WO2023132351A1

WO2023132351A1 - Hot-rolled steel sheet and method for producing same

Info

Publication number: WO2023132351A1
Application number: PCT/JP2023/000119
Authority: WO
Inventors: 栄作桜田; 隆安富
Original assignee: 日本製鉄株式会社
Priority date: 2022-01-07
Filing date: 2023-01-06
Publication date: 2023-07-13
Also published as: JPWO2023132351A1

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% to 96.0% of bainite, 70.0% or less of 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, the proportion of the line length that is in contact with residual austenite in the line length of large tilt angle grain boundaries of bainite having a crystal misorientation of 15° or more is not less than 3.0% but less than 12.0%.

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.
More specifically, the present invention relates to a hot-rolled steel sheet and a method for manufacturing the same, which reduces the difference in uniform elongation in the in-plane direction and enables the manufacture of complicated parts.
This application claims priority based on Japanese Patent Application No. 2022-001420 filed in Japan on January 7, 2022, the content of which is incorporated herein.

In recent years, the weight of automobile bodies has been reduced in order to reduce CO ₂ emissions. Hot-rolled steel sheets having a tensile strength of 780 MPa class are used for automobile chassis parts in order to reduce the weight of automobile bodies. In order to further reduce the weight of automobile bodies, various technological developments related to high-strength hot-rolled steel sheets are underway.

For example, Patent Document 1 discloses a steel sheet of 590 to 980 MPa in which the hole expandability is improved by classifying the metal structure of the steel sheet and optimizing the area ratio and arrangement.

WO2018/138887 WO2018/151273

When manufacturing lightweight parts using high-strength hot-rolled steel, it is necessary to compensate for the decrease in rigidity caused by thinning the parts with cross-sectional shapes. High ductility is required for hot-rolled steel sheets that are applied to parts having complicated cross-sectional shapes. For example, in order to obtain a tensile strength of 1100 MPa or more, it is necessary to use bainite as the main phase.

As a method of improving ductility with bainite as the main phase, there is a method of using retained austenite. The present inventors have studied a method of applying a hot-rolled steel sheet containing bainite as a main phase to a part having a complicated cross-sectional shape. By using bainite as the main phase, ductility and hole expandability can be improved, and end face cracks caused by stretch flange deformation can be eliminated. However, as shown in Fig. 1, the present inventors found that when a hot-rolled steel sheet containing bainite as a main phase is applied, for example, to a lower arm of automobile chassis parts, cracks occur at the curved overhang near the link. was found to occur. This crack was generated in a portion that underwent tensile deformation perpendicular to the crack after being subjected to stretch deformation.

As in the above example, when a hot-rolled steel sheet containing bainite as the main phase is applied to a part having a complicated cross-sectional shape for ensuring high rigidity, the present inventors have found that cracks occur. I found out. The inventors of the present invention have also found that the locations where the cracks occur are locations other than the flange portion and the burring end face, which have been conventionally considered problems.

As a result of the analysis of this cracked part, the inventors found that after receiving bending deformation, it locally contracted due to deformation in the direction perpendicular to the bending strain and broke. In order to suppress this fracture, it is necessary to maintain the work hardening ability against deformation in the orthogonal direction after receiving the prestrain of bending deformation, that is, uniform elongation is isotropically The inventors have found that it is important to increase

As a method for isotropically increasing the uniform elongation, for example, Patent Document 2 discloses a technique for reducing the ratio of absorbed energies in the L direction and the C direction.
However, in recent years, it is desired to increase the uniform elongation more isotropically.

An object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expansibility, and isotropically enhanced uniform elongation, and a method for producing the same.

In order to obtain a hot-rolled steel sheet having high strength, excellent ductility and hole expansibility, and isotropically enhanced uniform elongation, the present inventors particularly found that the first It is important to control the composition ratio of the two phases, that is, to control the ratio of the line lengths in contact with retained austenite among the line lengths of high-angle grain boundaries of bainite with a crystal orientation difference of 15° or more. I found out.

In addition, the present inventors have found that, in order to obtain the hot-rolled steel sheet, it is particularly important to control the finish rolling conditions and the heating conditions during reheating after coiling. .

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: 0.13 to 0.21%,
Si: 0.70 to 1.45%,
Mn: 1.95-2.55%,
P: 0.060% or less,
S: 0.005% or less,
N: 0.0070% or less,
Al: 0.020 to 0.430%,
Ti: 0.006-0.055%,
Nb: 0.006 to 0.040%,
Cr: 0.05 to 0.47%,
Mo: 0.020-0.120%,
B: 0.0008 to 0.0030%,
Cu: 0-0.40%,
Ni: 0 to 0.30%,
V: 0-0.30%, and Sn: 0-0.04%
and the balance consists of Fe and impurities,
The metal structure, in area %,
Bainite is 25.0 to 96.0%,
4.0 to 12.0% retained austenite,
Ferrite content is 5.0% or less,
martensite is 70.0% or less,
Perlite is 3.0% or less,
The average grain size of the prior austenite grains is 25.0 μm or less,
Among the line lengths of the high-angle grain boundaries of the bainite having a crystal orientation difference of 15° or more, the line lengths in contact with the retained austenite account for 3.0% or more and less than 12.0%.
(2) In the hot-rolled steel sheet according to (1) above, the chemical composition is, in mass%,
Cu: 0.01 to 0.40%,
Ni: 0.01 to 0.30%,
V: 0.01-0.30%, and Sn: 0.01-0.04%
It may contain one or more selected from the group consisting of
(3) 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) or (2) above,
A heating step of heating a slab having the chemical composition described in (1) or (2) 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 at a rough rolling temperature of 1070 to 1200 ° C. with a total rolling reduction of 75 to 85%;
A finish rolling step in which finish rolling is performed so that the final rolling temperature is 940 to 1020 ° C. and the final rolling reduction is 26 to 43%;
A cooling step of starting cooling within 2.0 seconds after the completion of the finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds after the completion of the finish rolling;
A winding process for winding,
A reheating step of heating such that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding;
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 isotropically enhanced uniform elongation, and a method for producing the same. .

FIG. 4 is a diagram showing a cracked portion of a lower arm manufactured from a hot-rolled steel sheet containing bainite as a main phase; Among the wire lengths of the high-angle grain boundaries of bainite having a crystal orientation difference of 15° or more, the ratio of wire lengths in contact with retained austenite, uniform elongation in the rolling direction, and uniform elongation in the direction perpendicular to the rolling direction in the examples It is a figure which shows the relationship with the difference with.

The hot-rolled steel sheet of the present embodiment and the method for manufacturing the same 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. In addition, the lower limit value and the upper limit value are included in the numerical limitation range described below between "-". Numerical values indicated as "less than" and "greater than" do not include the value within the numerical range.

First, the reasons for limiting the chemical composition of the hot-rolled steel sheet according to this embodiment will be described. In addition, all "%" about a chemical composition point out the thing of "mass %."

The chemical composition of the hot-rolled steel sheet according to the present embodiment is, in mass%, C: 0.13 to 0.21%, Si: 0.70 to 1.45%, Mn: 1.95 to 2.55%, P: 0.060% or less, S: 0.005% or less, N: 0.0070% or less, Al: 0.020 to 0.430%, Ti: 0.006 to 0.055%, Nb: 0. 006-0.040%, Cr: 0.05-0.47%, Mo: 0.020-0.120%, B: 0.0008-0.0030%, and the balance: Fe and impurities.
Each element will be described below.

C: 0.13-0.21%
C is an element necessary for increasing the strength. If the C content is less than 0.13%, the amount of ferrite may increase and the strength may deteriorate. Therefore, the C content is made 0.13% or more. Preferably, it is 0.15% or more and 0.16% or more.
On the other hand, when the C content exceeds 0.21%, the amount of martensite increases, and the hole expansibility may deteriorate. Therefore, the C content is made 0.21% or less. Preferably, it is 0.20% or less and 0.19% or less.

Si: 0.70-1.45%
Si is an element necessary for increasing the amount of retained austenite and enhancing uniform elongation. If the Si content is less than 0.70%, the amount of retained austenite decreases and the ductility deteriorates. Therefore, the Si content is set to 0.70% or more. Preferably, it is 0.80% or more and 0.90% or more.
On the other hand, when the Si content exceeds 1.45%, the amount of retained austenite increases, and the hole expansibility may deteriorate. Therefore, the Si content is set to 1.45% or less. Preferably, it is 1.30% or less and 1.20% or less.

Mn: 1.95-2.55%
Mn is an element necessary to obtain desired strength. If the Mn content is less than 1.95%, the amount of ferrite increases and the strength deteriorates. Therefore, the Mn content is set to 1.95% or more. It is preferably 2.00% or more and 2.10% or more.
On the other hand, if the Mn content exceeds 2.55%, the structure may become uneven due to Mn segregation, and the hole expansibility may deteriorate. Therefore, the Mn content is set to 2.55% or less. Preferably, it is 2.40% or less and 2.30% or less.

P: 0.060% or less P is an impurity element that is unavoidably mixed in the manufacturing process. If the P content is too high, slab cracking occurs during production due to embrittlement of grain boundaries. In order to stably suppress slab cracking during production, the P content is made 0.060% or less. Preferably, it is 0.050% or less and 0.040% or less. In order to reduce the incidence of slab cracking as much as possible, the P content is preferably 0.012% or less.
It should be noted that the lower the P content is, the more preferable it is, but since an extremely high cost is required to make the P content less than 0.002%, the P content may be 0.002% or more.

S: 0.005% or less S is an impurity element that is unavoidably mixed in the manufacturing process. If the S content is too high, MnS will form. When MnS is formed, martensite is generated around MnS, and as a result of an increase in the amount of martensite, the hole expansibility deteriorates. In order to prevent deterioration of the hole expansibility, the S content is made 0.005% or less. Preferably, it is 0.004% or less. In order to reduce the influence of MnS as much as possible, the S content is preferably 0.003% or less.
If the S content is less than 0.0005%, the desulfurization cost becomes extremely high, impairing economic efficiency. Therefore, the S content may be 0.0005% or more.

N: 0.0070% or less If the N content is too high, it may form nitrides during the manufacturing process and cause embrittlement cracking of the slab. Therefore, the smaller the N content, the better. In order to suppress embrittlement cracking of the slab, the N content is made 0.0070% or less. Preferably, it is 0.0050% or less.
If the N content is less than 0.0003%, the N removal cost will increase significantly. Therefore, the N content may be 0.0003% or more.

Al: 0.020-0.430%
Al is an impurity element that is inevitably mixed, but it is also an element that has a deoxidizing effect. Al is also an element that suppresses the formation of pearlite. If the Al content is less than 0.020%, the amount of pearlite increases and the hole expansibility may deteriorate. Therefore, the Al content is set to 0.020% or more. Preferably, it is 0.100% or more and 0.150% or more.
On the other hand, when the Al content exceeds 0.430%, AlN is formed during casting and slab cracking occurs. In addition, the formation of oxides in the casting nozzle during continuous casting may impair the manufacturability. Therefore, the Al content is set to 0.430% or less. Preferably, it is 0.350% or less.

Ti: 0.006-0.055%
Ti is an element necessary for maximizing the effect of B, which will be described later, by forming TiN. If the Ti content is less than 0.006%, B becomes BN, so the amount of ferrite increases and the strength deteriorates. Therefore, the Ti content is set to 0.006% or more. Preferably, it is 0.010% or more.
On the other hand, when the Ti content exceeds 0.055%, TiN causes slab cracking during continuous casting. Therefore, the Ti content is set to 0.055% or less. Preferably, it is 0.040% or less, 0.030% or less, or 0.025% or less.

Nb: 0.006-0.040%
Nb is an element necessary for refining the rolling structure and preferably controlling the average grain size of prior austenite grains. If the Nb content is less than 0.006%, the average grain size of the prior austenite grains cannot be preferably controlled, and the hole expansibility deteriorates. Therefore, the Nb content is made 0.006% or more. Preferably, it is 0.010% or more and 0.015% or more.
On the other hand, when the Nb content is more than 0.040%, the ratio of the line lengths in contact with the retained austenite among the line lengths of the large-angle grain boundaries of the bainite having a crystal orientation difference of 15° or more decreases. Uniform elongation cannot be increased isotropically. Therefore, the Nb content is set to 0.040% or less. Preferably, it is 0.030% or less and 0.022% or less.

Cr: 0.05-0.47%
Cr is an element necessary to reduce the amount of ferrite and obtain desired strength. If the Cr content is less than 0.05%, the amount of ferrite increases and the strength deteriorates. Therefore, the Cr content is set to 0.05% or more. Preferably, it is 0.10% or more and 0.20% or more.
On the other hand, when the Cr content exceeds 0.47%, the amount of martensite increases and the ductility deteriorates. Therefore, the Cr content is set to 0.47% or less. Preferably, it is 0.40% or less and 0.35% or less.

Mo: 0.020-0.120%
Mo is an element necessary to reduce the amount of ferrite and obtain desired strength. If the Mo content is less than 0.020%, the amount of ferrite increases and the strength deteriorates. Therefore, Mo content shall be 0.020% or more. Preferably, it is 0.040% or more and 0.060% or more.
On the other hand, if the Mo content exceeds 0.120%, the amount of martensite increases and the ductility deteriorates. Therefore, Mo content shall be 0.120% or less. Preferably, it is 0.100% or less and 0.080% or less.

B: 0.0008 to 0.0030%
B is an element that suppresses the formation of ferrite. If the B content is less than 0.0008%, the amount of ferrite may increase and the strength may deteriorate. Therefore, the B content is made 0.0008% or more. Preferably, it is 0.0009% or more and 0.0010% or more.
On the other hand, if the B content exceeds 0.0030%, the hot deformation resistance becomes high, making it difficult to thread the sheet. Therefore, the B content is set to 0.0030% or less. Preferably, it is 0.0020% or less and 0.0015% or less.

The balance of the chemical composition of the hot-rolled steel sheet may be Fe and impurities. Examples of impurities include elements that are inevitably mixed from steel raw materials or scraps and/or during the steelmaking process and that are permissible within a range that does not impair the properties of the hot-rolled steel sheet according to the present embodiment.

The chemical composition of the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements instead of part of Fe. The content is 0% when the following optional elements are not contained.

Cu: 0.01-0.40%
Cu is an element that is incorporated as a tramp element. Cu increases the strength by forming a solid solution, improves uniform elongation and hole expansibility, and isotropically enhances uniform elongation. In order to ensure this effect, the Cu content may be 0.01% or more.
On the other hand, when the Cu content exceeds 0.40%, Cu embrittlement cracking occurs during hot working, making production difficult. Therefore, the Cu content is set to 0.40% or less.

Ni: 0.01-0.30%
Ni increases the strength by forming a solid solution, improves uniform elongation and hole expansibility, and isotropically enhances uniform elongation. In order to ensure this effect, the Ni content may be 0.01% or more.
On the other hand, when the Ni content exceeds 0.30%, cracks occur during continuous casting. Therefore, the Ni content is set to 0.30% or less.

V: 0.01-0.30%
V increases the strength by forming a solid solution, improves uniform elongation and hole expansibility, and isotropically enhances uniform elongation. In order to ensure this effect, the V content may be 0.01% or more.
On the other hand, when the V content exceeds 0.30%, cracks occur during continuous casting. Therefore, the V content is set to 0.30% or less.

Sn: 0.01-0.04%
Sn increases the strength by forming a solid solution, improves uniform elongation and hole expansibility, and isotropically enhances uniform elongation. In order to ensure this effect, the Sn content may be 0.01% or more.
On the other hand, when the Sn content exceeds 0.04%, intergranular embrittlement cracking occurs during hot working. Therefore, the Sn content is set to 0.04% 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 %, 25.0 to 96.0% bainite, 4.0 to 12.0% retained austenite, 5.0% or less ferrite, marten A bainite high-angle grain boundary line having a site content of 70.0% or less, a pearlite content of 3.0% or less, an average grain size of prior austenite grains of 25.0 μm or less, and a crystal orientation difference of 15° or more. Among the lengths, the ratio of wire lengths in contact with retained austenite is 3.0% or more and less than 12.0%.

In the present embodiment, in a cross section perpendicular to the plate surface, at a depth position of 1/4 of the plate thickness from the surface (a region from 1/8 of the plate thickness to 3/8 of the plate thickness from the surface) Define metallographic structure. The reason is that the metallographic structure at this position shows the typical metallographic structure of the hot-rolled steel sheet. Each rule will be explained below.

Bainite area ratio: 25.0 to 96.0%
Bainite increases the strength, uniform elongation, and hole expansion ratio of hot-rolled steel sheets. In order to obtain desired strength, uniform elongation and hole expansion rate, the area ratio of bainite is set to 25.0% or more. Preferably, it is 30.0% or more, 40.0% or more, 50.0% or more, or 60.0% or more.
Based on the relationship with the area ratio of retained austenite, the area ratio of bainite is set to 96.0% or less. Preferably, it is 94.0% or less and 92.0% or less.

Area ratio of retained austenite: 4.0 to 12.0%
Retained austenite enhances the uniform elongation of the hot rolled steel sheet. In order to increase uniform elongation and obtain excellent ductility, the area ratio of retained austenite is set to 4.0% or more. Preferably, it is 6.0% or more and 8.0% or more.
On the other hand, if the area ratio of retained austenite exceeds 12.0%, the hole expansibility deteriorates. Therefore, the area ratio of retained austenite is set to 12.0% or less. Preferably, it is 10.0% or less and 8.0% or less.

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

Area ratio of martensite: 70.0% or less Martensite is a metal structure effective for increasing the strength of a hot-rolled steel sheet, but ductility deteriorates as the area ratio increases. If the area ratio of martensite exceeds 70.0%, the ductility of the hot-rolled steel sheet significantly deteriorates. Therefore, the area ratio of martensite is set to 70.0% or less. Preferably, it is 60.0% or less, 50.0% or less, or 40.0% or less. Since the hot-rolled steel sheet according to the present embodiment can ensure strength by including a desired amount of bainite, the area ratio of martensite may be 0.0%.

Area ratio of pearlite: 3.0% or less When pearlite is contained in the metal structure, the hole expansibility of the hot-rolled steel sheet deteriorates. If the area ratio of pearlite is more than 3.0%, the expansibility is significantly deteriorated. Further, the higher the area ratio of pearlite, the worse the uniform elongation. Therefore, the area ratio of pearlite is set to 3.0% or less. In order to sufficiently suppress the decrease in uniform elongation, the area ratio of pearlite is preferably 1.0% or less, more preferably 0.0%.

The method for measuring the area ratio of the metal structure will be described below.

Method for measuring area ratio of ferrite Measured using crystal orientation information obtained by EBSD analysis. For the measurement, an EBSD analyzer 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 apparatus is 1.0×10 ⁻⁴ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13 to 15, the electron beam irradiation level is 62, and the WD is 15 mm.

The sample to be observed is positioned at a depth of 1/4 of the plate thickness from the surface (1/4 of the plate thickness from the surface) from each of the 1/4 position, 1/2 position, and 3/4 position in the width direction of the hot-rolled steel sheet. 8 depth to 3/8 depth of the plate thickness) is sampled so as to have a width direction size of 10 mm in the direction perpendicular to the rolling direction so that the region can be observed. The cross section perpendicular to the rolling direction was roughly polished with #1000, mirror-polished with a polishing liquid in which diamond powder with a particle size of 1 to 3 μm was dispersed, and then electrolytically polished to remove polishing strain on the surface. and In this electropolishing, in order to remove the mechanical polishing distortion of the observation surface, it is sufficient to polish the observation surface by a minimum of 20 μm and a maximum of 50 μm. Considering the sagging of the end portion, the thickness is preferably 30 μm or less.

For the obtained observation sample, at a depth position of 1/4 of the plate thickness from the surface (a region from 1/8 of the plate thickness to 3/8 of the plate thickness from the surface), 100 μm in the plate thickness direction and EBSD analysis is performed over a range of 200 μm in the orthogonal direction. The crystal orientation information obtained by EBSD is data recording the crystal orientation measured at intervals of 0.05 to 0.5 μm and the measurement coordinates. Using the crystal orientation information obtained by the EBSD analysis, the area ratio of the region determined to be bcc is calculated by the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device.

Next, from the figure determined as the bcc phase on the inverse pole color map output by OIM, the boundary where the orientation difference between the adjacent measurement points is 15° or more is defined as the grain boundary, and the GAM value is calculated for each grain. do. Crystal grains with a GAM value of 0.5 or less are regarded as ferrite and pearlite. The sum of the area ratios of ferrite and pearlite is obtained by calculating the average value of the area ratios of ferrite and pearlite crystal grains at each position in the width direction. The area ratio of ferrite is obtained by subtracting the area ratio of pearlite, which will be described later, from the total area ratio of ferrite and pearlite obtained.

Method for measuring the area ratio of martensite Using the crystal orientation information obtained by measuring the area ratio of ferrite, an IQ (Image Quality) value is obtained by the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. A measurement point where the value of is 8000 or more is a martensite measurement point. The area ratio of martensite is obtained by calculating the ratio of martensite measurement points to the total number of measurement points. By this method, the average value of the area ratio of martensite at each position in the width direction is calculated to obtain the area ratio of martensite. Note that the IQ value may change depending on the state of the surface of the observation sample and the accuracy of measurement. In order to eliminate these influences, a CI (Confidence Index) value of 0.8 or more, which indicates the reliability of the crystal orientation information, should be used. If the CI value is less than 0.8, electropolishing is performed again by the above method, and the working distance between the sample and the EBSD pattern detector is adjusted, and the acceleration voltage and irradiation current level are adjusted. Alternatively, adjust the gain or exposure of the detector and acquire data so that the CI value is 0.8 or more.

Method for Measuring Area Ratio of Retained Austenite Using the crystal orientation information obtained by measuring the area ratio of ferrite, the region determined as the phase of fcc is regarded as retained austenite. The area ratio of retained austenite is obtained by calculating the average value of the area ratio of retained austenite at each width direction position.

Method for Measuring Area Ratio of Pearlite Pearlite is a structure in which lamellar carbides are arranged in ferrite. Pearlite is determined to have the same bcc as ferrite even when the crystal orientation information obtained by EBSD analysis is measured using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. This is because the ferrite in the pearlite is included in the spot diameter of the electron beam. Therefore, only the area ratio of pearlite is measured not by the EBSD analysis but by observing the corroded sample with an optical microscope.

Using the same sample as the one used to measure the area ratio of ferrite, a metallographic photograph is obtained by photographing the structure revealed by the nital etchant. It is assumed that the metal structure photograph is taken in a range of 100 μm in the plate thickness direction and 200 μm in the orthogonal direction. Using this metallographic photograph, the area ratio of pearlite is measured. Crystal grains containing lamellar carbides are regarded as pearlite. The area ratio of pearlite is obtained by calculating the average value of the area ratio of pearlite at each position in the width direction.

Method of measuring the area ratio of bainite Using the crystal orientation information obtained by measuring the area ratio of ferrite, the measurement points determined to be ferrite and pearlite, the measurement points determined to be martensite, and the measurement points determined to be retained austenite. The bainite measurement points are the measurement points except for the points marked with . The area ratio of bainite is obtained by calculating the ratio of the bainite measurement points shown in all the measurement points.

Average grain size of prior austenite grains: 25.0 µm or less When the grain size of prior austenite grains is large, the local elongation decreases in the dual-phase structure. As a result, the hole expansibility of the hot-rolled steel sheet deteriorates. If the average grain size of the prior austenite grains is more than 25.0 μm, the deterioration of the hole expansibility becomes remarkable. 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. Therefore, the average grain size of the prior austenite grains may be 7.0 μm or more.

Method for measuring the grain size of prior austenite grains The samples to be observed were obtained from 1/4, 1/2, and 3/4 positions in the width direction of the hot-rolled steel sheet. It is a thick section, and a sample is taken so that a position 1/4 of the plate thickness from the surface (a region from 1/8 of the plate thickness to 3/8 of the plate thickness from the surface) 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 in a 200 μm region in a 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.

Of the line lengths of large-angle grain boundaries of bainite with a crystal orientation difference of 15° or more, the ratio of line lengths in contact with retained austenite: 3.0% or more and less than 12.0% The austenitic morphology is characterized by being subjected to strain during inter-rolling, and even after cooling and coiling, the morphology is inherited to form a predetermined metal structure. Therefore, any metallographic structure retains not a little features of the rolling direction-derived texture and extended structure morphology produced by hot rolling.

However, when retained austenite is in contact with the grain boundaries of bainite, which is the main phase, the retained austenite undergoes stress-induced transformation and behaves as a hard structure at locations where deformation is strong, suppressing local deformation. In addition, retained austenite plays a role of supporting deformation as a soft structure at locations where deformation is weak. As a result, uniform elongation can be enhanced isotropically.

As a result of intensive studies by the present inventors, among the line lengths of the high-angle grain boundaries of bainite having a crystal orientation difference of 15° or more, the proportion of line lengths in contact with retained austenite is 3.0% or more. It has been found that the difference in uniform elongation in the rolling direction and in the direction perpendicular thereto can be reduced, and the uniform elongation can be isotropically enhanced. As a result, the present inventors have found that it is possible to suppress breakage of the pressed part caused by deformation in the direction perpendicular to the bending strain. Therefore, the proportion of line lengths in contact with retained austenite among the line lengths of large-angle grain boundaries of bainite having a crystal orientation difference of 15° or more is set to 3.0% or more. Preferably, it is 5.0% or more and 8.0% or more.

On the other hand, when the ratio of the line length in contact with the retained austenite is 12.0% or more among the line lengths of the large-angle grain boundaries of the bainite having a crystal orientation difference of 15° or more, the uniform elongation is isotropically increased. can't Therefore, the proportion of line lengths in contact with retained austenite among the line lengths of large-angle grain boundaries of bainite having a crystal orientation difference of 15° or more is less than 12.0%. It is preferably 11.0% or less and 10.0% or less.

Method for measuring the proportion of line lengths in contact with retained austenite among the line lengths of large-angle grain boundaries of bainite with a crystal orientation difference of 15° or more Using crystal orientation information obtained by measuring the area ratio of ferrite Then, on the inverse pole color map output by OIM with the point group determined to be bainite, the boundary where the orientation difference between the adjacent measurement points is 15 ° or more from the diagram determined as the bcc phase is defined as the grain boundary, Create a bainite grain boundary map. Next, the group of points determined to be fcc is drawn on the bainite grain boundary map, and the proportion of line lengths in which the bainite grain boundary and fcc are in contact with each other among all the line lengths of the bainite grain boundaries is determined. This operation is performed for all the samples collected, and the average value is obtained to obtain the ratio of the line length in contact with the retained austenite among the line lengths of the large angle grain boundaries of bainite with a crystal orientation difference of 15 ° 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 is possible to contribute to the weight reduction of the vehicle body. Although the upper limit of the tensile strength is not particularly defined, it may be 1400 MPa or less.

Further, the hot-rolled steel sheet according to the present embodiment has a uniform elongation of 5.0% or more, and a difference between the uniform elongation in the rolling direction and the uniform elongation in the direction perpendicular to the rolling direction of 2.0% or less. good too.

　Tensile strength and uniform elongation are determined according to the test method described in JIS Z 2241:2011 by preparing No. 5 test pieces described in JIS Z 2241:2011 from hot-rolled steel sheets.

Note that the uniform elongation is obtained for each of the rolling direction and the direction perpendicular to the rolling direction. When cutting from a coil, the longitudinal direction of the coil is the rolling direction. In addition, when judging the rolling direction from a cut sheet, the surface of the hot-rolled steel sheet has fine unevenness caused by Si scale along the rolling direction, so the rolling direction is judged from the scale pattern. A direction parallel to the long side of the scale pattern may be regarded as the rolling direction.

The hot-rolled steel sheet according to this embodiment may have a hole expansion ratio of 30% or more. The hole expansion ratio is measured by conducting a hole expansion test in accordance with JIS Z 2256:2010.

Next, a method for manufacturing a hot-rolled steel sheet according to this embodiment will be described. A preferable method for manufacturing a hot-rolled steel sheet according to the present embodiment sequentially performs the following steps.

A heating step of heating a slab having the chemical composition described above in a temperature range of 1220 to 1300° C. for 40 minutes or longer.
A rough rolling step in which rough rolling is performed at a rough rolling temperature of 1070 to 1200° C. at a total rolling reduction of 75 to 85%.
A finish rolling step in which finish rolling is performed so that the final rolling temperature is 940 to 1020° C. and the final rolling reduction is 26 to 43%.
A cooling step of starting cooling within 2.0 seconds after completion of the finish rolling and cooling to a temperature range of 300 to 480° C. within 17.0 seconds after completion of the finish rolling.
Winding process for winding.
A reheating step of heating such that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding;
An air-cooling process for 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.

Heating Step A slab having the chemical composition described above is heated in a temperature range of 1220 to 1300° C. for 40 minutes or longer. If the heating temperature is lower than 1220° C., the solutionization does not progress, the amount of ferrite increases, and the strength 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 is higher than 1300° C., the austenite grains become coarse during heating, resulting in deterioration of the hole expansibility. 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 less.

If the holding time in the temperature range of 1220 to 1300° C. is less than 40 minutes, the solutionization does not progress, the amount of ferrite increases, and the strength 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. Thus, the heating conditions in the heating process affect the phase composition of the final structure. Therefore, a holding time of about 30 minutes is generally not preferable for productivity, and a holding time of 40 minutes or longer is necessary so that the temperature at any point of the slab satisfies the above.
Although the upper limit of the retention time is not particularly limited, it may be 200 minutes or less.

Rough rolling step Rough rolling is performed at a total rolling reduction of 75 to 85%. The total rolling reduction in the rough rolling step is expressed by (1−t _r /t _s )×100(%) using the slab thickness: t _s and the strip thickness t _r at the end of rough rolling. If the total rolling reduction is less than 75%, the rolling strain is insufficient, and hot austenite recrystallization does not proceed. Therefore, the average grain size of the austenite grains is coarsened to exceed 25.0 μm, and as a result, the hole expansibility is deteriorated. Therefore, rough rolling is performed at a total rolling reduction of 75% or more. Preferably it is 77% or more.

On the other hand, if the total rolling reduction is more than 85%, the rolling strain is too high, and although austenite recrystallization progresses, grain growth is large, and the average grain size of the austenite grains is coarsened to exceed 25.0 μm. deteriorates. Therefore, rough rolling is performed with a total reduction of 85% or less. Preferably it is 83% or less.

The rough rolling temperature in the rough rolling step is not particularly limited, but is preferably 1070° C. or higher from the viewpoint of hot deformation resistance. Further, from the viewpoint of obtaining a desired ferrite fraction, the rough rolling temperature is preferably 1070° C. or higher.
From the viewpoint of reducing flaws due to scale entrapment, the rough rolling temperature is preferably 1200° C. or less. Also from the viewpoint of obtaining the desired average grain size of the prior austenite grains, the rough rolling temperature is preferably 1200° C. or less.
The term "rough rolling temperature" as used herein means the temperature at the delivery side of the last stage of rough rolling.

Finish Rolling Process Finish rolling is performed at a final rolling temperature of 940 to 1020° C. and a final rolling reduction of 26 to 43%. If the final rolling temperature is less than 940° C., recrystallization of austenite does not proceed and grain refinement does not occur, and as a result, the average grain size of prior austenite grains exceeds 25.0 μm. Therefore, the final rolling temperature should be 940° C. or higher. Preferably, it is 960°C or higher.
The final rolling temperature means the surface temperature of the steel sheet on the exit side of the final pass of finish rolling.

On the other hand, if the final rolling temperature exceeds 1020°C, although austenite recrystallization progresses, the grains become coarse and the hole expandability deteriorates. Therefore, the final rolling temperature is set to 1020° C. or lower. It is preferably 1000° C. or less.

The final reduction in finish rolling affects the recrystallization of austenite. When the final rolling reduction is less than 26%, recrystallization of austenite does not progress and grain refinement does not occur, and as a result, the average grain size of prior austenite grains exceeds 25.0 μm. Therefore, the final rolling reduction is set to 26% or more. Preferably it is 30% or more.
The final rolling reduction is expressed by (1−t ₁ /t ₀ )×100 (%) using the plate thickness on the entry side of the final pass of finish rolling: t ₀ and the plate thickness on the exit side of the final pass of finish rolling t ₁ . be.

On the other hand, if the final rolling reduction is more than 43%, although austenite recrystallization progresses, the grains become coarser, resulting in deterioration in hole expansibility. Therefore, the final rolling reduction is set to 43% or less. Preferably it is 40% or less.

The rolling start temperature of finish rolling (steel plate surface temperature at the first pass entry side of finish rolling) is preferably 1040°C or higher in order to reduce deformation resistance. In addition, the temperature is preferably 1100° C. or less in order to reduce flaws due to entrapment of scale.

Cooling Step If the time from the completion of finish rolling to the start of cooling exceeds 2.0 seconds, the grain growth of recrystallized austenite grains proceeds and the hole expandability 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 finish rolling is completed, it is necessary to inject cooling water directly below the finish rolling mill, and excessive cooling of the rolls increases the driving force for transformation nucleation. As a result, the proportion of line lengths in contact with the retained austenite among the line lengths of the high-angle grain boundaries of the bainite having a crystal orientation difference of 15° or more increases. Therefore, it is preferable to set the time until the start of cooling to 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 strength deteriorates. Therefore, after the start of cooling, the steel sheet is cooled to a temperature range of 300 to 480° C. within 17.0 seconds after completion of finish rolling. 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 set to 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 hole expansibility 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, resulting in deterioration of ductility and hole expansibility. 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 heating amount ΔT satisfies the following formula (1). The temperature here means the surface temperature of the steel sheet on the outermost periphery of the coil.
This reheating step is a particularly important step for preferably controlling the proportion of wire lengths in contact with retained austenite among wire lengths of high-angle grain boundaries of bainite having a crystal misorientation of 15° or more. The reheating process after winding promotes C diffusion at the large-angle grain boundaries of bainite with a crystal orientation difference of 15° or more corresponding to the prior austenite grain boundary, and bainite with a crystal orientation difference of 15° or more. Increase the proportion of wire lengths in contact with retained austenite in the wire lengths of high-angle grain boundaries.

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 (°C). The unit of ΔT is degrees Celsius.

When the amount of heating ΔT is equal to or less than the left side of the above formula (1), the amount of retained austenite can be preferably controlled, but the residual The proportion of wire length in contact with austenite is reduced. When the amount of heating ΔT is equal to or greater than the right side, the proportion of line lengths in contact with retained austenite among the line lengths of the high-angle grain boundaries of bainite having a crystal orientation difference of 15° or more increases.

Also, if the time required for ΔT heating exceeds 30 minutes, a carbide reaction occurs near the bainite interface, which reduces the amount of retained austenite.

It should be noted that the coil should be heated so that the amount of heating ΔT satisfies the above formula (1), and it is not necessary to hold the coil at that temperature for a long time. The retention time may be, for example, 1 to 3000 seconds.

Air-cooling process After heating so that the heating amount ΔT satisfies the above formula (1), air-cooling is performed to a temperature range of 200°C or less. Air cooling is preferably carried out to a temperature range of 100° C. or less in consideration of safety during coil transportation.

Next, examples of the present invention will be described, but the conditions in the examples are examples of conditions adopted to confirm the feasibility and effect of the present invention. 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.

By performing continuous casting, slabs having the chemical compositions shown in Tables 1A and 1B were obtained. Steel no. For I, L, S and X, slab cracking was observed during casting, and production was discontinued at that point.

Next, coils made of hot-rolled steel sheets were manufactured under the conditions shown in Tables 2A to 2C. In addition, the rolling start temperature of the finish rolling is 1040 to 1100 ° C., the holding time at the ΔT heating temperature is 1 to 3000 seconds, and after holding at the ΔT heating temperature, it is air-cooled to a temperature range of 100 ° C. or less.
In addition, test No. No. 53 had high deformation resistance in hot working, so production after finish rolling was discontinued.

Specimens for metallographic observation, tensile test and hole-expanding test were cut out from the outermost periphery of the obtained coil. Two test pieces were taken from each of the 1/4 position, 1/2 position, and 3/4 position in the width direction of the hot-rolled steel sheet, and the tensile strength, uniform elongation, hole expansion ratio, uniform elongation in the rolling direction and rolling direction The difference from the uniform elongation in the direction perpendicular to the The test method was the same as the method described above.
Moreover, metal structure observation was performed from the obtained test piece by the above-mentioned method. The results obtained are shown in Tables 3A-3C.

In Tables 3A to 3C, the "percentage of wire lengths in contact with retained austenite" refers to "the ratio of wire lengths in contact with retained austenite among the line lengths of high-angle grain boundaries of bainite with a crystal misorientation of 15° or more. The "uniform elongation difference" indicates the "difference between the uniform elongation in the rolling direction and the uniform elongation in the direction perpendicular to the rolling direction".

As shown in Tables 3A to 3C, it can be seen that the hot-rolled steel sheets according to the examples of the present invention had high strength, excellent ductility and hole expansibility, and isotropically increased uniform elongation.
On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples are inferior in at least one of the above properties.

In order to avoid cracks occurring in the portion subjected to tensile deformation perpendicular to the crack after being subjected to stretch deformation, the difference between the uniform elongation in the rolling direction and the uniform elongation in the direction perpendicular to the rolling direction should be 2.0. % or less. According to FIG. 2, among the line lengths of the high-angle grain boundaries of bainite with a crystal orientation difference of 15° or more, the proportion of line lengths in contact with retained austenite is 3.0% or more and less than 12.0%. , the difference between the uniform elongation in the rolling direction and the uniform elongation in the direction perpendicular to the rolling direction can be controlled to 2.0% or less.

Claims

The chemical composition, in mass %,
C: 0.13 to 0.21%,
Si: 0.70 to 1.45%,
Mn: 1.95-2.55%,
P: 0.060% or less,
S: 0.005% or less,
N: 0.0070% or less,
Al: 0.020 to 0.430%,
Ti: 0.006-0.055%,
Nb: 0.006 to 0.040%,
Cr: 0.05 to 0.47%,
Mo: 0.020-0.120%,
B: 0.0008 to 0.0030%,
Cu: 0-0.40%,
Ni: 0 to 0.30%,
V: 0-0.30%, and Sn: 0-0.04%
and the balance consists of Fe and impurities,
The metal structure, in area %,
Bainite is 25.0 to 96.0%,
4.0 to 12.0% retained austenite,
Ferrite content is 5.0% or less,
martensite is 70.0% or less,
Perlite is 3.0% or less,
The average grain size of the prior austenite grains is 25.0 μm or less,
The ratio of the line length in contact with the retained austenite is 3.0% or more and less than 12.0% among the line lengths of the large-angle grain boundaries of the bainite having a crystal orientation difference of 15° or more. hot-rolled steel sheet.
The chemical composition, in mass %,
Cu: 0.01 to 0.40%,
Ni: 0.01 to 0.30%,
V: 0.01-0.30%, and Sn: 0.01-0.04%
The hot-rolled steel sheet according to claim 1, containing one or more selected from the group consisting of:
A method for manufacturing a hot-rolled steel sheet according to claim 1 or 2,
a heating step of heating a slab having the chemical composition according to claim 1 or 2 in a temperature range of 1220 to 1300° C. for 40 minutes or more;
A rough rolling step in which rough rolling is performed at a rough rolling temperature of 1070 to 1200 ° C. with a total rolling reduction of 75 to 85%;
A finish rolling step in which finish rolling is performed so that the final rolling temperature is 940 to 1020 ° C. and the final rolling reduction is 26 to 43%;
A cooling step of starting cooling within 2.0 seconds after the completion of the finish rolling and cooling to a temperature range of 300 to 480 ° C. within 17.0 seconds after the completion of the finish rolling;
A winding process for winding,
A reheating step of heating such that the heating amount ΔT satisfies the following formula (1) within 30 minutes after the winding;
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.