KR20240128926A - hot rolled steel plate - Google Patents

Abstract

This hot-rolled steel sheet has a desired chemical composition and metal structure, an average spherical radius of alloy carbides in ferrite is 0.5 nm or more and less than 5.0 nm, an average number density is 3.5×10 ¹⁶ pieces/cm ³ or more, an E value indicating the periodicity of the metal structure is 10.7 or more, an I value indicating the uniformity of the metal structure is 1.020 or more, a standard deviation of the Mn concentration is 0.60 mass% or less, and a tensile strength is 980 MPa or more.

Description

hot rolled steel plate

The present invention relates to a hot-rolled steel sheet. Specifically, it relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like and used, and particularly, to a hot-rolled steel sheet that has high strength and excellent ductility, fatigue properties, and shear workability.

This application claims priority from Japanese Patent Application No. 2022-015116, filed in Japan on February 2, 2022, the contents of which are incorporated herein.

In recent years, from the perspective of protecting the global environment, many fields are focusing on reducing carbon dioxide emissions. Automobile manufacturers are also actively developing technologies to reduce vehicle weight for the purpose of reducing fuel consumption. However, because they also focus on improving collision resistance to ensure passenger safety, reducing vehicle weight is not easy.

In order to achieve both lightweight body and crashworthiness, thinning of parts by using high-strength steel plates is being considered. For this reason, steel plates that combine high strength and excellent formability are strongly desired. In order to meet these demands, several technologies have been proposed in the past. Since automobile parts have various processing methods, the required formability varies depending on the applied parts, but among them, ductility is positioned as an important indicator of formability.

In addition, automobile parts are formed by press forming, and the blank plates of the press forming are often manufactured by high-productivity shear processing. In the blank plates manufactured by shear processing, the end surface precision after shear processing must be excellent. For example, if the appearance of the end surface (shear end surface) after shear processing becomes a secondary shear surface that is shear surface-fracture surface-shear surface, the precision of the shear end surface is significantly deteriorated.

For example, patent document 1 discloses a high-strength steel plate having a tensile strength of 980 MPa or more and excellent ductility and elongation flangeability, in which a second phase composed of retained austenite and/or martensite is finely dispersed within crystal grains.

Patent Document 2 discloses a technology for controlling the burr height after punching by controlling the ratio d _s /d _b of the surface ferrite grain size d _s and the internal ferrite crystal grain d _b to 0.95 or less.

Japanese Patent Publication No. 2005-179703 Japanese Patent Publication No. 10-168544

J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596 D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546 K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII.5, Graphics Gems IV. P.S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp.474-485

The technologies disclosed in Patent Documents 1 and 2 are both technologies that improve either the ductility or the end face properties after shear processing. However, Patent Documents 1 and 2 do not mention a technology that achieves both of these properties.

Additionally, high-strength steel plates sometimes require better fatigue properties.

The present invention has been made in consideration of the above problems of the prior art, and aims to provide a hot-rolled steel sheet having high strength as well as excellent ductility, fatigue properties and shear workability.

The gist of the present invention is as follows.

(1) A hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%,

C: 0.050 to 0.250%,

Si: 0.05 to 3.00%,

Mn: 1.00 to 4.00%,

One or more of Ti, Nb and V: 0.060 to 0.500% in total,

sol.Al: 0.001 to 2.000%,

P: 0.100% or less,

S: 0.0300% or less,

N: 0.1000% or less,

O: 0.0100% or less,

Cu: 0 to 2.00%,

Cr: 0 to 2.00%,

Mo: 0 to 1.00%,

Ni: 0 to 2.00%,

B: 0 to 0.0100%,

Ca: 0 to 0.0200%,

Mg: 0 to 0.0200%,

REM: 0 to 0.1000%,

Bi: 0 to 0.020%,

One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and

Sn: Contains 0 to 0.05%,

The remainder consists of Fe and impurities,

Metal organization,

In % of area,

Retained austenite is less than 3.0%,

Ferrite is 15.0% or more and less than 60.0%,

Perlite is less than 5.0%,

The average spherical radius of the alloy carbide among the above ferrites is 0.5 nm or more and less than 5.0 nm, and the average number density is 3.5×10 ¹⁶ pieces/cm ³ or more,

The E value indicating the periodicity of the above metal structure is 10.7 or more,

The I value indicating the uniformity of the metal structure is 1.020 or more,

The standard deviation of Mn concentration is 0.60 mass% or less,

The tensile strength is 980 MPa or more.

(2) The hot-rolled steel plate described in (1) above has the chemical composition, in mass%,

Cu: 0.01 to 2.00%,

Cr: 0.01 to 2.00%,

Mo: 0.01 to 1.00%,

Ni: 0.02 to 2.00%,

B: 0.0001 to 0.0100%,

Ca: 0.0005 to 0.0200%,

Mg: 0.0005 to 0.0200%,

REM: 0.0005 to 0.1000%, and

Bi: 0.0005 to 0.020%

It may contain one or more kinds selected from the group consisting of:

According to the above aspect of the present invention, a hot-rolled steel sheet having high strength as well as excellent ductility, fatigue properties and shear workability can be obtained.

The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile parts, machine structural parts, and even construction parts.

Fig. 1 is an example of a shear end surface of a hot-rolled steel plate according to an example of the present invention.
Figure 2 is an example of a shear cross-section of a hot-rolled steel plate for a comparative example.

The chemical composition and metal structure of the hot-rolled steel sheet according to the present embodiment are described in more detail below. However, the present invention is not limited to the composition disclosed in the present embodiment, and various changes are possible without departing from the spirit of the present invention.

The numerical ranges indicated below with “within” include the lower and upper limits. The numerical values indicated as “less than” or “exceeding” are not included in the numerical range. In the following description, the % for the chemical composition of hot-rolled steel sheets is mass % unless otherwise specified.

Chemical composition

The hot-rolled steel sheet according to the present embodiment contains, in mass%, C: 0.050 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, one or more of Ti, Nb and V: 0.060 to 0.500% in total, sol.Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the remainder: Fe and impurities. Each element is described in detail below.

C: 0.050 to 0.250%

C increases the area ratio of the hard phase and increases the strength of ferrite by combining with precipitation strengthening elements such as Ti, Nb, and V. If the C content is less than 0.050%, the desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more.

On the other hand, if the C content exceeds 0.250%, the area ratio of ferrite decreases, thereby lowering the ductility of the hot-rolled steel sheet. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, 0.180% or less, or 0.150% or less.

Si: 0.05 to 3.00%

Si has the function of promoting the formation of ferrite to improve the ductility of hot-rolled steel sheets and the function of strengthening ferrite by solid solution to increase the strength of hot-rolled steel sheets. In addition, Si has the function of sounding steel by deoxidation (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.05%, the effect by the above function cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, and more preferably 0.80% or more.

On the other hand, if the Si content exceeds 3.00%, the surface properties and chemical treatment properties of the steel plate, as well as the ductility and weldability, are significantly deteriorated, and the _A3 transformation point is significantly increased. As a result, it becomes difficult to perform hot rolling stably. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, and more preferably 2.00% or less or 1.50% or less.

Mn: 1.00 to 4.00%

Mn has the function of suppressing ferrite transformation and increasing the strength of hot-rolled steel sheets. If the Mn content is less than 1.00%, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more, and more preferably 1.50% or more.

On the other hand, if the Mn content exceeds 4.00%, the hard phase becomes a periodic band shape due to segregation of Mn, making it difficult to obtain the desired shear workability. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.50% or less, and more preferably 3.00% or less or 2.50% or less.

One or more of Ti, Nb and V: 0.060 to 0.500% in total

Ti, Nb, and V are finely precipitated in steel as carbides and nitrides, and improve the strength of steel by precipitation strengthening. In addition, they are essential elements for obtaining desired fatigue characteristics. If the total content of Ti, Nb, and V is less than 0.060%, these effects cannot be obtained. Therefore, the total content of Ti, Nb, and V is set to 0.060% or more. In addition, it is not necessary to contain all of Ti, Nb, and V, and any one of them may be contained, as long as the content is 0.060% or more. The total content of Ti, Nb, and V is preferably 0.080% or more, and more preferably 0.100% or more.

On the other hand, if the total content of Ti, Nb and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the total content of Ti, Nb and V is set to 0.500% or less. Preferably, it is 0.300% or less, more preferably, it is 0.250% or less, and even more preferably, it is 0.200% or less.

sol.Al: 0.001 to 2.000%

Al, like Si, has the effect of making steel sound by deoxidation, and also has the effect of promoting the formation of ferrite, thereby increasing the ductility of hot-rolled steel sheets. If the sol.Al content is less than 0.001%, the effect of the above effect cannot be obtained. Therefore, the sol.Al content is set to 0.001% or more. The sol.Al content is preferably 0.010% or more, and more preferably 0.020% or more or 0.030% or more.

On the other hand, if the sol.Al content exceeds 2,000%, the above effect is saturated and it is economically undesirable, so the sol.Al content is set to 2,000% or less. The sol.Al content is preferably 1,500% or less, more preferably 1,000% or less, and still more preferably 0.500% or less.

Also, sol.Al means acid-available Al, and indicates the dissolved Al that exists in the steel in a solid solution state.

P: 0.100% or less

P is an element that is generally contained as an impurity, but it is also an element that has the function of increasing the strength of hot-rolled steel sheets by solid solution strengthening. Therefore, P may be actively contained. However, P is an element that is easily segregated, and if the P content exceeds 0.100%, the ductility deteriorates significantly due to grain boundary segregation. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content does not need to be specifically specified, but may be 0%. From the viewpoint of refining cost, the P content is preferably set to 0.001%.

S: 0.0300% or less

S is an element contained as an impurity, and forms sulfide inclusions in steel, thereby reducing the ductility of hot-rolled steel sheets. If the S content exceeds 0.0300%, the ductility of hot-rolled steel sheets is significantly reduced. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less. The lower limit of the S content does not need to be specifically specified, but may be 0%. From the viewpoint of refining costs, the S content is preferably set to 0.0001%.

N: 0.1000% or less

Nitrogen is an element contained in steel as an impurity, and has the effect of lowering the ductility of hot-rolled steel sheets. When the N content exceeds 0.1000%, the ductility of the hot-rolled steel sheet is significantly lowered. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100% or less or 0.0050% or less. The lower limit of the N content does not need to be specifically specified, but may be 0%. When the N content contains one or more of Ti, Nb, and V to further refine the metal structure, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more, in order to promote precipitation of carbonitrides.

O: 0.0100% or less

O, when contained in large amounts in steel, forms coarse oxides that become the starting point of fracture, causing brittle fracture or hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, more preferably 0.0055% or less or 0.0050% or less. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more or 0.0010% or more.

The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, impurities mean those mixed in from ores, scrap, or the manufacturing environment as raw materials, and/or those permitted within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.

The hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements instead of some of Fe. The lower limit of the content when the optional elements are not contained is 0%. The optional elements are described in detail below.

Cu: 0.01 to 2.00%

Cr: 0.01 to 2.00%

Mo: 0.01 to 1.00%

Ni: 0.02 to 2.00%

B: 0.0001 to 0.0100%

Cu, Cr, Mo, Ni and B all have the function of increasing the ??quenching property of hot-rolled steel sheets. In addition, Cu and Mo precipitate as carbides in steel and have the function of increasing the strength of hot-rolled steel sheets. In addition, Ni has the function of effectively suppressing grain boundary cracking of the slab caused by Cu when containing Cu. Therefore, one or more of these elements may be contained.

As described above, Cu has the function of increasing the ??quenching property of hot-rolled steel sheets and the function of precipitating as carbides in steel at low temperatures to increase the strength of hot-rolled steel sheets. In order to obtain the effect of the above function more reliably, the Cu content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, more preferably 1.00% or less.

As described above, Cr has the function of increasing the ??quenching property of hot-rolled steel sheets. In order to obtain the effect of the above function more reliably, it is preferable that the Cr content be 0.01% or more, and more preferably 0.05% or more. However, if the Cr content exceeds 2.00%, the chemical treatment property of the hot-rolled steel sheet is significantly reduced. Therefore, the Cr content is set to 2.00% or less.

As described above, Mo has the function of increasing the ??quenching property of hot-rolled steel sheets and the function of precipitating as carbides in steel to increase the strength of the hot-rolled steel sheets. In order to obtain the effect of the above effect more reliably, the Mo content is preferably 0.01% or more, more preferably 0.02% or more. However, even if the Mo content exceeds 1.00%, the effect of the above effect is saturated and is not economically desirable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less, more preferably 0.20% or less.

As described above, Ni has the function of increasing the ??quenching property of hot-rolled steel sheets. In addition, when containing Cu, Ni has the function of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to obtain the effect of the above function more reliably, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is not economically desirable to contain it in large quantities. Therefore, the Ni content is set to 2.00% or less.

As described above, B has the function of increasing the ??quenchability of hot-rolled steel sheets. In order to obtain the effect of this function more reliably, it is preferable that the B content be 0.0001% or more, and more preferably 0.0002% or more. However, if the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, so the B content is set to 0.0100% or less. It is preferable that the B content be 0.0050% or less.

Ca: 0.0005 to 0.0200%

Mg: 0.0005 to 0.0200%

REM: 0.0005 to 0.1000%

Bi: 0.0005 to 0.020%

Ca, Mg, and REM all have the function of increasing the ductility of hot-rolled steel sheets by adjusting the shape of inclusions in the steel to a desirable shape. In addition, Bi has the function of increasing the ductility of hot-rolled steel sheets by refining the solidification structure. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effect by the above-mentioned action, it is preferable that at least one of Ca, Mg, REM, and Bi be 0.0005% or more. However, if the Ca content or Mg content exceeds 0.0200%, or if the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which may rather lower the ductility of the hot-rolled steel sheet. In addition, even if the Bi content exceeds 0.020%, the effect by the above-mentioned action becomes saturated, which is not economically desirable. Therefore, the Ca content and Mg content are set to 0.0200% or less, the REM content to 0.1000% or less, and the Bi content to 0.020% or less. The Bi content is preferably 0.010% or less.

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

One or more of Zr, Co, Zn and W: 0 to 1.00% in total

Sn: 0 to 0.05%

With respect to Zr, Co, Zn and W, the inventors of the present invention 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. Therefore, one or two or more of Zr, Co, Zn and W may be contained in a total amount of 1.00% or less.

In addition, the inventors of the present invention confirmed that even if a small amount of Sn is contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. However, since scratches may occur during hot rolling when a large amount of Sn is contained, the Sn content is set to 0.05% or less.

The chemical composition of the hot-rolled steel sheet described above can be measured by a general analysis method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, sol.Al can be measured by ICP-AES using the filtrate after the sample is decomposed by heating with acid. C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas melting-thermal conductivity method, and O can be measured using the inert gas melting-nondispersive infrared absorption method.

Metallographic structure of hot rolled steel plate

Next, the metal structure of the hot-rolled steel plate according to the present embodiment will be described.

The hot-rolled steel sheet according to the present embodiment has a metal structure having, in area %, less than 3.0% of retained austenite, 15.0% or more and less than 60.0% of ferrite, less than 5.0% of pearlite, an average spherical radius of alloy carbides in the ferrite of 0.5 nm or more and less than 5.0 nm, an average number density of 3.5×10 ¹⁶ pieces/cm ³ or more, an E value representing the periodicity of the metal structure of 10.7 or more, an I value representing the uniformity of the metal structure of 1.020 or more, and a standard deviation of the manganese concentration of 0.60 mass% or less.

Since the hot-rolled steel plate according to the present embodiment has the above metal structure, high strength, excellent ductility, fatigue properties, and shear workability can be obtained.

In addition, in this embodiment, the standard deviation of the metallographic structure fraction, the average spherical radius and average number density of alloy carbides, the E value, the I value, and the Mn concentration at a cross-section parallel to the rolling direction, at a depth of 1/4 of the plate thickness from the surface (an area of a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface) and at a central position in the plate width direction is specified. This is because the metallographic structure at this position represents a representative metallographic structure of the steel plate.

Area fraction of retained austenite: less than 3.0%

Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite has the function of increasing the ductility of hot-rolled steel sheets by transformation-induced plasticity (TRIP). On the other hand, since retained austenite transforms into high-carbon martensite during shearing, it inhibits stable crack initiation and causes the formation of secondary shear surfaces. If the area ratio of the retained austenite is 3.0% or more, the above-described function becomes apparent, and the shearing workability of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the retained austenite is set to less than 3.0%. The area ratio of the retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since the smaller the amount of retained austenite, the more preferable it is, the area ratio of the retained austenite may be 0%.

Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement. In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction.

In the measurement of the area ratio of retained austenite by X-ray diffraction in the present embodiment, first, in a cross-section parallel to the rolling direction at a depth of 1/4 the plate thickness of the hot-rolled steel plate (an area from the surface to a depth of 1/8 the plate thickness to a depth of 3/8 the plate thickness) and also at the center position in the plate width direction, the integrated intensities of a total of six peaks of α (110), α (200), α (211), γ (111), γ (200), and γ (220) are obtained using the Co-Kα line, and the volume ratio of retained austenite is calculated using the intensity averaging method. The obtained volume ratio of retained austenite is regarded as the area ratio of retained austenite.

Area ratio of ferrite: 15.0% or more, less than 60.0%

Ferrite is a structure that is formed when fcc transforms into bcc at relatively high temperatures. Since ferrite has a high work hardening rate, it has the function of improving the strength-ductility balance of hot-rolled steel sheets. In order to obtain the above function, the area ratio of ferrite is set to 15.0% or more. It is preferably 20.0% or more, more preferably 25.0% or more, and still more preferably 30.0% or more.

On the other hand, since ferrite has low strength, if the area ratio is excessive, the desired strength cannot be obtained. Therefore, the ferrite area ratio is set to less than 60.0%. It is preferably 50.0% or less, and more preferably 45.0% or less.

Area ratio of pearlite: less than 5.0%

Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrites, and is a softer metal structure than bainite or martensite. If the area ratio of pearlite is 5.0% or more, carbon is consumed by the cementite contained in the pearlite, and the strength of the remaining structures, martensite and bainite, decreases, so that the desired strength cannot be obtained. Therefore, the area ratio of pearlite is set to less than 5.0%. The area ratio of pearlite is preferably 3.0% or less. In order to improve the elongation flangeability of a hot-rolled steel sheet, it is desirable to reduce the area ratio of pearlite as much as possible, and it is more desirable that the area ratio of pearlite is 0%.

In addition, the steel plate according to the present embodiment includes a hard structure composed of one or more types of bainite, martensite and tempered martensite having a total area ratio of 32.0% or more and less than 85.0% as a residual structure other than retained austenite, ferrite and pearlite.

The area ratio of the metal structure is measured by the following method. The plate thickness cross-section parallel to the rolling direction is finished to a mirror finish, and is polished for 8 minutes using colloidal silica containing no alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. At an arbitrary position in the longitudinal direction of the sample cross-section, an area of 50 µm in length, a position at a depth of 1/4 the plate thickness from the surface (an area of 1/8 the plate thickness from the surface to a depth of 3/8 the plate thickness from the surface), and also an area at the center of the plate width direction is measured at a measurement interval of 0.1 µm by electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the vacuum inside the EBSD analysis device is set to 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.

In addition, a reflection electron image is photographed in the same field of view. First, grains in which ferrite and cementite are precipitated in layers are specified from the reflection electron image, and the area ratio of the grains is calculated to obtain the area ratio of pearlite. Then, for grains excluding grains determined to be pearlite, the obtained crystal orientation information is used for grains, and an area with a Grain Average Misorientation value of 1.0° or less is determined to be ferrite, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. At this time, the Grain Tolerance Angle is set to 15°, and the area ratio of ferrite is obtained by obtaining the area of the area determined to be ferrite.

Average spherical radius of alloy carbide in ferrite: 0.5 nm or more and less than 5.0 nm

The hot-rolled steel sheet according to the present embodiment has excellent fatigue properties because the average spherical radius and average number density of alloy carbides in the ferrite are preferably controlled. If the average spherical radius of the alloy carbides in the ferrite is less than 0.5 nm, the strength of the ferrite against repeated deformation cannot be sufficiently increased, and the desired fatigue strength cannot be obtained. Therefore, the average spherical radius of the alloy carbides in the ferrite is set to 0.5 nm or more. The average spherical radius of the alloy carbides in the ferrite is preferably 1.0 nm or more. On the other hand, if the average spherical radius of the alloy carbides in the ferrite is 5.0 nm or more, the strength of the ferrite cannot be sufficiently increased, and due to the difference in hardness between crystal grains, a crack occurs from the edge of the shearing tool during the extreme shearing process, a fracture surface is formed, and then a shear surface is formed again. As a result, a secondary shear surface is easily formed, and the desired shearing workability cannot be obtained in the hot-rolled steel sheet. Therefore, the average spherical radius of the alloy carbide in the ferrite is less than 5.0 nm. The average spherical radius of the alloy carbide in the ferrite is preferably 4.0 nm or less, 3.0 nm or less, 2.0 nm or less, and more preferably less than 1.5 nm.

Average number density of alloy carbides in ferrite: 3.5×10 ¹⁶ /cm ³ or more

If the average number density of alloy carbides in the ferrite is less than 3.5×10 ¹⁶ /cm ³ , the strength against repeated deformation of the ferrite cannot be sufficiently increased, and the desired fatigue strength cannot be obtained. Therefore, the average number density of alloy carbides in the ferrite is set to 3.5×10 ¹⁶ /cm ³ or more. The average number density of alloy carbides in the ferrite is preferably 5.0×10 ¹⁶ /cm ³ or more, 10.0×10 ¹⁶ /cm ³ or more, or 20.0×10 ¹⁶ /cm ³ or more.

The upper limit of the average number density of alloy carbides in ferrite is not specifically stipulated, but the more the number, the more desirable it is. However, in the chemical composition and metal structure of the hot-rolled steel sheet according to the present embodiment, it is difficult to make the average number density of alloy carbides in ferrite exceed 1.0×10 ¹⁹ pieces/cm ³ . Therefore, the average number density of alloy carbides in ferrite may be 1.0×10 ¹⁹ pieces/cm ³ or less.

Additionally, in the present embodiment, the alloy carbide refers to a carbide containing one or more of Ti, Nb, Mo, and V.

The spherical radius and number density of alloy carbides in ferrite are measured by a three-dimensional atom probe. In the three-dimensional atom probe measurement, the laser wavelength (λ) is set to 355 nm, the laser power is set to 30 pJ, and the temperature of the needle-shaped test piece is set to 50 K. The device used for the three-dimensional atom probe measurement is not particularly limited. The three-dimensional atom probe measurement device is, for example, a product name LEAP4000XHR manufactured by Ametek Co., Ltd.

For the ferrite grains within the above-described EBSD observation field in which the area ratio of each organization is measured, a FIB (focused ion beam) device is used to collect a sample. The collected sample is processed into a needle shape by a known method, and by using a three-dimensional atom probe, the spherical radius and number density of fine precipitates ranging from less than 1 nm to several tens of nm in spherical radius can be accurately measured. The number density of the precipitates can be obtained by dividing the number of precipitates included in the area measured by the three-dimensional atom probe by the volume of the measurement area for the precipitates identified as alloy carbides by the method described below.

The total number of atoms of alloy elements (Ti, Nb, Mo, V, C) included in the entire precipitate within the measurement area is divided by the atomic density of the alloy carbide to obtain the total volume of precipitates within the measurement area. The volume of precipitates is obtained by dividing the total volume of precipitates by the number of precipitates. From the obtained volume of precipitates, the radius equivalent to a sphere is calculated assuming that the precipitate is spherical.

By performing the above-described method with five or more measurement data having a measurement area volume of 30000 nm ³ or more, the average number density and the average spherical radius are obtained. In addition, the area where Ga introduced during FIB processing is less than 0.025 at% is set as the observation area, and the area where Ga is mixed in at 0.025 at% or more is excluded from the measurement area. To confirm the amount of Ga, the amount of Ga in the longitudinal direction of the needle sample can be confirmed by the 1D Concentration Profile function of data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).

In addition, whether the observed precipitate is an alloy carbide or not is identified by using the Cluster Analysis function of the analysis software IVAS 3.6.14 on the data acquired by the 3D atom probe. For the analysis, dmax＝1.2nm, Order＝10, Nmin＝10, L＝0.5nm, and d erosion＝0.5nm are used as analysis parameters, and the precipitate recognized as a cluster is determined to be an alloy carbide.

E value: 10.7 or higher

I value: 1.020 or higher

In order to suppress the occurrence of a secondary shear plane, it is important to form a fracture plane after a sufficiently formed shear plane, and it is necessary to suppress early occurrence of cracks from the edge of the tool during shear processing. To this end, it is important that the periodicity of the metal structure is low and the uniformity of the metal structure is high. In this embodiment, the occurrence of a secondary shear plane is suppressed by controlling the E (Entropy) value indicating the periodicity of the metal structure and the I (Inverse difference normalized) value indicating the uniformity of the metal structure.

The E value indicates the periodicity of the metal structure. When the luminance is arranged periodically due to the formation of a band-shaped structure, that is, when the periodicity of the metal structure is high, the E value decreases. In the present embodiment, since it is necessary to have a metal structure with low periodicity, it is necessary to increase the E value. When the E value is less than 10.7, a secondary shear surface is likely to occur. With the periodically arranged structure as a starting point, a crack occurs from the edge of the shear tool during the extreme early stage of shearing processing, a fracture surface is formed, and then a shear surface is formed again. It is estimated that a secondary shear surface is likely to occur due to this. Therefore, the E value is set to 10.7 or more. It is preferably 10.8 or more, and more preferably 11.0 or more. The higher the E value, the more preferable it is, and the upper limit is not specifically defined, but it may be 13.0 or less, 12.5 or less, or 12.0 or less.

The I value indicates the uniformity of the metal structure, and increases as the area of the region having a constant brightness increases. A higher I value means that the metal structure has a high uniformity. In the present embodiment, since it is necessary to have a highly uniform metal structure, it is necessary to increase the I value. If the I value is less than 1.020, due to the influence of the hardness distribution caused by the difference in precipitates and element concentrations within the crystal grains, a crack occurs from the edge of the shear tool during the extreme early stage of shearing processing, a fracture surface is formed, and then a shear surface is formed again. It is estimated that this makes it easy for a secondary shear surface to occur. Therefore, the I value is set to 1.020 or more. It is preferably 1.025 or more, and more preferably 1.030 or more. The higher the I value, the more preferable it is, and the upper limit is not specifically defined, but it may be 1.200 or less, 1.150 or less, or 1.100 or less.

The E and I values can be obtained by the following methods.

In this embodiment, the shooting area of the SEM image taken to calculate the E value and the I value is set to a position at a depth of 1/4 of the plate thickness from the steel plate surface in the plate thickness cross-section parallel to the rolling direction (an area at a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface), and also at the center position in the plate width direction. For shooting the SEM image, a SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies, Ltd. is used, the emitter is set to tungsten, and the acceleration voltage is set to 1.5 kV. Under the above settings, the SEM image is output at a magnification of 1000 times and a grayscale of 256 levels.

Next, the obtained SEM image is cropped into an area of 880 × 880 pixels, and a smoothing process with a tile grid size of 8 × 8 is performed on the image, which is obtained by cutting out the obtained SEM image into an area of 880 × 880 pixels, with a contrast enhancement limitation magnification of 2.0, as described in Non-Patent Document 3. The SEM image after the smoothing process is rotated counterclockwise every degree from 0 to 179 degrees, excluding 90 degrees, and images are created every degree, thereby obtaining 179 images in total. Next, for each of these 179 images, the GLCM method described in Non-Patent Document 1 is used to collect the frequency values of the luminance between adjacent pixels in the form of a matrix.

The matrix of 179 frequency values collected by the above method is expressed as p _k (k = 0 ... 89, 91, ... 79), where k is the rotation angle from the original image. For each image, the generated p _k is summed for all k (k = 0 ... 89, 91 ... 179), and then a 256 × 256 matrix P is generated by normalizing so that the sum of each component becomes 1. In addition, the E value and the I value are each calculated using the following equations (1) and (2) described in Non-Patent Document 2. In the following equations (1) and (2), the value of the ith row and the jth column of the matrix P is expressed as P _ij .

Standard deviation of Mn concentration: 0.60 mass% or less

The standard deviation of the Mn concentration at a depth of 1/4 of the plate thickness from the surface of the hot-rolled steel sheet according to the present embodiment (a region of a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface) and at a central position in the plate width direction is 0.60 mass% or less. Thereby, the hard phase can be uniformly distributed, and cracks can be prevented from occurring from the edge of the shearing tool during the extreme shearing process. As a result, the occurrence of a secondary shear surface can be suppressed. The standard deviation of the Mn concentration is preferably 0.50 mass% or less, and more preferably 0.47 mass% or less. The lower limit of the standard deviation of the Mn concentration is preferably a smaller value from the viewpoint of suppressing excessive burrs, but due to restrictions of the manufacturing process, the practical lower limit is 0.10 mass%.

After the cross-section of the plate thickness parallel to the rolling direction of the hot-rolled steel plate is polished to a mirror finish, a region at a depth of 1/4 the plate thickness from the surface (a region of 1/8 the plate thickness from the surface to 3/8 the plate thickness from the surface) and also at the center position in the plate width direction is measured using an electron probe microanalyzer (EPMA), and the standard deviation of the Mn concentration is measured. The measurement conditions are an acceleration voltage of 15 kV, a magnification of 5000 times, and a distribution shape in a range of 20 µm in the sample rolling direction and 20 µm in the sample plate thickness direction. More specifically, the measurement interval is 0.1 µm, and the Mn concentration at 40,000 or more points is measured. Then, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all the measurement points.

Tensile properties

Among the mechanical properties of hot-rolled steel plates, tensile strength characteristics (tensile strength, total elongation) are evaluated in accordance with JIS Z 2241:2011. The test specimen shall be the No. 5 test specimen of JIS Z 2241:2011. The tensile test specimen shall be taken from 1/4 of the end portion in the width direction of the plate, and the direction perpendicular to the rolling direction shall be the longitudinal direction.

The hot rolled steel sheet according to the present embodiment has a tensile (maximum) strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more. By setting the tensile strength to 980 MPa or more, the applicable parts are not limited and it can greatly contribute to weight reduction of the car body. The upper limit does not need to be specifically limited, but from the viewpoint of suppressing mold wear, it may be 1780 MPa.

It is preferable that the total elongation is 10.0% or more, and the product of the tensile strength and the total elongation (TS × El) is 13000 MPa·% or more. It is more preferable that the total elongation is 11.0% or more, and it is still more preferable that it is 13.0% or more. In addition, the product of the tensile strength and the total elongation is more preferably 14000 MPa·% or more, and it is still more preferable that it is 15000 MPa·%MPa or more.

By making the total elongation 10.0% or higher and the product of the tensile strength and the total elongation 13000 MPa·% or higher, the applicable parts are not limited and it can greatly contribute to reducing the weight of the vehicle body.

Fatigue characteristics

If softening occurs during repeated deformation, fatigue life may be significantly reduced. Therefore, it is desirable that softening does not occur during repeated deformation. Whether softening occurs during repeated deformation can be judged by the following method.

A test piece is taken in accordance with JIS Z 2275-1978 at a position 1/4 in the width direction of the hot-rolled steel plate such that the rolling direction and the perpendicular direction (C direction) become the longitudinal direction. Using this test piece, a plane bending fatigue test is performed in accordance with JIS Z 2275-1978. The plane bending fatigue test is performed with a cyclic stress at which the number of repeated fractures is 1×10 ⁵ times or more and less than 3×10 ⁵ times, a cyclic stress at which the number of repeated fractures is 3×10 ⁵ times or more and less than 3×10 ⁶ times, and a cyclic stress at which the number of repeated fractures is 3×10 ⁶ times or more and less than 1×10 ⁷ times. The change in the cyclic stress is evaluated by measuring the torque during the fatigue test or the value of the strain gauge attached to the test piece. In the plane bending fatigue test at each cyclic stress, the cyclic hardening ratio is obtained. Repeated hardening rate is defined as (minimum repeating stress for more than 100 repetitions/repeated stress for 100 repetitions). If the minimum repeating hardening rate for each repeating stress is 1.00 or more, it can be judged that the hot rolled steel sheet has excellent fatigue properties because no repeat softening occurs.

Plate thickness

The plate thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 0.5 to 8.0 mm. If the plate thickness of the hot-rolled steel sheet is less than 0.5 mm, it becomes difficult to secure the rolling completion temperature and the rolling load becomes excessive, which may make hot rolling difficult. Therefore, the plate thickness of the hot-rolled steel sheet according to the present embodiment may be 0.5 mm or more. Preferably, it is 1.2 mm or more or 1.4 mm or more.

On the other hand, if the plate thickness exceeds 8.0 mm, it becomes difficult to refine the metal structure, and it may become difficult to obtain the metal structure described above. Therefore, the plate thickness may be 8.0 mm or less. Preferably, it is 6.0 mm or less.

Plating layer

The hot-rolled steel sheet according to the present embodiment having the chemical composition and metal structure described above may be used as a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrozinc plating, electrozinc Zn-Ni alloy plating, etc. Examples of the hot-dip plating layer include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, etc. The amount of plating adhesion is not particularly limited, and may be used in the same manner as in the past. In addition, it is also possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, applying and drying a silicate-based chromium-free chemical conversion treatment solution) after plating.

Manufacturing conditions

A suitable method for manufacturing a hot-rolled steel sheet according to the present embodiment having the chemical composition and metal structure described above is as follows.

In order to obtain a hot-rolled steel sheet according to the present embodiment, it is effective to perform heating of a slab under predetermined conditions, then hot rolling, accelerated cooling to a predetermined temperature range, and then complete cooling, and control the cooling history until coiling.

In a suitable method for manufacturing a hot-rolled steel plate according to the present embodiment, the following processes (1) to (10) are performed sequentially. In addition, the temperature of the slab and the temperature of the steel plate in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate. In addition, the stress refers to the stress applied in the rolling direction of the steel plate.

(1) After maintaining the slab at a temperature range of 700 to 850℃ for more than 900 seconds, it is further heated and maintained at a temperature range of 1100℃ or higher for more than 6000 seconds.

(2) Hot rolling is performed in a temperature range of 850 to 1100℃, resulting in a total thickness reduction of 90% or more.

(3) The rolling one stage prior to the final stage is performed at a temperature of 900℃ or higher and less than 1010℃, and after the rolling one stage prior to the final stage of hot rolling and also before the rolling of the final stage, a stress of 170kPa or higher is applied to the steel sheet.

(4) Hot rolling is completed so that the reduction ratio in the final stage of hot rolling is 8% or more and the rolling completion temperature Tf is 900℃ or more and less than 1010℃.

(5) In a temperature range of 840℃ or higher and less than 900℃, a pressure-lowering process is performed that results in a total thickness reduction of 5% or higher and less than 8%.

(6) The stress applied to the steel sheet after the final rolling of hot rolling and before the first rolling under pressure, and the stress applied to the steel sheet after the final rolling under pressure and until the steel sheet is cooled to 800°C shall be less than 200 kPa.

(7) After completion of the pressure-releasing process, accelerated cooling is performed to a temperature range of 680 to 730°C at an average cooling rate of 50°C/sec or higher.

(8) In a temperature range of 680 to 730°C, slow cooling is performed for 2.0 seconds or longer at an average cooling rate of less than 5°C/s.

(9) Cool to a temperature range of 350℃ or less at an average cooling rate of 50℃/s or more.

(10) Coil in a temperature range of 350℃ or lower.

By employing the above manufacturing method, it is possible to stably manufacture a hot-rolled steel sheet having high strength, excellent ductility, fatigue properties and shear workability. That is, by appropriately controlling the slab heating conditions and hot rolling conditions, Mn segregation is reduced and the austenite before transformation is equiaxed, and together with the cooling conditions after hot rolling described below, it is possible to stably manufacture a hot-rolled steel sheet having a desired metal structure.

(1) Slab temperature and holding time when providing slabs for hot rolling

The slab provided for hot rolling can be a slab obtained by continuous casting, a slab obtained by casting or crushing, etc., and if necessary, a slab that has been subjected to additional hot working or cold working can be used. It is preferable that the slab provided for hot rolling is maintained at a temperature range of 700 to 850°C for 900 seconds or longer when heating the slab, and then further heated and maintained at a temperature range of 1100°C or higher for 6000 seconds or longer. In addition, the upper limit of the heating temperature when heating the slab is not particularly limited, but may be 1350°C or lower from the viewpoint of heat efficiency.

In addition, when maintaining in a temperature range of 700 to 850°C, the steel plate temperature may be varied in this temperature range or may be kept constant. In addition, when maintaining in a temperature range of 1100°C or higher, the steel plate temperature may be varied in a temperature range of 1100°C or higher or may be kept constant.

In the austenite transformation in the temperature range of 700 to 850°C, Mn is distributed between ferrite and austenite, and by extending the transformation time, Mn can diffuse within the ferrite region. Thereby, Mn micro-segregation that is unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. Therefore, it is preferable to maintain the temperature range of 700 to 850°C for 900 seconds or longer. In addition, by maintaining the temperature range of 1100°C or higher for 6000 seconds or longer, the standard deviation of the Mn concentration can be significantly reduced.

Hot rolling is preferably performed using a reverse mill or a tandem mill as multi-pass rolling. In particular, from the viewpoint of industrial productivity and stress load on the steel sheet during rolling, it is more preferable to perform hot rolling using a tandem mill for at least the last two stages.

(2) Reduction ratio of hot rolling: A total thickness reduction of 90% or more in the temperature range of 850 to 1100℃

By performing hot rolling such that a total thickness reduction of 90% or more is achieved in a temperature range of 850 to 1100°C, mainly recrystallized austenite grains are refined, and accumulation of strain energy into non-recrystallized austenite grains is promoted. In addition, while austenite recrystallization is promoted, atomic diffusion of Mn is promoted, so that the standard deviation of the Mn concentration can be reduced. Therefore, it is preferable to perform hot rolling such that a total thickness reduction of 90% or more is achieved in a temperature range of 850 to 1100°C.

In addition, the total plate thickness reduction in the temperature range of 850 to 1100℃ can be expressed as {(t ₀ - t ₁ )/t ₀ } × 100(%), where the inlet plate thickness before the first rolling in the rolling in this temperature range is t ₀ and the exit plate thickness after the final rolling in the rolling in this temperature range is t ₁ .

(3) Rolling temperature one stage before the final stage: 900℃ or higher, less than 1010℃; Stress after rolling one stage before the final stage of hot rolling and before rolling of the final stage: 170kPa or higher

It is preferable that the rolling one stage prior to the final stage is performed at 900°C or higher and less than 1010°C, and that the stress applied to the steel sheet after the rolling one stage prior to the final stage of hot rolling and before the rolling of the final stage is 170 kPa or higher. Thereby, the number of crystal grains having a crystal orientation of {110}<001> among the recrystallized austenite after the rolling one stage prior to the final stage can be reduced. Since {110}<001> is a crystal orientation that is difficult to recrystallize, recrystallization by the pressing of the final stage can be effectively promoted by suppressing the formation of this crystal orientation. As a result, the band-shaped structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value increases. When the stress applied to the steel sheet is less than 170 kPa, there are cases where the desired E value cannot be obtained. The stress applied to the steel sheet is more preferably 190 kPa or higher. The upper limit of the stress applied to the steel plate is not specifically limited, but can be set to 350 kPa or less. The stress applied to the steel plate is tension in the rolling direction and can be controlled by adjusting the roll rotation speed during tandem rolling.

(4) Reduction ratio in the final stage of hot rolling: 8% or more, hot rolling completion temperature Tf: 900℃ or more, less than 1010℃

It is preferable that the reduction ratio in the final stage of hot rolling be 8% or more, and the hot rolling completion temperature Tf be 900°C or more. By setting the reduction ratio in the final stage of hot rolling to 8% or more, recrystallization by the reduction in the final stage can be promoted. As a result, the band-shaped structure of the hot rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value increases. By setting the hot rolling completion temperature Tf to 900°C or more, the excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, the formation of ferrite in the final structure (metal structure of the hot rolled steel sheet after manufacturing) can be suppressed, and the desired strength can be obtained. In addition, the upper limit of the reduction ratio in the final stage of hot rolling is not particularly limited, but can be 40% or less. In addition, by setting Tf to less than 1010℃, the coarsening of austenite grain size can be suppressed, and the periodicity of the metal structure can be reduced to obtain a desired E value.

(5) In a temperature range of 840℃ or higher and less than 900℃, a pressure-lowering process is performed that results in a total thickness reduction of 5% or higher and less than 8%.

After the final stage of hot rolling, it is preferable to perform a mild pressing in a temperature range of 840℃ or higher and less than 900℃, so that the total thickness of the plate is reduced by 5% or higher and less than 8%. As a result, the average spherical radius and the average number density of alloy carbides in the ferrite can be controlled to desired values.

Pressing may be performed, for example, at the final stage of a finishing mill, or by introducing new pressing equipment between the finishing mill and the cooling bed. Pressing may be performed in multiple stages by multiple rolls.

In addition, the total plate thickness reduction under pressure can be expressed as {(t 0 - t 1 )/t _{0 }} × 100(%), where the inlet plate thickness before the first rolling under pressure is t ₀ and the outlet plate thickness after the final rolling under pressure is _{t 1} .

(6) Stress applied to the steel sheet after the final rolling of hot rolling and before the first rolling under pressure, and stress applied to the steel sheet after the final rolling of hot rolling and before the steel sheet is cooled to 800°C: less than 200 kPa.

It is preferable that the stress applied to the steel sheet after the final rolling of the hot rolling and before the first rolling under pressure, and the stress applied to the steel sheet after the final rolling under pressure and until the steel sheet is cooled to 800°C are each less than 200 kPa. By setting the stress applied to the steel sheet at the above-mentioned locations to less than 200 kPa, the recrystallization of austenite preferentially progresses in the rolling direction, and the increase in the periodicity of the metal structure can be suppressed. As a result, the desired E value can be obtained. The stress applied to the steel sheet at the above-mentioned locations is more preferably 180 kPa or less.

(7) After completion of the pressure-releasing process, accelerated cooling to a temperature range of 680 to 730°C at an average cooling rate of 50°C/sec or more.

In order to suppress the growth of austenite grains refined by hot rolling, after completion of the low-pressure rolling, it is preferable to perform accelerated cooling to a temperature range of 730°C or lower at an average cooling rate of 50°C/sec or higher. By performing accelerated cooling to a temperature range of 730°C or lower, the formation of ferrite or pearlite with a small amount of precipitation strengthening can be suppressed. Thereby, the strength of the hot-rolled steel sheet is improved.

In addition, the average cooling rate referred to here refers to the value obtained by dividing the temperature drop of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling facility) to the completion of accelerated cooling (when the steel plate is taken out from the cooling facility) by the time required from the start of accelerated cooling to the completion of accelerated cooling.

There is no specific upper limit for the cooling rate, but if the cooling rate is fast, the cooling equipment becomes large, which increases the equipment cost. Therefore, considering the equipment cost, the average cooling rate is preferably 300°C/sec or less. In addition, in order to obtain the desired amount of ferrite, the cooling stop temperature of the accelerated cooling should be 680°C or higher.

In order to achieve the average cooling rate as described above, cooling with a large average cooling rate can be performed after completion of the pressure-lowering process, for example, by spraying cooling water onto the surface of the steel plate.

(8) In a temperature range of 680 to 730℃, slow cooling is performed for 2.0 seconds or longer at an average cooling rate of less than 5℃/s.

In a temperature range of 680 to 730°C, precipitation-strengthened ferrite can be sufficiently precipitated by performing slow cooling for 2.0 seconds or longer at an average cooling rate of less than 5°C/s. As a result, the strength and ductility of the hot-rolled steel sheet can be achieved at the same time. In addition, the average cooling rate referred to here means a value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature of accelerated cooling to the stop temperature of slow cooling by the time required from the stop of accelerated cooling to the stop of slow cooling.

The time for performing complete cooling is preferably 3.0 seconds or longer. The upper limit of the time for performing complete cooling is determined by the equipment layout, but it can be set to less than about 10.0 seconds. In addition, the lower limit of the average cooling speed of complete cooling is not specifically set, but since heating without cooling entails a large investment in equipment, it can be set to 0℃/s or longer.

(9) Average cooling rate to coiling temperature: 50℃/sec or more

In order to suppress the area ratio of pearlite and obtain the desired strength, it is preferable to set the average cooling rate from the cooling stop temperature of the complete cooling to the coiling temperature to 50°C/sec or more. As a result, the parent structure can be made hard.

In addition, the average cooling rate referred to here refers to the temperature drop of the steel sheet from the cooling stop temperature of slow cooling with an average cooling rate of less than 5°C/s to the coiling temperature, divided by the time required from the stop of slow cooling with an average cooling rate of less than 5°C/s to coiling.

(10) Coiling temperature: 350℃ or less

It is preferable that the coiling temperature be 350℃ or lower. By setting the coiling temperature to 350℃ or lower, the amount of precipitation of iron carbide can be reduced, and also the variation in hardness distribution within the hard phase can be reduced. As a result, the desired I value can be obtained. In addition, the lower limit of the coiling temperature is not particularly limited, but can be room temperature.

Example

Next, the effects of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and achieves the purpose of the present invention.

Steel having the chemical compositions shown in Tables 1 and 2 was melted and slabs having a thickness of 240 to 300 mm were manufactured by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Tables 5-1 to 6-2 were obtained under the manufacturing conditions shown in Tables 3-1 to 4-2.

In addition, the average cooling rate of the complete cooling was set to less than 5℃/s. In addition, since the coiling temperature described in Table 4-1 and Table 4-2 has a measurement lower limit of 50℃, the actual coiling temperature in the example described as 50℃ is 50℃ or lower. In addition, except for Production No. 7, the rolling one stage before the final stage of hot rolling was performed at 900℃ or higher and less than 1010℃.

In addition, for the hot-rolled steel plate of production No. 46, it was manufactured under the condition of not performing the above-mentioned mild pressure. For production No. 46, the load stress until the steel plate was cooled to 800°C after the final stage of hot rolling was set to 170 kPa.

For the obtained hot-rolled steel sheet, the area ratio of the metal structure, E value, I value, standard deviation of Mn concentration, average spherical radius and average number density of alloy carbides in ferrite, tensile strength TS, and total elongation El were obtained by the above-described method. In addition, fatigue properties were evaluated by performing a plane bending fatigue test by the above-described method. The obtained measurement results are shown in Tables 5-1 to 6-2.

In addition, the residual structure was judged to be a hard structure, and to be one or more types of bainite, martensite, and tempered martensite, based on the chemical composition and manufacturing method of this lecture.

Method for evaluating the properties of hot rolled steel plates

Tensile properties

If the tensile strength TS is 980 MPa or more, and further the total elongation El is 10.0% or more, and further the tensile strength TS × total elongation El is 13000 MPa·％ or more, it is considered a hot-rolled steel sheet having high strength and excellent ductility and is judged to be passed. If even one of them is not satisfied, it is considered a hot-rolled steel sheet not having high strength and excellent ductility and is judged to be failed.

Fatigue characteristics

By performing a plane bending fatigue test by the method described above, the cyclic hardening ratio at each cyclic stress was obtained. When the minimum value of the cyclic hardening ratio at each cyclic stress is 1.00 or more, it is judged as a hot-rolled steel sheet having excellent fatigue properties as no cyclic softening occurs and is judged as passed. On the other hand, when the minimum value of the cyclic hardening ratio at each cyclic stress is less than 1.00, it is judged as a hot-rolled steel sheet not having excellent fatigue properties and is judged as failed.

Shear workability (secondary shear surface evaluation)

The shear workability of hot-rolled steel plates was evaluated by a punching test.

For each example, three punching holes were made with a hole diameter of 10 mm, a clearance of 0%, and a punching speed of 3 m/s. Next, a plate thickness cross-section perpendicular to the rolling direction of the punching holes and a plate thickness cross-section parallel to the rolling direction were each embedded in resin, and the cross-sectional shape was photographed with a scanning electron microscope. In the obtained observation photographs, a shear section as shown in FIG. 1 or FIG. 2 can be observed. In addition, FIG. 1 is an example of a shear section of a hot-rolled steel sheet according to an example of the present invention, and FIG. 2 is an example of a shear section of a hot-rolled steel sheet according to a comparative example. In FIG. 1, the shear section is sagging-shear section-fracture section-burr. On the other hand, in FIG. 2, the shear section is sagging-shear section-fracture section-shear section-fracture section-burr. Here, the sag is the area of the smooth surface of the R shape, the shear surface is the area of the punching end surface separated by shear deformation, the fracture surface is the area of the punching end surface separated by a crack that occurred near the edge of the blade, and the burr is the area having a protrusion protruding from the lower surface of the hot-rolled steel plate.

Among the obtained shear cross-sections, if, for example, shear cross-section-fracture cross-section-shear cross-section as shown in Fig. 2 was observed on two sides perpendicular to the rolling direction and two sides parallel to the rolling direction, it was determined that a secondary shear cross-section was formed. For each punching hole, four sides, a total of twelve sides, were observed, and if none of the sides showed a secondary shear cross-section, it was determined that the hot-rolled steel sheet had excellent shear workability and passed, and “None” was noted in the table. On the other hand, if even one secondary shear cross-section was formed, it was determined that the hot-rolled steel sheet did not have excellent shear workability and failed, and “Present” was noted in the table.

[Table 1]

[Table 2]

[Table 3-1]

[Table 3-2]

[Table 4-1]

[Table 4-2]

[Table 5-1]

[Table 5-2]

[Table 6-1]

[Table 6-2]

Looking at Tables 5-1 to 6-2, it can be seen that the hot-rolled steel sheet according to the present invention has high strength, as well as excellent ductility, fatigue properties, and shear workability.

Meanwhile, it can be seen that the hot-rolled steel plate for the comparative example does not have any one or more of the above characteristics.

According to the above aspect of the present invention, a hot-rolled steel sheet having high strength and excellent ductility, fatigue properties and shear workability can be provided.

The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile parts, machine structural parts, and even construction parts.

Claims (2)

Chemical composition, in mass%,
C: 0.050 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00 to 4.00%,
One or more of Ti, Nb and V: 0.060 to 0.500% in total,
sol.Al: 0.001 to 2.000%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Cu: 0 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.00%,
Ni: 0 to 2.00%,
B: 0 to 0.0100%,
Ca: 0 to 0.0200%,
Mg: 0 to 0.0200%,
REM: 0 to 0.1000%,
Bi: 0 to 0.020%,
One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and
Sn: Contains 0 to 0.05%,
The remainder consists of Fe and impurities,
Metal organization,
In % of area,
Retained austenite is less than 3.0%,
Ferrite is 15.0% or more and less than 60.0%,
Perlite is less than 5.0%,
The average spherical radius of the alloy carbide among the above ferrites is 0.5 nm or more and less than 5.0 nm, and the average number density is 3.5×10 ¹⁶ pieces/cm ³ or more,
The E value indicating the periodicity of the above metal structure is 10.7 or more,
The I value indicating the uniformity of the metal structure is 1.020 or more,
The standard deviation of Mn concentration is 0.60 mass% or less,
Hot rolled steel sheet characterized by a tensile strength of 980 MPa or more. In the first paragraph,
The above chemical composition, in mass%,
Cu: 0.01 to 2.00%,
Cr: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
Ni: 0.02 to 2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005 to 0.0200%,
REM: 0.0005 to 0.1000%, and
Bi: 0.0005 to 0.020%
A hot-rolled steel sheet characterized by containing one or more types selected from the group consisting of:

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