WO2024009812A1 - Hot-rolled steel sheet - Google Patents

Abstract

This hot-rolled steel sheet has a desired chemical composition and has a metal structure which contains, in terms of area%, at least 10% of bainite, at least 10% of tempered martensite, a total of 70-96% of bainite and tempered martensite, at most 20% of fresh martensite, 4-12% of retained austenite, at most 5% of ferrite, and at most 5% of pearlite, and in which: the average grain diameter of prior austenite grains is at most 20.0 μm; the average aspect ratio of prior austenite grains is at most 3.00; and the area ratio of an aggregate of fresh martensite having a major diameter of at least 30 μm and retained austenite is at most 5%.

Description

hot rolled steel plate

The present invention relates to a hot rolled steel plate.
This application claims priority based on Japanese Patent Application No. 2022-109509 filed in Japan on July 7, 2022, the contents of which are incorporated herein.

In recent years, the weight of automobiles and mechanical parts has been reduced. By ensuring rigidity by designing parts into optimal shapes, it is possible to reduce the weight of automobiles and mechanical parts. Furthermore, in blank molded parts such as press molded parts, weight reduction is possible by reducing the thickness of the part material.

However, in order to ensure the strength characteristics of parts such as static fracture strength and yield strength while reducing the plate thickness, it is necessary to use high-strength materials. In particular, consideration has begun to be given to the application of steel plates of over 780 MPa class to automobile suspension parts such as lower arms, trail links, and knuckles. These automobile suspension parts are manufactured by subjecting steel plates to burring, stretch flanging, bending, and the like. Therefore, the steel sheets used for these automobile suspension parts are required to have excellent formability, particularly ductility and hole expandability.

For example, Patent Document 1 states that C/(Mo+Ti), which is the ratio of the amount of C in atomic % to the total amount of Mo and Ti, is 0.5 to 3.0, and the metal structure is two phases of ferrite and bainite. A high-strength steel sheet having a strength of 700 MPa or more is disclosed, which has a structure in which carbides containing Ti and Mo are dispersed and precipitated in a ferrite phase.

Patent Document 2 discloses a high-strength hot-rolled steel sheet with small anisotropy in stretch flangeability, characterized in that 88% or more of the steel structure consists of a bainite structure.

Japanese Patent No. 3891030 Japanese Patent No. 3197571

In order to further increase strength for the purpose of thinning parts, hot-rolled steel sheets with a high C content and containing retained austenite may be used. In such a hot-rolled steel sheet, hydrogen embrittlement cracking is likely to occur at the cut end face, which may deteriorate the hydrogen embrittlement resistance.

In Patent Documents 1 and 2, it is necessary to further increase the strength, and hydrogen embrittlement resistance at the cut end surface is not considered.

The present invention was made in view of the above circumstances, and an object of the present invention is to provide a hot rolled steel sheet having high strength, excellent ductility, hole expandability, and hydrogen embrittlement resistance.

The present inventors have conducted extensive studies on a method for improving the hydrogen embrittlement resistance of the cut end surface of a hot rolled steel sheet. As a result, it was found that by reducing the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more, the hydrogen embrittlement resistance of the cut end surface of a hot rolled steel sheet could be improved.

The present inventors have discovered that in order to manufacture the above-mentioned hot rolled steel sheet, it is necessary to control the grain size and shape of prior austenite grains in the rough rolling process and finish rolling process, and to strictly control the temperature history after the coiling process. We have found that controlling is effective.

The gist of the present invention based on the above findings is as follows.
(1) The hot rolled steel sheet according to one aspect of the present invention has a chemical composition in mass %,
C: 0.13-0.23%,
Si: 0.70-1.79%,
Mn: 1.79-3.00%,
P: 0.060% or less,
S: 0.005% or less,
N: 0.0070% or less,
O: 0.010% or less,
Al: 0.010-0.430%,
Ti: 0.006 to 0.055%,
Nb: 0.005-0.040%,
B: 0.0001 to 0.0030%,
Cr: 0-0.660%,
Mo: 0 to 0.300%,
Cu: 0 to 1.000%,
Ni: 0-1.000%,
Sn: 0-0.100%,
Ca: 0-0.0200%,
As: 0 to 0.100%,
Bi: 0 to 0.020%,
Mg: 0 to 0.0200%,
Zr: 0 to 0.400%,
V: 0 to 0.200%,
REM: 0-0.1000%,
Co: 0 to 0.2000%,
Contains W: 0 to 0.2000% and Zn: 0 to 0.2000%,
The remainder consists of Fe and impurities,
In metallographic structure,
In area%,
Bainite: 10% or more,
Tempered martensite: 10% or more,
Total of bainite and tempered martensite: 70-96%,
Fresh martensite: 20% or less,
Retained austenite: 4-12%,
Ferrite: 5% or less,
Perlite: 5% or less,
Average grain size of prior austenite grains: 20.0 μm or less,
Average aspect ratio of prior austenite grains: 3.00 or less,
The area ratio of aggregates of fresh martensite and retained austenite having a major axis of 30 μm or more is 5% or less.
(2) In the hot rolled steel sheet according to (1) above, the pearlite may be less than 3% in the metal structure.
(3) The hot-rolled steel sheet according to (1) or (2) above is a region of the bainite in which the average value of nanoindentation hardness at a load of 5000 μN and a loading rate of 500 μN/s is 4.7 GPa or less. may be 30% or less.
(4) The hot rolled steel sheet according to any one of (1) to (3) above, wherein the chemical composition is expressed in mass%,
Cr: 0.020-0.660%,
Mo: 0.001-0.300%,
Cu: 0.001 to 1.000%,
Ni: 0.001 to 1.000%,
Sn: 0.001 to 0.100%,
Ca: 0.0005-0.0200%,
As: 0.001 to 0.100%,
Bi: 0.001 to 0.020%,
Mg: 0.0005-0.0200%,
Zr: 0.001-0.400%,
V: 0.001-0.200%,
REM: 0.0005-0.1000%,
Co: 0.0005-0.2000%,
W: 0.0005 to 0.2000%, and Zn: 0.0005 to 0.2000%
It may contain one or more types from the group consisting of.

According to the above aspect of the present invention, it is possible to provide a hot rolled steel sheet having high strength, as well as excellent ductility, hole expandability, and hydrogen embrittlement resistance.

FIG. 3 is a diagram for explaining a method of approximating a crystal grain with an ellipsoid.

Hereinafter, the hot rolled steel sheet according to this embodiment will be described in detail. However, the present invention is not limited to only the configuration disclosed in this embodiment, and various changes can be made without departing from the spirit of the present invention.

The numerically limited ranges described below with "~" in between include the lower limit and the upper limit. Numerical values indicated as "less than" or "greater than" do not include the value within the numerical range. All "%" regarding chemical composition refers to "mass %".

The hot-rolled steel sheet according to this embodiment has a chemical composition in mass %: C: 0.13 to 0.23%, Si: 0.70 to 1.79%, Mn: 1.79 to 3.00%. , P: 0.060% or less, S: 0.005% or less, N: 0.0070% or less, O: 0.010% or less, Al: 0.010 to 0.430%, Ti: 0.006 to 0.055%, Nb: 0.005 to 0.040%, B: 0.0001 to 0.0030%, and the remainder: Fe and impurities.
Each element will be explained in detail below.

C: 0.13-0.23%
C is an element that improves the tensile strength of hot rolled steel sheets. If the C content is less than 0.13%, the area ratio of ferrite becomes too high, making it impossible to obtain the desired tensile strength in the hot rolled steel sheet. Therefore, the C content is set to 0.13% or more. The C content is preferably 0.14% or more, more preferably 0.16% or more.
On the other hand, if the C content exceeds 0.23%, the area ratio of fresh martensite becomes too high, and the hole expandability and hydrogen embrittlement resistance of the hot rolled steel sheet deteriorate. Therefore, the C content is set to 0.23% or less. The C content is preferably 0.21% or less, more preferably 0.20% or less.

Si: 0.70-1.79%
Si is an element that stabilizes retained austenite. If the Si content is less than 0.70%, a desired amount of retained austenite cannot be obtained, and the ductility of the hot rolled steel sheet deteriorates. Therefore, the Si content is set to 0.70% or more. The Si content is preferably 0.90% or more, more preferably 1.00% or more.
On the other hand, if the Si content exceeds 1.79%, the area ratio of retained austenite becomes too high, and the hole expandability of the hot rolled steel sheet deteriorates. Therefore, the Si content is set to 1.79% or less. The Si content is preferably 1.60% or less, more preferably 1.40% or less, even more preferably 1.30% or less.

Mn: 1.79-3.00%
Mn is an element necessary to improve the strength of hot rolled steel sheets. If the Mn content is less than 1.79%, the area ratio of ferrite becomes too high, making it impossible to obtain the desired tensile strength. Therefore, the Mn content is set to 1.79% or more. The Mn content is preferably 2.00% or more, more preferably 2.20% or more.
On the other hand, if the Mn content exceeds 3.00%, the ductility of the hot rolled steel sheet deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.60% or less, more preferably 2.40% or less.

P: 0.060% or less P is an element that segregates in the center of the thickness of the hot rolled steel sheet. Moreover, P is also an element that makes the weld part brittle. If the P content exceeds 0.060%, slab cracking is likely to occur, making it difficult to perform casting. Therefore, the P content is set to 0.060% or less. The P content is preferably 0.020% or less, more preferably 0.015% or less.
The lower the P content, the better, and preferably 0%. However, if the P content is reduced excessively, the cost for removing P will increase significantly, so the P content may be set to 0.001% or more.

S: 0.005% or less S is an element that embrittles slabs when it exists as a sulfide in steel. S is also an element that deteriorates the formability of hot rolled steel sheets. If the S content exceeds 0.005%, the hole expandability of the hot rolled steel sheet deteriorates. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less, more preferably 0.003% or less.
The lower the S content, the better, and preferably 0%. However, if the S content is reduced excessively, the cost for removing S will increase significantly, so the S content may be set to 0.001% or more.

N: 0.0070% or less N is an element that forms coarse nitrides in steel and embrittles the slab. If the N content exceeds 0.0070%, the risk of slab cracking increases significantly. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0050% or less, more preferably 0.0035% or less.
The lower the N content, the better, and preferably 0%. However, if the N content is reduced excessively, the cost for removing N will increase significantly, so the N content may be set to 0.0005% or more.

O: 0.010% or less When O is contained in a large amount in steel, it forms coarse oxides that serve as starting points for fracture, and deteriorates the hydrogen embrittlement resistance of hot rolled steel sheets. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.008% or less, more preferably 0.006% or less.
In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.001% or more.

Al: 0.010-0.430%
Al is an element that acts as a deoxidizer and improves the cleanliness of steel. If the Al content is less than 0.010%, a sufficient deoxidizing effect cannot be obtained and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions deteriorate the hole expandability of the hot rolled steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.040% or more, more preferably 0.100% or more.
On the other hand, if the Al content exceeds 0.430%, slab cracking tends to occur, making casting difficult. Therefore, the Al content is set to 0.430% or less. The Al content is preferably 0.400% or less, 0.350% or less, and more preferably 0.200% or less.

Ti: 0.006-0.055%
Ti is an element that increases the strength of hot rolled steel sheets by forming fine nitrides in steel. If the Ti content is less than 0.006%, the desired tensile strength cannot be obtained in the hot rolled steel sheet. Therefore, the Ti content is set to 0.006% or more. The Ti content is preferably 0.010% or more, more preferably 0.020% or more.
On the other hand, if the Ti content exceeds 0.055%, the risk of slab cracking increases significantly. Therefore, the Ti content is set to 0.055% or less. The Ti content is preferably 0.040% or less, more preferably 0.030% or less.

Nb: 0.005-0.040%
Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb is also an element that increases the strength of hot rolled steel sheets by forming fine carbides. If the Nb content is less than 0.005%, prior austenite grains cannot be refined, and the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more increases, resulting in hot rolling. The hole expandability and hydrogen embrittlement resistance of the steel sheet deteriorate. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably 0.010% or more, more preferably 0.020% or more.
On the other hand, when the Nb content exceeds 0.040%, the aspect ratio of prior austenite grains becomes high, and the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more becomes high, and hot rolling The hole expandability and hydrogen embrittlement resistance of the steel sheet deteriorate. Therefore, the Nb content is set to 0.040% or less. The Nb content is preferably 0.030% or less.

B: 0.0001-0.0030%
B is an element that increases the strength of the hot rolled steel sheet by suppressing the formation of ferrite during the cooling process. If the B content is less than 0.0001%, the desired tensile strength cannot be obtained in the hot rolled steel sheet. Therefore, the B content is set to 0.0001% or more. The B content is preferably 0.0005% or more, more preferably 0.0010% or more.
On the other hand, if the B content exceeds 0.0030%, hot deformation resistance increases and hot rolling becomes difficult. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less.

The remainder of the chemical composition of the hot rolled steel sheet according to this embodiment may be Fe and impurities. In this embodiment, impurities are those mixed in from ore as a raw material, scrap, manufacturing environment, etc., or those that are allowed within a range that does not adversely affect the properties of the hot rolled steel sheet according to this embodiment. means.
The hot rolled steel sheet according to this embodiment may contain the following arbitrary elements in place of a part of Fe. The lower limit of the content when no arbitrary element is contained is 0%. Each arbitrary element will be explained below.

Cr:0.020~0.660%
Cr is an element that reduces the amount of ferrite and increases the strength of the hot rolled steel sheet. In order to reliably obtain this effect, the Cr content is preferably 0.020% or more. The Cr content is more preferably 0.050% or more, 0.100% or more, even more preferably 0.200% or more.
On the other hand, when the Cr content is more than 0.660%, the ductility of the hot rolled steel sheet deteriorates. Therefore, the Cr content is set to 0.660% or less. The Cr content is preferably 0.500% or less, more preferably 0.400% or less.

Mo: 0.001-0.300%
Mo is an element that increases the strength of hot rolled steel sheets by forming fine carbides in steel. In order to reliably obtain this effect, the Mo content is preferably 0.001% or more.
On the other hand, when the Mo content exceeds 0.300%, the ductility of the hot rolled steel sheet deteriorates. Therefore, the Mo content is set to 0.300% or less.

Cu: 0.001-1.000%
Cu has the effect of increasing the hardenability of steel and the effect of precipitating as carbides in steel at low temperatures to increase the strength of hot rolled steel sheets. In order to more reliably obtain the effects of the above action, the Cu content is preferably 0.001% or more.
However, if the Cu content exceeds 1.000%, intergranular cracking may occur in the slab. Therefore, the Cu content is set to 1.000% or less.

Ni: 0.001-1.000%
Ni has the effect of increasing the hardenability of the steel sheet and increasing the strength of the hot rolled steel sheet. Further, when containing Cu, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. In order to more reliably obtain the effects of the above action, the Ni content is preferably 0.001% or more.
Since Ni is an expensive element, it is economically undesirable to contain a large amount of Ni. Therefore, the Ni content is set to 1.000% or less.

Sn: 0.001-0.100%
Sn increases the strength of the hot-rolled steel sheet by forming a solid solution in the steel, and also increases the ductility and hole expandability. In order to reliably exhibit this effect, the Sn content may be set to 0.001% or more.
On the other hand, if the Sn content exceeds 0.100%, grain boundary embrittlement cracking occurs in hot conditions. Therefore, the Sn content is set to 0.100% or less.

Ca: 0.0005-0.0200%
Ca has the effect of improving the formability of the hot rolled steel sheet by controlling the shape of inclusions into a preferable shape. In order to more reliably obtain the effects of the above action, the Ca content is preferably 0.0005% or more.
On the other hand, when the Ca content exceeds 0.0200%, inclusions are excessively generated in the steel, which deteriorates the ductility of the hot rolled steel sheet. Therefore, the Ca content is set to 0.0200% or less.

As: 0.001-0.100%
By lowering the austenite single-phase temperature, As makes prior austenite grains finer and contributes to improving the hydrogen embrittlement resistance of the hot rolled steel sheet. In order to reliably obtain this effect, it is preferable that the As content be 0.001% or more.
On the other hand, since the above effect is saturated even if a large amount of As is contained, the As content is set to 0.100% or less.

Bi:0.001~0.020%
Bi has the effect of improving the formability of the hot rolled steel sheet by making the solidification structure finer. In order to more reliably obtain the effect of this action, it is preferable that the Bi content is 0.001% or more.
On the other hand, even if the Bi content exceeds 0.020%, the effects of the above action will be saturated, which is economically unfavorable. Therefore, the Bi content is set to 0.020% or less.

Mg: 0.0005-0.0200%
Mg has the effect of improving the formability of the hot rolled steel sheet by controlling the shape of inclusions into a preferable shape. In order to more reliably obtain the effects of the above action, the Mg content is preferably 0.0005% or more.
On the other hand, when the Mg content exceeds 0.0200%, inclusions are excessively generated in the steel, which deteriorates the ductility of the hot rolled steel sheet. Therefore, the Mg content is set to 0.0200% or less.

Zr: 0.001-0.400%
Zr is an element that contributes to inclusion control, particularly to fine dispersion of inclusions, and increases the strength of the hot rolled steel sheet. In order to reliably obtain this effect, the Zr content is preferably 0.001% or more.
On the other hand, if a large amount of Zr is contained, the surface quality of the hot rolled steel sheet may be significantly deteriorated. Therefore, the Zr content is set to 0.400% or less.

V:0.001~0.200%
V is an element that increases the strength of hot rolled steel sheets by forming fine carbides in the steel. In order to reliably obtain this effect, the V content is preferably 0.001% or more.
On the other hand, if the V content exceeds 0.200%, the hole expandability of the hot rolled steel sheet deteriorates. Therefore, the V content is set to 0.200% or less.

REM: 0.0005-0.1000%
REM has the effect of improving the formability of a hot rolled steel sheet by controlling the shape of inclusions into a preferable shape. In order to more reliably obtain the effects of the above action, the REM content is preferably 0.0005% or more.
On the other hand, when the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which deteriorates the ductility of the hot rolled steel sheet. Therefore, the REM content is set to 0.1000% or less.
Here, REM refers to a total of 17 types of rare earth elements including Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are added industrially in the form of mischmetal.

Co:0.0005~0.2000%
Co is an element that contributes to inclusion control, particularly to fine dispersion of inclusions, and increases the strength of the hot rolled steel sheet. In order to reliably obtain this effect, the Co content is preferably 0.0005% or more.
On the other hand, if a large amount of Co is contained, the surface quality of the hot rolled steel sheet may be significantly deteriorated. Therefore, the Co content is set to 0.2000% or less.

W: 0.0005-0.2000%
W is an element that contributes to inclusion control, particularly to fine dispersion of inclusions, and increases the strength of the hot rolled steel sheet. In order to reliably obtain this effect, the W content is preferably 0.0005% or more.
On the other hand, if a large amount of W is contained, the surface quality of the hot rolled steel sheet may be significantly deteriorated. Therefore, the W content is set to 0.2000% or less.

Zn: 0.0005-0.2000%
Zn is an element that contributes to inclusion control, particularly to fine dispersion of inclusions, and increases the strength of hot rolled steel sheets. In order to reliably obtain this effect, the Zn content is preferably 0.0005% or more.
On the other hand, if a large amount of Zn is contained, the surface quality of the hot rolled steel sheet may be significantly deteriorated. Therefore, the W content is set to 0.2000% or less.

The chemical composition of the hot-rolled steel sheet described above may be analyzed using a spark discharge emission spectrometer or the like. Note that C and S are determined by burning in an oxygen stream using a gas component analyzer or the like, and then measuring by an infrared absorption method. Further, for N, a value identified by melting a test piece taken from a hot-rolled steel plate in a helium stream and measuring it by a thermal conductivity method is used.

Next, the metal structure of the hot rolled steel sheet according to this embodiment will be explained.
The hot rolled steel sheet according to the present embodiment has a metal structure in which, in terms of area %, bainite: 10% or more, tempered martensite: 10% or more, total of bainite and tempered martensite: 70 to 96%, fresh martensite : 20% or less, retained austenite: 4 to 12%, ferrite: 5% or less, pearlite: 5% or less, average grain size of prior austenite grains: 20.0 μm or less, average aspect ratio of prior austenite grains: 3.00 or less The area ratio of aggregates of fresh martensite and retained austenite having a major axis of 30 μm or more is 5% or less.

In this embodiment, the metallographic structure at a position of 1/4 of the thickness of a hot rolled steel plate (an area from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) is defined. The reason is that the metal structure at this position shows a typical metal structure of a hot rolled steel sheet.
Each regulation will be explained below.

Bainite: 10% or more Bainite is a structure that increases the ductility of hot rolled steel sheets. If the area ratio of bainite is less than 10%, desired ductility cannot be obtained in the hot rolled steel sheet. Therefore, the area ratio of bainite is set to 10% or more. The area ratio of bainite is preferably 20% or more or 30% or more, more preferably 40% or more.
Although the upper limit of the area ratio of bainite is not particularly limited, it may be set to 96% or less in view of the relationship with the area ratio of retained austenite. The area ratio of bainite may be 80% or less or 75% or less.

Tempered martensite: 10% or more Tempered martensite is an effective structure for obtaining desired ductility, hole expandability, and hydrogen embrittlement resistance in a hot rolled steel sheet. If the area ratio of tempered martensite is less than 10%, desired ductility, hole expandability, and hydrogen embrittlement resistance cannot be obtained in the hot rolled steel sheet. Therefore, the area ratio of tempered martensite is set to 10% or more. The area ratio of tempered martensite is preferably 15% or more or 20% or more, more preferably 30% or more.
Although the upper limit of the area ratio of tempered martensite is not particularly limited, it may be 96% or less in view of the relationship with the area ratio of retained austenite. The area ratio of tempered martensite may be 80% or less, 60% or less, or 50% or less.

Total of bainite and tempered martensite: 70-96%
If the total area ratio of bainite and tempered martensite is less than 70%, desired ductility, hole expandability and/or hydrogen embrittlement resistance cannot be obtained in the hot rolled steel sheet. Therefore, the total area ratio of bainite and tempered martensite is 70% or more. The area ratio of bainite and tempered martensite is preferably 75% or more, more preferably 80% or more.
Although the upper limit of the total area ratio of bainite and tempered martensite is not particularly limited, it is set to 96% or less in relation to the area ratio of retained austenite. The total area ratio of bainite and tempered martensite may be 90% or less.

Fresh martensite: 20% or less Fresh martensite is a structure that increases the strength of hot rolled steel sheets, but it is also a structure that deteriorates hole expandability and hydrogen embrittlement resistance. If the area ratio of fresh martensite exceeds 20%, the hole expandability and hydrogen embrittlement resistance of the hot rolled steel sheet deteriorate. Therefore, the area ratio of fresh martensite is set to 20% or less. The area ratio of fresh martensite is preferably 15% or less, more preferably 10% or less, even more preferably 7% or less.
The lower limit of the area ratio of fresh martensite is not particularly limited, but may be set to 0%.

Retained austenite: 4-12%
Retained austenite is a structure that increases the ductility of hot rolled steel sheets. If the area ratio of retained austenite is less than 4%, desired ductility cannot be obtained in the hot rolled steel sheet. Therefore, the area ratio of retained austenite is set to 4% or more. The area ratio of retained austenite is preferably 6% or more, more preferably 7% or more.
On the other hand, if the area ratio of retained austenite exceeds 12%, the hole expandability of the hot rolled steel sheet deteriorates. Therefore, the area ratio of retained austenite is set to 12% or less. The area ratio of retained austenite is preferably 10% or less.

Ferrite: 5% or less If the area ratio of ferrite exceeds 5%, the strength of the hot rolled steel sheet deteriorates. Therefore, the area ratio of ferrite is set to 5% or less. The area ratio of ferrite is preferably 3% or less, more preferably 2% or less. The area ratio of ferrite may be 0%.

Pearlite: 5% or less When the area ratio of pearlite exceeds 5%, the strength of the hot rolled steel sheet deteriorates. Therefore, the area ratio of pearlite is set to 5% or less.
The area ratio of pearlite is preferably 3% or less. By setting the area ratio of pearlite to 3% or less, the TS-El balance in the hot rolled steel sheet can be further improved. That is, high strength and excellent ductility can be achieved at a higher level. Since it is preferable that the area ratio of pearlite be small, the area ratio of pearlite may be set to 0%.

The method for measuring the area ratio of each tissue will be explained below.
A cross section parallel to the rolling direction of a hot-rolled steel plate, at a position of 1/4 of the plate thickness from the surface (an area from 1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) and plate width. A test piece is taken so that the metal structure at the central position can be observed.
After polishing the cross section of the above test piece using #600 to #1500 silicon carbide paper, polish it to a mirror surface using a dilute solution such as alcohol or a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in pure water. Finish. Next, the strain introduced into the surface layer of the sample is removed by polishing at room temperature using colloidal silica that does not contain an alkaline solution. At any position in the longitudinal direction of the sample cross section, observe an area with a length of 100 μm and a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface (hereinafter referred to as the EBSD observation field). The crystal orientation information is obtained by measuring with an electron backscatter diffraction method at a measurement interval of 0.1 μm.

For the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. From the obtained crystal orientation information, use the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer to identify a region with an fcc crystal structure, and Calculate the area ratio. Thereby, the area ratio of retained austenite is obtained.

Next, those whose crystal structure is BCC are determined to be bainite, ferrite, pearlite, fresh martensite, and tempered martensite. Regarding these regions, using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, "Grain A region where the "Orientation Spread" is 1° or less is extracted as ferrite. The area ratio of ferrite is obtained by calculating the area ratio of the extracted ferrite.

Next, in the remaining region (region where "Grain Orientation Spread" exceeds 1°), under the condition that the 15° grain boundary is defined as the grain boundary, the maximum value of "Grain Average IQ" of the ferrite region is determined as Iα Then, the region where the value exceeds Iα/2 is extracted as bainite, and the region where the value is less than Iα/2 is extracted as “pearlite, fresh martensite, and tempered martensite.” The area ratio of bainite is obtained by calculating the area ratio of the extracted bainite.

Regarding the extracted "pearlite, fresh martensite, and tempered martensite", pearlite, fresh martensite, and tempered martensite are distinguished by the following method.
Note that the IQ may change depending on the condition of the surface of the observed sample and the accuracy at the time of measurement. In order to eliminate these influences, a material having a CI (Confidence Index) value representing reliability of crystal orientation information of 0.8 or more may be used. If the CI value is 0.8 or less, perform electrolytic polishing using the above method again, adjust the working distance between the sample and the EBSD pattern detector, or adjust the accelerating voltage and irradiation current level. , adjust the gain or exposure of the detector and acquire data so that the CI value is 0.8 or more.

In order to observe the same area as the EBSD measurement area with SEM, a Vickers indentation is made near the observation position. Thereafter, contamination on the surface layer is polished off, leaving the structure on the observation surface, and nital etching is performed. Next, the same field of view as the EBSD observation surface (EBSD observation field) is observed using a SEM at a magnification of 3000 times. Among the regions identified as "pearlite, fresh martensite, and tempered martensite" in the EBSD measurement, the regions that have a substructure within the grains and where cementite is precipitated in multiple variants are tempered. It is determined to be rehydrated martensite. The area where cementite is precipitated in a lamellar shape is determined to be pearlite. A region where the brightness is high and the underlying structure is not exposed by etching is determined to be fresh martensite. By calculating the respective area ratios, the area ratios of tempered martensite, pearlite, and fresh martensite are obtained.

To remove contamination from the surface layer of the observation surface, a method such as buffing using alumina particles with a particle size of 0.1 μm or less or Ar ion sputtering may be used.

Note that the rolling direction of a hot rolled steel sheet can be determined by the following method.
First, a test piece is taken so that the cross section of the hot rolled steel sheet can be observed. After finishing the thickness section of the collected test piece with mirror polishing, it is observed using an optical microscope. The observation range is the entire thickness of the plate, and areas with low brightness are determined to be inclusions. Among inclusions, the length of the major axis is 5 μm or more, and a direction parallel to the direction in which the inclusion extends is determined as the rolling direction.

Average grain size of prior austenite grains: 20.0 μm or less When the grain size of prior austenite grains is large, local elongation deteriorates, resulting in deterioration of the hole expandability of the hot rolled steel sheet. When the average grain size of the prior austenite grains exceeds 20.0 μm, the hole expandability of the hot rolled steel sheet deteriorates significantly. Therefore, the average grain size of the prior austenite grains is set to 20.0 μm or less. The average particle size of the prior austenite grains is preferably 15.0 μm or less, more preferably 13.0 μm or less.
The smaller the average grain size of the prior austenite grains, the better the hole expandability of the hot rolled steel sheet, but the effect is saturated even when the average grain size is less than 7.0 μm. Therefore, the average grain size of the prior austenite grains may be 7.0 μm or more.

Average aspect ratio of prior austenite grains: 3.00 or less When the average aspect ratio of prior austenite grains exceeds 3.00, the hole expandability of the hot rolled steel sheet deteriorates. Therefore, the average aspect ratio of prior austenite grains is set to 3.00 or less. The average aspect ratio of the prior austenite grains is preferably 2.50 or less, more preferably 2.00 or less, and even more preferably 1.50 or less.
Note that the aspect ratio of the prior austenite grains is a value obtained by dividing the long axis of the prior austenite grains by the short axis, and takes a value of 1.00 or more. The smaller the aspect ratio, the more equiaxed the crystal grain, and the larger the aspect ratio, the flatter the crystal grain.

Method for measuring the average grain size and average aspect ratio of prior austenite grains In a cross section parallel to the rolling direction of a hot-rolled steel plate, from the surface to the 1/4th of the sheet thickness (1/8th of the sheet thickness from the surface to the sheet thickness) A sample is taken so that a 3/8 depth area) can be observed. The structure of the cross section of the plate is revealed using a saturated aqueous solution of picric acid and a sodium dodecylbenzenesulfonate etchant. EBSD at least three locations at a depth of 1/4 of the plate thickness from the surface of this sample (region from 1/8 depth of the plate thickness to 3/8 depth of the plate thickness from the surface) at a magnification of 500x. The grain size of prior austenite grains is measured from a metallographic photograph taken in the observation field. The equivalent circle diameter is calculated for one of the prior austenite grains included in each observation field. Excluding old austenite grains where the entire former austenite grain is not included in the photographing field of view, such as the edge of the photographing field of view, perform the above operation for all the former austenite grains included in each observation field, and remove all the old austenite grains in each photographing field of view Find the equivalent circle diameter of the austenite grains. The average grain size of the prior austenite grains is obtained by calculating the average value of the equivalent circle diameters of the prior austenite grains obtained in each photographic field of view.
In addition, when prior austenite grains having a circular equivalent diameter of less than 2.0 μm are included, the above-mentioned measurement is performed excluding these grains.

In addition, the long axis and short axis of at least 20 prior austenite grains with an equivalent circle diameter of 2.0 μm or more included in each of the above-mentioned photographic fields are measured. The average long axis and average short axis of the prior austenite grains are obtained by calculating the average value of the long axis and short axis obtained by measuring each prior austenite grain. By calculating these ratios (average major axis/average minor axis), the average aspect ratio of prior austenite grains is obtained.

Area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more: 5% or less If the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more exceeds 5%, hot rolling In the steel plate, the hydrogen embrittlement resistance characteristics deteriorate, especially at the cut end surface. Therefore, the area ratio of fresh martensite and retained austenite aggregates having a major axis of 30 μm or more is 5% or less. In order to further improve hydrogen embrittlement resistance, the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more is preferably 2% or less.
Since the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more is preferably as low as possible, the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more may be 0%.

The aggregate of fresh martensite and retained austenite will be explained.
A certain fresh martensite or retained austenite is approximated as an ellipsoid (ellipsoid 1), and fresh martensite or retained austenite adjacent to the ellipsoid 1 is also approximated as an ellipsoid (ellipsoid 2). Next, the distance l between ellipsoid 1 and ellipsoid 2 is determined. When the major axis of the ellipsoid 1 is a, if the distance l between the ellipsoid 1 and the ellipsoid 2 is smaller than 1.5×a, then the ellipsoid 1 and the ellipsoid 2 adjacent to the ellipsoid 1 is considered to be one aggregate. Fresh martensite and retained austenite in the metallographic structure are approximated as ellipsoids, the major axis a of the ellipsoids and the distance l between adjacent ellipsoids are determined, and the aggregate of fresh martensite and retained austenite is calculated based on the above criteria. Identify. By determining the major axis of the identified aggregate of fresh martensite and retained austenite and calculating the area ratio of the aggregate with a major axis of 30 μm or more, the aggregate of fresh martensite and retained austenite with a major axis of 30 μm or more can be obtained. Obtain the area ratio.
Note that fresh martensite and retained austenite are determined by the same method as the method for measuring the area ratio of the structure described above.

A method of approximating fresh martensite and retained austenite as ellipsoids will be explained.
As shown in FIG. 1, the ellipsoid is approximated so that the sum of the area S _{out of} a region of crystal grains not included in the ellipsoid and the area S _in of a region not included in the ellipsoid is minimized. By approximating as an ellipsoid in this way, x0, y0, a, and b are obtained.

In bainite, the area where the average value of nanoindentation hardness at a load of 5000 μN and a loading rate of 500 μN/s is 4.7 GPa or less: 30% or less By reducing the area ratio of the low hardness region of bainite , the TS-El balance in the hot rolled steel sheet can be further improved. That is, high strength and excellent ductility can be achieved at a higher level. Therefore, the area in which the average value of nanoindentation hardness at a load of 5000 μN and a loading rate of 500 μN/s is 4.7 GPa or less may be set to 30% or less of bainite.

A method for measuring the ratio (percentage) of a region in bainite where the average value of nanoindentation hardness at a load of 5000 μN and a loading rate of 500 μN/s is 4.7 GPa or less will be described.
Bainite is identified by the same method as in the method for measuring the area ratio of the structure described above. Nanoindentation hardness is measured at a load of 5000 μN and a loading rate of 500 μN/s for all regions identified as bainite. The nanoindentation hardness of at least 10 points of one region identified as bainite is measured, and the average value of the nanoindentation hardness of that region is calculated. Next, the area ratio of the region where the average value of nanoindentation hardness is 4.7 GPa or less is calculated. By dividing the area ratio of the area where the average value of nanoindentation hardness is 4.7 GPa or less by the area ratio of bainite and multiplying by 100, the nano The ratio (percentage) of the area where the average value of indentation hardness is 4.7 GPa or less is obtained.
Note that TriboScope/TriboIndenter manufactured by Hysitron is used for the measurement.

Tensile strength: 1100 MPa or more The hot rolled steel sheet according to this embodiment may have a tensile strength of 1100 MPa or more. By setting the tensile strength to 1100 MPa or more, it can be suitably applied to various automobile suspension parts. The tensile strength may be 1150 MPa or more, 1200 MPa or more, or 1300 MPa or more.
The tensile strength is preferably as high as possible, but it may be 1450 MPa or less.

Uniform elongation: 5.5% or more The hot rolled steel sheet according to this embodiment may have a uniform elongation of 5.5% or more. By setting the uniform elongation to 5.5% or more, it can be suitably applied to automobile suspension parts. Uniform elongation is preferably 6.0% or more or 7.0% or more.
The upper limit is not particularly limited, but may be 20.0% or less.

The tensile strength and uniform elongation are measured by performing a tensile test in accordance with JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The tensile test piece is taken at the center in the width direction of the plate, and the longitudinal direction is perpendicular to the rolling direction.
Note that uniform elongation refers to "full elongation at maximum test force" as defined in JIS Z 2241:2011.

Hole expansion ratio: 30% or more The hot rolled steel sheet according to this embodiment may have a hole expansion ratio of 30% or more. By setting the hole expansion rate to 30% or more, it can be suitably applied to automobile suspension parts. The hole expansion rate is preferably 35% or more, 40% or more, or 45% or more. Although the upper limit is not particularly specified, it may be 70% or less.
The hole expansion rate is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

Hydrogen embrittlement resistance evaluation The hydrogen embrittlement resistance of hot rolled steel sheets is evaluated by the following method.
A 50 mm x 50 mm test piece is taken from a hot rolled steel plate. A punch hole is formed in the center of the test piece using a φ10 mm punch and a die of φ10 mm + 0.2× plate thickness. Next, the test piece is immersed in hydrochloric acid at pH 2, and the hydrogen embrittlement resistance is evaluated based on the presence or absence of cracks on the cut end surface. A test piece can be determined to have excellent hydrogen embrittlement resistance when no cracks occur on the cut end surface even after immersing the test piece in hydrochloric acid for 72 hours or more. Furthermore, if the test piece is immersed in hydrochloric acid under the same conditions, the hydrochloric acid is replaced with new one after 36 hours, and no cracks occur on the cut section even after 36 hours of immersion (total 72 hours of immersion), the test piece is hydrogen resistant. It can be determined that the embrittlement properties are superior.
Note that when the cut end surface is observed with an optical microscope and a crack with a length exceeding 100 μm is observed, it is determined that a crack has occurred.

The hot-rolled steel sheet according to this embodiment may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., and may be used as 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, electrolytic Zn--Ni alloy plating, and the like. 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. Ru. The amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is also possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, applying and drying a silicate-based chromium-free chemical conversion treatment liquid) after plating.

Next, a preferred method for manufacturing the hot rolled steel sheet according to this embodiment will be described.
Note that the temperatures described below refer to the surface temperatures of slabs or steel plates unless otherwise specified.

A preferred method for manufacturing the hot rolled steel sheet according to this embodiment is as follows:
a heating step of holding the slab having the above chemical composition in a temperature range of 1220 to 1300°C for 40 minutes or more;
A rough rolling step in which rough rolling is performed so that each rolling reduction ratio in the first to third passes is 10 to 30%, and each rolling reduction ratio in the fourth pass and thereafter is 15 to 50%;
A finish rolling step in which the final pass is rolled in a temperature range of 940 to 1020°C and a reduction rate of 25% or more;
A cooling step of starting cooling within 2.0 seconds after completion of finish rolling and cooling to a temperature range of 300 ° C to (Ms-10) ° C within 16.0 seconds;
After the cooling, winding is started in the temperature range of 300°C to (Ms-10)°C, and the winding is performed so that the residence time in the temperature range of 300 to 450°C after the start of winding is 100 seconds or more. A winding step of taking the material is sequentially performed.
In addition, in the finish rolling process, it is more preferable to perform the finish rolling so that the finish rolling start temperature is in the temperature range of 1060 to 1080°C.
In addition, in the winding step, it is more preferable to perform the winding so that the maximum temperature after the start of the winding is less than 500°C.
Further, in the cooling step and the winding step, it is more preferable that the cooling and winding are performed such that the total residence time in the temperature range of 450 to 500° C. is less than 2000 seconds.
Each step will be explained below.

Heating process If the heating temperature is less than 1220°C, solution treatment will not proceed, the amount of ferrite will increase, and the strength of the hot rolled steel sheet will deteriorate. Therefore, the heating temperature is preferably 1220°C or higher. The heating temperature is more preferably 1240°C or higher.
On the other hand, if the heating temperature exceeds 1300° C., the austenite grains become coarse during heating, and as a result, the hole expandability of the hot rolled steel sheet deteriorates. Therefore, the heating temperature is preferably 1300°C or less. From the viewpoint of energy cost reduction, the heating temperature is preferably 1280° C. or lower.

If the holding time in the temperature range of 1220 to 1300° C. is less than 40 minutes, solution treatment will not proceed, the amount of ferrite will increase, and the strength of the hot rolled steel sheet will deteriorate. Therefore, the holding time in the above temperature range is preferably 40 minutes or more. The holding time is more preferably 60 minutes or more, even more preferably 80 minutes or more.
The upper limit of the holding time is not particularly limited, but may be 200 minutes or less.

Note that the slab to be heated is not particularly limited except that it has the above-mentioned chemical composition. For example, it is possible to use a slab produced by melting molten steel having the above chemical composition using a converter or an electric furnace, and by a continuous casting method. Instead of the continuous casting method, an ingot method, a thin slab casting method, etc. may be adopted.

Rough rolling process If rolling is performed with a reduction ratio of less than 10% in the first to third passes, or if rolling is performed with a reduction ratio of less than 15% in the fourth pass or later, the prior austenite grains will become coarse. Therefore, it is preferable that each rolling reduction ratio in the first to third passes is 10% or more, and each rolling reduction ratio in the fourth and subsequent passes is 15% or more. More preferably, each rolling reduction in the first to third passes is 15% or more or 20% or more, and each rolling reduction in the fourth or subsequent passes is 20% or more or 25% or more.

In addition, if rolling is performed with a reduction rate of more than 30% in the 1st to 3rd passes, or if rolling is performed with a reduction rate of more than 50% in the 4th pass or later, finish rolling will be performed with prior austenite grains being uneven. Therefore, prior austenite grains that are elongated in a specific direction are likely to be formed after finish rolling. As a result, the average aspect ratio of prior austenite grains becomes high, and the area ratio of fresh martensite and retained austenite aggregates having a major axis of 30 μm or more becomes high. Therefore, it is preferable that each rolling reduction ratio in the first to third passes is 30% or less, and each rolling reduction ratio in the fourth and subsequent passes is 50% or less. More preferably, each rolling reduction in the first to third passes is 25% or less, and each rolling reduction in the fourth and subsequent passes is 40% or less.

Note that the rolling reduction rate of each pass can be expressed as {1-(t1/t0)}×100(%), where the inlet plate thickness of each pass is t0 and the outlet plate thickness of each pass is t1.

The rough rolling completion temperature (the exit temperature of the final pass of rough rolling) is not particularly limited, but is preferably 1070° C. or higher from the viewpoint of hot deformation resistance. Further, from the viewpoint of reducing flaws due to scale encroachment, the temperature is preferably 1200°C or lower.

Finish rolling process In the finish rolling process, if the rolling temperature or reduction rate in the final pass is too low, recrystallization will not proceed sufficiently, and the average grain size of the prior austenite grains, the average aspect ratio and/or major axis of the prior austenite grains will be 30 μm. The area ratio of aggregates of fresh martensite and retained austenite cannot be preferably controlled. Therefore, in the finish rolling process, it is preferable that the final pass of rolling be performed in a temperature range of 940° C. or higher and at a rolling reduction rate of 25% or higher.

The final pass rolling is more preferably performed at a temperature of 960°C or higher, more preferably 1010°C or lower, and more preferably at a rolling reduction of 30% or higher. The rolling reduction ratio in the final pass may be 50% or less.
Further, in the finish rolling process, it is more preferable to perform finish rolling such that the finish rolling start temperature (inlet temperature of the first stage of finish rolling) is in the temperature range of 1060 to 1080°C. By setting the finish rolling start temperature to a temperature range of 1060 to 1080°C, it is possible to further reduce the area ratio of aggregates of fresh martensite and retained austenite having a major axis of 30 μm or more. As a result, a hot-rolled steel sheet having better hydrogen embrittlement resistance can be manufactured.

Cooling Step If the time from the completion of finish rolling to the start of cooling is more than 2.0 seconds, the grain growth of austenite recrystallized grains progresses, and as a result, the old austenite grains become coarse. Therefore, it is preferable to start cooling within 2.0 seconds after finish rolling is completed.
Cooling here does not include air cooling, and the time until the start of cooling refers to the time after rolling in the final pass of finish rolling until water cooling starts.

Although cooling may be started immediately after completion of finish rolling, it is necessary to inject cooling water directly below the finish rolling mill, and the rolls are cooled excessively, making it difficult to control the final rolling temperature. Therefore, it is more preferable that the time until the start of cooling is 1.0 seconds or more.

If the cooling time from the completion of finish rolling to the temperature range of 300°C to (Ms-10)°C is longer than 16.0 seconds, excessive ferrite will be produced. Therefore, after the start of cooling, it is preferable to cool to a temperature range of 300° C. to (Ms-10)° C. within 16.0 seconds after finish rolling is completed. Although it is preferable that the cooling time be short, in order to cool to a desired temperature range in a short time, it is necessary to increase the volume density, which increases the load on the cooling nozzle, which impairs economic efficiency. Therefore, the cooling time to the temperature range of 300° C. to (Ms-10)° C. is more preferably 5.0 seconds or more or 7.0 seconds or more from the completion of finish rolling.

After cooling to a temperature range of 300°C to (Ms-10)°C, cooling is stopped. If the cooling stop temperature is less than 300°C, bainite is produced in excess. Therefore, the cooling stop temperature is set to 300°C or higher. The cooling stop temperature is preferably 320°C or higher.
On the other hand, when the cooling stop temperature exceeds Ms-10°C, the amount of bainite and tempered martensite decreases. Therefore, the cooling stop temperature is preferably set to Ms-10°C or lower. The cooling stop temperature is more preferably 400°C or lower.
Immediately after cooling is stopped, it is wound into a coil. In other words, the winding temperature and the cooling stop temperature are the same temperature.

Note that Ms (°C) can be determined by the following formula.
Ms=496×(1-0.62×C)×(1-0.0092×Mn)×(1-0.033×Si)×(1-0.045×Ni)×(1-0.07 ×Cr)×(1-0.029×Mo)×(1-0.018×W)×(1+0.012×Co)

Winding Process After the start of winding, when the hot-rolled steel sheet is wound into a coil, the temperature of the steel sheet rises due to heat generation due to transformation. In particular, the temperature tends to rise at the center of the coil and at the center in the board width direction of the outermost circumferential surface of the coil. By ensuring sufficient residence time in the temperature range of 300 to 450°C after starting winding, it is possible to reduce the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more. . If the residence time in the temperature range of 300 to 450° C. after the start of winding is less than 100 seconds, the area ratio of aggregates of fresh martensite and retained austenite with a major axis of 30 μm or more cannot be reduced. Therefore, the residence time in the temperature range of 300 to 450° C. after the start of winding is preferably 100 seconds or more.
Note that the temperature here refers to the surface temperature of the center portion of the outermost circumferential surface of the coil in the width direction of the coil.

The residence time in the temperature range of 300 to 450 °C is from the time when the winding is started after cooling to the temperature range of 300 °C to (Ms-10) °C until the time when the temperature decreases and reaches 300 °C. Alternatively, it refers to the time until the temperature rises due to heat generation due to transformation and reaches 450°C.
After the start of winding, even if the temperature rises to over 450℃ due to heat generation due to transformation and then decreases to a temperature range of 300 to 450℃, once the temperature rises to over 450℃, the residence time is not added.

In the winding step, it is more preferable to wind the film so that the maximum temperature after the start of winding is less than 500°C. After the start of winding, the amount of pearlite can be further reduced by controlling the temperature rise due to recuperation so that the surface temperature at the center in the width direction of the outermost circumferential surface of the coil is less than 500°C. As a result, the TS-El balance in the hot rolled steel sheet can be further improved. That is, high strength and excellent ductility can be achieved at a higher level.

In the cooling step and the winding step, it is more preferable to carry out the cooling and winding so that the total residence time in the temperature range of 450 to 500° C. is less than 2000 seconds. By performing cooling and winding so that the total residence time in the temperature range of 450 to 500° C. is less than 2000 seconds, it is possible to reduce the area ratio of the region of low hardness in bainite. Thereby, the TS-El balance in the hot rolled steel sheet can be further improved. That is, high strength and excellent ductility can be achieved at a higher level.
Note that the total residence time here refers to the residence time in the temperature range of 450 to 500°C during cooling, as well as the residence time in the temperature range of 450 to 500°C when the temperature rises due to heat generation due to transformation after the start of winding. It is the total amount of time.

With the manufacturing method including the steps described above, the hot rolled steel sheet according to the present embodiment can be stably manufactured.

Next, examples of the present invention will be described, and the conditions in the examples are examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example condition. The present invention may adopt various conditions as long as the objectives of the present invention are achieved without departing from the gist of the present invention.

By performing continuous casting, slabs having chemical compositions shown in Tables 1A to 2B were obtained. Steel No. Production of I, L, U, and Z was discontinued at that point because cracks were observed in the slabs during casting.

Next, coils made of hot rolled steel sheets were manufactured under the conditions shown in Tables 3A to 4C.
In addition, test No. As for No. 5, the production after finish rolling was discontinued because the deformation resistance in hot conditions was high.

Test pieces for metal structure observation, tensile test, hole expansion test, and hydrogen embrittlement resistance evaluation were cut from the outermost part of the obtained coil. Each test method was the same as the method described above.
Further, the metallographic structure of the obtained test piece was observed by the method described above. The results obtained are shown in Tables 5A to 5D.

Note that each item in Tables 5A to 5D indicates the following.
B: Bainite TM: Tempered martensite FM: Fresh martensite γr: Retained austenite α: Ferrite P: Pearlite Average grain size of prior γ grains: Average grain size of prior austenite grains Average aspect ratio of prior γ grains: Prior austenite grains average aspect ratio of

In addition, "FM and γr aggregate area ratio" in the table indicates that the area ratio of fresh martensite and retained austenite aggregates with a major axis of 30 μm or more is more than 2% and 5% or less. , was written as "B", when it was 2% or less, it was written as "A", and when it was more than 5%, it was written as "C". Here, "A" and "B" were determined to be passed, and "C" was determined to be failed.
In addition, if the area where the average value of nanoindentation hardness is 4.7 GPa or less at a load of 5000 μN and a loading rate of 500 μN/s is 30% or less of bainite, “area where the average value of nanoindentation hardness is 4.7 GPa or less” is 30% or less" was written as "OK", and when this condition was not met, it was written as "NG".

If the tensile strength was 1100 MPa or more, it was determined that the hot rolled steel sheet had high strength and passed. On the other hand, when the tensile strength was less than 1100 MPa, it was determined that the hot rolled steel sheet did not have high strength and was rejected.

When the uniform elongation was 5.5% or more, it was determined that the hot rolled steel sheet had excellent ductility and passed. On the other hand, when the uniform elongation was less than 5.5%, it was determined that the hot rolled steel sheet did not have excellent ductility and was rejected.

If the hole expansion rate was 30% or more, it was determined that the hot rolled steel sheet had excellent hole expansion properties and was passed. On the other hand, when the hole expansion rate was less than 30%, it was determined that the hot rolled steel sheet did not have excellent hole expansion properties and was rejected.

In the hydrogen embrittlement resistance evaluation, if no cracks occur on the cut end surface even after immersing the test piece in hydrochloric acid for 72 hours or more, the hot rolled steel sheet is judged to have excellent hydrogen embrittlement resistance and is passed. , is indicated as "B" in the table. On the other hand, if the test piece is immersed in hydrochloric acid for more than 72 hours and cracks occur on the cut end surface, the hot-rolled steel sheet does not have excellent hydrogen embrittlement resistance and is judged to be rejected. ” was written.
Furthermore, if the test piece is immersed in hydrochloric acid under the same conditions, the hydrochloric acid is replaced with fresh one after 36 hours, and no cracks occur on the cut section even after immersion for a further 36 hours (72 hours of immersion in total), It was determined that the hot-rolled steel sheet had better hydrogen embrittlement resistance, and was marked as "A" in the table.

As shown in Tables 5A to 5D, it can be seen that the hot rolled steel sheets according to the examples of the present invention have high strength, as well as excellent ductility, hole expandability, and hydrogen embrittlement resistance.
On the other hand, it can be seen that the hot rolled steel sheet according to the comparative example is inferior in any one or more of the above characteristics.

According to the above aspect of the present invention, it is possible to provide a hot rolled steel sheet having high strength, as well as excellent ductility, hole expandability, and hydrogen embrittlement resistance.

Claims (5)

1. The chemical composition is in mass%,
   C: 0.13-0.23%,
   Si: 0.70-1.79%,
   Mn: 1.79-3.00%,
   P: 0.060% or less,
   S: 0.005% or less,
   N: 0.0070% or less,
   O: 0.010% or less,
   Al: 0.010-0.430%,
   Ti: 0.006 to 0.055%,
   Nb: 0.005-0.040%,
   B: 0.0001 to 0.0030%,
   Cr: 0-0.660%,
   Mo: 0 to 0.300%,
   Cu: 0 to 1.000%,
   Ni: 0-1.000%,
   Sn: 0-0.100%,
   Ca: 0-0.0200%,
   As: 0 to 0.100%,
   Bi: 0 to 0.020%,
   Mg: 0 to 0.0200%,
   Zr: 0 to 0.400%,
   V: 0 to 0.200%,
   REM: 0-0.1000%,
   Co: 0 to 0.2000%,
   Contains W: 0 to 0.2000% and Zn: 0 to 0.2000%,
   The remainder consists of Fe and impurities,
   In metallographic structure,
   In area%,
   Bainite: 10% or more,
   Tempered martensite: 10% or more,
   Total of bainite and tempered martensite: 70-96%,
   Fresh martensite: 20% or less,
   Retained austenite: 4-12%,
   Ferrite: 5% or less,
   Perlite: 5% or less,
   Average grain size of prior austenite grains: 20.0 μm or less,
   Average aspect ratio of prior austenite grains: 3.00 or less,
   A hot-rolled steel sheet characterized in that the area ratio of aggregates of fresh martensite and retained austenite having a major axis of 30 μm or more is 5% or less.
2. In the metal structure,
   The hot rolled steel sheet according to claim 1, wherein the pearlite content is less than 3%.
3. 3. The bainite according to claim 1 or 2, wherein a region in which the average value of nanoindentation hardness at a load of 5000 μN and a loading rate of 500 μN/s is 4.7 GPa or less is 30% or less. Hot rolled steel plate.
4. The chemical composition is in mass%,
   Cr: 0.020-0.660%,
   Mo: 0.001-0.300%,
   Cu: 0.001 to 1.000%,
   Ni: 0.001 to 1.000%,
   Sn: 0.001 to 0.100%,
   Ca: 0.0005-0.0200%,
   As: 0.001 to 0.100%,
   Bi: 0.001 to 0.020%,
   Mg: 0.0005-0.0200%,
   Zr: 0.001-0.400%,
   V: 0.001-0.200%,
   REM: 0.0005-0.1000%,
   Co: 0.0005-0.2000%,
   W: 0.0005 to 0.2000%, and Zn: 0.0005 to 0.2000%
   The hot-rolled steel sheet according to claim 1 or 2, characterized in that it contains one or more of the group consisting of:
5. The chemical composition is in mass%,
   Cr: 0.020-0.660%,
   Mo: 0.001-0.300%,
   Cu: 0.001 to 1.000%,
   Ni: 0.001 to 1.000%,
   Sn: 0.001 to 0.100%,
   Ca: 0.0005-0.0200%,
   As: 0.001 to 0.100%,
   Bi: 0.001 to 0.020%,
   Mg: 0.0005-0.0200%,
   Zr: 0.001-0.400%,
   V: 0.001-0.200%,
   REM: 0.0005-0.1000%,
   Co: 0.0005-0.2000%,
   W: 0.0005 to 0.2000%, and Zn: 0.0005 to 0.2000%
   The hot-rolled steel sheet according to claim 3, characterized in that it contains one or more of the group consisting of:

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