Hot-rolled steel sheet and method for manufacturing the same

Abstract

The present invention provides a hot-rolled steel sheet that has low strength before heat treatment and allows for control of the hardness after heat treatment within a desired range, even when the high-temperature holding time during heat treatment is shortened. The structural composition is appropriately controlled, with a ferrite area ratio of 40% to 80%, a pearlite area ratio of 20% to 60%, an average ferrite particle size of 5 μm to 20 μm, a number ratio of ferrite particles where 30% or more of the circumference is adjacent to pearlite of 40% to 100%, and a pseudo-pearlite area ratio of 30% to 100%.

Description

Technical Field

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

Background Art

[0002] Steel sheets are often used for members such as structures. Such members are produced, 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, the steel sheet (hereinafter also referred to as a steel sheet for heat treatment) that is the material of the member (hereinafter also referred to as a heat-treated member) produced through the above heat treatment is required to have high formability, that is, low strength before heat treatment, while obtaining a desired hardness after heat treatment.

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

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

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] Incidentally, in recent years, from the perspective of protecting the global environment, the entire industrial sector has been seeking ways to further reduce CO2 emissions. In the heat treatment described above, hardness is increased by rapid cooling after heating. Therefore, in the heat treatment described above, it is necessary to heat the steel plate to the austenite region. Furthermore, in the heat treatment described above, after heating the steel plate to the austenite region, it is held in the austenite region to allow sufficient transformation from ferrite to austenite to occur. Since a large amount of energy is required for holding in the austenite region, shortening the holding time in the austenite region (hereinafter also referred to as the high-temperature holding time) can be expected to significantly reduce the amount of energy used and, consequently, CO2 emissions.

[0007] However, in the technologies described in Patent Documents 1 to 3, when heat treatment is performed by shortening the high-temperature holding time, 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 to 550 MPa. The target range for the hardness after heat treatment is preferably a Vickers hardness of 370 to 550. The measurement procedures for yield strength and other parameters, as well as the heat treatment conditions, are as described in the examples below. [Means for solving the problem]

[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 investigations. As a result, the inventors discovered that in order to achieve the above objectives, it is important to control the microstructure of the hot-rolled steel sheet as follows. - The ferrite is made into a fine particle, and then pearlite, which serves as a carbon source, is placed adjacent to the ferrite. • In perlite, ensure a certain proportion of pseudo-perlite, in which cementite dissolves rapidly. Specifically, the microstructure of hot-rolled steel sheets, Ferrite area ratio: 40% to 80% Perlite area ratio: 20% to 60% Average particle size of ferrite: 5 μm to 20 μm. Of the ferrite, the percentage of ferrite particles adjacent to pearlite with a circumference of 30% or more: 40% to 100% and Area percentage of pseudo-perlite within perlite: 30% to 100% It is important to create an organization that is such as (hereinafter also referred to as an FP finely dispersed organization). This invention was completed based on the above findings and further considerations.

[0012] That is, the main structure of the present invention is as follows.

[0013] 1. In mass %, C: 0.16% or more and 0.32% or less, Si: 0.85% or less, Mn: 0.50% or more and 2.50% or less, P: 0.04% or less, S: 0.0060% or less, Al: 0.01% or more and 0.08% or less, N: 0.0150% or less, Ti: 0.005% or more and 0.070% or less, Ca: 0.0001% or more and 0.0060% or less, Cr: 0.03% or more and 1.50% or less and B: 0.0002% or more and 0.0050% or less and having a component composition in which the balance is Fe and unavoidable impurities, Area ratio of ferrite: 40% or more and 80% or less, Area ratio of pearlite: 20% or more and 60% or less, Average grain size of the ferrite: 5 μm or more and 20 μm or less, Among the ferrite, the ratio of the number of ferrite grains whose perimeter is 30% or more adjacent to the pearlite: 40% or more and 100% or less and Among the pearlite, the area ratio of pseudo-pearlite: 30% or more and 100% or less A hot-rolled steel sheet having such a structure.

[0014] 2. The hot-rolled steel sheet according to 1 above, wherein the component composition further has at least one of the following groups A to D in mass %. · 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. ·D group 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 heating step of heating a steel material having the component composition described in 1 or 2 above, A hot rolling process is performed on the steel material after the heating process to obtain a hot-rolled steel sheet by rough rolling and finish rolling. A cooling step for cooling the hot-rolled steel sheet after the hot-rolling process, A winding step in which the hot-rolled steel sheet after the cooling step is wound up, Equipped with, Heating temperature of the steel material in the aforementioned heating process: 1150°C or less. The completion temperature of rough rolling in the aforementioned hot rolling process: 950°C or less. The completion temperature for finish rolling in the aforementioned hot rolling process: 790°C or higher and 920°C or lower. The exit speed of the finish rolling in the aforementioned hot rolling process: 400 m / min or more. Average cooling rate in the aforementioned cooling process: 40°C / second or more. The cooling stop temperature in the above cooling process: Bs point -20°C or higher and less than 600°C and Winding temperature during the aforementioned winding process: Bs point -20°C or higher and less than 600°C This is a method for manufacturing hot-rolled steel sheets.

[0016] 4. A heating step of heating a steel material having the component composition described in 1 or 2 above, A hot rolling process is performed on the steel material after the heating process to obtain a hot-rolled steel sheet by rough rolling and finish rolling. A first cooling step for cooling the hot-rolled steel sheet after the hot-rolling process, A winding step in which the hot-rolled steel sheet after the first cooling step is wound up, A second cooling step for cooling the hot-rolled steel sheet after the winding step, Equipped with, Heating temperature of the steel material in the aforementioned heating process: 1150°C or less. The completion temperature of rough rolling in the aforementioned hot rolling process: 950°C or less. The completion temperature for finish rolling in the aforementioned hot rolling process: 790°C or higher and 920°C or lower. The exit speed of the finish rolling in the aforementioned hot rolling process: 400 m / min or more. Average cooling rate in the first cooling step described above: 40°C / second or more. The cooling stop temperature in the first cooling step is 600°C or higher and 680°C or lower. The winding temperature in the aforementioned winding process: 600°C or higher and 680°C or lower. Average cooling rate up to 200°C in the second cooling step: 50°C / hour or more This is a method for manufacturing hot-rolled steel sheets. [Effects of the Invention]

[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 heat-treated components, the high-temperature holding time can be shortened, which is expected to lead to a significant reduction in energy consumption and, consequently, CO2 emissions, making it extremely advantageous from an industrial perspective. [Brief explanation of the drawing]

[0018] [Figure 1] This is a schematic diagram showing an example of the form of a perlite block that falls under the category of pseudo-perlite. [Modes for carrying out the invention]

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

[0020] [1] Hot rolled steel plate 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% or more and 0.32% or less 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 higher, preferably 0.17% or higher. On the other hand, if the C content exceeds 0.32%, controlling the morphology of cementite in pearlite becomes difficult, and the above-mentioned fine FP 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 also be 0%. In addition, 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% or more and 2.50% or less 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 components 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 also 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% or more and 0.08% or less Al is sometimes 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 are formed, which reduces the toughness of the heat-treated component. Therefore, the Al content is 0.08% or less, preferably 0.07% or less.

[0027] N: 0.0150% or less Nitric oxide (N) is a harmful element that reduces the toughness of heat-treated components 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 also 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 due to solid-solution nitrogen by bonding with nitrogen. 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. Therefore, 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-system inclusions and improves 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 saturates. 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. ·B group At least one selected from Cu: 0.5% or less and Ni: 0.5% or less. ·C group At least one selected from Mg: 0.01% or less, REM: 0.1% or less, and Co: 0.01% or less. ·D group 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 also 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] Elements in group C are elements that can change the morphology of inclusions and are expected to contribute to improving the toughness of heat-treated members. 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 La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71, scandium (sc) with atomic number 21, and yttrium (y) 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% to 80% Ferrite has a soft structure. In order to ensure the formability required for heat-treated steel sheets, it is necessary to lower the strength, particularly to a yield strength of 550 MPa or less. Therefore, 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 discussed 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% to 60% 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. Furthermore, the P area ratio is 60% or less, preferably 50% or less.

[0041] Here, the term "perlite" includes both perlite in the strict sense and pseudo-perlite. Perlite in the strict sense is a lamellar structure in which ferrite (hereinafter also called lamellar ferrite to distinguish it from the aforementioned ferrite) and cementite are arranged alternately in layers. Pseudo-perlite is a structure that has lamellar ferrite and cementite, but in which they are not arranged in a completely layered manner, in particular, a structure in which cementite with an aspect ratio of 5 or less is dispersed. The determination of whether something is perlite in the strict sense or pseudo-perlite is made, for example, at the perlite block level. As shown in Figure 1, a perlite block consists of three or more cementites with a major axis of 0.2 μm or more, where the shortest distance between each cementite is 0.75 μm or less, and lamellar ferrite located between the cementites. The boundary of a pearlite block is defined, for example, by connecting the outermost parts of the cementite (the parts adjacent to other structures such as ferrite) with the shortest distance so that all the cementite constituting the pearlite block is included. If a pearlite block contains three or more cementite particles with an aspect ratio of 5 or less, and two or fewer cementite particles with an aspect ratio greater than 5, then the pearlite block is determined to be pseudo-pearlite. In all other cases, the pearlite block is determined to be pearlite in the narrow sense. Furthermore, in this disclosure, when simply referred to as pearlite, it means pearlite in the broad sense, that is, pearlite in the narrow sense and pseudo-pearlite.

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

[0043] The F area ratio and P area ratio can be measured according to standard 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. Then, using image analysis software (Photoshop Elements and Image J), ​​the F area ratio and P area ratio are calculated from the total area of ​​the regions occupied by ferrite and pearlite in the 10 fields of SEM image. Furthermore, the area ratio of the remaining tissue is determined by subtracting the F area ratio and P area ratio obtained as described above from 100%.

[0044] Average particle size of ferrite (hereinafter also referred to as F particle size): 5 μm or more and 20 μm or less The grain size of the ferrite particle (F) 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). Specifically, if the F grain size is less than 5 μm, the yield strength increases excessively, and the formability decreases. Therefore, 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. Therefore, 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 grain size can be measured, for example, by the sectioning method using the same SEM image used to measure the F area ratio and P area ratio. Specifically, 10 test lines with an actual length of 35 μm each are drawn in the longitudinal direction (thickness direction) and transverse direction (rolling direction) of the SEM image at regular intervals, for example, 3 μm apart. Then, the average length of the line segments of the test lines passing through each ferrite (crystal grain) in the above SEM image is calculated and given as the F grain size.

[0046] Of the ferrite, the percentage of ferrite particles whose circumference is adjacent to pearlite for 30% or more (also called the FP adjacency ratio): 40% to 100% During the heat treatment described above, pearlite transforms into austenite earlier than ferrite. Therefore, in order to shorten the time required for transformation to complete, that is, to shorten the high-temperature holding time during heat treatment, and to control the hardness after heat treatment within the desired range, it is important to promote the ferrite-to-austenite transformation. To achieve this, it is effective to refine the ferrite and place pearlite, which serves as a carbon source, adjacent to the ferrite. Accordingly, the FP adjacent ratio is 40% or more, preferably 50% or more. Furthermore, since a higher FP adjacent ratio is desirable, the upper limit of the FP adjacent ratio may be 100%, and the FP adjacent ratio is preferably 80% or less.

[0047] The FP adjacency ratio can be calculated, for example, by selecting 200 arbitrary ferrite (crystal grains) using SEM images used to measure the F area ratio and P area ratio, and dividing the number of ferrites whose perimeter is adjacent to pearlite by 30% or more by the number of ferrites selected (200).

[0048] The area ratio of pseudo-perlite within perlite (hereinafter also referred to as the pseudo-P ratio): 30% to 100% When shortening the high-temperature holding time during heat treatment, it is important to ensure a certain proportion of pseudo-pearlite, which has a high cementite dissolution rate, in order to control the hardness after heat treatment within a 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 surface area adjacent to lamellar ferrite compared to cementite in pearlite in the strict sense, and thus has a higher dissolution rate during heat treatment. Thus, pseudo-pearlite contributes more effectively to promoting the ferrite-to-austenite transformation during heat treatment than pearlite in the strict sense. For this reason, the pseudo-pearlite proportion is 30% or more, preferably 40% or more. Furthermore, since a higher pseudo-pearlite proportion is desirable, the upper limit of the pseudo-pearlite proportion may be 100%, and the pseudo-pearlite proportion is preferably 75% or less.

[0049] The pseudo-perlite ratio can be calculated, for example, in the SEM images 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. [Percentage of pseudo-perlite (%)] = [Total area of ​​the region occupied by pseudo-perlite (μm²)] 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 an embodiment 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. 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). 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. 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 (℃ / sec)] = ([Cooling start temperature (℃)] - [Cooling stop temperature (℃)]) / [Cooling time (seconds)]

[0052] (First Embodiment) A method for manufacturing a hot-rolled steel sheet according to the first embodiment of the present invention is: A heating step of heating a steel material having the aforementioned component composition, A hot rolling process is performed on the steel material after the heating process to obtain a hot-rolled steel sheet by rough rolling and finish rolling. A cooling step for cooling the hot-rolled steel sheet after the hot-rolling process, A winding step in which the hot-rolled steel sheet after the cooling step is wound up, Equipped with, Heating temperature of the steel material in the aforementioned heating process: 1150°C or less. The completion temperature of rough rolling in the aforementioned hot rolling process: 950°C or less. The completion temperature for finish rolling in the aforementioned hot rolling process: 790°C or higher and 920°C or lower. The exit speed of the finish rolling in the aforementioned hot rolling process: 400 m / min or more. Average cooling rate in the aforementioned cooling process: 40°C / second or more. The cooling stop temperature in the above cooling process: Bs point -20°C or higher and less than 600°C and Winding temperature during the aforementioned winding process: Bs point -20°C or higher and less than 600°C That is the case. The following describes each step.

[0053] ·Heating process First, a 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℃ or less When the heating temperature exceeds 1150°C, the austenite in the steel material becomes coarser. This reduces the nucleation sites for the austenite-to-ferrite transformation, making it easier for ferrite to nucleate unevenly. As a result, the proportion of adjacent fission products (FPs) decreases. It can also lead to further coarsening of the ferrite. Therefore, 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] ·Hot rolling process Next, the steel material heated in the heating process (hereinafter also referred to as the rolled material) is subjected to rough rolling and finish rolling to produce hot-rolled steel sheet.

[0056] Rough rolling completion temperature: 950℃ or lower To obtain the desired FP adjacency ratio, it is effective to increase the reduction ratio in the unrecrystallized austenite temperature range during finish rolling. To achieve this, lowering the completion temperature of rough rolling is effective. Therefore, the completion temperature of rough rolling is 950°C or lower, preferably 930°C or lower. The lower limit of the completion temperature of rough rolling is not particularly limited. From the viewpoint of controlling the completion temperature of finish rolling, which will be described later, to a predetermined range, it is preferable that the completion temperature of rough rolling 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] Finishing rolling completion temperature: 790°C to 920°C To ensure sufficient rolling in the unrecrystallized austenite temperature range during finish rolling, it is necessary to lower the completion temperature of the finish rolling to suppress the progression of recrystallization. Therefore, the completion temperature of the finish rolling is 920°C or lower, preferably 870°C or lower. On the other hand, if the completion temperature of the finish rolling is below 790°C, it becomes two-phase rolling, which may significantly reduce formability. Therefore, the completion temperature of the 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-rolling speed: 400 m / min or more In order to ensure sufficient rolling in the unrecrystallized austenite temperature range during finish rolling, it is necessary to increase the rolling speed during finish rolling to suppress the progression of recrystallization. Therefore, the exit rolling speed during finish rolling is 400 m / min or more, preferably 500 m / min or more. There is no particular upper limit to the exit rolling speed during finish rolling. For example, due to equipment constraints, the exit rolling speed during 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] ·Cooling process Next, the hot-rolled steel sheet obtained in the hot-rolling process is cooled.

[0063] Average cooling rate: 40℃ / sec or more In order to obtain the above-mentioned fine FP dispersion structure, particularly the desired FP adjacency ratio, in the final hot-rolled steel sheet 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 FP adjacency ratio decreases, and coarse ferrite is formed. For this reason, the average cooling rate is 40°C / second or higher, preferably 50°C / second or higher. There is no particular upper limit to the average cooling rate. For example, an average cooling rate of 150°C / second or lower is preferred. There is no particular limit to the cooling method in the cooling process. Cooling in the cooling process can be performed, for example, by water cooling. If the average cooling rate is 40°C / second or higher, cooling with oil, a fan, or a metal salt may also 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, excessive bainite and other materials are formed, and the above-mentioned fine FP dispersion structure cannot be obtained. As a result, the desired pre-heat treatment strength cannot be obtained. Therefore, 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 becomes 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. Therefore, the cooling stop temperature is less than 600°C, preferably 580°C or lower.

[0065] Here, point Bs can be calculated using the following formula. Bs point (℃)=732-200[%C]+220[%Si]-86[%Mn]-40[%Ni]-41[%Cr]-40[%Mo] In the formula, [% element (C, Si, Mn, Ni, Cr, and Mo)] represents the mass percentage of each element in the composition of the steel material. If an element is not present, its [% element] value is 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] ·Winding process After the cooling process, the hot-rolled steel sheet is rolled up.

[0068] Winding temperature: Bs point -20℃ or higher and less than 600℃ 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, excessive bainite and other materials are formed, and the above-mentioned fine FP dispersion structure cannot be obtained. As a result, the desired pre-heat treatment strength cannot be obtained. Therefore, 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 becomes 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. Therefore, 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 general 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 is: A heating step of heating a steel material having the aforementioned component composition, A hot rolling process is performed on the steel material after the heating process to obtain a hot-rolled steel sheet by rough rolling and finish rolling. A first cooling step for cooling the hot-rolled steel sheet after the hot-rolling process, A winding step in which the hot-rolled steel sheet after the first cooling step is wound up, A second cooling step for cooling the hot-rolled steel sheet after the winding step, Equipped with, Heating temperature of the steel material in the aforementioned heating process: 1150°C or less. The completion temperature of rough rolling in the aforementioned hot rolling process: 950°C or less. The completion temperature for finish rolling in the aforementioned hot rolling process: 790°C or higher and 920°C or lower. The exit speed of the finish rolling in the aforementioned hot rolling process: 400 m / min or more. Average cooling rate in the first cooling step described above: 40°C / second or more. The cooling stop temperature in the first cooling step is 600°C or higher and 680°C or lower. The winding temperature in the aforementioned winding process: 600°C or higher and 680°C or lower. Average cooling rate up to 200°C in the second cooling step: 50°C / hour or more The process is as follows. The steps described below will be explained. Note that the heating process and the hot rolling process, and other processes similar to those described above for manufacturing hot-rolled steel sheets according to the first embodiment of the present invention, will not be explained.

[0072] • First cooling process 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 process (hereinafter also referred to as the first cooling rate): 40°C / second or higher In order to obtain the above-mentioned fine FP dispersion structure, particularly the desired FP adjacency ratio, in the final hot-rolled steel sheet product, it is effective to increase the cooling rate in the cooling process. Here, if the first cooling rate is less than 40°C / second, the FP adjacency ratio decreases. Also, coarse ferrite is formed. For this reason, the first cooling rate is 40°C / second or higher, preferably 50°C / second or higher. The upper limit of the first cooling rate is not particularly limited. For example, the first cooling rate is preferably 150°C / second or lower.

[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 and winding processes (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, excessive bainite may be 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 FP 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 first cooling stop temperature is between 600°C and 680°C. Furthermore, the first cooling stop temperature is preferably 670°C or lower.

[0075] ·Winding process After the cooling process, the hot-rolled steel sheet is rolled up.

[0076] Winding temperature: 600℃ or more and 680℃ or less As mentioned above, to obtain the desired pseudo-P ratio, it is effective to set the winding temperature near the Bs point during the winding process. 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, excessive bainite may be formed, and the desired pre-heat treatment strength may not be obtained. On the other hand, if the winding temperature exceeds 680°C, the desired FP 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 is formed. For this reason, the winding temperature is between 600°C and 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 rolled up.

[0078] Average cooling rate up to 200°C in the second cooling process (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 between 600°C and 680°C, the desired pseudo-P ratio can be obtained by increasing the second cooling rate. Therefore, the second cooling rate is 50°C / hour or more, preferably 60°C / hour or more. 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 between 0°C and 200°C. The stopping temperature for cooling in the second cooling step may be, for example, room temperature. [Examples]

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

[0082] Using the hot-rolled steel sheets thus obtained, the F area ratio, P area ratio, F particle size, FP 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 sheets, 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 criteria for the strength before heat treatment were then determined according to the following criteria. [Judgment criteria] Passing criteria: Yield strength between 350 MPa and 550 MPa. Failure: Yield strength 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. The Vickers hardness was then measured at the 1 / 4 thickness point of the L-section of the heat-treated test specimens using a Vickers hardness test in accordance with JIS Z 2244-1:2020. The test force was 1 kgf, and the holding time was 15 seconds. The Vickers hardness test was performed five times for each hot-rolled steel sheet, and the average of the five Vickers hardness measurements was taken as the post-heat-treated hardness of the hot-rolled steel sheet. The pass / fail criteria for the post-heat-treated hardness were then determined according to the following criteria. [Heat treatment conditions] ·Heating conditions Average heating rate from room temperature to 950°C: 100°C / second ·Retention conditions Holding temperature: 950°C, Holding time at holding temperature: 10 seconds • Hardening conditions Oil cooling (cooling by immersion in oil set to 70°C) [Judgment criteria] Passing grade: Vickers hardness between 370 and 550. Failure: Vickers hardness less than 370 or greater than 550

[0086] [Table 1]

[0087] [Table 2]

[0088] [Table 3]

[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 250mm 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 underwent temper rolling with an elongation rate of 0.1-0.5% and pickling to produce hot-rolled steel sheets with a thickness of 1.6-7.2mm.

[0092] Using the hot-rolled steel sheets thus obtained, the F area ratio, P area ratio, F particle size, FP 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] [Table 4]

[0095] [Table 5]

[0096] [Table 6]

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

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. Area ratio of ferrite: 40% or more and 80% or less, Perlite area ratio: 20% to 60% The average particle size of the ferrite is 5 μm or more and 20 μm or less. Of the ferrites, the number ratio of ferrites adjacent to the pearlite with a circumference of 30% or more is 40% to 100% and Of the aforementioned perlite, the area ratio of pseudo-perlite is 30% to 100%. A method for producing a hot-rolled steel sheet having the following structure: The method described above is A heating step of heating a steel material having the component composition described in claim 1 or 2, A hot rolling process is performed on the steel material after the heating process to obtain a hot-rolled steel sheet by rough rolling and finish rolling. A cooling step for cooling the hot-rolled steel sheet after the hot-rolling process, A winding step in which the hot-rolled steel sheet after the cooling step is wound up, Equipped with, The heating temperature of the steel material in the aforementioned heating process: 1150°C or less. The completion temperature of rough rolling in the aforementioned hot rolling process: 950°C or less. The completion temperature for finish rolling in the aforementioned hot rolling process: 790°C or higher and 920°C or lower. The exit speed of the sheet during the finish rolling process in the aforementioned hot rolling step: 400 m / min or more. Average cooling rate in the aforementioned cooling process: 40°C / second or more. The cooling stop temperature in the above cooling process: Bs point -20°C or higher and less than 600°C and Winding temperature in the aforementioned winding process: Bs point -20°C or higher and less than 600°C This is a method for manufacturing hot-rolled steel sheets.

4. Area ratio of ferrite: 40% or more and 80% or less, Perlite area ratio: 20% to 60% The average particle size of the ferrite is 5 μm or more and 20 μm or less. Of the ferrites, the number ratio of ferrites adjacent to the pearlite with a circumference of 30% or more is 40% to 100% and Of the aforementioned perlite, the area ratio of pseudo-perlite is 30% to 100%. A method for producing a hot-rolled steel sheet having the following structure: The method described above is A heating step of heating a steel material having the component composition described in claim 1 or 2, A hot rolling process is performed on the steel material after the heating process to obtain a hot-rolled steel sheet by rough rolling and finish rolling. A first cooling step for cooling the hot-rolled steel sheet after the hot-rolling process, A winding step in which the hot-rolled steel sheet after the first cooling step is wound up, A second cooling step is performed to cool the hot-rolled steel sheet after the winding step, Equipped with, The heating temperature of the steel material in the aforementioned heating process: 1150°C or less. The completion temperature of rough rolling in the aforementioned hot rolling process: 950°C or less. The completion temperature for finish rolling in the aforementioned hot rolling process: 790°C or higher and 920°C or lower. The exit speed of the sheet during the finish rolling process in the aforementioned hot rolling step: 400 m / min or more. Average cooling rate in the first cooling step: 40°C / second or more. The cooling stop temperature in the first cooling step is 600°C or higher and 680°C or lower. The winding temperature in the aforementioned winding process: 600°C or higher and 680°C or lower. Average cooling rate up to 200°C in the second cooling step: 50°C / hour or more This is a method for manufacturing hot-rolled steel sheets.

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