JP7680692B2 - Hot rolled steel plate - Google Patents

Description

The present invention relates to a hot rolled steel sheet.
This application claims priority based on Japanese Patent Application No. 2021-168623, filed in Japan on October 14, 2021, the contents of which are incorporated herein by reference.

In recent years, efforts have been made to reduce the weight of automobile bodies by using high-strength steel plates in order to improve fuel efficiency and collision safety. However, increasing the strength of steel plates generally results in a decrease in toughness. Therefore, an important issue in the development of high-strength steel plates is how to increase strength without degrading toughness.

A commonly known method for improving toughness is to roll at low temperatures and impart a high cumulative strain to the unrecrystallized austenite state. However, increasing the reduction rate in the unrecrystallized austenite state raises the issue that the aspect ratio of the prior austenite grains increases, leading to increased anisotropy in toughness.

For example, Patent Document 1 discloses a hot-rolled steel sheet characterized in that in the center of the sheet thickness, which is a portion of the steel sheet partitioned between the 3/8th and 5/8ths of the sheet thickness position from the surface of the steel sheet, the sheet has a texture in which the average value of the X-ray random intensity ratio of the {100}<011> to {223}<110> orientation group on the sheet surface is 6.5 or less and the X-ray random intensity ratio of the {332}<113> crystal orientation is 5.0 or less, the total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and the microstructure has an average crystal grain size of 12.0 μm or less.

Japanese Patent No. 5621942

However, from the standpoint of improving fuel efficiency and collision safety of automobiles, the technology disclosed in Patent Document 1 above has room for further improvement in terms of reducing the anisotropy of toughness in high-strength steel plates.

The present invention has been made in consideration of the above-mentioned situation, and aims to provide a hot-rolled steel sheet having high strength, excellent toughness, and reduced toughness anisotropy.

The inventors have investigated the relationship between the texture and mechanical properties of hot-rolled steel sheets and have found that the anisotropy of toughness can be further reduced even in hot-rolled steel sheets with a tensile strength of 1180 MPa or more. The inventors have found that different textures develop on the surface and inside of rolled steel sheets. The inventors have also found that in order to reduce the anisotropy of toughness, it is more effective to control the texture in the austenite region than the texture of martensite after quenching. Furthermore, the inventors have found that in order to obtain a texture with a desired crystal orientation, it is effective to preferably control the hot-rolling conditions.

The gist of the present invention, based on the above findings, is as follows:

(1) A hot-rolled steel sheet according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.100-0.500%,
Si: 0.100-3.000%,
Mn: 0.50-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 1.000% or less,
N: 0.0100% or less,
Ti: 0 to 0.20%,
Nb: 0 to 0.100%,
Ca: 0-0.0060%,
Mo: 0 to 0.50%,
Cr: 0-1.00%,
V: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50% and Sn: 0 to 0.050%
with the remainder being Fe and impurities,
The metal structure in the region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface is, in terms of area%,
90-100% martensite;
and 0 to 10% remaining structure;
In the texture in the region from the surface to a depth of 1/8 of the plate thickness from the surface,
The pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups is 2.0 or more;
In the texture in the region from the surface to a depth of 1/8 of the sheet thickness to a depth of 1/2 of the sheet thickness,
The pole density of the {110}<112> orientation is 5.0 or less,
The tensile strength is 1180 MPa or more.
(2) The hot-rolled steel sheet according to the above (1), wherein the chemical composition is, in mass%,
Ti: 0.02-0.20%,
Nb: 0.010-0.100%,
Ca: 0.0001-0.0060%,
Mo: 0.01-0.50%,
Cr: 0.01-1.00%,
V: 0.01-0.50%,
Cu: 0.01 to 0.50%,
Ni: 0.01 to 0.50%, and Sn: 0.001 to 0.050%
may contain one or more selected from the group consisting of:

According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent toughness, and reduced toughness anisotropy.

Hereinafter, the hot-rolled steel sheet according to this embodiment will be specifically described.
First, the reasons for limiting the chemical composition of the hot-rolled steel sheet according to this embodiment will be described. Note that the numerical ranges described with "to" include the lower and upper limits. Numerical values indicated as "less than" and "more than" do not include the numerical range. Also, all % in the chemical composition means mass %.

The chemical composition of the hot-rolled steel sheet according to this embodiment is, in mass%, C: 0.100-0.500%, Si: 0.100-3.000%, Mn: 0.50-3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 1.000% or less, N: 0.0100% or less, and the balance: Fe and impurities. Each element will be described in detail below.

C: 0.100-0.500%
C is an important element for improving the strength of a hot-rolled steel sheet. If the C content is less than 0.100%, the strength of the hot-rolled steel sheet decreases. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.150% or more, 0.170% or more, 0.200% or more, or 0.220% or more.
On the other hand, if the C content exceeds 0.500%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.500% or less. The C content is preferably 0.450% or less, 0.400% or less, or 0.370% or less.

Si: 0.100-3.000%
Si is an element that has the effect of improving the strength of a hot-rolled steel sheet. If the Si content is less than 0.100%, the strength of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 0.100% or more. The Si content is preferably 0.200% or more, 0.300% or more, 0.400% or more, or 0.500% or more. The Si content is more preferably more than 1.000%, and even more preferably 1.100% or more.
On the other hand, if the Si content exceeds 3.000%, the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Si content is set to 3.000% or less. The Si content is preferably 2.700% or less, 2.500% or less, or 2.300% or less.

Mn: 0.50-3.00%
Mn is an effective element for improving the strength of hot-rolled steel sheets by improving hardenability and solid solution strengthening. If the Mn content is less than 0.50%, the strength of the hot-rolled steel sheet decreases. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 1.00% or more, 1.20% or more, or 1.50% or more.
On the other hand, if the Mn content exceeds 3.00%, MnS is generated, which increases the anisotropy of the toughness of the hot-rolled steel sheet. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.30% or less, or 2.00% or less.

P: 0.100% or less P is an impurity element, and the lower the P content, the better. If the P content exceeds 0.100%, the workability and weldability of the hot-rolled steel sheet deteriorate significantly, and the fatigue properties also deteriorate. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070% or less, 0.050% or less, or 0.030% or less.
Although there is no particular restriction on the lower limit of the P content, an excessive reduction in the P content increases the production costs, and therefore the P content may be set to 0.001% or more, or 0.005% or more.

S: 0.0100% or less S is an impurity element, and the lower the S content, the better. If the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which increases the anisotropy of the toughness of the hot-rolled steel sheet, is generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
Although there is no particular restriction on the lower limit of the S content, an excessive reduction in the S content increases the production costs, and therefore the S content may be set to 0.0005% or more, or 0.0010% or more.

Al: 1.000% or less Al acts as a deoxidizer in the steelmaking stage and is an effective element for improving the cleanliness of steel. However, if the Al content exceeds 1.000%, alumina precipitates in clusters, which deteriorates the toughness of the hot-rolled steel sheet. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.700% or less, 0.500% or less, or 0.400% or less.
Although the lower limit of the Al content is not particularly specified, an excessive reduction in the Al content increases the production cost, so the Al content may be 0.001% or more, or 0.005% or more.

N: 0.0100% or less N is an impurity element. If the N content exceeds 0.0100%, coarse Ti nitrides are formed at high temperatures, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less.
Although there is no particular restriction on the lower limit of the N content, an excessive reduction in the N content increases the production cost, and therefore the N content may be set to 0.0010% or more.

The hot-rolled steel sheet according to the present embodiment may contain the above elements, with the balance being Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and/or during the steelmaking process, or elements that are permissible within a range that does not impair the properties of the hot-rolled steel sheet according to the present embodiment.

In order to improve various properties, the hot-rolled steel sheet according to this embodiment may contain the optional elements shown below in place of part of Fe. In order to reduce alloy costs, it is not necessary to intentionally include these optional elements in the steel, so the lower limit of the content of these optional elements is 0%.

Ti: 0.02~0.20%
Ti is an effective element for suppressing the recrystallization and grain growth of austenite between stands in hot rolling. By suppressing the recrystallization of austenite between stands, it is possible to accumulate more strain. As a result, it is possible to favorably control the texture of the hot rolled steel sheet. To reliably obtain the above effects, it is preferable that the Ti content is 0.02% or more.
On the other hand, if the Ti content exceeds 0.20%, inclusions due to TiN are formed, and the toughness of the hot-rolled steel sheet is deteriorated. Therefore, the Ti content is set to 0.20% or less.

Nb: 0.010-0.100%
Nb is an effective element for suppressing the recrystallization and grain growth of austenite between stands in hot rolling. By suppressing the recrystallization of austenite between stands, it is possible to accumulate more strain. As a result, it is possible to favorably control the texture of the hot rolled steel sheet. In order to reliably obtain the above effects, it is preferable that the Nb content is 0.010% or more.
On the other hand, if the Nb content exceeds 0.100%, the effect is saturated, so the Nb content is set to 0.100% or less.

Ca: 0.0001-0.0060%
Ca is an element that has the effect of dispersing a large number of fine oxides during deoxidation of molten steel and refining the structure of the hot-rolled steel sheet. Ca is also an element that fixes S in the steel as spherical CaS and suppresses the generation of elongated inclusions such as MnS, thereby reducing the anisotropy of the toughness of the hot-rolled steel sheet. To reliably obtain these effects, the Ca content is preferably 0.0001% or more.
On the other hand, when the Ca content exceeds 0.0060%, the above effect is saturated, so the Ca content is set to 0.0060% or less.

Mo: 0.01~0.50%
Mo is an element effective in strengthening the precipitation of ferrite. To reliably obtain this effect, the Mo content is preferably 0.01% or more.
On the other hand, if the Mo content exceeds 0.50%, the cracking sensitivity of the slab increases, making the slab difficult to handle, so the Mo content is set to 0.50% or less.

Cr:0.01~1.00%
Cr is an element effective in improving the strength of a hot-rolled steel sheet. In order to reliably obtain this effect, the Cr content is preferably 0.01% or more.
On the other hand, if the Cr content exceeds 1.00%, the ductility of the hot-rolled steel sheet deteriorates, so the Cr content is set to 1.00% or less.

V:0.01~0.50%
V improves the strength of the hot-rolled steel sheet by strengthening the steel through precipitates and by refining ferrite grains. To reliably obtain this effect, the V content is preferably 0.01% or more.
On the other hand, if the V content exceeds 0.50%, a large amount of carbonitrides precipitates, deteriorating the formability of the hot-rolled steel sheet. Therefore, the V content is set to 0.50% or less.

Cu: 0.01~0.50%
Cu is an element that dissolves in steel and contributes to improving the strength of the steel. Cu also improves hardenability. To reliably obtain these effects, the Cu content is preferably 0.01% or more.
On the other hand, if the Cu content exceeds 0.50%, the surface properties of the hot-rolled steel sheet may deteriorate, and the chemical conversion treatability and corrosion resistance may be deteriorated. Therefore, the Cu content is set to 0.50% or less.

Ni: 0.01~0.50%
Ni is an element that dissolves in steel and contributes to increasing the strength of the steel. Ni also improves hardenability. To reliably obtain these effects, the Ni content is preferably 0.01% or more.
On the other hand, Ni is expensive as an alloy, so a large Ni content increases the cost. In addition, if the Ni content exceeds 0.50%, the weldability of the hot-rolled steel sheet may deteriorate. Therefore, the Ni content is set to 0.50% or less.

Sn: 0.001-0.050%
Sn has the effect of suppressing internal oxidation and the effect of improving strength. In order to reliably obtain these effects, the Sn content is preferably 0.001% or more.
On the other hand, if a large amount of Sn is contained, defects may occur during hot rolling, so the Sn content is set to 0.050% or less.

The above-mentioned chemical compositions may be measured by a general analytical method. For example, they may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.
When the hot-rolled steel sheet has a plating layer on the surface, the plating layer on the surface may be removed by mechanical grinding before the chemical composition is analyzed.

Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, a metal structure in a region from the surface to a depth of 1/8 of the plate thickness from the surface is composed of 90 to 100% martensite and 0 to 10% of a remaining structure, in terms of area %, and in the texture in the region from the surface to a depth of 1/8 of the plate thickness from the surface, the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation group is 2.0 or more, and in the texture in the region from the surface to a depth of 1/8 of the plate thickness to a depth of 1/2 of the plate thickness from the surface, the pole density of the {110}<112> orientation is 5.0 or less.

In this embodiment, the area percentages of martensite and the remaining structure in the region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness are specified because the metal structure at this position shows a typical metal structure of a hot-rolled steel plate. Each specification will be explained in detail below.

Area ratio of martensite: 90-100%
If the area ratio of martensite is less than 90%, the strength of the hot-rolled steel sheet is deteriorated and the desired strength cannot be obtained. Therefore, the area ratio of martensite is set to 90% or more. The area ratio of martensite is preferably 92% or more, 95% or more, or 97% or more, and more preferably 100%.

In this embodiment, martensite refers to fresh martensite and tempered martensite. In this embodiment, there is no need to distinguish between fresh martensite and tempered martensite, so both are collectively referred to as martensite.

Note that tempered martensite is fresh martensite that has been tempered, and has a lower dislocation density than fresh martensite. The preferred manufacturing method for the hot-rolled steel sheet according to the present embodiment described below does not include a heat treatment for tempering after quenching, but tempered martensite may be generated during cooling after hot rolling or by reheating after coiling.

Area ratio of remaining structure: 0 to 10%
The metal structure of the hot-rolled steel sheet according to the present embodiment may contain bainite as a remaining structure. If the area ratio of the remaining structure exceeds 10%, the strength of the hot-rolled steel sheet decreases and the desired strength cannot be obtained. Therefore, the area ratio of the remaining structure is set to 10% or less. The area ratio of the remaining structure is preferably 8% or less, 5% or less, or 3% or less, and more preferably 0%.

The area ratio of each structure is obtained by the following method.
A test piece for microstructure observation is taken from a 1/4 position of the thickness of the hot-rolled steel sheet (a region from 1/8 depth of the thickness from the surface to 3/8 depth of the thickness from the surface) and the center position of the sheet width so that the thickness cross section parallel to the rolling direction becomes the observation surface. The observation surface is mirror-polished and then corroded with 3% by volume nital solution. Using an optical microscope and a scanning electron microscope (SEM), three fields of view are photographed at a magnification of 2000 times for the observation surface after corrosion. Each photographed field of view is 500 μm × 500 μm. Image analysis is performed on the photographed photograph to calculate the area ratio of each structure. The area ratio of each structure is obtained by calculating the average value of the area ratios obtained for the three fields of view.

Martensite is a structure having substructures such as blocks and packets within the grains, and can be distinguished from other metal structures by electron channeling contrast images taken with a scanning electron microscope.
Bainite is defined as a structure that is a collection of lath-shaped crystal grains and does not contain Fe-based carbides with a major axis of 20 nm or more, and is not martensite, and also as a structure that contains Fe-based carbides with a major axis of 20 nm or more and has a single variant, i.e., is elongated in the same direction. Here, Fe-based carbides elongated in the same direction refer to those in which the difference in the elongation direction of the Fe-based carbides is within 5°.

Average grain size of prior austenite grains: more than 5.0 μm, 30.0 μm or less In the hot-rolled steel sheet according to this embodiment, the average grain size of prior austenite grains may be more than 5.0 μm, 30.0 μm or less at the 1/4 position of the sheet thickness (region from the surface to 1/8 depth of the sheet thickness to 3/8 depth of the sheet thickness from the surface). By making the average grain size of prior austenite grains more than 5.0 μm, it is possible to stably obtain a predetermined texture required in this embodiment, and to further reduce the anisotropy of the toughness of the hot-rolled steel sheet. The average grain size of prior austenite grains is preferably 6.0 μm or more, 7.0 μm or more, 8.0 μm or more, or 9.0 μm or more.
On the other hand, if the average grain size of the prior austenite grains exceeds 30.0 μm, the desired strength may not be obtained. Therefore, the average grain size of the prior austenite grains is preferably 30.0 μm or less.

The average grain size of the prior austenite grains is obtained by the following method.
A test piece for microstructure observation is taken from a 1/4 position of the thickness of a hot-rolled steel plate (a region from 1/8 depth of the thickness from the surface to 3/8 depth of the thickness from the surface) and a central position of the plate width, so that a plate thickness cross section parallel to the rolling direction is the observation surface. After mirror polishing, the observation surface is corroded with 3 volume % nital solution, and the metal structure is observed with a scanning electron microscope (SEM). Three fields of view are photographed by SEM observation, in which about 10,000 crystal grains are observed in one field of view. The photographs obtained are subjected to image analysis using image analysis software (WinROOF) to calculate the average grain size of the prior austenite grains. For one of the prior austenite grains included in the observation field of view, the average value of the shortest diameter and the longest diameter is calculated, and the average value is regarded as the grain size of the prior austenite grain. The above operation is carried out for all prior austenite grains, except for prior austenite grains not entirely included in the photographed field of view, such as at the ends of the photographed field of view, to determine the grain size of all prior austenite grains in the photographed field of view. The average grain size of the prior austenite grains in the photographed field of view is obtained by dividing the sum of the grain sizes of the prior austenite grains obtained by dividing it by the total number of prior austenite grains whose grain sizes have been measured. This operation is carried out for all photographed fields of view, and the average grain size of the prior austenite grains in all photographed fields of view is calculated to obtain the average grain size of the prior austenite grains.

Pole density of {001}<110>, {111}<110> and {112}<110> orientation group in the texture of the surface to 1/8 depth of the sheet thickness from the surface: 2.0 or more If the pole density of {001}<110>, {111}<110> and {112}<110> orientation group in the texture of the surface to 1/8 depth of the sheet thickness from the surface (hereinafter sometimes referred to as the surface layer region) is less than 2.0, it is not possible to suppress the occurrence of microcracks in the surface layer region. As a result, the anisotropy of the toughness of the hot-rolled steel sheet increases. Therefore, the pole density of {001}<110>, {111}<110> and {112}<110> orientation group in the texture of the surface layer region is set to 2.0 or more. It is preferably 2.2 or more, 2.5 or more, or 2.7 or more.
The upper limit of the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface region is not particularly specified, but may be 9.0 or less, 8.0 or less, 7.0 or less, or 5.0 or less from the viewpoint of suppressing deterioration of ductility.

Pole density of the {110}<112> orientation in the texture in the region from 1/8 of the sheet thickness from the surface to 1/2 of the sheet thickness from the surface: 5.0 or less If the pole density of the {110}<112> orientation in the texture in the region from 1/8 of the sheet thickness from the surface to 1/2 of the sheet thickness from the surface (hereinafter sometimes referred to as the inner region) exceeds 5.0, the anisotropy of the toughness of the hot-rolled steel sheet increases. Therefore, the pole density of the {110}<112> orientation in the texture in the inner region is set to 5.0 or less. It is preferably 4.6 or less, 4.2 or less, or 4.0 or less.
The lower limit of the pole density of the {110}<112> orientation in the texture of the inner region is not particularly specified, but may be set to 2.0 or more, or 2.5 or more, from the viewpoint of suppressing deterioration in strength.

The pole density is measured using a device that combines a scanning electron microscope and an EBSD analyzer, and OIM Analysis (registered trademark) manufactured by AMETEK. The pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface region, and the pole density of {110}<112> in the texture of the inner region are obtained from the crystal orientation distribution function (ODF: Orientation Distribution Function) that displays the three-dimensional texture, which is calculated using the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and spherical harmonic functions.

The measurement range is from the surface to 1/8 of the plate thickness from the surface for the surface region, and from 1/8 of the plate thickness from the surface to 1/2 of the plate thickness from the surface for the internal region. The measurement pitch is 5 μm/step.

{hkl} represents a crystal plane parallel to the rolling surface, and <uvw> represents a crystal direction parallel to the rolling direction. In other words, {hkl}<uvw> indicates a crystal with {hkl} oriented in the normal direction to the sheet surface and <uvw> oriented in the rolling direction.

The rolling direction of the hot-rolled steel sheet can be determined by the following method.
First, a test piece is taken so that the thickness cross section of the hot-rolled steel sheet can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope. The observation range is the entire thickness of the sheet, and areas with dark brightness are judged to be inclusions. For inclusions with a major axis length of 40 μm or more, the direction parallel to the direction in which the inclusion extends is judged to be the rolling direction.

Tensile strength: 1180 MPa or more From the viewpoint of improving the collision safety of automobiles and the like or reducing the vehicle body weight, the hot-rolled steel sheet according to this embodiment has a tensile strength of 1180 MPa or more. Preferably, the tensile strength is 1250 MPa or more, 1300 MPa or more, 1350 MPa or more, or 1400 MPa or more.
The upper limit of the tensile strength is not particularly specified, but it is preferably 2000 MPa or less, 1600 MPa or less, 1500 MPa or less, or 1400 MPa or less.

Tensile strength is measured in accordance with JIS Z 2241:2011. The test specimen is JIS Z 2241:2011 No. 5 test specimen, and the test direction is perpendicular to the rolling direction.

The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult.
Furthermore, if the sheet thickness exceeds 8.0 mm, it may be difficult to control the texture, and it may be difficult to obtain the above-mentioned texture. Therefore, the sheet thickness may be 8.0 mm or less.

In addition, the hot-rolled steel sheet according to this embodiment may have a plating layer on the surface. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanizing layer, an electrolytic zinc plating layer, and an alloyed hot-dip galvanizing layer.

Next, a preferred manufacturing method for the hot-rolled steel sheet according to this embodiment will be described. The preferred manufacturing method for the hot-rolled steel sheet according to this embodiment includes the following steps (a) to (d). Note that the temperature in the following description refers to the surface temperature of the steel sheet unless otherwise specified.

(a) A heating step of heating a slab having the above-mentioned chemical composition to a temperature range of 1100°C or more and less than 1350°C.
(b) A finish rolling step in which the heated slab is finish rolled using a rolling mill having a plurality of stands, the finish rolling step satisfying the following conditions (I) to (V):
(I) The finish rolling start temperature is set to 800° C. or higher.
(II) In each of the last four stands among the plurality of stands, rolling is performed so that σ represented by the following formula (1) is 40 to 80.
σ=exp(0.753+3000/T)×ε ^0.21 ×ε' ^0.13 ...(1)
Here, T is the temperature (° C.) immediately before entering each stand, ε is the equivalent plastic strain, and ε' is the strain rate.
(III) The inter-pass time between each of the last four stands is between 0.2 and 10.0 seconds.
(IV) The cumulative rolling reduction of the last four stands is 60% or more.
(V) The finish rolling completion temperature is set to 800 to 950°C.
(c) A cooling step in which cooling is started within 1.0 second after the completion of finish rolling, and cooling is performed to a temperature range of 300°C or less so that the average cooling rate in the temperature range from the finish rolling completion temperature to 300°C is 100°C/s or more.
(d) A winding step in which the film is cooled and then wound.
Each step will be described below.

(a) Heating step In the heating step, the slab having the above-mentioned chemical composition is preferably heated to a temperature range of 1100° C. or more and less than 1350° C. The method for producing the slab is not particularly limited, and a common method can be applied in which molten steel having the above-mentioned chemical composition is melted in a converter or the like, and a slab is formed by a casting method such as continuous casting. Note that an ingot-blooming method may also be used.

In the slab, most of the carbonitride-forming elements such as Ti are present as coarse carbonitrides with a non-uniform distribution in the slab. The non-uniformly distributed coarse precipitates (carbonitrides) deteriorate the various properties of the hot-rolled steel sheet (e.g., tensile strength, toughness, hole expansion property, etc.). Therefore, the slab is heated before hot rolling to dissolve the coarse precipitates. In order to sufficiently dissolve the coarse precipitates before hot rolling, it is preferable to set the heating temperature of the slab to 1100°C or higher. However, if the heating temperature of the slab is too high, it will cause the occurrence of surface defects and a decrease in yield due to scale-off. Therefore, it is preferable to set the heating temperature of the steel material to less than 1350°C.

The slab is heated to a temperature range of 1100°C or higher and less than 1350°C and held for a specified time, but if the holding time exceeds 4800 seconds, the amount of scale generation increases. As a result, scale bite and other problems are more likely to occur in the subsequent finish rolling process, and the surface quality of the hot-rolled steel sheet may deteriorate. Therefore, it is preferable to hold the slab for 4800 seconds or less in the temperature range of 1100°C or higher and less than 1350°C.

Rough rolling process Between the heating process and the finish rolling process, the slab may be subjected to rough rolling. The conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be obtained.

(b) Finish Rolling Step In the finish rolling step, the heated slab is finish rolled using a rolling mill having multiple stands. At this time, it is preferable to satisfy the following conditions (I) to (V).
It is preferable to carry out descaling before the finish rolling or during rolling between the rolling stands in the finish rolling.

(I) Finish rolling start temperature: 800°C or higher The finish rolling start temperature (the entry temperature of the first pass of finish rolling) is preferably 800°C or higher. If the finish rolling start temperature is lower than 800°C, rolling in some of the rolling stands (especially the first half of the stands) will be performed at the two-phase temperature of ferrite + austenite. As a result, processed structures may remain after finish rolling, deteriorating the strength and toughness of the hot-rolled steel sheet. Therefore, the finish rolling start temperature is preferably 800°C or higher.
The finish rolling start temperature is preferably 1100° C. or lower in order to suppress coarsening of austenite and to favorably control the textures in the surface layer region and the inner region.

(II) In each of the last four stands, σ represented by the following formula (1): 40 to 80
σ=exp(0.753+3000/T)・ε ^0.21・ε' ^0.13 ...(1)
Here, T is the temperature (° C.) immediately before entering each stand (i.e., the entry temperature), ε is the equivalent plastic strain, and ε' is the strain rate.
The fact that σ is between 40 and 80 for each of the last four stands can be rephrased as saying that the σ of the fourth-to-last stand, the σ of the third-to-last stand, the σ of the second-to-last stand, and the σ of the final stand are all between 40 and 80.

If even one stand has a σ of less than 40, the strain required for the development of the texture in the surface region may not be appropriately applied in each of the last four stands. As a result, the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups may not be appropriately controlled in the texture in the region from the surface to a depth of 1/8 of the sheet thickness from the surface. Therefore, it is preferable that the σ in each of the last four stands is 40 or more.
In addition, if there is even one stand with σ exceeding 80, the texture of the inner region cannot be favorably controlled, and the anisotropy of the toughness of the hot-rolled steel sheet may increase. Therefore, it is preferable that σ in each of the last four stands is 80 or less.

The equivalent plastic strain, ε, can be calculated by ε = (2/√3) x (h/H), where h is the entry thickness and H is the exit thickness. Furthermore, ε', the strain rate, can be calculated by ε' = ε/t, where t (s) is the rolling time. Furthermore, the rolling time t refers to the time during which the steel plate comes into contact with the rolling rolls and strain is applied to the steel plate.

(III) Interpass time between the last four stands: 0.2 to 10.0 seconds If there is even one interpass between the last four stands whose interpass time exceeds 10.0 seconds, recovery and recrystallization between the passes will proceed. As a result, it becomes difficult to accumulate strain, and the desired structure may not be obtained in the hot-rolled steel sheet. Therefore, it is preferable that the interpass time between the last four stands is 10.0 seconds or less.
It is preferable that the interpass time between the last four stands is short, but there are limitations to the installation space of each stand and the rolling speed in shortening the interpass time. Furthermore, if the interpass time between the last four stands is less than 0.2 seconds, the number of unrecrystallized grains increases significantly, and the desired texture may not be obtained. Therefore, it is preferable to set the interpass time to 0.2 seconds or more.
Furthermore, the fact that the inter-pass times between each of the last four stands are 0.2 to 10.0 seconds can be rephrased as meaning that the inter-pass times between the fourth-to-last stand and the third-to-last stand, the inter-pass times between the third-to-last stand and the second-to-last stand, and the inter-pass times between the second-to-last stand and the final stand are all 0.2 to 10.0 seconds.

(IV) Cumulative reduction rate of the last four stands: 60% or more If the cumulative reduction rate of the last four stands is less than 60%, the dislocation density introduced into the non-recrystallized austenite may be small. If the dislocation density introduced into the non-recrystallized austenite is small, it may be difficult to obtain a desired structure, and the strength and toughness of the hot-rolled steel sheet may be deteriorated. Therefore, it is preferable that the cumulative reduction rate of the last four stands is 60% or more.
In addition, if the cumulative reduction rate of the last four stands exceeds 97%, the shape of the hot rolled steel sheet may deteriorate. Therefore, the cumulative reduction rate of the last four stands may be set to 97% or less.

The cumulative rolling reduction rate of the last four stands can be expressed as {1-(t1/t0)} x 100 (%), where t0 is the inlet thickness of the fourth-to-last stand and t1 is the outlet thickness of the final stand.

(V) Finish rolling completion temperature: 800 to 950°C
If the finish rolling end temperature (the temperature at the exit side of the final stand) is less than 800°C, the rolling will be performed at a two-phase temperature of ferrite + austenite. Therefore, the processed structure may remain after rolling, and the strength and toughness of the hot-rolled steel sheet may decrease. Therefore, the finish rolling end temperature is preferably 800°C or higher.
In addition, in the slab having the chemical composition according to this embodiment, the unrecrystallized austenite region is generally a temperature range of 950°C or less. Therefore, if the finish rolling completion temperature exceeds 950°C, the austenite grains grow, and the grain length of the martensite in the hot rolled steel sheet obtained after cooling becomes large. As a result, it becomes difficult to obtain a desired texture, and the strength and toughness of the hot rolled steel sheet may decrease. Therefore, it is preferable to set the finish rolling completion temperature to 950°C or less.

(c) Cooling Step In the cooling step, it is preferable to start cooling within 1.0 second after the completion of finish rolling, and to cool to a temperature range of 300°C or less so that the average cooling rate in the temperature range from the finish rolling completion temperature to 300°C is 100°C/s or more.

In this embodiment, it is preferable to install a cooling facility after the finish rolling facility and pass the steel sheet after finish rolling through the cooling facility to perform cooling. It is preferable that the cooling facility is capable of cooling the steel sheet at an average cooling rate of 100°C/s or more. An example of such a cooling facility is a water-cooling facility that uses water as a cooling medium.

The average cooling rate in the cooling step is defined as the temperature drop of the steel sheet from the start to the end of cooling divided by the time required from the start to the end of cooling. The start of cooling refers to the time when the steel sheet is introduced into the cooling equipment, and the end of cooling refers to the time when the steel sheet is removed from the cooling equipment.
In addition, the cooling equipment may have no air-cooling section or may have one or more air-cooling sections. In the present embodiment, either type of cooling equipment may be used. Even when using cooling equipment having an air-cooling section, it is sufficient that the average cooling rate from the start to the end of cooling is 100° C./s or more.

The reasons for limiting the cooling conditions are explained below. The cooling stop temperature is 300°C or less, and this condition will be explained in the winding process.

Cooling start time: within 1.0 second after the completion of finish rolling It is preferable to start cooling immediately after the completion of finish rolling. If the cooling start time exceeds 1.0 second, recrystallization progresses, and cooling is performed in a strain-released state, so that the desired texture may not be obtained in the hot-rolled steel sheet. Therefore, it is preferable to start cooling within 1.0 second after the completion of finish rolling.

Average cooling rate in the temperature range from the finish rolling completion temperature to 300°C: 100°C/s or more If the average cooling rate in the temperature range from the finish rolling completion temperature to 300°C is less than 100°C/s, bainite and ferrite are likely to be formed, and the desired amount of martensite may not be obtained. Therefore, it is preferable that the average cooling rate in the temperature range from the finish rolling completion temperature to 300°C is 100°C/s or more.

(d) Winding process In the winding process, it is preferable to wind the steel sheet cooled to a temperature range of 300°C or less into a coil. Since the steel sheet is wound immediately after cooling, the winding temperature is almost equal to the cooling stop temperature. If the winding temperature exceeds 300°C, polygonal ferrite or bainite is generated, and the strength of the hot-rolled steel sheet may decrease. Therefore, it is preferable that the winding temperature is in the temperature range of 300°C or less.

After coiling, the hot-rolled steel sheet may be subjected to temper rolling in the usual manner, and may also be subjected to pickling to remove scale formed on the surface. Alternatively, the sheet may be subjected to plating treatments such as aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrolytic zinc plating, and alloyed hot-dip galvanizing, or chemical conversion treatments.

The preferred manufacturing method described above enables the stable production of hot-rolled steel sheet according to this embodiment.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example 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. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Molten steel having the chemical composition shown in Table 1 was melted in a converter, and slabs were obtained by continuous casting. These slabs were then heated under the conditions shown in Tables 2A and 2B, and subjected to rough rolling, and then to finish rolling under the conditions shown in Tables 2A and 2B. After the finish rolling was completed, the slabs were cooled and coiled under the conditions shown in Tables 3A and 3B, to obtain hot-rolled steel sheets having the thicknesses shown in Tables 3A and 3B.
In the heating step, the holding time at the heating temperatures shown in Tables 2A and 2B was 4800 seconds or less.

Cooling after finish rolling was by water cooling, and was performed by passing the steel plate through a water cooling facility that did not have an air cooling section in between. The average cooling rates in Tables 3A and 3B are the temperature drop of the steel plate from the time the water cooling facility was introduced to the time it was removed, divided by the time required for the steel plate to pass through the water cooling facility.

Test pieces were taken from the obtained hot-rolled steel sheets, and the area ratio of each structure and the pole density of the texture were measured and a tensile test was performed by the above-mentioned methods.
The results obtained are shown in Tables 4A and 4B.

If the obtained tensile strength was 1180 MPa or more, it was judged to have high strength and to have passed the test. On the other hand, if the obtained tensile strength was less than 1180 MPa, it was judged to have insufficient strength and to have failed the test.

As a toughness evaluation of hot-rolled steel sheets, Charpy impact tests were performed to measure the ductile-brittle transition temperature. The ductile-brittle transition temperature was measured in accordance with JIS Z 2242:2018, using a 2.5 mm subsize V-notch test piece and a Charpy impact test with a C-direction notch. The temperature at which the brittle fracture surface ratio was 50% was defined as the ductile-brittle transition temperature. In addition, for hot-rolled steel sheets with a final thickness of less than 2.5 mm, the measurement was performed at the full thickness.

If the obtained ductile-brittle transition temperature was -50°C or lower, it was judged to have excellent toughness and to have passed the test. On the other hand, if the obtained ductile-brittle transition temperature was above -50°C, it was judged to have poor toughness and to have failed the test.

Furthermore, the anisotropy of toughness was evaluated by the following method. In accordance with JIS Z 2242:2018, the absorbed energy of the C-direction notch and the absorbed energy of the L-direction notch were measured by Charpy impact testing using 2.5 mm sub-size V-notch test pieces. The Charpy impact testing was performed at -60°C. The difference between the absorbed energy of the L-direction notch and the absorbed energy of the C-direction notch was calculated, and if the difference was ±15 J or less, the anisotropy of toughness was deemed to have been reduced and the specimen was judged to have passed. On the other hand, if the difference between the absorbed energy of the L-direction notch and the absorbed energy of the C-direction notch exceeded ±15 J, the anisotropy of toughness was deemed to have not been reduced and the specimen was judged to have failed.

From Tables 4A and 4B, it can be seen that the hot-rolled steel sheets according to the examples of the present invention have high strength and excellent toughness, and the anisotropy of the toughness is reduced. On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples have deteriorated in one of the characteristics.

Claims (2)

The chemical composition, in mass%, is
C: 0.100-0.500%,
Si: 0.100-3.000%,
Mn: 0.50-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 1.000% or less,
N: 0.0100% or less,
Ti: 0 to 0.20%,
Nb: 0 to 0.100%,
Ca: 0-0.0060%,
Mo: 0 to 0.50%,
Cr: 0-1.00%,
V: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50% and Sn: 0 to 0.050%
with the remainder being Fe and impurities,
The metal structure in the region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface is, in terms of area%,
90-100% martensite;
and 0 to 10% remaining structure;
In the texture in the region from the surface to a depth of 1/8 of the plate thickness from the surface,
The pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups is 2.0 or more;
In the texture in the region from the surface to a depth of 1/8 of the sheet thickness to a depth of 1/2 of the sheet thickness,
The pole density of the {110}<112> orientation is 5.0 or less,
A hot-rolled steel sheet having a tensile strength of 1180 MPa or more. The chemical composition, in mass%,
Ti: 0.02-0.20%,
Nb: 0.010-0.100%,
Ca: 0.0001-0.0060%,
Mo: 0.01-0.50%,
Cr: 0.01-1.00%,
V: 0.01-0.50%,
Cu: 0.01 to 0.50%,
Ni: 0.01 to 0.50%, and Sn: 0.001 to 0.050%
The hot-rolled steel sheet according to claim 1, further comprising one or more selected from the group consisting of: