolled steel sheet according to an embodiment of the present invention includes:
A hot rolling process comprising heating a slab having the chemical composition described above in connection with a high-strength hot rolled steel sheet to a temperature of 1220 to 1300°C and subjecting it to rough rolling and finish rolling, the process comprising: The exit temperature of the finish rolling is 1100 to 1200°C, the entry temperature (F0) of the finish rolling is 1000 to 1100°C, the exit temperature (FT) of the finish rolling is 940 to 1000°C, and the a hot rolling process in which the total rolling reduction is 85 to 95%;
The finish rolled steel plate is heated in the temperature range from the exit temperature (FT) of the finish rolling to the martensitic transformation start temperature Ms + 50°C at an average cooling rate of not less than critical cooling rate Vc + 10°C/s and not more than 60°C/s. A cooling step in which secondary cooling is performed in the temperature range from Ms + 50°C to 200°C at an average cooling rate of 50 to 120°C/s, the vertical cooling ratio of the upper surface to the lower surface of the steel plate in the primary cooling. is 0.8 to 1.2, and the vertical cooling ratio of the upper surface to the lower surface of the steel plate in the secondary cooling is 0.8 to 1.2, and the secondary cooled steel plate is heated to 50 to 100°C. The feature is that it includes a winding process. Each step will be explained in detail below.

[Hot rolling process]
[Heating the slab]
First, a slab having the chemical composition described above in connection with hot rolled steel sheet is heated. The slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method. The slabs used contain relatively high amounts of alloying elements in order to obtain high strength steel sheets. For this reason, it is necessary to heat the slab to dissolve the alloying elements in the slab before hot rolling. If the heating temperature is less than 1220° C., the alloying elements will not be fully dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is 1220°C or higher, preferably 1230°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1300° C. or lower from the viewpoint of the capacity of the heating equipment and productivity.

[Rough rolling]
In this method, the heated slab is subjected to rough rolling before finish rolling in order to adjust the plate thickness and the like. In the rough rolling, in order to ensure the desired sheet bar dimensions and to be able to adjust the total rolling reduction within the desired range in the temperature range of 940°C or higher in the finish rolling, the exit temperature of the rough rolling is set at 1100 to 1100°C. The temperature is 1200°C, preferably 1150 to 1200°C. If the exit temperature of rough rolling is less than 1100°C, it becomes difficult to obtain an exit temperature of 940°C or higher in finish rolling following rough rolling. Moreover, when the exit temperature of rough rolling exceeds 1200° C., crystal grains may become coarse and the toughness of the obtained hot rolled steel sheet may decrease.

[Finish rolling]
The rough rolled slab is then subjected to finish rolling. As mentioned above, since the slab used contains a relatively large amount of alloying elements, it is necessary to increase the rolling load during hot rolling. For this reason, hot rolling is performed at high temperature and under high pressure. Specifically, the entry temperature (F0) of finish rolling is 1000 to 1100°C, the exit temperature (FT) of finish rolling is 940 to 1000°C, and the finishing temperature is 1000 to 1100°C. The total rolling reduction ratio is 85 to 95%. In particular, the exit temperature of finish rolling is important in terms of controlling the metallographic structure of the steel sheet. More specifically, if the exit temperature during finish rolling is low, the metal structure may become non-uniform and formability may deteriorate. For this reason, the exit temperature of finish rolling is set to 940° C. or higher. On the other hand, in order to suppress coarsening of austenite, the exit temperature of finish rolling is set to 1000° C. or less.

[Cooling process]
[Primary cooling]
In the next cooling process, the finish-rolled steel plate is first cooled at a critical cooling rate of Vc+10°C/s or more, 60°C/s, in the temperature range from the finish rolling exit temperature (FT) to the martensitic transformation start temperature Ms+50°C. Primary cooling is performed at the following average cooling rate. By performing primary cooling in this temperature range at an average cooling rate of critical cooling rate Vc+10°C/s or more and 60°C/s or less, martensitic transformation is promoted, and the final metal structure is ferrite, upper bainite, etc. It is possible to reduce the total amount of at least one of pearlite and pearlite to 5% by area or less, and to make the metal structure uniform in the width direction. If the average cooling rate of primary cooling is less than the critical cooling rate Vc + 10°C/s, the total amount of at least one of ferrite, upper bainite, and pearlite will exceed 5 area%, and the desired strength may not be achieved. be. On the other hand, if the average cooling rate of primary cooling is more than 60° C./s, the cooling rate is so fast that it becomes difficult to uniformly cool the steel plate in the width direction, resulting in uneven cooling in the width direction. In this case, the desired metallographic structure may not be obtained in the width direction in the finally obtained hot rolled steel sheet, and/or the tensile strength may vary widely in the width direction. The shape collapses and warpage occurs in the width direction of the steel plate, making it impossible to achieve sufficient flatness. Therefore, the average cooling rate of the primary cooling is greater than or equal to the critical cooling rate Vc+10°C/s and less than or equal to 60°C/s, preferably greater than or equal to the critical cooling rate Vc+12°C/s and less than or equal to 60°C/s.

In this manufacturing method, the Ms point (° C.) is determined by the following formula 1.
Ms=823-350[C]-40[Mn]-35[V]-20[Cr]-17[Ni]-10[Cu]-10[Mo]-10[W]+15[Co]+30[Al ]-273 ...Formula 1
Here, [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], [W], [Co] and [Al] are the respective elements in the steel. The content (mass%) is 0 if the element is not contained. Further, the critical cooling rate Vc (° C.) is also a hardenability index at which the martensite area ratio is 90% or more, and can be expressed by the following formulas 2 and 3.
When the amount of solid solution B≧0.0005% by mass,
logVc=2.94-0.75×(2.7[C]+0.4[Si]+[Mn]+0.45[Ni]+0.8[Cr]+2[Mo])...Formula 2
When the amount of solid solution B is <0.0005% by mass,
logVc=3.69-0.75×(2.7[C]+0.4[Si]+[Mn]+0.45[Ni]+0.8[Cr]+[Mo])...Formula 3
Here, [C], [Si], [Mn], [Ni], [Cr] and [Mo] are the contents (mass%) of each element in the steel, and are 0 if no element is contained. It is. Moreover, the solid solution B amount (mass %) corresponds to the amount obtained by subtracting the B amount consumed to form boron nitride (BN) from the B content contained in the steel. On the other hand, the amount of solid solution N (mass %) that can form BN can be reduced by including Ti in the steel and fixing it as TiN. Therefore, the amount of solid solution B can be calculated using equations 4 and 5 below.
Solid solution B amount = 10.81 × ([B] / 10.81 - solid solution N amount / 14.01) ... Formula 4
However, when [N]/14.01-[Ti]/47.88>0,
Solid solution N amount = 14.01 × ([N]/14.01-[Ti]/47.88) ... Formula 5
When [N]/14.01−[Ti]/47.88≦0, the amount of solid solute N is 0.
Here, [B], [N], and [Ti] are the content (mass%) of each element in the steel, and are 0 when no element is contained.

In primary cooling, in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces. Such cooling is performed such that the vertical cooling ratio of the upper surface of the steel plate to the lower surface of the steel plate is 0.8 to 1.2. More specifically, the amount of cooling water injected onto the upper surface of the steel plate is This is done so that the amount of cooling water is 0.8 to 1.2 times the amount of cooling water injected to the lower surface. By uniformly cooling the upper and lower surfaces of the steel plate in this manner, it is possible to significantly suppress or reduce the occurrence of uneven cooling. As a result, it is possible to achieve uniformity of the metallographic structure and reduction of strength variations in the entire width direction, and related to this, it is possible to achieve sufficient flatness without causing warpage in the width direction of the hot rolled steel sheet. It becomes possible to achieve this. If the upper and lower cooling ratio is less than 0.8 or more than 1.2, the metal structure cannot be uniformly distributed in the width direction due to uneven cooling, that is, the edges in the width direction At all positions from 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position, the metal structure at 1/4 plate thickness position is expressed as area% of tempered marten. Site: It becomes impossible to organize more than 95% of the sites. As a result, it becomes impossible to sufficiently reduce the strength variation in the width direction to a level where the difference between the maximum value and the minimum value among the tensile strengths at all positions in the width direction is 30 MPa or less.

Here, the above-mentioned upper and lower cooling ratio does not mean the ratio of the amount of cooling water on the entire upper surface and the amount of cooling water on the entire lower surface in the section from FT to (Ms+50)°C. More specifically, in this manufacturing method, the section from FT to (Ms+50)°C is divided into sections every 10 m, and the upper and lower cooling ratios are calculated for each section from the amount of cooling water on the top surface and the amount of cooling water on the bottom surface. The upper and lower cooling ratios of each section calculated in this way are all controlled within the range of 0.8 to 1.2. By controlling the upper and lower cooling ratios not for each divided section but for the entire section, it is very difficult to sufficiently suppress the occurrence of cooling unevenness due to local overcooling, for example. However, by controlling the upper and lower cooling ratios for each section, it is possible to reduce local overcooling and reliably suppress the occurrence of uneven cooling. Moreover, such control of the upper and lower cooling ratios for each section can be performed by any appropriate means. For example, but not limited to, each section has a plurality of cooling water nozzles arranged above and below the steel plate along the traveling direction of the steel plate, so these cooling water nozzles can be appropriately controlled based on on/off control. By injecting, it is possible to relatively easily control the upper and lower cooling ratio of each section within the range of 0.8 to 1.2.

[Secondary cooling]
The primarily cooled steel plate is then secondarily cooled in a temperature range from Ms+50°C to 200°C at an average cooling rate of 50 to 120°C/s. By performing secondary cooling at an average cooling rate of 50 to 120°C/s in the temperature range from Ms+50°C to 200°C, which may correspond to the end temperature of martensitic transformation, the residence time in the temperature range where autotempering progresses is sufficient. This makes it possible to promote auto-tempering. Therefore, it is possible to sufficiently temper the martensite-transformed structure by the previous primary cooling, and to uniformly distribute tempered martensite in the width direction in the final metal structure in an amount of 95% or more by area. If the average cooling rate of secondary cooling is less than 50° C./s, it may not be possible to obtain a desired metal structure in the width direction of the steel plate, and strength variations in the width direction may not be sufficiently reduced. On the other hand, if the average cooling rate of secondary cooling exceeds 120°C/s, autotempering cannot be promoted, and fresh martensite may remain in an amount exceeding 5 area% in the final metal structure. be. In addition to or in place of this, due to such rapid cooling, the controllability of the amount of water used for cooling deteriorates, and uneven cooling occurs in the width direction due to local overcooling of parts. . In this case, the tensile strength of the finally obtained hot rolled steel sheet increases in the width direction, the shape of the hot rolled steel sheet collapses, and the steel sheet warps in the width direction, resulting in insufficient strength. It becomes impossible to achieve flatness.

In secondary cooling, as in the case of primary cooling, in addition to controlling the average cooling rate, it is extremely important to cool the steel plate evenly on its upper and lower surfaces. As in the case of primary cooling, such cooling is performed such that the vertical cooling ratio of the upper surface of the steel plate to the lower surface of the steel plate is 0.8 to 1.2. The amount of cooling water sprayed is 0.8 to 1.2 times the amount of cooling water injected onto the lower surface of the steel plate. By uniformly cooling the upper and lower surfaces of the steel plate in this manner, it is possible to significantly suppress or reduce the occurrence of uneven cooling. As a result, it is possible to achieve uniformity of the metallographic structure and reduction of strength variations in the entire width direction, and related to this, it is possible to achieve sufficient flatness without causing warpage in the width direction of the hot rolled steel sheet. It becomes possible to achieve this. If the upper and lower cooling ratio is less than 0.8 or more than 1.2, the metal structure cannot be uniformly distributed in the width direction due to uneven cooling, that is, the edges in the width direction At all positions from 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position, the metal structure at 1/4 plate thickness position is expressed as area% of tempered marten. Site: It becomes impossible to organize more than 95% of the sites. As a result, it becomes impossible to sufficiently reduce the strength variation in the width direction to a level where the difference between the maximum value and the minimum value among the tensile strengths at all positions in the width direction is 30 MPa or less.

Here, the above-mentioned upper and lower cooling ratio does not mean the ratio of the amount of cooling water on the entire upper surface and the amount of cooling water on the entire lower surface in the range from (Ms+50)° C. to 200° C. More specifically, in this manufacturing method, the section from (Ms + 50) °C to 200 °C is divided into sections every 10 m, and the upper and lower cooling ratios are calculated from the amount of cooling water on the upper surface and the amount of cooling water on the lower surface for each section, The upper and lower cooling ratios of each section calculated in this way are all controlled within the range of 0.8 to 1.2. By controlling the upper and lower cooling ratios not for each divided section but for the entire section, it is very difficult to sufficiently suppress the occurrence of cooling unevenness due to local overcooling, for example. However, by controlling the upper and lower cooling ratios for each section, it is possible to reduce local overcooling and reliably suppress the occurrence of uneven cooling. Moreover, such control of the upper and lower cooling ratios for each section can be performed by any appropriate means. Although not particularly limited, as in the case of primary cooling, for example, each section has a plurality of cooling water nozzles arranged above and below the steel plate along the direction of movement of the steel plate, so these cooling water By appropriately injecting the nozzle based on on/off control, it is possible to relatively easily control the upper and lower cooling ratio of each section within the range of 0.8 to 1.2.

[Winding process]
The secondarily cooled steel plate is finally wound up at 50 to 100°C in a winding process. If the coiling temperature is too low, the hot-rolled steel sheet may become hard and brittle, and excessive water cooling or the like will be required, resulting in a decrease in productivity. Therefore, the winding temperature is 50°C or higher, preferably 80°C or higher.

According to the hot-rolled steel sheet manufactured by the above manufacturing method, when the total width in the direction perpendicular to the rolling direction and the plate thickness direction is W, a 1/10W position, a 3/10W position from the end in the width direction, At all positions, 5/10W position, 7/10W position, and 9/10W position, the tempered martensite in the metal structure at the 1/4 plate thickness position is 95% or more in area%, and martensite is the main While achieving high strength due to the phase structure, more specifically, a tensile strength of 980 MPa or more, the difference between the maximum and minimum tensile strengths at all positions in the width direction is surely 30 MPa or less. can be controlled. Therefore, it becomes possible to significantly reduce strength variations in the width direction, and in connection with this, it becomes possible to achieve sufficient flatness without causing warpage in the width direction of the hot rolled steel sheet. Therefore, the high-strength hot-rolled steel sheet manufactured by the above-mentioned manufacturing method has uniform characteristics in the width direction and has very good flatness in spite of its high strength. In addition, the above-mentioned high strength and flatness can be achieved in hot-rolled steel sheets that have not been subjected to flattening treatment using a leveler, for example, in hot-rolled steel sheets immediately after production, so such flattening treatment ( Pre-processing) does not consume part of the steel sheet's inherent ductility. Therefore, it is possible to reduce the risk of forming defects during pressing of the steel plate, and it is also possible to significantly improve productivity. Therefore, it goes without saying that the high-strength hot-rolled steel sheet is particularly useful in the automobile field, but can also be used very effectively in other fields.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples in any way.

In the following examples, high-strength hot-rolled steel sheets according to embodiments of the present invention were manufactured under various conditions, and the characteristics and flatness in the width direction of the obtained high-strength hot-rolled steel sheets were investigated.

First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1, these slabs were heated under the conditions shown in Table 2, and then hot rolled. Hot rolling was carried out by performing rough rolling and finish rolling, and the exit temperature of rough rolling, the entry temperature (F0), exit temperature (FT), and total rolling reduction of finish rolling were as shown in Table 2. Met. Next, under the conditions shown in Table 2, the finish-rolled steel plate was first cooled in a temperature range from the finish rolling exit temperature (FT) to the martensitic transformation start temperature Ms + 50°C, and then Ms + 50°C. Secondary cooling was performed in a temperature range from 200°C to 200°C.

In primary cooling and secondary cooling, the section from FT to (Ms+50)℃ and the section from (Ms+50)℃ to 200℃ are divided into sections every 10m, and the amount of cooling water on the top surface and the amount on the bottom surface are determined for each section. The upper and lower cooling ratios were calculated from the amount of cooling water, and cooling was performed so that the upper and lower cooling ratios of each section calculated in this manner were controlled within a predetermined range. The upper and lower cooling ratios in primary cooling and secondary cooling in Table 2 indicate the one with the largest absolute value of the difference from the cooling ratio 1 among the upper and lower cooling ratios of each section in primary cooling and secondary cooling. Finally, the secondary cooled steel plate was rolled up under the conditions shown in Table 2 to obtain a hot rolled steel plate having a thickness of about 2.3 to 3.2 mm and a total width of 1200 mm.

The properties of the obtained hot rolled steel sheet were measured and evaluated by the following methods.

[Prior austenite grain size in metal structure]
The prior austenite grain size in the metal structure was determined as follows. First, a 200 μm x 200 μm area in the L cross section of a steel piece sampled from the surface of a hot rolled steel sheet at a position 1/4 of the sheet thickness was analyzed by SEM/EBSD. More specifically, a predetermined crystal orientation transformation was performed on the martensite structure obtained by SEM/EBSD to obtain an image in which the prior austenite grains were reconstructed, and then the equivalent circle diameter was determined from the prior austenite grains in the image. . This operation was performed for a total of 10 prior austenite grains, and the obtained 10 circle equivalent diameters were averaged to determine the prior austenite grain size.

[Strength variation in width direction]
First, a test was conducted in the direction parallel to the rolling direction at each of the 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the widthwise end of the hot rolled steel plate. A No. 5 tensile test piece according to JIS Z2241:2011 with the direction is taken. Next, five tensile strength values were obtained by conducting a tensile test based on JIS Z2241:2011 using these tensile test pieces, and finally, the width direction was determined by calculating the difference between the maximum and minimum values. The strength variation was determined.

[Tensile strength of hot rolled steel plate]
The minimum value among the above five tensile strength values was determined as the tensile strength of the hot rolled steel sheet.

[Evaluation of flatness]
Evaluation of flatness was performed as follows. First, the obtained hot-rolled steel plate is placed on a surface plate so that at least a part of one of the plate surfaces (lower surface) is in contact with the surface plate, and then the height of the hot-rolled steel sheet from the surface plate is the highest. The distance from the surface plate at a high position to the lower surface of the hot-rolled steel plate was measured, and the obtained measured value was determined as the maximum warp height H (mm) of the hot-rolled steel plate. As for evaluation of flatness, a case where the maximum warp height H was within 10 mm was judged as a pass, and a case where the maximum warp height H exceeded 10 mm was judged as a failure.

A case where the tensile strength of the hot-rolled steel sheet was 980 MPa or more and the flatness evaluation was acceptable was evaluated as a high-strength hot-rolled steel sheet with improved flatness. The results are shown in Table 3.

Referring to Tables 1 to 3, in Comparative Examples 2 and 10, the average cooling rate of secondary cooling was high, and the upper and lower cooling ratio of secondary cooling was not appropriate, so martensite was not sufficiently formed by auto-tempering during cooling. The percentage of fresh martensite (fM) becomes high, and the metal structure cannot be uniformly distributed in the width direction due to uneven cooling, resulting in a decrease in tensile strength in the width direction. The dispersion became noticeable. As a result, sufficient flatness could not be achieved. In Comparative Example 8, the average cooling rate of primary cooling was high, and the upper and lower cooling ratios of primary cooling and secondary cooling were not appropriate, resulting in uneven cooling, which made it difficult to obtain the desired metal structure in the width direction. It was not possible to distribute it evenly. In connection with this, it was not possible to achieve a tensile strength of 980 MPa or more, and the variation in tensile strength in the width direction became significant. As a result, sufficient flatness could not be achieved. In Comparative Example 9, the average cooling rate of the primary cooling was high and the upper and lower cooling ratio of the primary cooling was not appropriate, resulting in uneven cooling, which made it impossible to uniformly distribute the desired metal structure in the width direction. However, the tensile strength in the width direction varied significantly. As a result, sufficient flatness could not be achieved. In Comparative Example 11, because the upper and lower cooling ratios of primary cooling and secondary cooling were not appropriate, it was not possible to uniformly distribute the desired metal structure in the width direction due to uneven cooling. Relatedly, the variation in tensile strength in the width direction also became significant. As a result, sufficient flatness could not be achieved. In Comparative Example 13, since the C content was high, the variation in tensile strength in the width direction could not be controlled within a predetermined range, and the flatness decreased. In Comparative Example 14, the desired tensile strength could not be achieved because the C content was low. It is considered that in Comparative Example 15, auto-tempering during cooling of the steel sheet was suppressed because the Si content was high. As a result, the proportion of fresh martensite (fM) in the metal structure increased, and related to this, it was not possible to sufficiently reduce the variation in tensile strength in the width direction, and the flatness decreased. In Comparative Example 16, since the Mn content was high, it is considered that martensite was not sufficiently tempered even by auto-tempering during cooling of the steel sheet due to improvement in hardenability. As a result, the proportion of fresh martensite (fM) in the metal structure increased, and related to this, it was not possible to sufficiently reduce the variation in tensile strength in the width direction, and the flatness decreased. Comparative Example 17 could not achieve the desired tensile strength because the Mn content was low. It is also believed that because the Mn content was low, hardenability was insufficient and a relatively large amount of soft phases such as ferrite were formed during cooling. As a result, the structure with tempered martensite as the main phase cannot be uniformly distributed in the width direction, and the shape of the steel sheet collapses due to expansion of the steel sheet due to transformation into ferrite, etc. It is thought that the flatness has decreased. In Comparative Example 18, since the Ti content was high, a relatively large amount of soft phases such as ferrite were generated from unrecrystallized austenite during the cooling process. In connection with this, it was not possible to form a desired metal structure in the width direction, and as a result, it was not possible to sufficiently reduce the variation in tensile strength in the width direction, resulting in a decrease in flatness.

In Comparative Example 19, because the average cooling rate of primary cooling was low, the total amount of at least one of ferrite, upper bainite, and pearlite exceeded 5 area %, and the desired tensile strength could not be achieved. In Comparative Example 20, since the average cooling rate of the primary cooling was high, it was not possible to uniformly distribute the desired metal structure in the width direction due to the occurrence of cooling unevenness. Dispersion in tensile strength also became significant. As a result, sufficient flatness could not be achieved. In Comparative Examples 21 and 22, the desired metal structure could not be uniformly distributed in the width direction due to the occurrence of uneven cooling because the upper and lower cooling ratio of the primary cooling was not appropriate. The variation in tensile strength in the width direction also became significant. As a result, sufficient flatness could not be achieved. In Comparative Example 23, because the average cooling rate of secondary cooling was low, it was not possible to obtain the desired metallographic structure in the width direction of the steel plate, and related to this, the variation in tensile strength in the width direction was also significant. . As a result, sufficient flatness could not be achieved. In Comparative Example 24, because the average cooling rate of secondary cooling was high, martensite could not be sufficiently tempered by auto-tempering during cooling, resulting in a high proportion of fresh martensite (fM) and uneven cooling. Due to this, the metal structure could not be uniformly distributed in the width direction, and the tensile strength in the width direction varied significantly. As a result, sufficient flatness could not be achieved. In Comparative Examples 25 and 26, the desired metal structure could not be uniformly distributed in the width direction due to the occurrence of uneven cooling because the upper and lower cooling ratio of secondary cooling was not appropriate. The variation in tensile strength in the width direction also became significant. As a result, sufficient flatness could not be achieved.

In contrast, the hot-rolled steel sheets according to all the invention examples have a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, especially the cooling process, At all positions: 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position, the tempered martensite in the metal structure at the plate thickness 1/4 position is calculated by area%. With a tensile strength of 95% or more, it was possible to achieve a tensile strength of 980 MPa or more due to the structure having tempered martensite as the main phase. In addition, it is possible to reliably control the difference between the maximum and minimum tensile strengths at all positions in the width direction to 30 MPa or less, and therefore it is possible to significantly reduce strength variations in the width direction. In this regard, sufficient flatness could be achieved without causing warpage in the width direction of the hot rolled steel sheet. Table 3 shows the part where the minimum tensile strength was obtained and the part where the maximum tensile strength was obtained among the 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the end in the width direction. Only the metal structure of the part where tensile strength was obtained is specifically shown. However, in Invention Examples 1, 3 to 7, and 12, in all these positions, the metal structure at the 1/4 position of the plate thickness has an area ratio of tempered martensite: 95% or more and fresh martensite: 5%. The following and at least one of ferrite, upper bainite, and pearlite: the total amount was 5% or less.

Claims (3)

1. In mass%,
   C: 0.050-0.100%,
   Si: 0.010-0.200%,
   Mn: 1.00-2.50%,
   Ti: 0.001 to 0.120%,
   Al: 0.001-0.050%,
   B: 0.0005-0.0050%,
   P: 0.100% or less,
   S: 0.050% or less,
   N: 0.0050% or less,
   O: 0 to 0.0050%,
   Cu: 0 to 0.20%,
   Ni: 0 to 0.20%,
   Sn: 0 to 0.10%,
   Cr: 0-0.40%,
   Mo: 0-0.20%,
   Nb: 0 to 0.05%,
   V: 0-0.10%,
   As: 0 to 0.100%,
   Zr: 0 to 0.100%,
   Ca: 0-0.0050%,
   Mg: 0-0.100%,
   Bi: 0 to 0.020%,
   Co: 0 to 0.20%,
   W: 0-0.20%,
   Zn: 0-0.20%,
   It has a chemical composition consisting of REM: 0 to 0.1000%, and the balance: Fe and impurities,
   When the total width in the direction perpendicular to the rolling direction and the plate thickness direction is W, 1/10W position, 3/10W position, 5/10W position, 7/10W position, and 9/10W position from the end in the width direction At all positions, the metal structure at the 1/4 position of the plate thickness is expressed as area%,
   Tempered martensite: 95% or more,
   Fresh martensite: 5% or less, and at least one of ferrite, upper bainite, and pearlite: 5% or less in total,
   A high-strength hot-rolled steel sheet, characterized in that the difference between the maximum and minimum tensile strengths at all positions in the width direction is 30 MPa or less.
2. The chemical composition is in mass%,
   O: 0.0001 to 0.0050%,
   Cu: 0.001 to 0.20%,
   Ni: 0.001 to 0.20%,
   Sn: 0.001 to 0.10%,
   Cr: 0.001-0.40%,
   Mo: 0.001-0.20%,
   Nb: 0.001 to 0.05%,
   V: 0.001 to 0.10%,
   As: 0.001 to 0.100%,
   Zr: 0.0001 to 0.100%,
   Ca: 0.0001-0.0050%,
   Mg: 0.0001-0.100%,
   Bi: 0.0001-0.020%,
   Co: 0.001 to 0.20%,
   W: 0.001-0.20%,
   Zn: 0.001~0.20%, and REM: 0.0001~0.1000%
   The high-strength hot-rolled steel sheet according to claim 1, characterized in that it contains at least one of the following.
3. The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the prior austenite grain size in the metal structure is 40 μm or less.