KR20240065114A - hot rolled steel plate - Google Patents

Abstract

This hot-rolled steel sheet has a predetermined chemical composition and metal structure, and in the texture of the surface layer region, the polar density of {001}<110>, {111}<110>, and {112}<110> orientation groups is 2.0. As above, in the texture of the internal region, the pole density in the {110}<112> orientation is 5.0 or less, and the tensile strength is 1180 MPa or more.

Description

hot rolled steel plate

The present invention relates to hot rolled steel sheets.

This application claims priority based on Patent Application No. 2021-168623 filed in Japan on October 14, 2021, and uses the content here.

In recent years, for the purpose of improving the fuel efficiency and collision safety of automobiles, weight reduction has been made by applying high-strength steel plates. However, when steel sheets are strengthened, toughness generally deteriorates. Therefore, in the development of high-strength steel sheets, it is an important task to achieve high strength without deteriorating toughness.

Generally, in order to improve toughness, there is a known method of improving toughness by rolling at low temperature and applying high cumulative strain in the state of unrecrystallized austenite. However, there is a problem that increasing the reduction ratio in the state of unrecrystallized austenite increases the aspect ratio of the old austenite particles and increases the anisotropy of toughness.

For example, in Patent Document 1, at the center of the plate thickness, which is a portion of the steel plate divided into a 3/8 thickness position and a 5/8 thickness position from the steel plate surface, {100}<011> to {223} of the plate surface. The average value of the A hot-rolled steel sheet is disclosed, which is characterized by having a microstructure in which the total area ratio of nite is more than 85% and the average crystal grain size is 12.0 μm or less.

Japanese Patent No. 5621942 Publication

However, in the technology disclosed in Patent Document 1, there is room for further improvement in reducing the anisotropy of toughness in high-strength steel sheets from the viewpoint of improving fuel efficiency and collision safety of automobiles.

The present invention has been made in view of the above-described circumstances, and its purpose is to provide a hot-rolled steel sheet with high strength and excellent toughness, and with reduced anisotropy of toughness.

As a result of investigating the relationship between the texture and mechanical properties of hot-rolled steel sheets, the present inventors discovered 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 present inventors found that rolled steel sheets develop different textures on the surface and inside. Additionally, the present inventors found that in order to reduce the anisotropy of toughness, it is more effective to control the texture in the austenite region rather than the texture of martensite after rapid cooling. Additionally, the present inventors have found that it is effective to appropriately control hot rolling conditions in order to obtain a texture having a desired crystal orientation.

The gist of the present invention made based on the above knowledge is as follows.

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

C: 0.100 to 0.500%,

Si: 0.100 to 3.000%,

Mn: 0.50 to 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 to 0.0060%,

Mo: 0 to 0.50%,

Cr: 0 to 1.00%,

V: 0 to 0.50%,

Cu: 0 to 0.50%,

Ni: 0 to 0.50%, and

Sn: 0 to 0.050%

Contains, the balance consists of Fe and impurities,

The metal structure in the area from 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 is expressed in area%,

90 to 100% martensite,

Consists of 0 to 10% of residual tissue,

In the texture of the surface or the area at a depth of 1/8 of the plate thickness from the surface,

{001}<110>, {111}<110> and {112}<110> National Guards have a pole density of 2.0 or more;

In the texture of the area from a depth of 1/8 of the plate thickness from the surface to a depth of 1/2 of the plate thickness from the surface,

The polar density of the {110}<112> orientation is 5.0 or less,

Tensile strength is more than 1180MPa.

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

Ti: 0.02 to 0.20%,

Nb: 0.010 to 0.100%,

Ca: 0.0001 to 0.0060%,

Mo: 0.01 to 0.50%,

Cr: 0.01 to 1.00%,

V: 0.01 to 0.50%,

Cu: 0.01 to 0.50%,

Ni: 0.01 to 0.50% and

Sn: 0.001 to 0.050%

It may contain one or two or more types selected from the group consisting of.

According to the above aspect according to the present invention, a hot rolled steel sheet having high strength and excellent toughness and reduced anisotropy of toughness can be provided.

Hereinafter, the hot rolled steel sheet according to this embodiment will be described in detail.

First, the reason for limiting the chemical composition of the hot rolled steel sheet according to this embodiment will be explained. In addition, the numerical limitation range described between "to" includes the lower limit and the upper limit. Numerical values indicated as “less than” or “exceeding” are not included in the numerical range. In addition, % for chemical composition all means mass %.

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

C: 0.100 to 0.500%

C is an important element for improving the strength of hot rolled steel sheets. 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 to 3.000%

Si is an element that has the effect of improving the strength of hot rolled steel sheets. 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. Moreover, the Si content is more preferably greater than 1.000%, and even more preferably is 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 to 3.00%

Mn is an element effective in improving the strength of hot rolled steel sheets by improving hardenability and strengthening solid solution. 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, when 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 more preferable. If the P content exceeds 0.100%, the workability and weldability of the hot rolled steel sheet deteriorate significantly, and 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.

The lower limit of the P content is not specifically specified, but if the P content is excessively reduced, the manufacturing cost increases, so the P content may be 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 more preferable. 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.

The lower limit of the S content is not specifically specified, but if the S content is excessively reduced, the manufacturing cost increases, so the S content may be 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 in improving the cleanliness of steel. However, if the Al content exceeds 1.000%, alumina precipitated in clusters is generated, and the toughness of the hot rolled steel sheet deteriorates. 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.

The lower limit of the Al content is not specifically specified, but if the Al content is excessively reduced, the manufacturing cost increases, 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 is more than 0.0100%, coarse Ti nitride is formed at high temperature, 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.

The lower limit of the N content is not specifically defined, but if the N content is excessively reduced, the manufacturing cost increases, so the N content may be 0.0010% or more.

The hot-rolled steel sheet according to this embodiment may contain the above elements, and the remainder may consist of Fe and impurities. Examples of impurities include elements that are unavoidably mixed from steel raw materials or scrap and/or during the steelmaking process, or elements that are allowed as long as they do not impair the characteristics of the hot rolled steel sheet according to the present embodiment.

In order to improve various characteristics, the hot rolled steel sheet according to this embodiment may contain any element shown below instead of part of Fe. In order to reduce alloy costs, there is no need to intentionally contain these arbitrary elements in the steel, so the lower limit of the content of these arbitrary elements is all 0%.

Ti: 0.02 to 0.20%

Ti is an effective element for suppressing austenite recrystallization and grain growth between hot rolling stands. By suppressing recrystallization of austenite between stands, strain can be further accumulated. As a result, the texture of the hot rolled steel sheet can be preferably controlled. In order to reliably obtain the above effect, it is preferable that the Ti content is 0.02% or more.

On the other hand, if the Ti content is more than 0.20%, inclusions originating from TiN are generated, and the toughness of the hot rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.20% or less.

Nb: 0.010 to 0.100%

Nb is an effective element for suppressing recrystallization and grain growth of austenite between hot rolling stands. By suppressing recrystallization of austenite between stands, strain can be further accumulated. As a result, the texture of the hot rolled steel sheet can be preferably controlled. In order to reliably obtain the above effect, it is preferable that the Nb content is 0.010% or more.

On the other hand, when the Nb content exceeds 0.100%, the effect is saturated. Therefore, the Nb content is set to 0.100% or less.

Ca: 0.0001 to 0.0060%

Ca is an element that has the effect of dispersing many fine oxides during deoxidation of molten steel and refining the structure of the hot rolled steel sheet. In addition, Ca is also an element that fixes S in the steel into spherical CaS, suppresses the generation of stretching inclusions such as MnS, and reduces the anisotropy of the toughness of the hot rolled steel sheet. In order to reliably obtain these effects, it is preferable that the Ca content is 0.0001% or more.

On the other hand, when the Ca content exceeds 0.0060%, the above effect is saturated. Therefore, the Ca content is set to 0.0060% or less.

Mo: 0.01 to 0.50%

Mo is an element effective in strengthening the precipitation of ferrite. To reliably obtain this effect, the Mo content is preferably set to 0.01% or more.

On the other hand, if the Mo content exceeds 0.50%, the cracking susceptibility of the slab increases and handling of the slab becomes difficult. Therefore, the Mo content is set to 0.50% or less.

Cr: 0.01 to 1.00%

Cr is an element effective in improving the strength of hot rolled steel sheets. To ensure this effect, the Cr content is preferably set to 0.01% or more.

On the other hand, when the Cr content exceeds 1.00%, the ductility of the hot rolled steel sheet deteriorates. Therefore, the Cr content is set to 1.00% or less.

V: 0.01 to 0.50%

V improves the strength of the hot rolled steel sheet by strengthening by precipitates and refining the ferrite particles. To reliably obtain this effect, the V content is preferably set to 0.01% or more.

On the other hand, if the V content exceeds 0.50%, a large amount of carbonitride precipitates and the formability of the hot rolled steel sheet deteriorates. Therefore, the V content is set to 0.50% or less.

Cu: 0.01 to 0.50%

Cu is an element that is dissolved in steel and contributes to improving the strength of steel. Additionally, Cu is also an element that improves compatibility. In order to reliably obtain these effects, it is preferable that the Cu content is 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 chemical treatment properties and corrosion resistance may deteriorate. Therefore, the Cu content is set to 0.50% or less.

Ni: 0.01 to 0.50%

Ni is an element that is dissolved in steel and contributes to increasing the strength of steel. Additionally, Ni is also an element that improves compatibility. In order to reliably obtain these effects, it is preferable that the Ni content is 0.01% or more.

On the other hand, since Ni has a high alloying cost, containing a large amount of Ni causes an increase in cost. Additionally, 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 to 0.050%

Sn has the effect of suppressing internal oxidation and improving strength. In order to reliably obtain this effect, it is preferable that the Sn content is 0.001% or more.

On the other hand, if Sn is contained in a large amount, flaws may occur during hot rolling. Therefore, the Sn content is set to 0.050% or less.

The above-mentioned chemical composition may be measured by a general analysis method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Additionally, C and S can be measured using the combustion-infrared absorption method, and N can be measured using the 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 can be removed by mechanical grinding and then the chemical composition can be analyzed.

Next, the metal structure of the hot rolled steel sheet according to this embodiment will be described.

The hot rolled steel sheet according to the present embodiment has a metal structure in a region from the surface to a depth of 1/8 of the sheet thickness to a depth of 3/8 of the sheet thickness from the surface of 90 to 100% martensite in area percent. and 0 to 10% of the residual structure, and in the aggregate structure of the surface or the area at a depth of 1/8 of the plate thickness from the surface, {001}<110>, {111}<110>, and {112 }<110> The polar density of the orientation is 2.0 or more, and in the texture of the region from a depth of 1/8 of the plate thickness from the surface to a depth of 1/2 of the plate thickness from the surface, the {110}<112> orientation The polar density is less than 5.0.

In addition, in this embodiment, the area % of martensite and residual structure in the region from the surface to the depth of 1/8 of the sheet thickness to the depth of 3/8 of the sheet thickness from the surface is defined at this position. This is because the metal structure represents the representative metal structure of hot rolled steel sheets. Below, each regulation is explained in detail.

Area ratio of martensite: 90 to 100%

If the area ratio of martensite is less than 90%, the strength of the hot rolled steel sheet deteriorates 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, since there is no need to distinguish between fresh martensite and tempered martensite, both are collectively referred to as martensite.

In addition, tempered martensite is fresh martensite tempered, and has a lower dislocation density than fresh martensite. In the suitable manufacturing method of the hot rolled steel sheet according to this embodiment described later, heat treatment for the purpose of tempering after rapid cooling is not included, but tempered martensite may be generated during cooling after hot rolling or by reheating after coiling.

Area ratio of residual tissue: 0 to 10%

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

The area ratio of each tissue is obtained by the following method.

A position of 1/4 of the sheet thickness of a hot rolled steel sheet (an area from a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface) and a cross section of the sheet thickness parallel to the rolling direction from the central position of the sheet width. A test piece for tissue observation is collected so that this observation surface is used. After mirror polishing the observation surface, it is corroded with 3% by volume Nital solution. Using an optical microscope and a scanning electron microscope (SEM), three views of the observed surface after corrosion are photographed at a magnification of 2000 times. Each shooting field of view is 500㎛×500㎛. Image analysis is performed on the captured photographs, and the area ratio of each tissue is calculated. Additionally, the area ratio of each tissue is obtained by calculating the average value of the area ratios obtained for the three fields of view.

Since martensite is a structure that has substructures such as blocks and packets within the grain, it is possible to distinguish it from other metal structures according to an electron channeling contrast image using a scanning electron microscope.

It is a lath-type crystal grain set, a non-martensite structure that does not contain Fe-based carbides with a long diameter of 20 nm or more inside the structure, and a Fe-based carbide with a long diameter of 20 nm or more inside the structure, and the Fe The structure in which the carbide-based carbide has a single variant, that is, the Fe-based carbide extended in the same direction, is regarded as bainite. Here, Fe-based carbides extending in the same direction refer to those in which the difference in the stretching directions of the Fe-based carbides is within 5°.

Average particle size of old austenite particles: greater than 5.0㎛, less than 30.0㎛

In the hot rolled steel sheet according to the present embodiment, at a position of 1/4 of the sheet thickness (an area from a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface), the average of the old austenite grains The particle size may be greater than 5.0 μm or less than 30.0 μm. By setting the average particle size of the old austenite particles to more than 5.0 μm, the predetermined texture obtained in this embodiment can be stably obtained, and the anisotropy of the toughness of the hot rolled steel sheet can be further reduced. The average particle size of the prior austenite particles 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 particle size of the old austenite particles is more than 30.0 μm, the desired strength may not be obtained. Therefore, it is preferable that the average particle size of the old austenite particles is 30.0 μm or less.

The average particle size of prior austenite particles is obtained by the following method.

At the position of 1/4 of the sheet thickness of a hot rolled steel sheet (an area from a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the sheet thickness from the surface), there is also a cross section of the sheet thickness parallel to the rolling direction from the central position of the sheet width. A test piece for tissue observation is collected to serve as an observation surface. After mirror polishing the observation surface, it is corroded with 3% by volume Nital solution, and the metal structure is observed with a scanning electron microscope (SEM). The area where approximately 10,000 crystal grains are observed in one field of view is photographed in three fields of view using SEM observation. The obtained photograph is subjected to image analysis using image analysis software (WinROOF), and the average grain size of the old austenite grains is calculated. For one of the old austenite grains included in the observation field, the average value of the shortest diameter and the longest diameter is calculated, and the average value is taken as the grain size of the old austenite grain. The above-described operation is performed on all prior austenite particles, excluding the former austenite grains in which the entire crystal grain, such as the edge of the imaging field of view, is not included in the imaging field of view, and the grain size of all prior austenite grains in the imaging field of view is determined. Save. The average grain size of the prior austenite grains in the imaging field of view is obtained by calculating the sum of the grain sizes of the obtained prior austenite grains divided by the total number of prior austenite grains whose particle diameters were measured. This operation is performed for every photographed field of view, and the average grain size of the prior austenite grains in all photographed fields is calculated to obtain the average grain size of the prior austenite grains.

Polar density of {001}<110>, {111}<110>, and {112}<110> defense groups in the texture of the surface or the area at a depth of 1/8 of the plate thickness from the surface: 2.0 or more

{001}<110>, {111}<110>, and {112}<110 in the texture of the surface or a region from the surface to a depth of 1/8 of the plate thickness (hereinafter sometimes referred to as the surface layer region) > If the polar density of the defense force is less than 2.0, the occurrence of microcracks in the surface layer region cannot be suppressed. As a result, the anisotropy of the toughness of the hot rolled steel sheet increases. Therefore, the polar density of the {001}<110>, {111}<110>, and {112}<110> defense groups in the aggregate structure of the surface region is set to 2.0 or more. Preferably it is 2.2 or more, 2.5 or more, or 2.7 or more.

The upper limit of the polar density of the {001}<110>, {111}<110>, and {112}<110> defense groups in the aggregate structure of the surface region is not specifically specified, but is 9.0 or less from the viewpoint of suppressing deterioration of ductility. , may be 8.0 or less, 7.0 or less, or 5.0 or less.

Polar density in the {110}<112> orientation in the texture of the region from a depth of 1/8 of the plate thickness from the surface to a depth of 1/2 of the plate thickness from the surface: 5.0 or less

The polar density of the {110}<112> orientation in the texture of the region from the surface to a depth of 1/8 of the sheet thickness to the depth of 1/2 of the sheet thickness from the surface (hereinafter sometimes referred to as the internal region) is If it 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 of the internal region is set to 5.0 or less. Preferably it is 4.6 or less, 4.2 or less, or 4.0 or less.

The lower limit of the polar density of the {110}<112> orientation in the texture of the internal region is not specifically specified, but may be 2.0 or more or 2.5 or more from the viewpoint of suppressing strength deterioration.

The extreme density uses a device combining a scanning electron microscope and an EBSD analysis device and OIM Analysis (registered trademark) manufactured by AMETEK. The aggregate texture of the surface region is determined from the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and the crystal orientation distribution function (ODF: Orientation Distribution Function), which represents the three-dimensional texture, calculated using a spherical harmonic function. The pole densities of the {001}<110>, {111}<110>, and {112}<110> defense forces in , and the pole densities of {110}<112> in the collective organization of the internal region are obtained.

In addition, the measurement range is from the surface to a depth of 1/8 of the sheet thickness from the surface for the surface area, and for the internal area, from a depth of 1/8 of the sheet thickness from the surface to 1/2 of the sheet thickness from the surface. This is the area of depth. The measurement pitch is 5㎛/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> represents a crystal with {hkl} oriented in the plate surface normal direction and <uvw> oriented in the rolling direction.

Additionally, the rolling direction of the hot rolled steel sheet can be determined by the following method.

First, a test piece is collected so that the thickness cross section of the hot rolled steel sheet can be observed. After finishing the plate thickness cross section of the collected test piece by mirror polishing, it is observed using an optical microscope. The observation range is the entire plate thickness, and areas with dark luminance are determined to be inclusions. For inclusions with a major axis length of 40 μm or more, the direction parallel to the direction in which the inclusion is extending is determined as the rolling direction.

Tensile strength: more than 1180MPa

The hot-rolled steel sheet according to this embodiment has a tensile strength of 1180 MPa or more from the viewpoint of improving crash safety of automobiles or other vehicles or reducing the weight of vehicle bodies. Preferably, it 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 is preferably 2000 MPa or less, 1600 MPa or less, 1500 MPa or less, or 1400 MPa or less.

Tensile strength is measured based on JIS Z 2241:2011. The test piece is No. 5 in JIS Z 2241:2011, and the test direction is perpendicular to the rolling direction.

The plate thickness of the hot rolled steel sheet according to this 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 becomes difficult to secure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult.

Additionally, when the plate thickness exceeds 8.0 mm, control of the texture becomes difficult, and it may become difficult to obtain the texture described above. Therefore, the plate thickness may be 8.0 mm or less.

Additionally, 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 electric galvanizing layer, and an alloyed hot-dip galvanizing layer.

Next, a suitable manufacturing method for the hot rolled steel sheet according to this embodiment will be described. A manufacturing method suitable for the hot rolled steel sheet according to this embodiment includes the following steps (a) to (d). In addition, the temperature in the following description refers to the surface temperature of the steel sheet unless otherwise specified.

(a) A heating process in which a slab having the above-mentioned chemical composition is heated to a temperature range of 1100°C or more and less than 1350°C.

(b) This is a finish rolling process in which the heated slab is finish rolled using a rolling mill having a plurality of stands, and satisfies the following conditions (I) to (V).

(Ⅰ) The finish rolling start temperature is 800°C or higher.

(II) In each of the last four stands among the plurality of stands, rolling is performed so that σ expressed by the following formula (1) is 40 to 80.

Here, T is the temperature (°C) just before entering each stand, ε is the equivalent plastic strain, and ε' is the strain rate.

(Ⅲ) The time between passes between each of the last four stands shall be 0.2 to 10.0 seconds.

(Ⅳ) The cumulative reduction ratio of the last four stands shall be 60% or more.

(Ⅴ) The finish rolling completion temperature is 800 to 950°C.

(c) A cooling process that starts cooling within 1.0 seconds after completion of finish rolling and cools to a temperature range of 300°C or lower 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 process in which winding is performed after cooling.

Hereinafter, each process will be described.

(a) Heating process

In the heating process, it is preferable to heat the slab having the above-mentioned chemical composition to a temperature range of 1100°C or more and less than 1350°C. The manufacturing method of the slab does not need to be 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 used to form a slab by a casting method such as continuous casting. Additionally, the incorporation-disintegration method may be used.

In a slab, most of the carbonitride forming elements such as Ti are distributed unevenly in the slab and exist as coarse carbonitrides. Coarse precipitates (carbonitrides) present in a non-uniform distribution deteriorate various properties (e.g., tensile strength, toughness, hole expandability, etc.) of the hot rolled steel sheet. Therefore, the slab before hot rolling is heated to dissolve the coarse precipitates into solid solution. In order to sufficiently dissolve these coarse precipitates before hot rolling, the heating temperature of the slab is preferably set to 1100°C or higher. However, if the heating temperature of the slab becomes too high, surface flaws may occur or yield decreases due to scale-off. Therefore, it is preferable that the heating temperature of the steel material is lower than 1350°C.

The slab is heated to a temperature range of 1100°C or higher and below 1350°C and held for a predetermined period of time. However, if the holding time exceeds 4800 seconds, the amount of scale generated increases. As a result, scale entrapment and the like are likely to occur during the subsequent finish rolling process, and the surface quality of the hot rolled steel sheet may deteriorate. Therefore, it is preferable that the holding time in the temperature range of 1100°C or more and less than 1350°C is 4800 seconds or less.

Rough rolling process

Between the heating process and the finish rolling process, rough rolling may be performed on the slab. Rough rolling is sufficient as long as the desired sheet bar size can be obtained, and the conditions are not particularly limited.

(b) Finish rolling process

In the finish rolling process, the heated slab is finish rolled using a rolling mill having a plurality of stands. At this time, it is desirable to satisfy conditions (I) to (V) described below.

Additionally, it is preferable to perform descaling before finish rolling or during rolling between the rolling stands of finish rolling.

(Ⅰ) Finish rolling start temperature: 800℃ or higher

The finish rolling start temperature (entrance temperature of the first pass of finish rolling) is preferably 800°C or higher. If the finish rolling start temperature is less than 800°C, rolling in some of the plurality of rolling stands (particularly the entire stand) is performed at the two-phase temperature of ferrite + austenite. As a result, the processed structure may remain after finish rolling, and the strength and toughness of the hot rolled steel sheet may deteriorate. Therefore, it is preferable that the finish rolling start temperature is 800°C or higher.

In addition, the finish rolling start temperature is preferably set to 1100°C or lower in order to suppress coarsening of austenite and to preferably control the texture of the surface region and internal region.

(II) In each of the last four stands, σ expressed by the following formula (1): 40 to 80

Here, T is the temperature (°C) immediately before entering each stand (i.e., entrance temperature), ε is the equivalent plastic strain, and ε' is the strain rate.

The fact that σ is 40 to 80 in each of the last four stands means that σ of the fourth-to-last stand, σ of the third-to-last stand, σ of the second-to-last stand, and σ of the final stand are all 40. It can be rephrased as 80.

If there is even one stand with σ less than 40, the strain necessary for the development of the texture of the surface layer may not be appropriately provided to each of the last four stands. As a result, the pole density of the {001}<110>, {111}<110> and {112}<110> defense groups is preferably controlled in the aggregate structure of the surface or the area at a depth of 1/8 of the plate thickness from the surface. There are cases where it cannot be done. Therefore, it is desirable that 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 internal region cannot be preferably controlled, and the anisotropy of the toughness of the hot rolled steel sheet may increase. Therefore, it is desirable that σ in each of the last four stands is 80 or less.

In addition, ε, which is the equivalent plastic strain, can be obtained by ε = (2/√3) × (h/H) when the entry plate thickness is h and the exit plate thickness is H. Additionally, ε', which is the strain rate, can be obtained by ε'=ε/t when the rolling time is t(s). In addition, the rolling time t refers to the time during which the steel sheet and the rolling roll come into contact and strain is applied to the steel sheet.

(III) Inter-pass time between each of the last four stands: 0.2 to 10.0 seconds.

Among the last four stands, if there is at least one pass with an inter-pass time exceeding 10.0 seconds, recovery and re-determination between passes proceeds. As a result, accumulation of strain becomes difficult, and the desired structure may not be obtained in the hot rolled steel sheet. Therefore, it is desirable that the inter-pass time between each of the last four stands is 10.0 seconds or less.

It is desirable that the inter-pass time between each of the last four stands is shorter, but there are restrictions on shortening the inter-pass time in terms of the installation space and rolling speed of each stand. Additionally, if the inter-pass time between the last four stands is less than 0.2 seconds, unrecrystallized grains increase significantly, and the desired texture may not be obtained. Therefore, it is desirable to set it to 0.2 seconds or more.

In addition, the pass-to-pass time between each of the last four stands is 0.2 to 10.0 seconds, which means the pass-to-pass time between the fourth-to-last stand and the third-to-last stand, and the pass-to-pass time between the third-to-last stand and the second-to-last stand. It can be said that the pass time, the pass time between the second-to-last stand and the final stand, is all 0.2 to 10.0 seconds.

(Ⅳ) Cumulative reduction ratio of the last four stands: 60% or more

If the cumulative reduction ratio of the last four stands is less than 60%, the dislocation density introduced into the unrecrystallized austenite may be small. If the dislocation density introduced into unrecrystallized austenite becomes small, it becomes difficult to obtain the desired structure, and the strength and toughness of the hot rolled steel sheet may deteriorate. Therefore, it is desirable that the cumulative reduction ratio of the last four stands is 60% or more.

Additionally, if the cumulative reduction ratio of the last four stands exceeds 97%, the shape of the hot rolled steel sheet may deteriorate. Therefore, the cumulative reduction ratio of the last four stands may be 97% or less.

Additionally, the cumulative reduction ratio of the last four stands is {1-(t1/t0)}×100 when the entrance plate thickness of the fourth-to-last stand is t0 and the outlet plate thickness of the final stand is t1. It can be expressed as (%).

(V) Finish rolling completion temperature: 800 to 950°C

If the finish rolling finish temperature (exit temperature of the final stand) is less than 800°C, rolling is performed at the temperature of the two-phase region of ferrite + austenite. Therefore, the strength and toughness of the hot rolled steel sheet may decrease because the processed structure remains after rolling. Therefore, it is preferable that the finish rolling completion temperature is 800°C or higher.

Additionally, in the slab having the chemical composition according to the present embodiment, the unrecrystallized austenite region is generally in a temperature range of 950°C or lower. Therefore, when the finish rolling completion temperature exceeds 950°C, austenite grains grow and the martensite grain length of the hot rolled steel sheet obtained after cooling increases. As a result, it becomes difficult to obtain the desired texture, and the strength and toughness of the hot rolled steel sheet may decrease. Therefore, it is preferable that the finish rolling completion temperature is 950°C or lower.

(c) Cooling process

In the cooling process, it is preferable to start cooling within 1.0 seconds after completion of finish rolling and cool to a temperature range of 300°C or lower 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. do.

In this embodiment, it is preferable to install a cooling facility at the rear of the finish rolling facility and cool the steel sheet after finish rolling by passing it through this cooling facility. It is desirable that the cooling equipment be equipment that can cool the steel sheet at an average cooling rate of 100°C/s or more. Examples of such cooling equipment include water cooling equipment using water as a cooling medium.

The average cooling rate in the cooling process is the temperature drop of the steel sheet from the start of cooling to the end of cooling divided by the time required from the start of cooling to the end of cooling. The start of cooling is taken as the time of introduction of the steel sheet into the cooling facility, and the time of completion of cooling is taken as the time of delivery of the steel sheet from the cooling facility.

Additionally, cooling facilities include facilities that do not have an air-cooling section in the middle or facilities that have one or more air-cooling sections in the middle. In this embodiment, any cooling equipment may be used. Even when cooling equipment with an air cooling section is used, the average cooling rate from the start of cooling to the end of cooling is sufficient as long as 100°C/s or more.

Hereinafter, the reason for the limitation of cooling conditions will be explained. In addition, the cooling stop temperature is 300°C or lower, and this condition is explained in the winding process.

Cooling start time: Within 1.0 seconds after completion of finish rolling

After finish rolling is completed, it is desirable to start cooling immediately. If the cooling start time exceeds 1.0 seconds, recrystallization proceeds and cooling is performed in a state in which strain is released, and the desired texture may not be obtained in the hot rolled steel sheet. Therefore, it is desirable to start cooling within 1.0 seconds after completion of finish rolling.

Average cooling rate in the temperature range from the finish rolling completion temperature to 300℃: 100℃/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 tend to form, and the desired amount of martensite may not be obtained. Therefore, it is desirable 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 lower into a coil. Since coiling of the steel sheet is performed immediately after cooling, the coiling temperature is almost equal to the cooling stop temperature. If the coiling temperature exceeds 300°C, polygonal ferrite or bainite is generated, so the strength of the hot rolled steel sheet may decrease. Therefore, it is preferable that the coiling temperature is set to a temperature range of 300°C or lower.

In addition, after coiling, the hot rolled steel sheet may be temper rolled according to a conventional method, or may be pickled to remove scale formed on the surface. Alternatively, plating treatment such as aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electro-galvanizing, alloyed hot-dip galvanizing, etc., or chemical conversion treatment may be performed.

By using the preferred manufacturing method described above, the hot rolled steel sheet according to this embodiment can be stably manufactured.

Example

Next, examples of the present invention will be described. However, the conditions in the examples are examples of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to these examples of conditions. no. The present invention can adopt various conditions as long as the purpose of the present invention is achieved without departing from the gist of the present invention.

Molten steel with the chemical composition shown in Table 1 was melted in a converter, and a slab was obtained by continuous casting. Next, these slabs were heated under the conditions shown in Tables 2A and 2B, rough rolling was performed, and then finish rolling was performed under the conditions shown in Tables 2A and 2B. After finishing the finish rolling, it was cooled and coiled under the conditions shown in Tables 3A and 3B to obtain hot-rolled steel sheets with the thicknesses shown in Tables 3A and 3B.

In addition, in the heating process, the holding time at the heating temperature shown in Tables 2A and 2B was 4800 seconds or less.

In addition, cooling after finish rolling was performed by water cooling, and was performed by passing the steel sheet through water cooling equipment without an air cooling section in the middle. The average cooling rate in Tables 3A and 3B is the temperature drop of the steel sheet from the time of introduction of the water cooling equipment to the time of extraction of the water cooling equipment divided by the time required for the steel sheet to pass through the water cooling equipment.

A test piece was taken from the obtained hot-rolled steel sheet, and the area ratio of each structure and the ultimate density of the aggregate structure were measured and a tensile test was performed by the method described above.

The obtained results are shown in Tables 4A and 4B.

When the obtained tensile strength was 1180 MPa or more, it was judged to have high strength and was judged as passing. On the other hand, when the obtained tensile strength was less than 1180 MPa, it was judged to be disqualified as it did not have high strength.

As an evaluation of the toughness of the hot rolled steel sheet, the ductile-brittle transition temperature was measured by performing a Charpy impact test. The ductile-brittle transition temperature was measured using a C-direction notch Charpy impact test using a V-notch test piece of 2.5 mm subsize in accordance with JIS Z 2242:2018. The temperature at which the brittle fracture ratio was 50% was designated as the ductile-brittle transition temperature. In addition, for hot rolled steel sheets with a final plate thickness of less than 2.5 mm, the total thickness was measured.

When the obtained ductile-brittle transition temperature was -50°C or lower, the test was judged to have excellent toughness and passed. On the other hand, when the obtained ductile-brittle transition temperature exceeded -50°C, it was judged to be disqualified due to poor toughness.

Additionally, 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 a Charpy impact test using a V-notch test piece of 2.5 mm subsize. The Charpy impact test was conducted 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 when the difference was ±15 J or less, the anisotropy of toughness was determined to be reduced and the test was judged as passing. On the other hand, when the difference between the absorbed energy of the L-direction notch and the absorbed energy of the C-direction notch exceeded ±15J, the anisotropy of toughness was not reduced and was judged to be rejected.

[Table 1]

[Table 2A]

[Table 2B]

[Table 3A]

[Table 3B]

[Table 4A]

[Table 4B]

Looking at Tables 4A and 4B, it was found that the hot rolled steel sheet according to the example of the present invention had high strength and excellent toughness, and that the anisotropy of toughness was reduced. On the other hand, it was found that some of the properties of the hot rolled steel sheet according to the comparative example were deteriorated.

Claims (2)

Chemical composition, in mass%,
C: 0.100 to 0.500%,
Si: 0.100 to 3.000%,
Mn: 0.50 to 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 to 0.0060%,
Mo: 0 to 0.50%,
Cr: 0 to 1.00%,
V: 0 to 0.50%,
Cu: 0 to 0.50%,
Ni: 0 to 0.50%, and
Sn: 0 to 0.050%
Contains, the balance consists of Fe and impurities,
The metal structure in the area from 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 is expressed in area%,
90 to 100% martensite,
Consists of 0 to 10% of residual tissue,
In the texture of the surface or the area at a depth of 1/8 of the plate thickness from the surface,
{001}<110>, {111}<110> and {112}<110> defense forces have a polar density of 2.0 or more;
In the texture of the area from a depth of 1/8 of the plate thickness from the surface to a depth of 1/2 of the plate thickness from the surface,
The polar density of the {110}<112> orientation is 5.0 or less,
A hot-rolled steel sheet characterized by a tensile strength of 1180 MPa or more. According to paragraph 1,
The chemical composition is expressed in mass%,
Ti: 0.02 to 0.20%,
Nb: 0.010 to 0.100%,
Ca: 0.0001 to 0.0060%,
Mo: 0.01 to 0.50%,
Cr: 0.01 to 1.00%,
V: 0.01 to 0.50%,
Cu: 0.01 to 0.50%,
Ni: 0.01 to 0.50% and
Sn: 0.001 to 0.050%
A hot rolled steel sheet characterized in that it contains one or two or more types selected from the group consisting of.