Hot rolled steel plate

Abstract

This hot-rolled steel sheet has a prescribed chemical composition and metallographic structure; has an extreme density for the orientation group {001}<110>, {111}<110>, and {112}<110> in the texture in a surface layer region of at least 2.0; has an extreme density for the {110}<112> orientation in the texture in an internal region of not more than 5.0; and has a tensile strength of at least 1180 MPa.

Description

[Technical field]

[0001] The present invention relates to a hot rolled steel sheet. This application claims priority based on Japanese Patent Application No. 2021-168623, filed on October 14, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, automobiles have been made lighter by using high-strength steel plates in order to improve fuel efficiency and collision safety. However, increasing the strength of steel plates generally leads to a decrease in toughness. Therefore, in the development of high-strength steel plates, it is important to increase the strength without decreasing the toughness.

[0003] Generally, a method for improving toughness is known in which rolling is performed at a low temperature to impart a large accumulated strain to the unrecrystallized austenite state, but increasing the rolling reduction in the unrecrystallized austenite state increases the aspect ratio of the prior austenite grains, which increases the anisotropy of toughness.

[0004] For example, in Patent Document 1, in the center of the plate thickness, which is a portion of the steel plate defined by a 3 / 8 thickness position and a 5 / 8 thickness position from the surface of the steel plate, the {100} <011> ~{223} <110> The average X-ray random intensity ratio of the orientation group is 6.5 or less, and {332} <113> The present invention discloses a hot-rolled steel sheet characterized by having a texture in which the X-ray random intensity ratio of the crystal orientation is 5.0 or less, a total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and a microstructure in which the average grain size is 12.0 μm or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent No. 5621942 Summary of the Invention [Problem to be solved by the invention]

[0006] However, from the viewpoint of improving fuel economy and collision safety of automobiles, the technique disclosed in the above Patent Document 1 has room for further improvement in reducing the anisotropy of toughness in high-strength steel plates.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a hot-rolled steel sheet having high strength and excellent toughness with reduced anisotropy of toughness. [Means for solving the problem]

[0008] The present 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 having a tensile strength of 1180 MPa or more. The present inventors have found that different textures develop on the surface and inside of rolled steel sheets. The present 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 present inventors have found that in order to obtain a texture having a desired crystal orientation, it is effective to preferably control the hot-rolling conditions.

[0009] The gist of the present invention, which has been made based on the above findings, is as follows.

[0010] (1) A hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%, C: 0.100~0.500%, Si: 0.100 to 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 to 0.0060%, Mo: 0 to 0.50%, Cr: 0-1.00%, V: 0 to 0.50%, Cu: 0-0.50%, 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, The remaining structure is 0 to 10%. In the texture in the region from the surface to a depth of 1 / 8 of the sheet thickness from the surface, {001} <110> , {111} <110> and {112} <110> The pole density of the orientation group 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, {110} <112> The pole density of the orientation is 5.0 or less, The tensile strength is 1180MPa or more. (2) The hot-rolled steel sheet according to the above (1) has a chemical composition, in mass%, Ti: 0.02 to 0.20%, Nb: 0.010~0.100%, 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% The resin may contain one or more selected from the group consisting of: Effect of the Invention

[0011] According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, excellent toughness, and reduced anisotropy of toughness. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] 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 %.

[0013] The hot-rolled steel sheet according to the present embodiment has a chemical composition, in mass%, of 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.

[0014] 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.

[0015] Silicon: 0.100 to 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.

[0016] 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.

[0017] 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.

[0018] 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 increase 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 lower limit for the S content, excessively reducing the S content increases the production costs, so the S content may be 0.0005% or more, or 0.0010% or more.

[0019] 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 there is no particular restriction on the lower limit of the Al content, an excessively reduced Al content increases the production costs, and therefore the Al content may be set to 0.001% or more, or 0.005% or more.

[0020] 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 lower limit for 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.

[0021] 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, and elements that are permissible within a range that does not impair the properties of the hot-rolled steel sheet according to the present embodiment.

[0022] 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 a portion of Fe. In order to reduce alloy costs, it is not necessary to intentionally contain these optional elements in the steel, so the lower limit of the content of each of these optional elements is 0%.

[0023] Ti: 0.02 to 0.20% Ti is an element that is effective in 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 be 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, so the Ti content is set to 0.20% or less.

[0024] 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. 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 becomes saturated, so the Nb content is set to 0.100% or less.

[0025] Ca: 0.0001 to 0.0060% Ca is an element that disperses many fine oxides during deoxidation of molten steel and has the effect of refining the structure of hot-rolled steel sheets. Ca also fixes S in steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and reduces the anisotropy of the toughness of hot-rolled steel sheets. To reliably obtain these effects, the Ca content is preferably 0.0001% or more. On the other hand, if the Ca content exceeds 0.0060%, the above effect saturates, so the Ca content is set to 0.0060% or less.

[0026] 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 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.

[0027] Cr: 0.01 to 1.00% Cr is an element effective in improving the strength of hot-rolled steel sheets. 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.

[0028] V: 0.01 to 0.50% V improves the strength of hot-rolled steel sheet by strengthening 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, so the V content is set to 0.50% or less.

[0029] Cu: 0.01 to 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.

[0030] Ni: 0.01 to 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 in alloying, so adding a large amount of Ni increases the cost. Also, 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.

[0031] Sn: 0.001 to 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.

[0032] 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.

[0033] 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, the metal structure in the region from the surface to a depth of 1 / 8 of the sheet thickness from the surface is composed of 90 to 100% martensite and 0 to 10% of the remainder, in terms of area %, and the texture in the region from the surface to a depth of 1 / 8 of the sheet thickness from the surface is composed of {001} <110> , {111} <110> and {112} <110> The pole density of the orientation group is 2.0 or more, and in the texture in the region from the surface to the depth of 1 / 8 of the sheet thickness to the depth of 1 / 2 of the sheet thickness, the {110} <112> The orientation pole density is 5.0 or less.

[0034] In this embodiment, the area percentages of martensite and the remaining structure in the region from the surface to 1 / 8 of the sheet thickness depth to 3 / 8 of the sheet thickness depth from the surface are specified because the metal structure at this position shows a typical metal structure of a hot-rolled steel sheet. Each specification will be described in detail below.

[0035] Martensite area ratio: 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%.

[0036] In the present embodiment, martensite refers to fresh martensite and tempered martensite. Since there is no need to distinguish between fresh martensite and tempered martensite in the present embodiment, both are collectively referred to as martensite.

[0037] In addition, tempered martensite is obtained by tempering fresh martensite, and has a lower dislocation density than fresh martensite. In a preferred manufacturing method of the hot-rolled steel sheet according to the present embodiment described later, no heat treatment for tempering is performed after quenching, but tempered martensite may be generated during cooling after hot rolling or by reheating after coiling.

[0038] Area ratio of remaining tissue: 0-10% 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%.

[0039] 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 a hot-rolled steel plate (a region from 1 / 8 depth of the plate thickness from the surface to 3 / 8 depth of the plate thickness from the surface) and the center position of the plate width, so that the plate thickness cross section parallel to the rolling direction becomes the observation surface. The observation surface is mirror-polished and then etched with 3 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 etching. Each photographed field is 500 μm × 500 μm. The photographed photographs are subjected to image analysis 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.

[0040] 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., the Fe-based carbides are elongated in the same direction. Here, Fe-based carbides elongated in the same direction are defined as those whose elongation directions differ by within 5°.

[0041] Average grain size of prior austenite grains: over 5.0 μm, 30.0 μm or less In the hot rolled steel sheet according to this embodiment, the average grain size of the prior austenite grains may be more than 5.0 μm and 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 the prior austenite grains more than 5.0 μm, it is possible to stably obtain the predetermined texture required in this embodiment, and it is possible to further reduce the anisotropy of the toughness of the hot rolled steel sheet. The average grain size of the 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, and therefore the average grain size of the prior austenite grains is preferably 30.0 μm or less.

[0042] 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 cross section of the thickness 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 an area where about 10,000 crystal grains are observed in one field of view. Image analysis is performed on the photographed images 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 performed on all prior austenite grains except for prior austenite grains where the entire crystal grain is not included in the photographed field of view, such as the end 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 obtained prior austenite grains by the total number of prior austenite grains whose grain sizes are measured. This operation is performed for each photographed field 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.

[0043] {001} in the texture in the region from the surface to 1 / 8 of the plate thickness <110> , {111} <110> and {112} <110> Pole density of orientation group: 2.0 or more The {001} <110> , {111} <110> and {112} <110> If the pole density of the orientation group 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 {001} <110> , {111} <110> and {112} <110> The pole density of the orientation group is 2.0 or more, preferably 2.2 or more, 2.5 or more, or 2.7 or more. The {001} texture in the surface region <110> , {111} <110> and {112} <110> The upper limit of the pole density of the orientation group is not particularly specified, but from the viewpoint of suppressing deterioration of ductility, it may be 9.0 or less, 8.0 or less, 7.0 or less, or 5.0 or less.

[0044] {110} in the texture in the region from 1 / 8 of the plate thickness to 1 / 2 of the plate thickness from the surface <112> Orientation pole density: 5.0 or less The {110} texture in the region from the surface to 1 / 8 of the plate thickness to 1 / 2 of the plate thickness (hereinafter sometimes referred to as the internal region) <112> If the pole density of the orientation exceeds 5.0, the toughness anisotropy of the hot-rolled steel sheet becomes high. Therefore, the {110} <112> The pole density of the orientation is 5.0 or less, preferably 4.6 or less, 4.2 or less, or 4.0 or less. {110} in the texture of the inner region <112> The lower limit of the orientation pole density is not particularly specified, but from the viewpoint of suppressing deterioration in strength, it may be set to 2.0 or more, or 2.5 or more.

[0045] The pole density was measured using a combination of a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by AMETEK. The orientation distribution function (ODF: Orientation Distribution Function) that represents the three-dimensional texture was calculated using the orientation data measured by the EBSD (Electron Back Scattering Diffraction) method and spherical harmonic functions, and the {001} <110> , {111} <110> and {112} <110> The pole density of the orientation group and the {110} <112> Find the pole density of .

[0046] 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.

[0047] {hkl} are crystal planes parallel to the rolling plane, <uvw>represents the crystal direction parallel to the rolling direction, i.e., {hkl} <uvw>are in the normal direction to the plate surface {hkl} and in the rolling direction {hkl}. <uvw>This indicates the crystal that is oriented in the direction.

[0048] 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 plate 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 plate, 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 extension direction of the inclusion is judged to be the rolling direction.

[0049] Tensile strength: 1180MPa 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 1250 MPa or more, 1300 MPa or more, 1350 MPa or more, or 1400 MPa or more. There is no particular upper limit for the tensile strength, but it is preferably 2000 MPa or less, 1600 MPa or less, 1500 MPa or less, or 1400 MPa or less.

[0050] The tensile strength is measured in accordance with JIS Z 2241: 2011. The test piece is a No. 5 test piece of JIS Z 2241: 2011, and the test direction is perpendicular to the rolling direction.

[0051] 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 becomes difficult to control the texture, and it may become difficult to obtain the above-mentioned texture. Therefore, the sheet thickness may be 8.0 mm or less.

[0052] The hot-rolled steel sheet according to the present embodiment may have a plating layer on its 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 a hot-dip galvannealed layer.

[0053] Next, a preferred method for producing the hot-rolled steel sheet according to the present embodiment will be described. The preferred method for producing the hot-rolled steel sheet according to the present 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.

[0054] (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 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) just 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 not less than 60%. (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) After cooling, the sheet is wound up in a winding process. Each step will be described below.

[0055] (a) Heating process 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 produced 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.

[0056] 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 (e.g., tensile strength, toughness, hole expandability, etc.) of the hot-rolled steel sheet. 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 causes 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.

[0057] The slab is heated to a temperature range of 1100°C or more 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 the like are likely to occur in the subsequent finish rolling process, and the surface quality of the hot-rolled steel sheet may deteriorate. Therefore, the holding time in the temperature range of 1100°C or more and less than 1350°C is preferably 4800 seconds or less.

[0058] Rough rolling process Between the heating step and the finish rolling step, 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.

[0059] (b) Finish rolling process In the finish rolling step, the heated slab is finish rolled using a rolling mill having multiple stands, which preferably satisfies 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.

[0060] (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 less 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.

[0061] (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) just before entering each stand (i.e., the entry temperature), ε is the equivalent plastic strain, and ε' is the strain rate. The σ of each of the last four stands being between 40 and 80 can be rephrased as 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 final stand are all between 40 and 80.

[0062] If there is even one stand with σ less than 40, the strain required for the development of the texture in the surface layer region may not be appropriately imparted in each of the last four stands. As a result, the texture in the region from the surface to 1 / 8 of the sheet thickness depth from the surface may be dominated by {001} <110> , {111} <110> and {112} <110> In some cases, it may not be possible to favorably control the pole density of the orientation group, so it is preferable 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 in the inner region cannot be controlled favorably, 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.

[0063] The equivalent plastic strain, ε, can be calculated by ε = (2 / √3) × (h / H), where h is the entry thickness and H is the exit thickness. Furthermore, the strain rate, ε', 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 sheet comes into contact with the rolling rolls and strain is applied to the steel sheet.

[0064] (III) Inter-pass time between each of the last four stands: 0.2 to 10.0 seconds If the interpass time between the last four stands 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. In addition, 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 inter-pass times between each of the last four stands being 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.

[0065] (IV) Cumulative reduction 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 deteriorate. Therefore, it is preferable that the cumulative reduction rate of the last four stands is 60% or more. 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.

[0066] The cumulative reduction ratio 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.

[0067] (V) Finish rolling completion temperature: 800~950℃ If the finish rolling end temperature (the temperature at the exit of the final stand) is less than 800°C, the rolling will be performed at a two-phase temperature range 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, it is preferable to set the finish rolling end temperature to 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.

[0068] (c) Cooling process 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.

[0069] In this embodiment, it is preferable to install a cooling facility after the finish rolling facility and pass the finish-rolled steel sheet through the cooling facility to perform cooling. The cooling facility is preferably a facility 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 using water as a cooling medium.

[0070] 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. Either type of cooling equipment may be used in this embodiment. Even when using cooling equipment with 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.

[0071] 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 coiling process.

[0072] Cooling start time: Within 1.0 seconds after finishing rolling is completed 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, which may make it impossible to obtain a desired texture in the hot-rolled steel sheet. Therefore, it is preferable to start cooling within 1.0 second after the completion of finish rolling.

[0073] Average cooling rate in the temperature range from the finish rolling 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.

[0074] (d) Winding process In the coiling process, it is preferable to coil the steel sheet cooled to a temperature range of 300°C or less. Since the steel sheet is coiled 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, which may reduce the strength of the hot-rolled steel sheet. Therefore, it is preferable to set the coiling temperature to a temperature range of 300°C or less.

[0075] After coiling, the hot-rolled steel sheet may be subjected to temper rolling according to a conventional method, or pickling to remove scale formed on the surface, or may be further subjected to plating treatment such as aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrogalvanizing, or alloyed hot-dip galvanizing, or chemical conversion treatment.

[0076] By the preferable manufacturing method described above, the hot rolled steel sheet according to this embodiment can be stably manufactured. EXAMPLES

[0077] Next, an embodiment of the present invention will be described, but 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 can 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.

[0078] 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 under the conditions shown in Tables 3A and 3B, and coiled 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.

[0079] The cooling after the finish rolling was by water cooling, and was performed by passing the steel sheet 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 values ​​obtained by dividing the temperature drop of the steel sheet from the time the water cooling facility was introduced to the time the water cooling facility was removed by the time the steel sheet needed to pass through the water cooling facility.

[0080] 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.

[0081] When the obtained tensile strength was 1180 MPa or more, it was judged to have high strength and to pass the test, whereas when the obtained tensile strength was less than 1180 MPa, it was judged to not have high strength and to fail the test.

[0082] To evaluate the toughness of hot-rolled steel sheets, a Charpy impact test was conducted to measure the ductile-brittle transition temperature. The ductile-brittle transition temperature was measured in accordance with JIS Z 2242:2018, using a 2.5mm sub-size 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 2.5mm, the measurement was performed over the entire thickness.

[0083] When the obtained ductile-brittle transition temperature was -50°C or lower, the toughness was judged to be excellent and the specimen was judged to pass the test. On the other hand, when the obtained ductile-brittle transition temperature was higher than -50°C, the toughness was judged to be poor and the specimen was judged to fail the test.

[0084] 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 L-direction notch were measured by Charpy impact testing using a 2.5 mm sub-size V-notch test piece. 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.

[0085] [Table 1]

[0086] [Table 2A]

[0087] [Table 2B]

[0088] [Table 3A]

[0089] [Table 3B]

[0090] [Table 4A]

[0091] [Table 4B]

[0092] 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, whereas the hot-rolled steel sheets according to the comparative examples are deteriorated in any of the properties.< / uvw> < / uvw> < / uvw>

Claims

1. 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: 0-0.050% 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.

2. 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: 0.001-0.050% The hot-rolled steel sheet according to claim 1, further comprising one or more selected from the group consisting of:

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