Hot-rolled steel sheet and method for producing same

Abstract

Provided is a hot-rolled steel sheet which has low strength before heat treatment and for which it is possible to control the hardness thereof after heat treatment to within a desired range, even when shortening the high temperature-holding time during the heat treatment. The present invention properly controls the component composition of said sheet so as to have a structure in which the percentage by area of ferrite is 40-80%, inclusive, the percentage by area of pearlite is 20-60%, inclusive, the average particle diameter of the ferrite is 5-20μm, inclusive, the percentage by number of ferrite adjacent to pearlite along at least 30% of its peripheral length, among all the ferrite, is 40-100%, inclusive, and the percentage by area of pseudo-pearlite, among all the pearlite, is 30-100%, inclusive.

Description

Hot-rolled steel sheet and method for producing the same

[0001] The present invention relates to a hot-rolled steel sheet and a method for producing the same.

[0002] Steel sheets are often used for members such as structures. Such members are manufactured, for example, by forming a steel sheet into a predetermined shape in a low-strength state, then heating and quenching (hardening) the formed steel sheet, and further performing tempering heat treatment as necessary to obtain a desired hardness. That is, for a steel sheet (hereinafter also referred to as a heat-treatment steel sheet) that is a raw material for a member (hereinafter also referred to as a heat-treated member) manufactured through the above heat treatment, it is required to have high formability, that is, low strength before the heat treatment, while obtaining a desired hardness after the heat treatment.

[0003] As a technique related to the above heat-treated members and heat-treatment steel sheets, Patent Document 1 discloses "a steel member containing 0.010 to 0.120% of Ti in mass%, and 0.005% or more of Ti precipitates as precipitates having a particle size of 20 nm or less in the structure."

[0004] Also, Patent Documents 2 and 3 disclose techniques related to the above heat-treated members and heat-treatment steel sheets.

[0005] International Publication No. 2018 / 186274, Japanese Unexamined Patent Application Publication No. 2019-44219, International Publication No. 2015 / 146173

[0006] By the way, in recent years, from the perspective of global environmental conservation, further reduction of CO ,

[0007] emissions has been sought in the entire industry. Here, in the above heat treatment, the hardness is increased by quenching after heating. Therefore, in the above heat treatment, it is necessary to heat the steel sheet to the austenite region. Also, in the above heat treatment, after heating the steel sheet to the austenite region, in order to sufficiently perform the transformation from ferrite to austenite, it is held in the austenite region. Since a large amount of energy is required for this holding in the austenite region, by shortening the holding time in the austenite region (hereinafter also referred to as the high-temperature holding time), a significant reduction in the amount of energy and thus in the CO ２ emissions can be expected.

[0007] However, in the technologies described in Patent Documents 1 to 3, if the high-temperature holding time is shortened during heat treatment, the desired hardness cannot be stably obtained in the heat-treated member. Therefore, there is a need for the development of a steel sheet, particularly a hot-rolled steel sheet, that has low strength before heat treatment and can control the hardness of the heat-treated member, i.e., the steel sheet after heat treatment (hereinafter also referred to as post-heat treatment hardness), to a desired range, even when the high-temperature holding time is shortened.

[0008] The present invention was developed to meet the above-mentioned requirements, and aims to provide a hot-rolled steel sheet that has low strength before heat treatment and can control the hardness after heat treatment to a desired range even when the high-temperature holding time during heat treatment is shortened, along with an advantageous manufacturing method thereof. In this specification, any numerical range expressed using "~" means a range that includes the numerical values ​​written before and after "~" as the lower limit and upper limit, respectively.

[0009] Here, the target range for the strength of the hot-rolled steel sheet before heat treatment (hereinafter also referred to as the strength before heat treatment) is preferably a yield strength of 350 MPa or more and 550 MPa or less. The target range for the hardness after heat treatment is preferably a Vickers hardness of 370 or more and 550 or less. The measurement procedures for yield strength and other parameters, as well as the heat treatment conditions, are as described in the examples below.

[0010] The inventors diligently conducted research to achieve the above objective. As a result, they obtained the following findings. Specifically, the thickness of the hot-rolled steel sheet is, for example, between 1.0 mm and 8.0 mm. Here, in order to control the hardness after heat treatment within the desired range when shortening the high-temperature holding time during heat treatment, it is necessary to accelerate the transformation from ferrite to austenite as much as possible during heating in the heat treatment. However, in the ferrite that remains until just before the transformation from ferrite to austenite (hereinafter also called the ferrite → austenite transformation) is completed, the carbon (C) in the ferrite becomes dilute, and the transformation temperature from ferrite to austenite rises. Therefore, when the high-temperature holding time during heat treatment is shortened, the sheet is rapidly cooled before the transformation from ferrite to austenite is completely finished. As a result, ferrite remains in the structure of the steel sheet after rapid cooling, i.e., the heat-treated member, and the hardness decreases.

[0011] Based on the above findings, the inventors conducted further studies. As a result, the inventors found that in order to achieve the above objectives, it is important to control the microstructure of the hot-rolled steel sheet as follows: - Finely mill the ferrite and place pearlite, which serves as a source of carbon, adjacent to the ferrite. - Ensure a certain amount of pseudo-pearlite, which has a fast cementite dissolution rate, in the pearlite. - Specifically, it is important to have a microstructure of the hot-rolled steel sheet such that the area ratio of ferrite is 40% to 80%, the area ratio of pearlite is 20% to 60%, the average particle size of ferrite is 5 μm to 20 μm, the number ratio of ferrite particles whose circumference is 30% or more adjacent to pearlite is 40% to 100%, and the area ratio of pseudo-pearlite among the pearlite is 30% to 100% (hereinafter also referred to as F-P fine dispersion microstructure). The present invention was completed by further studies based on the above findings.

[0012] In other words, the gist of the present invention is as follows:

[0013] 1. The composition is as follows, in mass%, C: 0.16% to 0.32%, Si: 0.85% or less, Mn: 0.50% to 2.50%, P: 0.04% or less, S: 0.0060% or less, Al: 0.01% to 0.08%, N: 0.0150% or less, Ti: 0.005% to 0.070%, Ca: 0.0001% to 0.0060%, Cr: 0.03% to 1.50%, and B: 0.0002% to 0.0050%, with the remainder being Fe and unavoidable impurities, with a ferrite area ratio of 40% to 80%, a pearlite area ratio of 20% to 60%, and an average particle size of the ferrite of 5 μm to 20 μm. A hot-rolled steel sheet having a structure in which, of the ferrite, 30% or more of the circumference is adjacent to the pearlite, with a number ratio of ferrite between 40% and 100%, and the area ratio of pseudo-pearlite among the pearlite is between 30% and 100%.

[0014] 2. The hot-rolled steel sheet according to claim 1, wherein the component composition, in mass%, further comprises at least one of the following groups A to D. - Group A: At least one selected from V: 0.2% or less, Nb: 0.07% or less, Mo: 0.3% or less, Zr: 0.05% or less, Hf: 0.05% or less, and W: 0.05% or less. - Group B: At least one selected from Cu: 0.5% or less and Ni: 0.5% or less. - Group C: At least one selected from Mg: 0.01% or less, REM: 0.1% or less, and Co: 0.01% or less. - Group D: At least one selected from Sb: 0.1% or less, Sn: 0.1% or less, As: 0.1% or less, Ta: 0.1% or less, Pb: 0.1% or less, Cs: 0.1% or less, Te: 0.1% or less, Bi: 0.1% or less, Zn: 0.1% or less, Ge: 0.1% or less, and Sr: 0.1% or less.

[0015] 3. A method for manufacturing a hot-rolled steel sheet, comprising: a heating step of heating a steel material having the component composition described in 1 or 2 above; a hot-rolling step of subjecting the steel material after the heating step to rough-rolling and finish-rolling to obtain a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step; and a winding step of winding the hot-rolled steel sheet after the cooling step, wherein the heating temperature of the steel material in the heating step is 1150°C or less, the completion temperature of the rough-rolling in the hot-rolling step is 950°C or less, the completion temperature of the finish-rolling in the hot-rolling step is 790°C or more and 920°C or less, the exit-side sheet-passing speed of the finish-rolling in the hot-rolling step is 400 m / min or more, the average cooling rate in the cooling step is 40°C / second or more, the cooling stop temperature in the cooling step is Bs point - 20°C or more and less than 600°C, and the winding temperature in the winding step is Bs point - 20°C or more and less than 600°C.

[0016] 4. A heating step of heating a steel material having the component composition described in 1 or 2 above; a hot rolling step of subjecting the steel material after the heating step to rough rolling and finish rolling to obtain a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot rolling step; a winding step of winding the hot-rolled steel sheet after the first cooling step; and a second cooling step of cooling the hot-rolled steel sheet after the winding step, wherein the heating temperature of the steel material in the heating step is 1150°C or less, the completion temperature of the rough rolling in the hot rolling step is 950°C or less, the completion temperature of the finish rolling in the hot rolling step is 790°C or more and 920°C or less, the exit speed of the sheet during finish rolling in the hot rolling step is 400 m / min or more, the average cooling rate in the first cooling step is 40°C / second or more, and the cooling stop temperature in the first cooling step is 600°C or more and 680°C or less. A method for manufacturing a hot-rolled steel sheet, wherein the winding temperature in the winding step is 600°C or higher and 680°C or lower, and the average cooling rate to 200°C in the second cooling step is 50°C / hour or higher.

[0017] According to the present invention, a hot-rolled steel sheet can be obtained that has low strength before heat treatment, and whose hardness after heat treatment can be controlled to a desired range even when the high-temperature holding time during heat treatment is shortened. When heat treatment is performed using the hot-rolled steel sheet of the present invention as a material to manufacture a heat-treated component, the high-temperature holding time can be shortened, thus reducing the amount of energy and, consequently, CO2. ２ Significant reductions in emissions are expected, making it extremely advantageous from an industrial perspective.

[0018] This is a schematic diagram showing an example of the form of a perlite block that falls under the category of pseudo-perlite.

[0019] The present invention will be described based on the following embodiments.

[0020] [1] Hot-rolled steel sheet First, the component composition of a hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that all units in the component composition are "mass%", and unless otherwise specified, they will be simply referred to as "%".

[0021] C: 0.16% to 0.32% Carbon (C) is an important element for ensuring the desired hardness after heat treatment. Furthermore, C is also important for suppressing the formation of a soft structure during rapid cooling after heat treatment. Therefore, the C content is 0.16% or more, preferably 0.17% or more. On the other hand, if the C content exceeds 0.32%, controlling the morphology of cementite in pearlite becomes difficult, and the above-mentioned F-P fine dispersion structure cannot be obtained. It also becomes difficult to obtain the desired pre-heat treatment strength. Therefore, the C content is 0.32% or less, preferably 0.29% or less.

[0022] Si: 0.85% or less. Si is an element that increases hardenability. Si has the effect of improving the stability of hardness after heat treatment. However, if the Si content is excessive, the surface quality deteriorates. For this reason, the Si content is 0.85% or less, preferably 0.75% or less. The Si content may be 0%. Also, Si may be present as an unavoidable impurity. The Si content is preferably 0.002% or more, more preferably 0.005% or more.

[0023] Mn: 0.50% to 2.50% Mn is an element that increases hardenability and contributes to an increase in hardness after heat treatment. Therefore, the Mn content is 0.50% or more, preferably 0.60% or more. On the other hand, if the Mn content exceeds 2.50%, the transformation behavior in the hot rolling process changes, and the desired microstructure cannot be obtained. Therefore, the Mn content is 2.50% or less, preferably 2.30% or less.

[0024] P: 0.04% or less. P is a harmful element that reduces the toughness of heat-treated members by segregating at grain boundaries. Therefore, it is preferable to reduce P as much as possible, but a P content of up to 0.04% is acceptable. Thus, the P content is 0.04% or less, preferably 0.03% or less. The P content may also be 0%. However, P may inevitably be mixed in during manufacturing, and excessive removal of P leads to increased costs. Therefore, the P content is preferably 0.002% or more.

[0025] S: 0.0060% or less. S is a harmful element that reduces the toughness of heat-treated members by forming wedge-shaped inclusions. Therefore, it is preferable to reduce S as much as possible, but an S content of up to 0.0060% is acceptable. Thus, the S content is 0.0060% or less, preferably 0.0040% or less. The S content may be 0%. However, S may be unavoidably introduced during manufacturing, and excessive removal of S leads to increased costs. Therefore, the S content is preferably 0.0001% or more.

[0026] Al: 0.01% to 0.08% Al may be added as a deoxidizing agent during the steelmaking process. Therefore, the Al content is 0.01% or more. On the other hand, if Al is included in excess, Al oxides will be formed, reducing the toughness of the heat-treated member. Therefore, the Al content is 0.08% or less, preferably 0.07% or less.

[0027] N: 0.0150% or less. N is a harmful element that reduces the toughness of heat-treated members by combining with Ti to form coarse TiN. Therefore, it is preferable to reduce N as much as possible, but an N content of up to 0.0150% is acceptable. Thus, the N content is 0.0150% or less, preferably 0.0090% or less, and more preferably 0.0070% or less. The N content may be 0%. However, N may inevitably be mixed in during manufacturing, and excessive removal of N leads to increased costs. Therefore, the N content is preferably 0.0005% or more.

[0028] Ti: 0.005% or more and 0.070% or less. Ti has the effect of suppressing room-temperature aging by solid-solution N by bonding with N. To obtain this effect, the Ti content is 0.005% or more, preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.070%, carbides containing coarse Ti will be dispersed, reducing the toughness of the heat-treated member. For this reason, the Ti content is 0.070% or less, preferably 0.060% or less.

[0029] Ca: 0.0001% or more and 0.0060% or less. Ca, by bonding with S, reduces wedge-shaped S-type inclusions and has the effect of improving the toughness of the heat-treated member. To obtain this effect, the Ca content is 0.0001% or more, preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0060%, the above effect becomes saturated. Therefore, the Ca content is 0.0060% or less, preferably 0.0050% or less.

[0030] Cr: 0.03% or more and 1.50% or less. Cr is an element that increases hardenability and contributes to an increase in hardness after heat treatment. Therefore, the Cr content is 0.03% or more, preferably 0.05% or more. On the other hand, if the Cr content exceeds 1.50%, the above effect saturates. Therefore, the Cr content is 1.50% or less, preferably 1.25% or less.

[0031] B: 0.0002% or more and 0.0050% or less. B is an element that increases hardenability and contributes to an increase in hardness after heat treatment. Therefore, the B content is 0.0002% or more, preferably 0.0005% or more. On the other hand, if the B content exceeds 0.0050%, the above effect saturates. Therefore, the B content is 0.0050% or less, preferably 0.0040% or less.

[0032] The basic elements (hereinafter also referred to as basic component elements) of the composition of a hot-rolled steel sheet according to one embodiment of the present invention have been described above. In addition to the basic component elements described above, a hot-rolled steel sheet according to one embodiment of the present invention may also have at least one of the following groups A to D as optional additive elements. - Group A: At least one selected from V: 0.2% or less, Nb: 0.07% or less, Mo: 0.3% or less, Zr: 0.05% or less, Hf: 0.05% or less, and W: 0.05% or less. - Group B: At least one selected from Cu: 0.5% or less and Ni: 0.5% or less. - Group C: At least one selected from Mg: 0.01% or less, REM: 0.1% or less, and Co: 0.01% or less. - Group D: At least one selected from Sb: 0.1% or less, Sn: 0.1% or less, As: 0.1% or less, Ta: 0.1% or less, Pb: 0.1% or less, Cs: 0.1% or less, Te: 0.1% or less, Bi: 0.1% or less, Zn: 0.1% or less, Ge: 0.1% or less, and Sr: 0.1% or less.

[0033] All elements in Group A suppress the ferrite-to-austenite transformation during heat treatment by forming precipitates. However, the content of each element in Group A is acceptable as long as it is within the above range. Therefore, the content of each element in Group A is preferably within the above range. The content of each element in Group A may be 0%. In addition, each element in Group A may be present as an unavoidable impurity. The content of each element in Group A may be 0.001% or more.

[0034] All elements in Group B are particularly effective in lowering the temperature of the austenite-ferrite transformation and increasing hardenability. However, if the content of Group B elements is excessive, adverse effects such as changes in the microstructure of the hot-rolled steel sheet and an excessive increase in strength may occur. Therefore, the content of each element in Group B is preferably within the above range. The content of each element in Group B may be 0%. In addition, each element in Group B may be present as an unavoidable impurity. The content of each element in Group B may be 0.001% or more.

[0035] The elements in group C are expected to improve the toughness of heat-treated members by changing the morphology of inclusions. However, the effect of each element in group C saturates even if included in large quantities. Furthermore, it may cause deterioration of various properties such as weldability. For this reason, the content of each element in group C is preferably within the above range. The content of each element in group C may be 0%. In addition, each element in group C may be included as an unavoidable impurity. The content of each element in group C may be 0.0001% or more. Note that REM is a collective term for 17 elements, consisting of the 15 lanthanide elements from atomic number 57 La (lanthanum) to atomic number 71 Lu (lutetium), Sc (scandium) with atomic number 21, and Y (yttrium) with atomic number 39. These 17 elements can be included individually or in combination. Note that the REM content refers to the total content of these 17 elements.

[0036] Elements in Group D may be present as unavoidable impurities depending on the raw materials used. However, the content of each element in Group D is acceptable as long as it is within the above range. Therefore, the content of each element in Group D is preferably within the above range. The content of each element in Group D may be 0%. Alternatively, the content of each element in Group D may be 0.0001% or more.

[0037] The remainder of the elements other than those mentioned above consists of Fe and unavoidable impurities. In other words, the component composition of a hot-rolled steel sheet according to one embodiment of the present invention consists of the above basic component elements, the above optional additive elements, Fe, and unavoidable impurities.

[0038] Next, the reasons for limiting the microstructure of the hot-rolled steel sheet according to one embodiment of the present invention will be explained. Note that the area ratio of each phase is the area ratio that each phase occupies to the entire microstructure.

[0039] Ferrite area ratio (hereinafter also referred to as F area ratio): 40% or more and 80% or less. Ferrite is a soft structure. Here, in order to ensure the formability required for heat-treated steel sheets, it is necessary to lower the strength, in particular, to a yield strength of 550 MPa or less. For this reason, the F area ratio is 40% or more, preferably 50% or more. In addition, in order to ensure the area ratio of pearlite, which will be described later, the F area ratio is 80% or less, preferably 75% or less.

[0040] Perlite area ratio (hereinafter also referred to as P area ratio): 20% or more and 60% or less. From the viewpoint of ensuring the formability required for heat-treated steel sheets, the P area ratio is 20% or more, preferably 25% or more. Also, the P area ratio is 60% or less, preferably 50% or less.

[0041] Here, pearlite includes pearlite in the narrow sense and pseudo-pearlite. Pearlite in the narrow sense is a lamellar structure in which ferrite (hereinafter also referred to as lamellar ferrite to distinguish it from the aforementioned ferrite) and cementite are alternately arranged in layers. Pseudo-pearlite has lamellar ferrite and cementite, but they are arranged in a state where they are not completely layered. In particular, it is an organization in which cementite with an aspect ratio of 5 or less is dispersed and arranged. The determination of whether it corresponds to pearlite in the narrow sense or pseudo-pearlite is performed, for example, in units of pearlite blocks. As shown in FIG. 1, a pearlite block is composed of three or more cementites with a major diameter of 0.2 μm or more and a shortest distance between each cementite of 0.75 μm or less, and lamellar ferrite located between the cementites. The boundary of the pearlite block is defined, for example, by connecting the outermost peripheral portions (portions adjacent to other tissues such as ferrite) of the cementites with the shortest distance so that all the cementites constituting the pearlite block are included. If a pearlite block contains three or more cementites with an aspect ratio of 5 or less and two or less cementites with an aspect ratio exceeding 5, the pearlite block is determined to be pseudo-pearlite. In other cases, the pearlite block is determined to be pearlite in the narrow sense. Also, in the present disclosure, when simply referring to pearlite, it means pearlite in the broad sense, that is, including both pearlite in the narrow sense and pseudo-pearlite.

[0042] The structure of the hot-rolled steel sheet according to an embodiment of the present invention may include a remaining structure other than ferrite and pearlite (hereinafter also simply referred to as the remaining structure). The area ratio of the remaining structure is preferably 2% or less, more preferably 1% or less. The area ratio of the remaining structure may be 0%. Examples of the remaining structure include martensite, bainite, and retained austenite.

[0043] The F area ratio and P area ratio can be measured according to conventional methods. For example, a sample is cut from a hot-rolled steel sheet so that the cross section parallel to the rolling direction (hereinafter also referred to as the L section) becomes the observation surface. Then, the observation surface of the sample is etched with 1 volume% nital. Next, the observation surface of the sample is photographed with a scanning electron microscope (SEM) at 2000x magnification for 10 fields, with the center being at the 1 / 4 thickness position in the thickness direction of the hot-rolled steel sheet. In the SEM image, ferrite is observed as gray contrast crystal grains in which no corrosion traces are observed within the crystal grains. Cementite is observed as white contrast precipitates in the SEM image. Perlite (perlite in the narrow sense and pseudo-perlite) is observed and defined as described above. Next, using image analysis software (Photoshop elements and Image J), ​​the F area ratio and P area ratio are calculated from the total area occupied by ferrite and pearlite in 10 fields of view of the SEM images. The area ratio of the remaining tissue is then calculated by subtracting the F area ratio and P area ratio obtained above from 100%.

[0044] The average grain size of ferrite (hereinafter also referred to as F grain size): 5 μm or more and 20 μm or less. The F grain size is related to the strength of the hot-rolled steel sheet before heat treatment and the time required for the ferrite-to-austenite transformation to be completed during heat treatment (hereinafter also referred to as the transformation completion time). That is, if the F grain size is less than 5 μm, the yield strength increases excessively and the formability decreases. For this reason, the F grain size is 5 μm or more, preferably 6 μm or more. On the other hand, if the F grain size exceeds 20 μm, the transformation temperature from ferrite to austenite rises and the transformation completion time increases. For this reason, when shortening the high-temperature holding time during heat treatment, it becomes impossible to control the hardness after heat treatment within the desired range. For this reason, the F grain size is 20 μm or less, preferably 18 μm or less.

[0045] The F particle size can be measured by the cutting method, for example, using the SEM photographs used for measuring the F area ratio and the P area ratio. That is, on the SEM photograph, draw 10 test lines each with an actual length of 35 μm in the vertical direction (plate thickness direction) and the horizontal direction (rolling direction) at a fixed interval, for example, an interval of 3 μm. Then, in the above SEM photograph, obtain the average value of the line segment lengths of the test lines passing through each ferrite (crystal grain) and set it as the F particle size.

[0046] Among the ferrites, the number ratio of ferrites whose perimeter is 30% or more adjacent to pearlite (also referred to as the F-P adjacent ratio): 40% or more and 100% or less. During the above heat treatment, pearlite transforms into austenite earlier than ferrite. Therefore, when shortening the required time for transformation completion, that is, shortening the high-temperature holding time during heat treatment, promoting the ferrite → austenite transformation is important for controlling the hardness after heat treatment within the desired range. For this purpose, it is effective to refine the ferrite and then arrange pearlite, which serves as a C supply source, at a position adjacent to the ferrite. Therefore, the F-P adjacent ratio is 40% or more, preferably 50% or more. Also, since a higher F-P adjacent ratio is desirable, the upper limit of the F-P adjacent ratio may be 100%, and the F-P adjacent ratio is preferably 80% or less.

[0047] The F-P adjacent ratio can be calculated, for example, by selecting any 200 ferrites (crystal grains) as the target using the SEM photograph used for measuring the F area ratio and the P area ratio, and dividing the number of ferrites whose perimeter is 30% or more adjacent to pearlite by the number of ferrites selected as the target (200).

[0048] In pearlite, the area ratio of pseudo-pearlite (hereinafter also referred to as the pseudo-P ratio): 30% or more and 100% or less. When shortening the high-temperature holding time in heat treatment, it is important to ensure a certain amount of pseudo-pearlite, which has a high cementite dissolution rate, in order to control the hardness after heat treatment within the desired range. As mentioned above, pseudo-pearlite contains lamellar ferrite and cementite, but these are not arranged in a completely layered manner, and some of the cementite is arranged in a fragmented state. Therefore, the cementite in pseudo-pearlite has a larger area adjacent to lamellar ferrite compared to cementite in pearlite in the strict sense, and therefore has a higher dissolution rate during heat treatment. Thus, pseudo-pearlite contributes more effectively to promoting the ferrite → austenite transformation during heat treatment than pearlite in the strict sense. For this reason, the pseudo-P ratio is 30% or more, preferably 40% or more. Also, since a higher pseudo-P ratio is desirable, the upper limit of the pseudo-P ratio may be 100%, and the pseudo-P ratio is preferably 75% or less.

[0049] The pseudo-perlite ratio can be calculated, for example, in an SEM image used to measure the F area ratio and P area ratio, by dividing the total area occupied by pseudo-perlite by the total area occupied by perlite (perlite in the narrow sense + pseudo-perlite), and multiplying by 100, as shown in the following formula: [Pseudo-perlite ratio (%)] = [Total area occupied by pseudo-perlite (μm)] ２ ) ÷ [Total area occupied by perlite (μm²) ２ )] × 100

[0050] The thickness of the hot-rolled steel sheet according to one embodiment of the present invention is preferably, for example, 1.0 mm or more and 8.0 mm or less. Furthermore, the hot-rolled steel sheet according to one embodiment of the present invention is suitable as a steel sheet for heat treatment. Examples of heat treatment include heating, holding, and rapid cooling (quenching) of the hot-rolled steel sheet after it has been formed into a predetermined shape, as described in the measurement of hardness after heat treatment in the examples described later. In addition, tempering may be performed during the heat treatment.

[0051] [2] Method for Manufacturing Hot-Rolled Steel Sheets Next, a method for manufacturing hot-rolled steel sheets according to embodiments of the present invention will be described. The method for manufacturing hot-rolled steel sheets according to one embodiment of the present invention can also be described as a preferred method for manufacturing hot-rolled steel sheets as described above. Unless otherwise specified, each temperature in each process refers to the surface temperature of the steel material and the steel sheet (hot-rolled steel sheet). Unless otherwise specified, the average cooling rate also refers to the surface temperature of the steel material and the steel sheet (hot-rolled steel sheet). Unless otherwise specified, the average cooling rate can be calculated by the following formula. The cooling start temperature may be, for example, the temperature at the end of the immediately preceding process. [Average cooling rate (°C / sec)] = ([Cooling start temperature (°C)] - [Cooling stop temperature (°C)]) / [Cooling time (seconds)]

[0052] (First Embodiment) A method for manufacturing a hot-rolled steel sheet according to the first embodiment of the present invention comprises: a heating step of heating a steel material having the above-mentioned component composition; a hot-rolling step of subjecting the steel material after the heating step to rough-rolling and finish-rolling to obtain a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step; and a winding step of winding the hot-rolled steel sheet after the cooling step, wherein the heating temperature of the steel material in the heating step is 1150°C or less, the completion temperature of the rough-rolling in the hot-rolling step is 950°C or less, the completion temperature of the finish-rolling in the hot-rolling step is 790°C or more and 920°C or less, the exit-side feed speed of the finish-rolling in the hot-rolling step is 400 m / min or more, the average cooling rate in the cooling step is 40°C / second or more, the cooling stop temperature in the cooling step is Bs point - 20°C or more and less than 600°C, and the winding temperature in the winding step is Bs point - 20°C or more and less than 600°C. Each step will be described below.

[0053] • Heating process: First, the steel material having the above-mentioned component composition is heated. The steel material can be obtained, for example, by melting steel having the above-mentioned component composition in a converter or electric furnace according to conventional methods, and then by continuous casting, ingot casting, or thin slab casting.

[0054] Heating temperature of steel material (hereinafter also simply referred to as heating temperature): 1150°C or lower. If the heating temperature exceeds 1150°C, the austenite in the steel material becomes coarser. This reduces the nucleation sites for austenite-ferrite transformation, making it easier for ferrite to nucleate unevenly. As a result, the F-P adjacent ratio decreases. It may also lead to coarsening of the ferrite. For this reason, the heating temperature should be 1150°C or lower, preferably 1120°C or lower. Furthermore, based on the conditions of the hot rolling process described later, the heating temperature should preferably be 1000°C or higher, more preferably 1050°C or higher.

[0055] - In the hot rolling process, the steel material (hereinafter also referred to as the rolled material) that has been heated in the heating process is subjected to rough rolling and finish rolling to produce hot-rolled steel sheet.

[0056] Rough rolling completion temperature: 950°C or lower To obtain the desired F-P adjacent ratio, it is effective to increase the reduction ratio in the unrecrystallized austenite temperature range during finish rolling. To achieve this, lowering the rough rolling completion temperature is effective. Therefore, the rough rolling completion temperature is 950°C or lower, preferably 930°C or lower. The lower limit of the rough rolling completion temperature is not particularly limited. From the viewpoint of controlling the finish rolling completion temperature, which will be described later, to a predetermined range, it is preferable that the rough rolling completion temperature be 860°C or higher.

[0057] Here, the completion temperature of rough rolling can also be defined as the temperature of the rolled material at the exit of the final stand (final pass) of rough rolling (the end of rough rolling).

[0058] Completion temperature of finish rolling: 790°C or higher and 920°C or lower. In order to adequately perform rolling in the unrecrystallized austenite temperature range during finish rolling, it is necessary to lower the completion temperature of finish rolling to suppress the progression of recrystallization. For this reason, the completion temperature of finish rolling is 920°C or lower, preferably 870°C or lower. On the other hand, if the completion temperature of finish rolling is below 790°C, it becomes two-phase rolling, and the formability may be significantly reduced. For this reason, the completion temperature of finish rolling is 790°C or higher, preferably 800°C or higher.

[0059] Here, the completion temperature of the finish rolling can also be defined as the temperature of the hot-rolled steel sheet at the exit of the final stand (final pass) of the finish rolling process (the end of the finish rolling).

[0060] End-side sheet feed speed in finish rolling: 400 m / min or more. In order to adequately perform rolling in the unrecrystallized austenite temperature range during finish rolling, it is necessary to increase the sheet feed speed in finish rolling to suppress the progression of recrystallization. For this reason, the end-side sheet feed speed in finish rolling is 400 m / min or more, preferably 500 m / min or more. There is no particular upper limit to the end-side sheet feed speed in finish rolling. For example, due to equipment constraints, the end-side sheet feed speed in finish rolling is preferably 1500 m / min or less.

[0061] The exit speed of the finish rolling process can also be defined as the speed at which the hot-rolled steel sheet moves out of the final stand (final pass) of the finish rolling process (at the end of the finish rolling).

[0062] Next, the hot-rolled steel sheet obtained in the hot-rolling process is cooled.

[0063] Average cooling rate: 40°C / second or higher. In order to obtain the above-mentioned fine F-P dispersion structure, and especially the desired F-P adjacency ratio, in the hot-rolled steel sheet that becomes the final product, it is effective to increase the cooling rate in the cooling process. Here, if the average cooling rate is less than 40°C / second, the F-P adjacency ratio decreases. Also, coarse ferrite is formed. For this reason, the average cooling rate is 40°C / second or higher, preferably 50°C / second or higher. The upper limit of the average cooling rate is not particularly limited. For example, an average cooling rate of 150°C / second or lower is preferred. The cooling method in the cooling process is not particularly limited. Cooling in the cooling process is performed by, for example, water cooling. Also, if the average cooling rate is 40°C / second or higher, cooling by oil, fan, or metal salt may be performed.

[0064] Cooling stop temperature: Bs point - 20°C or higher and less than 600°C. By setting the cooling stop temperature near the Bs point, the desired pseudo-P ratio can be obtained. However, if the cooling stop temperature is below Bs point - 20°C, bainite and the like will be excessively formed, and the above-mentioned fine F-P dispersion structure cannot be obtained. As a result, the desired pre-heat treatment strength cannot be obtained. For this reason, the cooling stop temperature is Bs point - 20°C or higher, preferably Bs point - 10°C or higher. On the other hand, if the cooling stop temperature is 600°C or higher, it will be necessary to control the cooling conditions after the winding process, as in the second embodiment described later, in order to obtain the desired pseudo-P ratio. For this reason, the cooling stop temperature is less than 600°C, preferably 580°C or lower.

[0065] Here, the Bs point can be calculated using the following formula: Bs point (°C) = 732 - 200 [%C] + 220 [%Si] - 86 [%Mn] - 40 [%Ni] - 41 [%Cr] - 40 [%Mo] In the formula, [% element (C, Si, Mn, Ni, Cr, and Mo)] means the content (mass %) of each element in the composition of the steel material. If an element is not present, the [% element] for that element should be 0.

[0066] Furthermore, the time from the end of the finish rolling in the hot rolling process to the start of the cooling process in the cooling process is preferably 3.0 seconds or less, and more preferably 2.0 seconds or less, from the viewpoint of suppressing austenite recrystallization.

[0067] - After the winding and cooling process, the hot-rolled steel sheet is wound up.

[0068] Winding temperature: Bs point - 20°C or higher and less than 600°C. By setting the winding temperature near the Bs point, the desired pseudo-P ratio can be obtained. However, if the winding temperature is below Bs point - 20°C, bainite and the like will be excessively formed, and the above-mentioned fine F-P dispersion structure cannot be obtained. As a result, the desired pre-heat treatment strength cannot be obtained. For this reason, the winding temperature is Bs point - 20°C or higher, preferably Bs point - 10°C or higher. On the other hand, if the winding temperature is 600°C or higher, it will be necessary to control the cooling conditions after the winding process, as in the second embodiment described later, in order to obtain the desired pseudo-P ratio. For this reason, the winding temperature is less than 600°C, preferably 580°C or lower.

[0069] Furthermore, the hot-rolled steel sheet may be subjected to pickling and temper rolling. The conditions for pickling and temper rolling are not particularly limited and should be followed according to conventional methods. The elongation rate of temper rolling is preferably, for example, 0.1 to 0.5%.

[0070] Other than the conditions mentioned above, there are no particular limitations, and conventional methods should be followed. For example, in the hot rolling process, it is preferable that the number of stands (passes) for rough rolling is 2 to 5, the number of stands (passes) for finish rolling is 5 to 7, and the total reduction ratio is 90% or more.

[0071] (Second Embodiment) A method for manufacturing a hot-rolled steel sheet according to a second embodiment of the present invention comprises: a heating step of heating a steel material having the above-mentioned composition; a hot-rolling step of subjecting the steel material after the heating step to rough-rolling and finish-rolling to obtain a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot-rolling step; a winding step of winding the hot-rolled steel sheet after the first cooling step; and a second cooling step of cooling the hot-rolled steel sheet after the winding step, wherein the heating temperature of the steel material in the heating step is 1150°C or less, the completion temperature of the rough-rolling in the hot-rolling step is 950°C or less, the completion temperature of the finish-rolling in the hot-rolling step is 790°C or more and 920°C or less, the exit speed of the sheet during finish-rolling in the hot-rolling step is 400 m / min or more, the average cooling rate in the first cooling step is 40°C / second or more, and the cooling stop temperature in the first cooling step is 600°C or more and 680°C or less. The winding temperature in the winding process is 600°C to 680°C, and the average cooling rate to 200°C in the second cooling process is 50°C / hour or more. The following describes each process. Processes similar to those for manufacturing hot-rolled steel sheets according to the first embodiment of the present invention, such as the heating process and the hot-rolling process, will not be described.

[0072] • First cooling step: In the first cooling step, the hot-rolled steel sheet obtained in the hot-rolling step is cooled.

[0073] Average cooling rate in the first cooling step (hereinafter also referred to as the first cooling rate): 40°C / second or more. In order to obtain the above-mentioned fine F-P dispersion structure, and in particular the desired F-P adjacency ratio, in the hot-rolled steel sheet that becomes the final product, it is effective to increase the cooling rate in the cooling step. Here, if the first cooling rate is less than 40°C / second, the F-P adjacency ratio decreases. Also, coarse ferrite is formed. For this reason, the first cooling rate is 40°C / second or more, preferably 50°C / second or more. The upper limit of the first cooling rate is not particularly limited. For example, the first cooling rate is preferably 150°C / second or less.

[0074] Cooling stop temperature in the first cooling process: 600°C or higher and 680°C or lower As described above, in order to obtain the desired pseudo-P ratio, it is effective to set the cooling stop temperature in the first cooling process between the hot rolling process and the winding process (hereinafter also referred to as the first cooling stop temperature) near the Bs point. However, depending on the component composition, it may be difficult to set the first cooling stop temperature near the Bs point. Here, even if the first cooling stop temperature is 600°C or higher, the desired pseudo-P ratio can be obtained by appropriately controlling the cooling conditions of the second cooling process after the winding process, which will be described later. Also, if the first cooling stop temperature is too low, bainite may be excessively formed, and the desired pre-heat treatment strength may not be obtained. On the other hand, if the first cooling stop temperature exceeds 680°C, the desired F-P adjacent ratio cannot be obtained even if the cooling conditions of the second cooling process after the winding process, which will be described later, are appropriately controlled. Also, coarse ferrite will be formed. For this reason, the first cooling stop temperature is 600°C or higher and 680°C or lower. Furthermore, the first cooling stop temperature is preferably 670°C or lower.

[0075] - After the winding and cooling process, the hot-rolled steel sheet is wound up.

[0076] Winding temperature: 600°C to 680°C As described above, in order to obtain the desired pseudo-P ratio, it is effective to set the winding temperature of the winding process near the Bs point. However, depending on the component composition, it may be difficult to set the winding temperature near the Bs point. Here, even if the winding temperature is 600°C or higher, the desired pseudo-P ratio can be obtained by appropriately controlling the cooling conditions of the second cooling process after the winding process, which will be described later. Also, if the winding temperature is too low, bainite will be excessively formed, and the desired strength before heat treatment may not be obtained. On the other hand, if the winding temperature exceeds 680°C, the desired F-P adjacent ratio cannot be obtained even if the cooling conditions of the second cooling process after the winding process, which will be described later, are appropriately controlled. In addition, coarse ferrite will be formed. For this reason, the winding temperature is 600°C to 680°C. Furthermore, the winding temperature is preferably 670°C or lower.

[0077] • Second cooling process: Next, the hot-rolled steel sheet is cooled after being coiled.

[0078] Average cooling rate up to 200°C in the second cooling step (hereinafter also referred to as the second cooling rate): 50°C / hour or more As described above, even if the first cooling stop temperature is 600°C or higher and 680°C or lower, the desired pseudo-P ratio can be obtained by increasing the second cooling rate. For this reason, the second cooling rate is 50°C / hour or higher, preferably 60°C / hour or higher. There is no particular upper limit to the second cooling rate. For example, from the viewpoint of reducing variations in the longitudinal properties of the hot-rolled steel sheet, 120°C / hour or less is preferred.

[0079] The starting temperature for cooling in the second cooling step is, for example, the winding temperature (the temperature of the hot-rolled steel sheet at the time of completion of winding). The stopping temperature for cooling in the second cooling step is preferably 0°C or higher and 200°C or lower. The stopping temperature for cooling in the second cooling step may be, for example, room temperature.

[0080] The present invention will be further explained by the following embodiments. However, the present invention is not limited to the following embodiments.

[0081] Example 1: A 250 mm thick steel material having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was subjected to a heating process, hot rolling process, cooling process, and winding process under the conditions shown in Table 2. After that, it was subjected to temper rolling with an elongation rate of 0.1 to 0.5% and pickling to produce hot-rolled steel sheets with a thickness of 1.6 to 7.2 mm.

[0082] Using the hot-rolled steel sheets thus obtained, the F area ratio, P area ratio, F particle size, F-P adjacency ratio, and pseudo-P ratio were measured according to the procedure described above. The results are shown in Table 3.

[0083] Furthermore, the strength before heat treatment and hardness after heat treatment were measured using the obtained hot-rolled steel sheets according to the following procedure. The results are shown in Table 3.

[0084] <Strength Before Heat Treatment> From the obtained hot-rolled steel sheet, JIS No. 5 tensile test specimens were taken so that the longitudinal direction was perpendicular to the rolling direction. Tensile tests were performed on the taken specimens in accordance with the provisions of JIS Z 2241:2011 to determine the yield strength. The crosshead speed was set to 10 mm / min. Tensile tests were performed five times on each hot-rolled steel sheet, and the average of the yield strengths measured over the five tests was taken as the yield strength of the hot-rolled steel sheet. The pass / fail status of the strength before heat treatment was determined according to the following criteria. [Criteria] Pass: Yield strength of 350 MPa or more and 550 MPa or less Fail: Yield strength of less than 350 MPa or greater than 550 MPa

[0085] <Hardness after heat treatment> Test specimens taken from the obtained hot-rolled steel sheets were subjected to heat treatment under the following conditions. Then, the Vickers hardness was measured at the 1 / 4 position of the plate thickness in the L-section of the heat-treated test specimen by a Vickers hardness test in accordance with JIS Z 2244-1:2020. The test force was 1 kgf, and the holding time of the test force was 15 seconds. The Vickers hardness test was performed five times for each hot-rolled steel sheet, and the average value of the Vickers hardness measured over the five tests was taken as the hardness after heat treatment of the hot-rolled steel sheet. The pass / fail status of the hardness after heat treatment was determined according to the following criteria. [Heat Treatment Conditions] ・Heating conditions: Average heating rate from room temperature to 950°C: 100°C / second ・Holding conditions: Holding temperature: 950°C, holding time at holding temperature: 10 seconds ・Quenching conditions: Oil cooling (cooling by immersion in oil set to 70°C) [Judgment Criteria] Pass: Vickers hardness of 370 or more and 550 or less Fail: Vickers hardness less than 370 or greater than 550

[0086]

[0087]

[0088]

[0089] As shown in Table 3, in all of the inventive examples, the desired strength before heat treatment and hardness after heat treatment were obtained.

[0090] On the other hand, in the comparative example, at least one of the following was unsatisfactory: pre-heat treatment strength and post-heat treatment hardness.

[0091] Example 2: A 250 mm thick steel material having the component composition shown in Table 4 (the remainder being Fe and unavoidable impurities) was subjected to a heating process, hot rolling process, first cooling process, winding process, and second cooling process under the conditions shown in Table 5. After that, it was subjected to temper rolling with an elongation rate of 0.1 to 0.5% and pickling to produce hot-rolled steel sheets with a thickness of 1.6 to 7.2 mm.

[0092] Using the hot-rolled steel sheets thus obtained, the F area ratio, P area ratio, F particle size, F-P adjacency ratio, and pseudo-P ratio were measured according to the procedure described above. The results are shown in Table 6.

[0093] Furthermore, the strength before heat treatment and hardness after heat treatment were measured using the obtained hot-rolled steel sheet in the same manner as in Example 1. The results are shown in Table 6.

[0094]

[0095]

[0096]

[0097] As shown in Table 6, in all of the inventive examples, the desired strength before heat treatment and hardness after heat treatment were obtained.

[0098] On the other hand, in the comparative example, at least one of the following was unsatisfactory: pre-heat treatment strength and post-heat treatment hardness.

Claims

1. The composition is such that, in mass%, C: 0.16% to 0.32%, Si: 0.85% or less, Mn: 0.50% to 2.50%, P: 0.04% or less, S: 0.0060% or less, Al: 0.01% to 0.08%, N: 0.0150% or less, Ti: 0.005% to 0.070%, Ca: 0.0001% to 0.0060%, Cr: 0.03% to 1.50%, and B: 0.0002% to 0.0050%, with the remainder being Fe and unavoidable impurities, the area ratio of ferrite: 40% to 80%, the area ratio of pearlite: 20% to 60%, and the average particle size of the ferrite: 5 μm to 20 μm. A hot-rolled steel sheet having a structure in which, of the ferrite, 30% or more of the circumference is adjacent to the pearlite, with a number ratio of ferrite between 40% and 100%, and the area ratio of pseudo-pearlite among the pearlite is between 30% and 100%.

2. The hot-rolled steel sheet according to claim 1, wherein the component composition, in mass%, further comprises at least one of the following groups A to D. - Group A: At least one selected from V: 0.2% or less, Nb: 0.07% or less, Mo: 0.3% or less, Zr: 0.05% or less, Hf: 0.05% or less, and W: 0.05% or less. - Group B: At least one selected from Cu: 0.5% or less and Ni: 0.5% or less. - Group C: At least one selected from Mg: 0.01% or less, REM: 0.1% or less, and Co: 0.01% or less. - Group D: At least one selected from Sb: 0.1% or less, Sn: 0.1% or less, As: 0.1% or less, Ta: 0.1% or less, Pb: 0.1% or less, Cs: 0.1% or less, Te: 0.1% or less, Bi: 0.1% or less, Zn: 0.1% or less, Ge: 0.1% or less, and Sr: 0.1% or less.

3. A method for manufacturing a hot-rolled steel sheet, comprising: a heating step of heating a steel material having the component composition described in claim 1 or 2; a hot-rolling step of subjecting the steel material after the heating step to rough-rolling and finish-rolling to obtain a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step; and a winding step of winding the hot-rolled steel sheet after the cooling step, wherein the heating temperature of the steel material in the heating step is 1150°C or less; the completion temperature of the rough-rolling in the hot-rolling step is 950°C or less; the completion temperature of the finish-rolling in the hot-rolling step is 790°C or more and 920°C or less; the exit-side sheet passing speed for the finish-rolling in the hot-rolling step is 400 m / min or more; the average cooling rate in the cooling step is 40°C / second or more; the cooling stop temperature in the cooling step is Bs point - 20°C or more and less than 600°C; and the winding temperature in the winding step is Bs point - 20°C or more and less than 600°C.

4. A heating step of heating a steel material having the component composition described in claim 1 or 2; a hot rolling step of subjecting the steel material after the heating step to rough rolling and finish rolling to obtain a hot-rolled steel sheet; a first cooling step of cooling the hot-rolled steel sheet after the hot rolling step; a winding step of winding the hot-rolled steel sheet after the first cooling step; and a second cooling step of cooling the hot-rolled steel sheet after the winding step, wherein the heating temperature of the steel material in the heating step is 1150°C or less, the completion temperature of the rough rolling in the hot rolling step is 950°C or less, the completion temperature of the finish rolling in the hot rolling step is 790°C or more and 920°C or less, the exit speed of the sheet during finish rolling in the hot rolling step is 400 m / min or more, the average cooling rate in the first cooling step is 40°C / second or more, and the cooling stop temperature in the first cooling step is 600°C or more and 680°C or less. A method for manufacturing a hot-rolled steel sheet, wherein the winding temperature in the winding step is 600°C or higher and 680°C or lower, and the average cooling rate to 200°C in the second cooling step is 50°C / hour or higher.

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