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 length of ferrite in the thickness direction of the sheet 12 μm or less, a standard deviation of ferrite in the thickness direction of the sheet 8 μm or less, an average length of pearlite in the thickness direction of the sheet 8 μm or less, and a standard deviation of pearlite in the thickness direction of the sheet 5 μm or less.

Description

Technical Field

[0005] , , ,

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

Background Art

[0002] <​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[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 disclosure, 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 and a tensile strength of 440 MPa to 600 MPa. The target range for the hardness after heat treatment is preferably a Vickers hardness of 360 to 530. 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, when shortening the high-temperature holding time in heat treatment, in order to control the hardness after heat treatment within the desired range, it is necessary to promote 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 in 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 objective, it is important to make the structure of the hot-rolled steel sheet a uniform structure in which ferrite and pearlite are finely dispersed in the thickness direction (hereinafter also referred to as FP fine dispersion structure), and in particular to control the area ratio of ferrite as follows. Ferrite area ratio: 40% to 80% Perlite area ratio: 20% to 60% Average length of ferrite in the thickness direction: 12 μm or less. Standard deviation of length in the thickness direction of ferrite: 8 μm or less. Average length of perlite in the thickness direction: 8 μm or less and Standard deviation of length in the thickness direction of perlite: 5 μm or less This invention was completed based on the above findings and further considerations.

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

[0013] 1. In mass%, C: not less than 0.17% and not more than 0.30%, Si: not more than 0.80%, Mn: not less than 0.80% and not more than 2.50%, P: not more than 0.04%, S: not more than 0.0060%, Al: not less than 0.01% and not more than 0.08%, N: not more than 0.0150%, Ti: not less than 0.005% and not more than 0.070%, Ca: not less than 0.0001% and not more than 0.0060%, Cr: not less than 0.03% and not more than 1.50% and B: not less than 0.0002% and not more than 0.0050% having a component composition in which the balance is Fe and inevitable impurities, area ratio of ferrite: not less than 40% and not more than 80%, area ratio of pearlite: not less than 20% and not more than 60%, average value of the length of the ferrite in the plate thickness direction: not more than 12 μm, standard deviation of the length of the ferrite in the plate thickness direction: not more than 8 μm, average value of the length of the pearlite in the plate thickness direction: not more than 8 μm and standard deviation of the length of the pearlite in the plate thickness direction: not more than 5 μm The 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 group selected from the following Group A to Group D in mass%. · Group A At least one selected from V: not more than 0.2%, Nb: not more than 0.05%, Mo: not more than 0.3%, Zr: not more than 0.05%, Hf: not more than 0.05% and W: not more than 0.05% · Group B At least one selected from Cu: not more than 0.5% and Ni: not more than 0.5% · Group C At least one selected from Mg: not more than 0.01%, REM: not more than 0.1% and Co: not more than 0.01% · 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 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 in the aforementioned heating step: 1200°C or less. The time from the end of rough rolling in the aforementioned hot rolling process to the start of finish rolling: 10 seconds or more and 120 seconds or less. Starting temperature for finish rolling in the aforementioned hot rolling process: 950°C or lower. The completion temperature for finish rolling in the aforementioned hot rolling process is 790°C or higher and 870°C or lower. Time from the end of finish rolling in the hot rolling process to the start of cooling in the cooling process: 3.0 seconds or less. Average cooling rate in the aforementioned cooling process: 40°C / second or more and Cooling stop temperature in the above cooling process: 590°C or higher and 680°C or lower This is a method for manufacturing hot-rolled steel sheets. [Effects of the Invention]

[0016] 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]

[0017] [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]

[0018] The present invention will be described based on the following embodiments.

[0019] [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 "%".

[0020] C: 0.17% or more and 0.30% 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 0.17% or higher, preferably 0.18% or higher. On the other hand, if the C content exceeds 0.30%, the average value and standard deviation of the length in the thickness direction of the pearlite become excessively large, making it difficult to control the hardness after heat treatment within the desired range when shortening the high-temperature holding time. It also becomes difficult to obtain the desired strength before heat treatment. Therefore, the C content is 0.30% or less, preferably 0.28% or less.

[0021] Si:0.80% or less Si is an element that increases hardenability. Si also has the effect of improving the stability of hardness after heat treatment. However, excessive Si content degrades surface quality. Therefore, the Si content is 0.80% or less, preferably 0.75% or less. The Si content may also be 0%. The Si content is preferably 0.002% or more, more preferably 0.005% or more.

[0022] Mn: 0.80% or more and 2.50% or less Mn is an element that increases hardenability and contributes to an increase in hardness after heat treatment. Furthermore, Mn is a useful element for obtaining the desired strength before heat treatment. Therefore, the Mn content is 0.80% or more, preferably 0.90% or more. On the other hand, if the Mn content exceeds 2.50%, the transformation behavior during 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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: 0.05% 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.

[0032] 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.

[0033] 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.

[0034] The elements in group C are expected to improve the toughness of heat-treated members by altering the morphology of inclusions. However, the effect of each element in group C saturates 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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 and a tensile strength of 600 MPa or less. For this reason, the F area ratio is 40% or more, preferably 45% 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.

[0039] 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.

[0040] 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 contains lamellar ferrite and cementite, but in which they are not arranged in a completely layered manner; specifically, it is 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.

[0041] 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.

[0042] 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%.

[0043] Average length of ferrite in the thickness direction (hereinafter also referred to as average F length): 12 μm or less To achieve the target characteristics described above, particularly when shortening the high-temperature holding time during heat treatment, and to control the hardness after heat treatment within the desired range, it is crucial to establish the microstructure of the hot-rolled steel sheet as the fine FP dispersion structure described above. Specifically, during heating in heat treatment, the temperature of the hot-rolled steel sheet gradually increases from the surface towards the center of the sheet thickness. Furthermore, ferrite has a lower carbon concentration than pearlite. In other words, ferrite transforms to austenite at a higher temperature than pearlite. Also, if coarse ferrite is formed, it transforms from pearlite to austenite, increasing the migration distance of the ferrite / austenite interface. Therefore, to shorten the high-temperature holding time during heat treatment and complete the transformation from ferrite to austenite, it is important to reduce the distance between the austenite nucleation sites in the thickness direction and the ferrite remaining until just before the transformation is complete. This makes it possible to establish the microstructure of the hot-rolled steel sheet as a fine FP dispersion structure. Therefore, it is effective to set the average F length to 12 μm or less, and to set the standard deviation of the length in the thickness direction of the ferrite to 8 μm or less, the average length in the thickness direction of the pearlite to 8 μm or less, and the standard deviation of the length in the thickness direction of the pearlite to 5 μm or less, as described later. The average F length is preferably 10 μm or less. The lower limit of the average F length is not particularly limited. The average F length is preferably 2 μm or more.

[0044] Standard deviation of length in the thickness direction of ferrite (hereinafter also referred to as F-length standard deviation): 8 μm or less As described above, in order to control the hardness after heat treatment to a desired range, particularly when shortening the high-temperature holding time during heat treatment, it is important to make the microstructure of the hot-rolled steel sheet a fine FP dispersion structure. Therefore, the F length standard deviation is 8 μm or less, preferably 5 μm or less. The lower limit of the F length standard deviation is not particularly limited. The F length standard deviation is preferably 0.5 μm or more.

[0045] Average length of perlite in the thickness direction (hereinafter also referred to as average P length): 8 μm or less As described above, in order to control the hardness after heat treatment to a desired range, particularly when shortening the high-temperature holding time during heat treatment, it is important to make the microstructure of the hot-rolled steel sheet a fine FP dispersion structure. Therefore, the average P length is 8 μm or less, preferably 7 μm or less. The lower limit of the average P length is not particularly limited. The average P length is preferably 1 μm or more.

[0046] Standard deviation of length in the thickness direction of perlite (hereinafter also referred to as P-length standard deviation): 5 μm or less As described above, in order to control the hardness after heat treatment to a desired range, particularly when shortening the high-temperature holding time during heat treatment, it is important to make the microstructure of the hot-rolled steel sheet a fine FP dispersion structure. Therefore, the P length standard deviation is 5 μm or less, preferably 4 μm or less. The lower limit of the P length standard deviation is not particularly limited. The P length standard deviation is preferably 0.5 μm or more.

[0047] The average F length, standard deviation of F length, average P length, and standard deviation of P length can be measured, for example, using the same SEM image used to measure the F area ratio and P area ratio, by the sectioning method. That is, 10 test lines with an actual length of 35 μm are drawn in the vertical direction (thickness direction) and 10 lines with a constant length of 3 μm are drawn in the horizontal direction (rolling direction) at regular intervals, for example, 3 μm apart. Then, the average value and standard deviation of the line segment length of the test lines passing through each ferrite (crystal grain) are determined in the above SEM image and are used as the average F length and standard deviation of F length. Similarly, the average value and standard deviation of the line segment length of the test lines passing through each pearlite is determined in the above SEM image and is used as the average P length and standard deviation of P length.

[0048] 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, rapid cooling (quenching), and tempering 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.

[0049] [2] Method for manufacturing hot-rolled steel sheets Next, a method for manufacturing hot-rolled steel sheets according to one 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 the hot-rolled steel sheets 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). 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)]

[0050] ·Heating process First, a steel material having the above-mentioned chemical composition is heated. The steel material can be obtained, for example, by melting steel having the above-mentioned chemical composition in a converter or electric furnace according to conventional methods, and then by continuous casting, ingot casting, or thin slab casting.

[0051] Heating temperature of steel material (hereinafter also simply referred to as heating temperature): 1200℃ or less In the hot rolling process described later, it is necessary to create a fine recrystallized austenite structure in the steel material immediately before finish rolling, and then to perform sufficient rolling in the non-recrystallized austenite temperature range during finish rolling. If the heating temperature exceeds 1200°C, the austenite in the steel material becomes coarser, and fine recrystallized austenite cannot be obtained immediately before finish rolling. As a result, the recrystallization behavior of austenite in the hot rolling process changes, and the fine FP dispersion structure described above cannot be obtained in the final hot-rolled steel sheet. Therefore, the heating temperature should be 1200°C or lower, preferably 1150°C or lower, and more preferably 1130°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, and more preferably 1050°C or higher.

[0052] ·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.

[0053] Time from the end of rough rolling to the start of finish rolling (hereinafter also referred to as the first intermediate residence time): 10 seconds or more and 120 seconds or less As described above, in the hot rolling process, it is necessary to make the microstructure of the rolled material into fine recrystallized austenite immediately before finish rolling. If the first intermediate residence time is less than 10 seconds, finish rolling will begin with the rolled material's microstructure partially containing unrecrystallized austenite. In the regions where this unrecrystallized austenite is located, the reduction amount during finish rolling will be large, and recrystallization will proceed easily. As a result, the fine FP dispersion structure described above will not be obtained in the final hot-rolled steel sheet. For this reason, the first intermediate residence time is 10 seconds or more, preferably 15 seconds or more. On the other hand, if the first intermediate residence time exceeds 120 seconds, the austenite in the rolled material's microstructure will become coarser. As a result, the fine FP dispersion structure described above will not be obtained in the final hot-rolled steel sheet. For this reason, the first intermediate residence time is 120 seconds or less, preferably 90 seconds or less.

[0054] Here, the first intermediate residence time can also be defined as the time it takes for the rolled material to reach the entry point of the (first) first stand (first pass) of finish rolling (the start of finish rolling) from the exit of the final stand (final pass) of rough rolling (the end of rough rolling).

[0055] Starting temperature for finish rolling: 950℃ or less To ensure sufficient rolling in the unrecrystallized austenite temperature range during finish rolling, it is necessary to lower the temperature during finish rolling to suppress the progression of recrystallization. Therefore, the starting temperature for finish rolling is 950°C or lower, preferably 920°C or lower. The lower limit of the starting temperature for finish 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, the starting temperature for finish rolling is preferably 830°C or higher.

[0056] Here, the starting temperature for finish rolling can also be defined as the temperature of the rolled material at the entry point of the (first) first stand (first pass) of the finish rolling process (the start time of the finish rolling).

[0057] Finishing rolling completion temperature: 790°C to 870°C To ensure sufficient rolling in the unrecrystallized austenite temperature range during finish rolling, it is necessary to lower the finishing rolling temperature to suppress the progression of recrystallization. Therefore, the finishing rolling completion temperature is 870°C or lower, preferably 860°C or lower. On the other hand, if the finishing rolling completion temperature is below 790°C, it becomes two-phase rolling, which may significantly reduce formability. Therefore, the finishing rolling temperature is 790°C or higher, preferably 800°C or higher.

[0058] 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).

[0059] ·Cooling process Next, the hot-rolled steel sheet obtained in the hot-rolling process is cooled.

[0060] Time from the end of the finish rolling process in the hot rolling stage to the start of the cooling process in the cooling stage (hereinafter also referred to as the second intermediate residence time): 3.0 seconds or less To obtain the above-mentioned fine FP dispersion structure in the final hot-rolled steel sheet, it is necessary to suppress the recrystallization of austenite in the hot-rolled steel sheet obtained during the hot-rolling process. Therefore, it is necessary to start cooling immediately after the completion of finish rolling. Accordingly, the second intermediate residence time is 3.0 seconds or less, preferably 2.0 seconds or less. The lower limit of the second intermediate residence time is not particularly limited and may be 0 seconds (i.e., cooling may be started immediately after the completion of finish rolling).

[0061] Average cooling rate: 40℃ / sec or more To obtain the above-mentioned fine FP dispersion structure in the final hot-rolled steel sheet, it is effective to increase the cooling rate in the cooling process. However, if the average cooling rate is less than 40°C / second, coarse ferrite will be formed, and the above-mentioned fine FP dispersion structure will not be obtained in the final hot-rolled steel sheet. Therefore, 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.

[0062] Cooling stop temperature: 590℃ or higher and 680℃ or lower If the cooling stop temperature is below 590°C, bainite, martensite, and retained austenite are excessively 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 590°C or higher, preferably 600°C or higher. On the other hand, if the cooling stop temperature exceeds 680°C, coarse ferrite is formed, and the above-mentioned fine FP dispersion structure cannot be obtained in the final hot-rolled steel sheet product. Therefore, the cooling stop temperature is 680°C or lower, preferably 670°C or lower.

[0063] The cooling method is not particularly limited and can include, for example, air cooling, water cooling, or oil cooling.

[0064] ·Winding process After the cooling process, the hot-rolled steel sheet is wound up. The winding conditions are not particularly limited and can be done according to conventional methods. The winding temperature is preferably, for example, 580 to 670°C.

[0065] 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%.

[0066] 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. [Examples]

[0067] The present invention will be further explained by the following embodiments. However, the present invention is not limited to the following embodiments.

[0068] 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.

[0069] Using the hot-rolled steel sheets thus obtained, the F area ratio, P area ratio, average F length, standard deviation of F length, average P length, and standard deviation of P length were measured according to the procedure described above. The results are shown in Table 3.

[0070] Furthermore, the strength (yield strength and tensile strength) before heat treatment and the 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.

[0071] <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 and tensile 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 values ​​of the yield strength and tensile strength measured over the five tests were taken as the yield strength and tensile strength of the hot-rolled steel sheet, respectively. The pass / fail criteria for the strength before heat treatment were then determined according to the following criteria. [Judgment criteria] Approval: Satisfies both the yield strength of 350 MPa to 550 MPa and the tensile strength of 440 MPa to 600 MPa. Failure: Does not satisfy at least one of the following conditions: yield strength of 350 MPa or more and 550 MPa or less, and tensile strength of 440 MPa or more and 600 MPa or less.

[0072] <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) • Tempering conditions Tempering temperature: 200°C, Tempering time (holding time at tempering temperature): 30 minutes, Cooling after tempering: Air cooling [Judgment criteria] Passing criteria: Vickers hardness between 360 and 530. Failure: Vickers hardness less than 360 or greater than 530

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[0076] As shown in Table 3, in all of the inventive examples, the desired strength before heat treatment and hardness after heat treatment were obtained.

[0077] 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: 0.17% or more and 0.30% or less, Si: 0.80% or less, Mn: 0.80% 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 length of the ferrite in the thickness direction: 12 μm or less. Standard deviation of the length of the ferrite in the thickness direction: 8 μm or less. The average length of the perlite in the thickness direction: 8 μm or less and Standard deviation of the length of the perlite in the thickness direction: 5 μm or less 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: 0.05% 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 length of the ferrite in the thickness direction: 12 μm or less. Standard deviation of the length of the ferrite in the thickness direction: 8 μm or less. The average length of the perlite in the thickness direction: 8 μm or less and Standard deviation of the length of the perlite in the thickness direction: 5 μm or less 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, Heating temperature in the aforementioned heating step: 1200°C or less. The time from the end of rough rolling in the aforementioned hot rolling process to the start of finish rolling: 10 seconds or more and 120 seconds or less. The starting temperature for finish rolling in the aforementioned hot rolling process is 950°C or lower. The completion temperature for finish rolling in the aforementioned hot rolling process: 790°C or higher and 870°C or lower. Time from the end of finish rolling in the hot rolling process to the start of cooling in the cooling process: 3.0 seconds or less. Average cooling rate in the aforementioned cooling process: 40°C / second or more and Cooling stop temperature in the above cooling process: 590°C or higher and 680°C or lower This is a method for manufacturing hot-rolled steel sheets.

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