WO2023132342A1

WO2023132342A1 - Hot-rolled steel sheet and method for producing same

Info

Publication number: WO2023132342A1
Application number: PCT/JP2023/000071
Authority: WO
Inventors: 隆安富; 栄作桜田; 玄紀虻川
Original assignee: 日本製鉄株式会社
Priority date: 2022-01-07
Filing date: 2023-01-05
Publication date: 2023-07-13
Also published as: JPWO2023132342A1

Abstract

A hot-rolled steel sheet according to the present invention has a specific chemical composition, while having a metal structure that contains, in area%, 25.0% or more of bainite, a total of 70.0% or less of fresh martensite and tempered martensite, 4.0% to 12.0% of residual austenite, 5.0% or less of ferrite and 3.0% or less of pearlite, with the average prior austenite grain size being 25.0 µm or less. With respect to the metal structure, if θ is the angle between the rolling direction and the respective major axis directions of the crystal grains of residual austenite, fresh martensite and tempered martensite, the proportion of the crystal grains having θ of 0° to 15°, the proportion of the crystal grains having θ of more than 15° but not more than 30°, the proportion of the crystal grains having θ of more than 30° but not more than 45°, the proportion of the crystal grains having θ of more than 45° but not more than 60°, the proportion of the crystal grains having θ of more than 60° but not more than 75° and the proportion of the crystal grains having θ of more than 75° but not more than 90° are 40% or less in percentage.

Description

Hot-rolled steel sheet and manufacturing method thereof

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet and a method for manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2022-001424 filed in Japan on January 7, 2022, the content of which is incorporated herein.

In recent years, efforts have been made to reduce the weight of automobiles and machine parts. It is possible to reduce the weight of automobiles and machine parts by ensuring rigidity by designing the parts to have an optimum shape. Furthermore, in blank molded parts such as press molded parts, it is possible to reduce the weight by reducing the plate thickness of the part material.

However, when trying to secure 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, the application of steel sheets of over 780 MPa class is beginning to be considered for automotive underbody parts such as lower arms, trail links and knuckles. These automotive underbody parts are manufactured by subjecting steel sheets to burring, stretch flanging, bending forming, and the like. Therefore, the steel sheets applied to these automobile chassis parts are required to have excellent formability, particularly excellent ductility and hole expansibility.

For example, in Patent Document 1, the metal structure is substantially a two-phase structure of ferrite and bainite, and carbides containing Ti and Mo are dispersed and precipitated in the ferrite phase. A steel plate is disclosed.

Japanese Patent Application Laid-Open No. 2003-321725

However, Patent Document 1 does not consider ductility and hole expansibility.

The automobile suspension parts described above are manufactured by performing multiple forming processes on steel plates. Therefore, steel sheets that are applied to automobile chassis parts are required to have excellent formability even after being subjected to a certain degree of prestrain in the preceding process. In multi-step forming, if a rapid change in strain path occurs in the process of changing the deformation path from the previous process to the subsequent process, deformation-induced transformation of retained austenite tends to progress rapidly, and the retained austenite fraction decreases. As a result, the work hardening rate in the latter stage of deformation in the subsequent forming process tends to decrease, and the inherent formability of the hot-rolled steel sheet may not be exhibited.

The present invention has been made in view of the above circumstances, and aims to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expansibility, and excellent formability after prestraining, and a method for producing the same. aim.

The inventors of the present invention have obtained the following findings as a result of creative studies on the method for obtaining the above-mentioned hot-rolled steel sheet and its manufacturing method.

In order to improve formability after prestraining, it is especially important that the uniform elongation does not decrease significantly even during deformation in which the deformation path changes rapidly. In order not to reduce the uniform elongation when the deformation path changes, the angle θ between the longitudinal direction of the grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction should be concentrated in a specific direction. It was found that it is important that the

　In order to obtain the metallographic structure described above, it is effective to strictly control the conditions for rough rolling and finish rolling.

The gist of the present invention made based on the above knowledge is as follows.
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%,
C: 0.11 to 0.23%,
Si: 0.70 to 1.80%,
Mn: 1.95-3.10%,
P: 0.060% or less,
S: 0.005% or less,
Al: 0.010 to 0.430%,
N: 0.0070% or less,
Ti: 0.006-0.055%,
Nb: 0.006 to 0.040%,
B: 0.0001 to 0.0030%,
Cr: 0-0.470%,
Mo: 0-0.120%,
V: 0 to 0.10%,
Cu: 0-0.40%,
Ni: 0 to 0.30%,
Ca: 0 to 0.0200%,
Mg: 0-0.0200%,
REM: 0 to 0.1000%,
Bi: 0 to 0.020%,
Total of Zr, Co, Zn and W: 0 to 1.00%, and Sn: 0 to 0.05%,
The balance consists of Fe and impurities,
The metal structure, in area %,
Bainite: 25.0% or more,
Total of fresh martensite and tempered martensite: 70.0% or less,
Retained austenite: 4.0 to 12.0%,
ferrite: 5.0% or less, and pearlite: 3.0% or less,
The average grain size of the prior austenite grains is 25.0 μm or less,
When the angle formed by the long axis direction of the crystal grains of the retained austenite, the fresh martensite and the tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 °, 15 The ratio of the crystal grains that are more than 30° and 45° or less, the ratio of the crystal grains that are more than 45° and 60° or less, the ratio of the crystal grains that are more than 60° and 75° or less The percentage of crystal grains and the percentage of said crystal grains having an angle of more than 75° and 90° or less are respectively 40% or less.
(2) In the hot-rolled steel sheet according to (1) above, the area ratio of the tempered martensite out of the total area ratio of the fresh martensite and the tempered martensite is 80.0% as a percentage. or more.
(3) The hot-rolled steel sheet according to (1) or (2) above, wherein the chemical composition is, in mass%,
Cr: 0.001 to 0.470%,
Mo: 0.001 to 0.120%,
V: 0.01 to 0.10%,
Cu: 0.01 to 0.40%,
Ni: 0.02 to 0.30%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005-0.1000% and Bi: 0.0005-0.020%,
You may contain 1 type(s) or 2 or more types out of the group which consists of.
(4) A method for manufacturing a hot-rolled steel sheet according to another aspect of the present invention is the method for manufacturing a hot-rolled steel sheet according to (1) above,
A heating step of holding a slab having the chemical composition described in (1) above 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 in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%;
Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, A finish rolling step in which the final pass is rolled at a rolling reduction of less than 15%;
A cooling step of starting cooling within 2.0 seconds after completion of finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds;
A winding step of winding in the temperature range;
A reheating step of heating so that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding step;
and an air-cooling step of air-cooling to a temperature range of 200° C. or less.
245−0.942×CT+0.00092×CT ² <ΔT<686−2.38×CT+0.0022×CT ² (1)
However, CT in the above formula (1) is the cooling stop temperature.

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

It is a figure for demonstrating the method of approximating a crystal grain with an ellipsoid.

The hot-rolled steel sheet according to this embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.

　The numerical limitation range described below with "~" in between includes the lower limit and the upper limit. Numerical values indicated as "less than" and "greater than" do not include the value within the numerical range. All "%" in chemical composition refer to "% by mass".

The hot-rolled steel sheet according to the present embodiment has a chemical composition in mass% of C: 0.11 to 0.23%, Si: 0.70 to 1.80%, Mn: 1.95 to 3.10%. , P: 0.060% or less, S: 0.005% or less, Al: 0.010 to 0.430%, N: 0.0070% or less, Ti: 0.006 to 0.055%, Nb: 0 0.006-0.040%, B: 0.0001-0.0030%, and the balance: containing Fe and impurities. Each element will be described in detail below.

C: 0.11-0.23%
C is an element necessary for obtaining the desired tensile strength of the hot-rolled steel sheet. Desired tensile strength cannot be obtained as C content is less than 0.11%. Therefore, the C content is made 0.11% or more. The C content is preferably 0.12% or more, 0.13% or more, or 0.15% or more.
On the other hand, if the C content exceeds 0.23%, the martensite area ratio becomes excessive, and the ductility and/or hole expandability of the hot-rolled steel sheet deteriorates. Therefore, the C content is made 0.23% or less. The C content is preferably 0.21% or less, more preferably 0.19% or less.

Si: 0.70-1.80%
Si is an element that stabilizes retained austenite. If the Si content is less than 0.70%, the 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 or 1.00% or more, more preferably 1.10% or more.
On the other hand, when the Si content exceeds 1.80%, the amount of retained austenite becomes too large, and the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 1.80% or less. The Si content is preferably 1.60% or less, more preferably 1.50% or less.

Mn: 1.95-3.10%
Mn is an element necessary for improving the strength of the hot-rolled steel sheet. If the Mn content is less than 1.95%, the ferrite area ratio becomes too high and the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.95% or more. The Mn content is preferably 2.00% or more, more preferably 2.10% or more.
On the other hand, when the Mn content exceeds 3.10%, the strength of the hot-rolled steel sheet becomes too high and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Mn content is set to 3.10% or less. The Mn content is preferably 3.00% or less, 2.80% or less, and more preferably 2.60% or less.

P: 0.060% or less P is an element that segregates in the thickness central portion of the hot-rolled steel sheet. P is also an element that embrittles the weld zone. If the P content exceeds 0.060%, slab cracking is likely to occur, making casting difficult. Therefore, the P content should be 0.060% or less. The P content is preferably 0.020% or less, more preferably 0.010% or less.
The lower the P content is, the more preferable it is, and 0% is preferable. Therefore, the P content may be 0.0005% or more.

S: 0.005% or less S is an element that embrittles the slab by existing as a sulfide. S is also an element that deteriorates the formability of the hot-rolled steel sheet. If the S content exceeds 0.005%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the S content is made 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, preferably 0%. Therefore, the S content may be 0.0005% or more.

Al: 0.010-0.430%
Al is an element that acts as a deoxidizing agent 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 formability of the hot-rolled steel sheet. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more, more preferably 0.030% or more.
On the other hand, if the Al content exceeds 0.430%, casting becomes difficult. Therefore, the Al content is set to 0.430% or less. The Al content is preferably 0.400% or less, 0.300% or less, and more preferably 0.100% or less.

N: 0.0070% or less N is an element that forms coarse nitrides in steel and deteriorates the hole expansibility of the hot rolled steel sheet. If the N content exceeds 0.0070%, the hole expansibility of the hot-rolled steel sheet deteriorates. Moreover, when a large amount of N is contained, the risk of slab cracking increases. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0050% or less or 0.0040% or less, more preferably 0.0035% or less.
The lower the N content is, the more preferable it is, preferably 0%. Therefore, the N content may be 0.0005% or more.

Ti: 0.006-0.055%
Ti is an element that increases the strength of a hot-rolled steel sheet by forming fine nitrides in the steel. Desired tensile strength cannot be obtained as Ti content is less than 0.006%. Therefore, the Ti content is set to 0.006% or more. The Ti content is preferably 0.010% or more, 0.020% or more, or 0.025% or more.
On the other hand, if the Ti content exceeds 0.055%, the hole expansibility of the hot-rolled steel sheet deteriorates. Moreover, when a large amount of Ti is contained, the risk of slab cracking increases. Therefore, the Ti content should be 0.055% or less. The Ti content is preferably 0.050% or less, more preferably 0.045% or less.

Nb: 0.006-0.040%
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 the hot-rolled steel sheet by forming fine carbides. If the Nb content is less than 0.006%, the prior austenite grains cannot be refined, and the hole expansibility of the hot rolled steel sheet deteriorates. Therefore, the Nb content is made 0.006% or more. The Nb content is preferably 0.010% or more or 0.013% or more, more preferably 0.015% or more.
On the other hand, if the Nb content exceeds 0.040%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the Nb content is set to 0.040% or less. The Nb content is preferably 0.035% or less, more preferably 0.030% or less.

B: 0.0001 to 0.0030%
B is an element that suppresses the formation of ferrite in the cooling process and increases the strength of the hot-rolled steel sheet. If the B content is less than 0.0001%, the desired tensile strength cannot be obtained. Therefore, the B content is made 0.0001% or more. The B content is preferably 0.0002% or more, 0.0005% or more, or 0.0010% or more.
On the other hand, if the B content exceeds 0.0030%, the hot deformation resistance increases, making hot rolling 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 steel sheet according to this embodiment may be Fe and impurities. In the present embodiment, the term "impurities" refers to ores used as raw materials, scraps, or impurities that are mixed in from the manufacturing environment or the like, or impurities that are allowed within a range that does not adversely affect the steel sheet according to the present embodiment.

The steel sheet according to the present embodiment may contain the following arbitrary elements instead of part of Fe. The lower limit of the content is 0% when the optional element is not included. Each arbitrary element will be described below.

Cr: 0-0.470%
Cr is an element that exhibits effects similar to those of Mn. The Cr content is preferably 0.001% or more in order to reliably obtain the effect of increasing the strength of the hot-rolled steel sheet by containing Cr.
On the other hand, when the Cr content is more than 0.470%, the martensite fraction increases and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the Cr content is set to 0.470% or less.

Mo: 0-0.120%
Mo is an element that increases the strength of a hot-rolled steel sheet by forming fine carbides in the steel. In order to reliably obtain this effect, the Mo content is preferably 0.001% or more.
On the other hand, if the Mo content exceeds 0.120%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, Mo content shall be 0.120% or less.

V: 0-0.10%
V is an element that increases the strength of the hot-rolled steel sheet by forming fine carbides in the steel. In order to reliably obtain this effect, the V content is preferably 0.01% or more.
On the other hand, if the V content exceeds 0.10%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the V content is set to 0.10% or less.

Cu: 0-0.40%
Cu has the effect of increasing the hardenability of the steel sheet and the effect of increasing the strength of the hot-rolled steel sheet by being precipitated as carbides in the steel at low temperatures. The Cu content is preferably 0.01% or more in order to more reliably obtain the effects of the above action.
However, when the Cu content exceeds 0.40%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 0.40% or less.

Ni: 0-0.30%
Ni has the effect of increasing the hardenability of the steel sheet and increasing the strength of the hot-rolled steel sheet. In addition, when Cu is contained, 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 0.02% or more.
Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content should be 0.30% or less.

Ca: 0-0.0200%
Mg: 0-0.0200%
REM: 0-0.1000%
All of Ca, Mg and REM have the effect of improving the formability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferred shape. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effects of the above action, it is preferable that at least one of Ca, Mg and REM is 0.0005% or more. However, when the Ca content or Mg content exceeds 0.0200%, or the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, deteriorating the ductility of the hot rolled steel sheet. Sometimes. Therefore, the Ca content and Mg content are set to 0.0200% or less, respectively, and the REM content is set to 0.1000% or less.

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

Bi: 0-0.020%
Bi has the effect of increasing the formability of the hot-rolled steel sheet by refining the solidified structure. In order to more reliably obtain the effect of this action, the Bi content is preferably 0.0005% or more. However, even if the Bi content exceeds 0.020%, the above effect is saturated, which is economically unfavorable. Therefore, the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.

Sum of Zr, Co, Zn and W: 0-1.00%
Sn: 0-0.05%
Regarding Zr, Co, Zn and W, the present inventors have confirmed that even if these elements are contained in a total amount of 1.00% or less, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. there is Therefore, one or more of Zr, Co, Zn and W may be contained in a total amount of 1.00% or less.
In addition, the present inventors have confirmed that even if a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. The Sn content should be 0.05% or less.

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

Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
The hot-rolled steel sheet according to the present embodiment has a metal structure in terms of area %, bainite: 25.0% or more, total of fresh martensite and tempered martensite: 70.0% or less, retained austenite: 4.0 to 12.0%, ferrite: 5.0% or less, pearlite: 3.0% or less, the average grain size of prior austenite grains is 25.0 μm or less, and the retained austenite, the fresh martensite and the When the angle formed by the major axis direction of the tempered martensite crystal grains and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15°, the crystal grains where θ is more than 15° and 30° or less , the proportion of the crystal grains that are more than 30° and 45° or less, the proportion of the crystal grains that are more than 45° and 60° or less, the proportion of the crystal grains that are more than 60° and 75° or less, and the proportion of the crystal grains that are more than 75° and 90 ° or less is respectively 40% or less in percentage.

In this embodiment, the metallographic structure is defined at the position of 1/4 of the thickness of the hot-rolled steel sheet (the region from 1/8 of the thickness from the surface to 3/8 of the thickness from the surface). The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.

Area ratio of bainite: 25.0% or more Bainite is a structure that increases the strength, ductility, and expansibility of the hot-rolled steel sheet. If the area ratio of bainite is less than 25.0%, desired strength, ductility and/or hole expansibility cannot be obtained. Therefore, the area ratio of bainite is set to 25.0% or more. Preferably, it is 30.0% or more, 40.0% or more, 55.0% or more, 60.0% or more, or 65.0% or more.
Although the upper limit of the area ratio of bainite is not particularly limited, it may be 96.0% or less in view of the relationship with the area ratio of retained austenite. The area ratio of bainite may be 90.0% or less or 85.0% or less.

Total area ratio of fresh martensite and tempered martensite: 70.0% or less Fresh martensite and tempered martensite are effective structures for increasing the strength of hot-rolled steel sheets. However, if the total area ratio of these structures exceeds 70.0%, the ductility and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, the total area ratio of fresh martensite and tempered martensite is set to 70.0% or less. Preferably, it is 40.0% or less, 20.0% or less, or 10.0% or less.
In addition, it is not necessary to contain both fresh martensite and tempered martensite, and only one of them may be contained. In addition, the total area ratio of fresh martensite and tempered martensite may be 0%, 1.0% or more, or 3.0% or more.

Of the total area ratio of fresh martensite and tempered martensite, the ratio of the area ratio of tempered martensite: 80.0% or more in percentage The ratio of the area ratio of the reverted martensite is preferably 80.0% or more in percentage. By setting the percentage of tempered martensite to 80.0% or more, fresh martensite adjacent to retained austenite can be prevented from lowering the stability of retained austenite. As a result, even when the deformation path changes, it is possible to further suppress the deterioration of the uniform elongation.
Although the upper limit is not particularly limited, it may be 100%.

Area ratio of retained austenite: 4.0 to 12.0%
Retained austenite is a structure that increases the ductility of hot-rolled steel sheets. If the area ratio of retained austenite is less than 4.0%, desired ductility may not be obtained, or excellent formability may not be obtained after prestraining. Therefore, the area ratio of retained austenite is set to 4.0% or more. Preferably, it is 5.0% or more or 6.0% or more.
On the other hand, if the area ratio of retained austenite exceeds 12.0%, the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of retained austenite is set to 12.0% or less. Preferably, it is 11.0% or less, 10.0% or less, or 8.0% or less.

Area ratio of ferrite: 5.0% or less When the area ratio of ferrite exceeds 5.0%, the strength of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of ferrite is set to 5.0% or less. It is preferably 3.0% or less or 2.0% or less, and may be 0.0%.

Area ratio of pearlite: 3.0% or less If the area ratio of pearlite exceeds 3.0%, the strength, ductility and/or hole expansibility of the hot-rolled steel sheet are deteriorated, or the hot-rolled steel sheet is excellent after prestraining. In some cases, it may not be possible to obtain moldability. Therefore, the area ratio of pearlite is set to 3.0% or less. It is preferably 2.0% or less or 1.0% or less, and may be 0.0%.

The method for measuring the area ratio of each tissue will be described below.
From the hot-rolled steel sheet, in the cross section parallel to the rolling direction, the position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) and width A test piece is taken so that the metal structure can be observed at the direction center position.

After polishing the cross section of the above test piece using silicon carbide paper of #600 to #1500, diamond powder with a particle size of 1 to 6 μm is applied to a mirror surface using a diluted solution such as alcohol or a liquid dispersed in pure water. Finish. Next, the sample is polished for 8 minutes with colloidal silica having a particle size of 0.25 μm that does not contain an alkaline solution at room temperature to remove the strain introduced to the surface layer of the sample. At any position in the longitudinal direction of the sample cross section, 50 μm in the longitudinal direction of the sample, the area from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface, at a measurement interval of 0.1 μm Crystallographic orientation information is obtained by electron backscatter diffractometry.

For the measurement, an EBSD apparatus composed 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 apparatus 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, using the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, a region with a crystal structure of fcc is identified, and the region of this region is identified. Calculate the area ratio. Thereby, the area ratio of retained austenite is obtained.

Next, those with a bcc crystal structure are judged to be bainite, ferrite, pearlite, fresh martensite, and tempered martensite. For these regions, using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device, "Grain Orientation Spread” is extracted as ferrite in the region of 1° or less. By calculating the area ratio of the extracted ferrite, the area ratio of ferrite is obtained.

Subsequently, the maximum value of the "Grain Average IQ" of the ferrite region under the condition that the 15° grain boundary is defined as the grain boundary in the remaining region (the region where the "Grain Orientation Spread" exceeds 1°) is Iα. , a region of more than Iα/2 is extracted as bainite, and a region of Iα/2 or less is extracted as “pearlite, fresh martensite and tempered martensite”. By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained.
When ferrite is not extracted in the observation field, the GAM "Grain Average Misorientation" function is used for the same field of view, and the "Grain Average Misorientation" is set to 0.5 under the condition that the 5° grain boundary is regarded as the grain boundary. A region of more than 50° and 0.75° or less is extracted as bainite, and a region of more than 0.75° is extracted as "pearlite, fresh martensite and tempered martensite". By calculating the area ratio of the extracted bainite, the area ratio of bainite is obtained.

Regarding the extracted "perlite, fresh martensite and tempered martensite", the perlite, fresh martensite and tempered martensite are distinguished by the following method.

In order to observe the same area as the EBSD measurement area with an SEM, a Vickers indentation is stamped near the observation position. After that, leaving the structure of the observation surface, contamination on the surface layer is removed by polishing, and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed with a SEM at a magnification of 3000 times. In the EBSD measurement, among the regions discriminated as "pearlite, fresh martensite and tempered martensite", the region that has a substructure in the grain and where cementite is precipitated with multiple variants is tempered. Judged as reverted martensite. A region in which cementite is precipitated in a lamellar shape is determined to be pearlite. A region with high brightness and in which the substructure is not revealed by etching is judged to be fresh martensite. By calculating the respective area ratios, the area ratios of tempered martensite, pearlite, and fresh martensite are obtained.

For removing contaminants 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.

Average grain size of prior austenite grains: 25.0 µm or less When the grain size of prior austenite grains is large, local elongation is degraded, resulting in poor hole expandability of the hot-rolled steel sheet. When the average grain size of the prior austenite grains exceeds 25.0 μm, the hole expansibility of the hot-rolled steel sheet significantly deteriorates. Therefore, the average grain size of prior austenite grains is set to 25.0 μm or less. It is preferably 20.0 μm or less and 15.0 μm or less.
The smaller the average grain size of the prior austenite grains, the better the hole expansibility of the hot-rolled steel sheet. Therefore, the average grain size of the prior austenite grains may be 7.0 μm or more.

Measurement method of the grain size of prior austenite grains It is a thickness cross section perpendicular to the rolling direction of the hot-rolled steel sheet, and the position of 1/4 of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/3 of the plate thickness The sample is taken so that the 8-deep region) can be observed. The texture of the thickness cross-section is revealed by an etchant in which a sodium dodecylbenzenesulfonate etchant is added to a picric acid saturated aqueous solution. 200 μm in the thickness direction at a magnification of 500 times for the 1/4 depth position of the plate thickness from the surface of this sample (region from 1/8 depth of plate thickness to 3/8 depth of plate thickness from surface) , the grain size of the prior austenite grains is measured from metallographic photographs taken at three locations within a range of 200 μm in the direction perpendicular to the rolling direction. An equivalent circle diameter is calculated for one of the prior austenite grains included in each observation field. Except for the old austenite grains where the entire old austenite grains are not included in the imaging field such as the edge of the imaging field, perform the above operation for all the old austenite grains contained in each observation field, and all the old austenite grains in each imaging field Obtain the circle-equivalent diameter of the austenite grains. By calculating the average value of the equivalent circle diameters of the prior austenite grains obtained in each imaging field, the average grain size of the prior austenite grains is obtained.

When the angle between the major axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 °, more than 15 ° 30 The ratio of the crystal grains that are 30° to 45°, the ratio of the crystal grains that are 45° to 60°, and the crystal grains that are 60° to 75° and the proportion of the grains that are more than 75° and 90° or less: 40% or less in terms of percentage respectively The long axis directions of the grains of retained austenite, fresh martensite and tempered martensite are concentrated in a specific direction Then, due to deformation in a plurality of steps, deformation-induced transformation of retained austenite tends to progress, and the retained austenite fraction tends to decrease. As a result, the formability of the hot-rolled steel sheet after prestraining is degraded. Therefore, in the present embodiment, the long axis direction of the crystal grains is appropriately dispersed. In the present embodiment, when the angle formed by the long axis direction of the crystal grains of retained austenite, fresh martensite, and tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 ° , the ratio of the crystal grains that are 15° to 30°, the ratio of the crystal grains that are 30° to 45°, the ratio of the crystal grains that are 45° to 60°, and the crystal grains that are 60° to 75° The ratio of certain crystal grains and the ratio of crystal grains having an angle of more than 75° to 90° or less (hereinafter sometimes simply referred to as the crystal grain ratio) shall each be 40% or less in percentage. Preferably, it is 38% or less, 35% or less or 30% or less.
Although the lower limit is not particularly limited, it may be 0% or more, 10% or more, or 15% or more. Moreover, it is good also as 20% or more.

The above crystal grain ratio is measured by the following method.
By the same method as when measuring the area ratio of each tissue, the 1/4 position of the plate thickness from the surface (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface) and the plate Retained austenite, fresh martensite and tempered martensite are identified in the metallographic structure at the center position in the width direction. Next, the grains of retained austenite, fresh martensite and tempered martensite are approximated by ellipsoids to determine the direction of their long axes. An angle θ (0 to 90°) formed by the longitudinal direction and the rolling direction of the hot-rolled steel sheet is calculated. The resulting θ was 0 to 15°, 15° to 30°, 30° to 45°, 45° to 60°, 60° to 75°, and 75° to 90° at intervals of 15°. Subdivide and create a histogram. By determining the existence ratio of each section as a percentage, the crystal grain ratio in each section is obtained.

A method of approximating the grains of retained austenite, fresh martensite, and tempered martensite with ellipsoids will be described.
As shown in FIG. 1, by approximating with an ellipsoid so that the sum of the area S _out of the crystal grain region not included in the ellipsoid and the area S _{in of} the region other than the crystal grain in the ellipsoid is minimized , x0, y0, a, b, θ. If the major axis length a of the ellipsoid is 1 μm or less, the crystal grains are not included in the measurement. If the value (a/b) obtained by dividing the length a of the major axis of the ellipsoid by the length b of the minor axis is less than 1.1, the crystal grain is not included in the measurement.

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

Tensile strength: 1100 MPa or more The hot-rolled steel sheet according to the present 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 automotive underbody parts. The tensile strength may be 1200 MPa or higher, or 1300 MPa or higher.
The higher the tensile strength, the better, but it may be 1400 MPa or less.

Uniform elongation: 5.0% or more The hot-rolled steel sheet according to the present embodiment may have a uniform elongation of 5.0% or more. By setting the uniform elongation to 5.0% or more, it can be suitably applied to automotive underbody parts. Preferably, it is 5.5% or more, 6.0% or more, or 7.0% or more.
Although the upper limit is not particularly limited, it may be 20.0% or less.

Tensile strength and uniform elongation are measured by performing a tensile test according to JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. The tensile test piece is taken at the central position in the sheet width direction, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
The uniform elongation means "total elongation at maximum test force" according to JIS Z 2241:2011.

Hole expansion rate: 30% or more The hot-rolled steel sheet according to the present embodiment may have a hole expansion rate of 30% or more. By setting the hole expansion ratio to 30% or more, it can be suitably applied to automotive underbody parts. Preferably, it is 35% or more, 40% or more, 45% or more or 50% or more. Although the upper limit is not particularly defined, it may be 80% or less.
The hole expansion rate is measured by conducting a hole expansion test in accordance with JIS Z 2256:2020.

Amount of decrease in uniform elongation after prestraining: 2.0% or less In order to improve formability even after prestraining, the uniform elongation should not decrease significantly even in deformation in which the deformation path suddenly changes. is important. In this embodiment, the amount of decrease in uniform elongation when the deformation path suddenly changes is evaluated by the following method.
A No. 5 test piece of JIS Z 2241:2011 is taken from a hot-rolled steel sheet. A tensile predeformation with a true strain of 0.03 is applied to this test piece in a direction parallel to the rolling direction. After that, small tensile test pieces are taken from the test piece so that the tensile direction is 0°, 45°, and 90° with respect to the tensile direction. A tensile test is performed using these tensile test pieces to obtain uniform elongation when the tensile directions are 0° direction, 45° direction and 90° direction. The amount of decrease in uniform elongation when the tensile direction is 45 ° and 90 ° relative to the uniform elongation when the tensile direction is 0 ° (uniform elongation in 0 ° direction - uniform elongation in 45 ° direction, (uniform elongation in 0° direction - uniform elongation in 90° direction) is calculated. When the amount of decrease in uniform elongation is 2.0% or less, it is judged that uniform elongation does not decrease significantly even in deformation in which the deformation path suddenly changes. In other words, it is judged that it has excellent moldability even after prestraining.
In addition, a tensile test is performed based on JISZ2241:2011.

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

Next, a preferred method for manufacturing the hot-rolled steel sheet according to this embodiment will be described.
The temperatures described below refer to the surface temperature of the slab or steel plate unless otherwise specified.

A preferred method for manufacturing the hot-rolled steel sheet according to the present embodiment is
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 in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%;
Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, A finish rolling step in which the final pass is rolled at a rolling reduction of less than 15%;
A cooling step of starting cooling within 2.0 seconds after completion of finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds;
A winding step of winding in the temperature range;
A reheating step of heating so that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding step;
and an air-cooling step of air-cooling to a temperature range of 200° C. or less.
245−0.942×CT+0.00092×CT ² <ΔT<686−2.38×CT+0.0022×CT ² (1)
However, CT in the above formula (1) is the cooling stop temperature.
Each step will be described below.

Heating Step If the heating temperature is lower than 1220° C., solution treatment does not proceed, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the heating temperature is set to 1220° C. or higher. It is preferably 1240° C. or higher.
On the other hand, if the heating temperature exceeds 1300° C., the austenite grains become coarse during heating, resulting in deterioration of the hole expansibility of the hot-rolled steel sheet. Therefore, the heating temperature is set to 1300° C. or less. From the viewpoint of energy cost, 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 does not progress, the amount of ferrite increases, and the strength of the hot-rolled steel sheet deteriorates. Therefore, the holding time in the above temperature range is set to 40 minutes or longer. It is preferably 60 minutes or more and 80 minutes or more.
Although the upper limit of the retention time is not particularly limited, it may be 200 minutes or less.

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

Rough Rolling Step When rolling is performed with a rolling reduction of less than 10% in the 1st to 3rd passes, or when rolling is performed with a rolling reduction of less than 15% in the 4th and subsequent passes, the prior austenite grains become coarse. Therefore, the rolling reduction ratios of the first to third passes are set to 10% or more, and the rolling reduction ratios of the fourth and subsequent passes are set to 15% or 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 pass or later is 20% or more or 25% or more.
Further, when rolling is performed with a rolling reduction of more than 30% in the 1st to 3rd passes, or when rolling is performed with a rolling reduction of more than 50% in the 4th and subsequent passes, finish rolling is performed in a state where the prior austenite grains are non-uniform. This facilitates the formation of prior austenite grains elongated in a specific direction after finish rolling. Crystal grains tend to concentrate in a specific direction, and as a result, the ratio of crystal grains increases, resulting in deterioration of the hole expansibility of the hot-rolled steel sheet. Therefore, the rolling reduction ratios of the 1st to 3rd passes are set to 30% or less, and the rolling reduction ratios of the 4th and subsequent passes are set to 50% or less. 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.

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

Although the temperature at which rough rolling is completed (temperature on the delivery side of the final pass of rough rolling) is not particularly limited, it is preferably 1070°C or higher from the viewpoint of hot deformation resistance. In addition, from the viewpoint of reducing flaws due to entrapment of scale, the temperature is preferably 1200° C. or lower.

Finish Rolling Step In the finish rolling step, if the rolling reduction and/or the rolling temperature in the following condition I or condition II are too low, recrystallization does not proceed sufficiently, and the average grain size of the prior austenite grains cannot be reduced. Also, if the rolling temperature in I or II below is too high, the prior austenite grains become coarse and the hole expandability of the hot-rolled steel sheet deteriorates. Therefore, in the finish rolling step, finish rolling is performed so as to satisfy either Condition I or Condition II.
Condition I: Final pass rolling is performed at a temperature range of 940 to 1020° C. and a rolling reduction of more than 25%.
Condition II: Rolling one pass before the final pass is performed at a temperature range of 940 to 1020° C. and a rolling reduction of more than 25%, and the final pass is rolled at a rolling reduction of less than 15%.

In condition I, the rolling one pass before the final pass is preferably performed at 960°C or higher, preferably at 1020°C or lower, and preferably at a rolling reduction of 30% or higher. The rolling reduction may be 60% or less. In Condition II, the rolling one pass before the final pass is preferably performed at a rolling reduction of 30% or more, and the upper limit may be 60% or less. The rolling in the final pass is preferably performed at a rolling reduction of less than 10%, and the lower limit may be 5% or more.

Cooling Step If the time from the completion of finish rolling to the start of cooling exceeds 2.0 seconds, grain growth of recrystallized austenite grains proceeds, and the hole expansibility of the hot-rolled steel sheet deteriorates. Therefore, cooling is started within 2.0 seconds after completion of finish rolling. Here, the time until the start of cooling means the time until the start of water cooling after rolling in the final pass of finish rolling.

　Although cooling may be started immediately after the finish rolling is completed, it is necessary to inject cooling water directly below the finish rolling mill, which impairs productivity. The time until the start of cooling is preferably 1.0 second or longer.

If the cooling time from the completion of finish rolling to the temperature range of 300 to 480°C exceeds 17.0 seconds, the area ratio of ferrite increases and the ductility and hole expansibility of the hot rolled steel sheet deteriorate. Therefore, after the start of cooling, the steel sheet is cooled to a temperature range of 300 to 480° C. within 14.0 seconds after finishing rolling is completed. A shorter cooling time is more preferable, but in order to cool to a desired temperature range in a short time, it is necessary to increase the mass density, which increases the load on the cooling nozzle and impairs economic efficiency. Therefore, the cooling time to the temperature range of 300 to 480° C. is preferably 5.0 seconds or more or 7.0 seconds or more after completion of finish rolling.

After cooling to a temperature range of 300-480° C., cooling is stopped. If the cooling stop temperature is less than 300°C, the amount of martensite increases and the ductility of the hot-rolled steel sheet deteriorates. Therefore, the cooling stop temperature is set to 300° C. or higher. Preferably, it is 320°C or higher.
On the other hand, when the cooling stop temperature is higher than 480°C, the amount of pearlite increases and the amount of retained austenite decreases, thereby deteriorating the strength, ductility and expansibility of the hot-rolled steel sheet. Therefore, the cooling stop temperature is set to 480° C. or lower. It is preferably 460° C. or less.

Winding process Immediately after cooling is stopped, the material is wound into a coil. That is, the winding temperature and the cooling stop temperature are the same temperature.

Reheating Step Within 30 minutes after winding, the film is heated so that the amount of heating ΔT in °C satisfies the above formula (1). The temperature here means the surface temperature of the steel sheet on the outermost circumference of the coil.

If the heating amount ΔT is equal to or less than the left side of the above formula (1), the retained austenite cannot be stabilized and the desired amount of retained austenite cannot be obtained. Therefore, the heating amount ΔT is set to exceed the left side of the above equation (1).
On the other hand, if the heating amount ΔT is equal to or larger than the right side of the above formula (1), a large amount of pearlite will be generated. Therefore, the heating amount ΔT is set to be less than the right side of the above equation (1).

When the time required for ΔT heating exceeds 30 minutes, a carbide reaction occurs near the bainite interface, which reduces the amount of retained austenite. Therefore, the time required for ΔT heating is preferably within 30 minutes.

After heating so that the heating amount ΔT satisfies the above formula (1), air cooling may be performed. Air cooling is preferably carried out to a temperature range of 100° C. or less in consideration of safety during coil transportation.

The hot-rolled steel sheet according to the present embodiment can be manufactured by the manufacturing method including the steps described above. The method for manufacturing a hot-rolled steel sheet according to the present embodiment may further include the following steps.

Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. The present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

Slabs having chemical compositions shown in Tables 1 and 2 were produced by continuous casting. Using the obtained slabs, steel sheets with a thickness of 3.0 mm were manufactured under the conditions shown in Tables 3A to 4B. After the reheating step, air cooling was performed to a temperature range of 100° C. or lower. Also, test no. No. 54 had high deformation resistance in hot working, so production after finish rolling was discontinued.
A blank in Tables 1 and 2 indicates that the element is not intentionally contained.

For the obtained steel sheet, the area ratio of each structure, the average grain size of the prior austenite grains, the angle formed by the major axis direction of the crystal grains of retained austenite, fresh martensite and tempered martensite and the rolling direction are determined by the above-described method. When θ is θ, the ratio of the crystal grains where θ is 0 to 15°, the ratio of the crystal grains where θ is more than 15° and 30° or less, and the ratio of the crystal grains where θ is more than 30° and 45° or less is 45°. Percentage of the crystal grains that are more than 60° or less, percentage of the crystal grains that are more than 60° and less than or equal to 75°, and percentage of the crystal grains that are more than 75° and 90° or less, tensile strength, uniform elongation, hole The expansion ratio and the amount of decrease in uniform elongation after prestraining were determined.
The results obtained are shown in Tables 5A-5C.

TM, P, etc. in Tables 5A to 5C respectively indicate the following.
TM: tempered martensite P: pearlite α: ferrite B: bainite FM: fresh martensite γr: retained austenite When the angle formed by the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15°, the ratio of the crystal grains where the ratio is more than 15° and 30° or less, the ratio of the crystal grains where θ is more than 30° and 45° or less Of the proportion of the crystal grains, the proportion of the crystal grains that are more than 45° and 60° or less, the proportion of the crystal grains that are more than 60° and 75° or less, and the proportion of the crystal grains that are more than 75° and 90° or less, Maximum value TM ratio: The ratio of the area ratio of tempered martensite to the total area ratio of fresh martensite and tempered martensite

When the tensile strength was 1100 MPa or more, it was judged to be a hot-rolled steel sheet with high strength and passed. On the other hand, when the tensile strength was less than 1100 MPa, the hot-rolled steel sheet did not have high strength and was determined to be unacceptable.

When the uniform elongation was 5.0% or more, the hot-rolled steel sheet was judged to have excellent ductility and was judged to pass. On the other hand, when the uniform elongation was less than 5.0%, the hot-rolled steel sheet did not have excellent ductility and was determined to be unacceptable.

When the hole expansion ratio was 30% or more, it was judged to be a hot-rolled steel sheet with excellent hole expansion properties and judged to be acceptable. On the other hand, when the hole expansion rate was less than 30%, the hot rolled steel sheet did not have excellent hole expandability and was determined to be unacceptable.

When the amount of decrease in uniform elongation after prestraining was 2.0% or less, the hot rolled steel sheet was judged to have excellent formability after prestraining and was judged to pass. On the other hand, when the amount of decrease in uniform elongation after prestraining was more than 2.0%, the hot rolled steel sheet did not have excellent formability after prestraining and was judged to be unacceptable.

Tables 5A and 5B show that the steel sheets according to the invention examples have high strength, excellent ductility and hole expansibility, and excellent formability after prestraining.
Among the total area ratios of fresh martensite and tempered martensite, among the total area ratios of fresh martensite and tempered martensite, hot-rolled steel sheets in which the area ratio of tempered martensite is 80.0% or more in terms of percentage are more excellent in hole expansion. It can be seen that the
On the other hand, it can be seen that the steel sheets according to the comparative examples are inferior in at least one of the above properties.

According to the above aspect of the present invention, it is possible to provide a steel sheet having high strength, excellent ductility and hole expansibility, and excellent formability after prestraining, and a method for producing the same.

Claims

The chemical composition, in mass %,
C: 0.11 to 0.23%,
Si: 0.70 to 1.80%,
Mn: 1.95-3.10%,
P: 0.060% or less,
S: 0.005% or less,
Al: 0.010 to 0.430%,
N: 0.0070% or less,
Ti: 0.006-0.055%,
Nb: 0.006 to 0.040%,
B: 0.0001 to 0.0030%,
Cr: 0-0.470%,
Mo: 0-0.120%,
V: 0 to 0.10%,
Cu: 0-0.40%,
Ni: 0 to 0.30%,
Ca: 0 to 0.0200%,
Mg: 0-0.0200%,
REM: 0 to 0.1000%,
Bi: 0 to 0.020%,
Total of Zr, Co, Zn and W: 0 to 1.00%, and Sn: 0 to 0.05%,
The balance consists of Fe and impurities,
The metal structure, in area %,
Bainite: 25.0% or more,
Total of fresh martensite and tempered martensite: 70.0% or less,
Retained austenite: 4.0 to 12.0%,
ferrite: 5.0% or less, and pearlite: 3.0% or less,
The average grain size of the prior austenite grains is 25.0 μm or less,
When the angle formed by the long axis direction of the crystal grains of the retained austenite, the fresh martensite and the tempered martensite and the rolling direction is θ, the ratio of the crystal grains where θ is 0 to 15 °, 15 The ratio of the crystal grains that are more than 30° and 45° or less, the ratio of the crystal grains that are more than 45° and 60° or less, the ratio of the crystal grains that are more than 60° and 75° or less A hot-rolled steel sheet, wherein the percentage of crystal grains and the percentage of said crystal grains having an angle of more than 75° and 90° or less are each 40% or less.
The heat according to claim 1, wherein the area ratio of the tempered martensite is 80.0% or more in percentage of the total area ratio of the fresh martensite and the tempered martensite. Rolled steel plate.
The chemical composition, in mass %,
Cr: 0.001 to 0.470%,
Mo: 0.001 to 0.120%,
V: 0.01 to 0.10%,
Cu: 0.01 to 0.40%,
Ni: 0.02 to 0.30%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005-0.1000% and Bi: 0.0005-0.020%,
The hot-rolled steel sheet according to claim 1 or 2, containing one or more of the group consisting of:
A method for manufacturing a hot-rolled steel sheet according to claim 1,
A heating step of holding a slab having the chemical composition according to claim 1 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 in the first to third passes is 10 to 30% and each rolling reduction in the fourth and subsequent passes is 15 to 50%;
Perform the final pass rolling at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, or perform rolling one pass before the final pass at a temperature range of 940 to 1020 ° C. and a reduction rate of more than 25%, A finish rolling step in which the final pass is rolled at a rolling reduction of less than 15%;
A cooling step of starting cooling within 2.0 seconds after completion of finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds;
A winding step of winding in the temperature range;
A reheating step of heating so that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding step;
and an air-cooling step of air-cooling to a temperature range of 200° C. or less.
245−0.942×CT+0.00092×CT 2 <ΔT<686−2.38×CT+0.0022×CT 2 (1)
However, CT in the above formula (1) is the cooling stop temperature.