Hot-rolled steel sheet and production method of same

Abstract

Provided is a hot-rolled steel sheet that has low strength before heat treatment, and the hardness thereof after heat treatment can be controlled within a desired range even when a high-temperature holding time in the heat treatment is shortened. In the present invention, the component composition is properly controlled to obtain a structure in which: the area ratio of ferrite is 40% to 80% inclusive; the area ratio of pearlite is 20% to 60% inclusive; the average value of the length of ferrite in the sheet thickness direction is 12 μm or less; the standard deviation of the length of ferrite in the sheet thickness direction is 8 μm or less; the average value of the length of pearlite in the sheet thickness direction is 8 μm or less; and the standard deviation of the length of pearlite in the sheet thickness direction is 5 μm or less.

Description

Hot-rolled steel sheet and method for producing the same

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

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

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

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

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

[0006] By the way, in recent years, from the perspective of global environmental conservation, further reduction of CO ２ emissions has been sought in the entire industrial sector. Here, in the above heat treatment, the hardness is increased by rapid cooling after heating. Therefore, in the above heat treatment, it is necessary to heat the steel sheet to the austenite region. Also, in the above heat treatment, after heating the steel sheet to the austenite region, in order to sufficiently perform the transformation from ferrite to austenite, it is held in the austenite region. Since a large amount of energy is required for this holding in the austenite region, by shortening the holding time in the austenite region (hereinafter also referred to as the high-temperature holding time), a significant reduction in the amount of energy and thus in CO ２ emissions can be expected.

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

[0008] The present invention was developed to meet the above-mentioned requirements, and aims to provide a hot-rolled steel sheet that has low strength before heat treatment and can control the hardness after heat treatment to a desired range even when the high-temperature holding time during heat treatment is shortened, along with an advantageous manufacturing method thereof. In this 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.

[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 F-P fine dispersion structure), and in particular to control the area ratio of ferrite as follows: Area ratio of ferrite: 40% or more and 80% or less, Area ratio of pearlite: 20% or more and 60% or less, Average length of ferrite in the thickness direction: 12 μm or less, Standard deviation of length of ferrite in the thickness direction: 8 μm or less, Average length of pearlite in the thickness direction: 8 μm or less and standard deviation of length of pearlite in the thickness direction: 5 μm or less. The present invention was completed by further studies based on the above findings.

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

[0013] 1. The composition is such that, in mass%, C: 0.17% to 0.30%, Si: 0.80% or less, Mn: 0.80% to 2.50%, P: 0.04% or less, S: 0.0060% or less, Al: 0.01% to 0.08%, N: 0.0150% or less, Ti: 0.005% to 0.070%, Ca: 0.0001% to 0.0060%, Cr: 0.03% to 1.50%, and B: 0.0002% to 0.0050%, with the remainder being Fe and unavoidable impurities, the area ratio of ferrite: 40% to 80%, the area ratio of pearlite: 20% to 60%, and the average length of the ferrite in the thickness direction: 12 μm or less. A hot-rolled steel sheet having a structure in which the standard deviation of the length of the ferrite in the thickness direction is 8 μm or less, the average value of the length of the pearlite in the thickness direction is 8 μm or less, and the standard deviation of the length of the pearlite in the thickness direction is 5 μm or less.

[0014] 2. The hot-rolled steel sheet according to claim 1, wherein the component composition, in mass%, further comprises at least one of the following groups A to D. - Group A: At least one selected from V: 0.2% or less, Nb: 0.05% 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.

[0015] 3. A method for manufacturing a hot-rolled steel sheet, comprising: a heating step of heating a steel material having the component composition described in 1 or 2 above; a hot-rolling step of subjecting the steel material after the heating step to rough-rolling and finish-rolling to obtain a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step; and a winding step of winding the hot-rolled steel sheet after the cooling step, wherein the heating temperature in the heating step is 1200°C or less; the time from the end of the rough-rolling in the hot-rolling step to the start of the finish-rolling step is 10 seconds or more and 120 seconds or less; the starting temperature for the finish-rolling in the hot-rolling step is 950°C or less; the completion temperature for the finish-rolling in the hot-rolling step is 790°C or more and 870°C or less; the time from the end of the finish-rolling in the hot-rolling step to the start of cooling in the cooling step is 3.0 seconds or less; the average cooling rate in the cooling step is 40°C / second or more and the cooling stop temperature in the cooling step is 590°C or more and 680°C or less.

[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 a heat-treated component, the high-temperature holding time can be shortened, thus reducing the amount of energy and, consequently, CO2. ２ Significant reductions in emissions are expected, making it extremely advantageous from an industrial perspective.

[0017] This is a schematic diagram showing an example of the form of a perlite block that falls under the category of pseudo-perlite.

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

[0019] [1] Hot-rolled steel sheet First, the component composition of a hot-rolled steel sheet according to one embodiment of the present invention will be described. Note that all units in the component composition are "mass%", and unless otherwise specified, they will be simply referred to as "%".

[0020] C: 0.17% or more and 0.30% or less. C is an important element for ensuring the desired hardness after heat treatment. It is also an important element for suppressing the formation of a soft structure during rapid cooling after heat treatment. Therefore, the C content is 0.17% or more, preferably 0.18% or more. 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 the high-temperature holding time is shortened. 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, if the Si content is excessive, the surface quality deteriorates. For this reason, the Si content is 0.80% or less, preferably 0.75% or less. The Si content may be 0%. The Si content is preferably 0.002% or more, more preferably 0.005% or more.

[0022] Mn: 0.80% to 2.50%. Mn is an element that increases hardenability and contributes to an increase in hardness after heat treatment. Mn is also 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 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.

[0023] P: 0.04% or less. P is a harmful element that reduces the toughness of heat-treated members by segregating at grain boundaries. Therefore, it is preferable to reduce P as much as possible, but a P content of up to 0.04% is acceptable. Thus, the P content is 0.04% or less, preferably 0.03% or less. The P content may also be 0%. However, P may inevitably be mixed in during manufacturing, and excessive removal of P leads to increased costs. Therefore, the P content is preferably 0.002% or more.

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

[0026] N: 0.0150% or less. N is a harmful element that reduces the toughness of heat-treated members by combining with Ti to form coarse TiN. Therefore, it is preferable to reduce N as much as possible, but an N content of up to 0.0150% is acceptable. Thus, the N content is 0.0150% or less, preferably 0.0090% or less, and more preferably 0.0070% or less. The N content may be 0%. However, N may inevitably be mixed in during manufacturing, and excessive removal of N leads to increased costs. Therefore, the N content is preferably 0.0005% or more.

[0027] Ti: 0.005% or more and 0.070% or less. Ti has the effect of suppressing room-temperature aging by solid-solution N by bonding with N. To obtain this effect, the Ti content is 0.005% or more, preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.070%, carbides containing coarse Ti will be dispersed, reducing the toughness of the heat-treated member. For this reason, the Ti content is 0.070% or less, preferably 0.060% or less.

[0028] Ca: 0.0001% or more and 0.0060% or less. Ca, by bonding with S, reduces wedge-shaped S-type inclusions and has the effect of improving the toughness of the heat-treated member. To obtain this effect, the Ca content is 0.0001% or more, preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0060%, the above effect becomes saturated. Therefore, the Ca content is 0.0060% or less, preferably 0.0050% or less.

[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. - 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.

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

[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% or more and 80% or less. Ferrite is a soft structure. In order to ensure the formability required for heat-treated steel sheets, it is necessary to lower the strength, in particular, to have 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 pearlite area ratio described 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% or more and 60% or less. From the viewpoint of ensuring the formability required for heat-treated steel sheets, the P area ratio is 20% or more, preferably 25% or more. Also, the P area ratio is 60% or less, preferably 50% or less.

[0040] Here, the term "perlite" includes both perlite in the narrow sense and pseudo-perlite. Perlite in the narrow 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 narrow 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, 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 conventional methods. For example, a sample is cut from a hot-rolled steel sheet so that the cross section parallel to the rolling direction (hereinafter also referred to as the L section) becomes the observation surface. Then, the observation surface of the sample is etched with 1 volume% nital. Next, the observation surface of the sample is photographed with a scanning electron microscope (SEM) at 2000x magnification for 10 fields, with the center being at the 1 / 4 thickness position in the thickness direction of the hot-rolled steel sheet. In the SEM image, ferrite is observed as gray contrast crystal grains in which no corrosion traces are observed within the crystal grains. Cementite is observed as white contrast precipitates in the SEM image. Perlite (perlite in the narrow sense and pseudo-perlite) is observed and defined as described above. Next, using image analysis software (Photoshop elements and Image J), ​​the F area ratio and P area ratio are calculated from the total area occupied by ferrite and pearlite in 10 fields of view of the SEM images. The area ratio of the remaining tissue is then calculated by subtracting the F area ratio and P area ratio obtained above from 100%.

[0043] Average value of the length of ferrite in the plate thickness direction (hereinafter also referred to as F length average value): 12 μm or less For the above target properties, especially when shortening the high-temperature holding time during heat treatment, in order to control the hardness after heat treatment within a desired range, it is important to make the structure of the hot-rolled steel sheet the above-mentioned F-P fine-dispersed structure. That is, during heating in heat treatment, the temperature of the hot-rolled steel sheet gradually rises from the surface towards the center of the plate thickness. Also, ferrite has a lower C concentration than pearlite. That is, ferrite has a higher transformation temperature to austenite than pearlite. Also, when coarse ferrite occurs, pearlite transforms to austenite, and the migration distance of the ferrite / austenite interface becomes longer. Therefore, in order 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 plate thickness direction and the ferrite remaining until just before the completion of transformation. Thereby, it becomes possible to make the structure of the hot-rolled steel sheet the F-P fine-dispersed structure. Therefore, it is effective to make the F length average value 12 μm or less, and the standard deviation of the length of ferrite in the plate thickness direction, which will be described later, 8 μm or less, the average value of the length of pearlite in the plate thickness direction 8 μm or less, and the standard deviation of the length of pearlite in the plate thickness direction 5 μm or less. The F length average value is preferably 10 μm or less. The lower limit of the F length average value is not particularly limited. The F length average value is preferably 2 μm or more.

[0044] Standard deviation of the length of ferrite in the plate thickness direction (hereinafter also referred to as F length standard deviation): 8 μm or less As described above, for the above target properties, especially when shortening the high-temperature holding time during heat treatment, in order to control the hardness after heat treatment within a desired range, it is important to make the structure of the hot-rolled steel sheet the F-P fine-dispersed 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 value of the length of pearlite in the plate thickness direction (hereinafter also referred to as P length average value): 8 μm or less As described above, when shortening the high-temperature holding time in the heat treatment for the above target characteristics, especially, in order to control the hardness after heat treatment within a desired range, it is important to make the structure of the hot-rolled steel sheet into a fine-dispersed F-P structure. Therefore, the P length average value is 8 μm or less, preferably 7 μm or less. The lower limit of the P length average value is not particularly limited. The P length average value is preferably 1 μm or more.

[0046] Standard deviation of the length of pearlite in the plate thickness direction (hereinafter also referred to as P length standard deviation): 5 μm or less As described above, when shortening the high-temperature holding time in the heat treatment for the above target characteristics, especially, in order to control the hardness after heat treatment within a desired range, it is important to make the structure of the hot-rolled steel sheet into a fine-dispersed F-P 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 F length average value, F length standard deviation, P length average value, and P length standard deviation may be measured by the cutting method, for example, using the SEM photograph used for measuring the F area ratio and P area ratio. That is, on the SEM photograph, draw 10 test lines with an actual length of 35 μm in the vertical direction (plate thickness direction) at regular intervals, for example, at intervals of 3 μm in the horizontal direction (rolling direction). Then, in the above SEM photograph, obtain the average value and standard deviation of the line segment lengths of the test lines passing through each ferrite (crystal grain), and use them as the F length average value and F length standard deviation. Similarly, in the above SEM photograph, obtain the average value and standard deviation of the line segment lengths of the test lines passing through each pearlite, and use them as the P length average value and P length standard deviation.

[0048] The plate thickness of the hot-rolled steel sheet according to an embodiment of the present invention is preferably, for example, 1.0 mm or more and 8.0 mm or less. Further, the hot-rolled steel sheet according to an embodiment of the present invention is suitable as a steel sheet for heat treatment. Examples of the heat treatment include a mode in which heating, holding, rapid cooling (quenching), and tempering are performed on the hot-rolled steel sheet after being formed into a predetermined shape, as described in the measurement of the 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 hot-rolled steel sheets as described above. Unless otherwise specified, each temperature in each process shall be the surface temperature of the steel material and the steel sheet (hot-rolled steel sheet). Unless otherwise specified, the average cooling rate shall also be the surface temperature of the steel material and the steel sheet (hot-rolled steel sheet). Unless otherwise specified, the average cooling rate can be calculated by the following formula. The cooling start temperature may be, for example, the temperature at the end of the immediately preceding process. [Average cooling rate (°C / sec)] = ([Cooling start temperature (°C)] - [Cooling stop temperature (°C)]) / [Cooling time (seconds)]

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

[0051] Heating temperature of steel material (hereinafter also simply referred to as heating temperature): 1200°C or less In the hot rolling process described later, it is necessary to make the structure of the steel material into fine recrystallized austenite immediately before finish rolling, and then to perform sufficient rolling in the unrecrystallized 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 above-mentioned F-P fine dispersion structure cannot be obtained in the hot-rolled steel sheet that becomes the final product. For this reason, the heating temperature is 1200°C or less, preferably 1150°C or less, and more preferably 1130°C or less. Also, the heating temperature is preferably 1000°C or higher, and more preferably 1050°C or higher, based on the conditions of the hot rolling process described later.

[0052] - In the hot rolling process, the steel material (hereinafter also referred to as the rolled material) that has been heated in the heating process is subjected to rough rolling and finish rolling to produce hot-rolled steel sheet.

[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 start 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 above-mentioned F-P fine dispersion structure 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 microstructure of the rolled material will become coarser. As a result, the above-mentioned F-P fine dispersion structure will not be obtained in the final hot-rolled steel sheet. Therefore, 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°C or lower. In order to adequately perform 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. For this reason, 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 of the finish rolling).

[0057] Completion temperature of finish rolling: 790°C or higher and 870°C or lower. In order to adequately perform 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. For this reason, the completion temperature of finish rolling is 870°C or lower, preferably 860°C or lower. On the other hand, if the completion temperature of finish rolling is below 790°C, it will become two-phase rolling, and the formability may be significantly reduced. For this reason, the finish 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] Next, the hot-rolled steel sheet obtained in the hot-rolling process is cooled.

[0060] The time from the end of the finish rolling in the hot rolling process to the start of cooling in the cooling process (hereinafter also referred to as the second intermediate residence time): 3.0 seconds or less. In order to obtain the above-mentioned F-P fine dispersion structure in the hot-rolled steel sheet that becomes the final product, it is necessary to suppress the recrystallization of austenite in the hot-rolled steel sheet obtained in the hot rolling process. For this reason, it is necessary to start cooling as soon as possible after the completion of the finish rolling. Thus, 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 start immediately after the completion of the finish rolling).

[0061] Average cooling rate: 40°C / second or higher. In order to obtain the above-mentioned F-P fine dispersion structure in the final hot-rolled steel sheet product, it is effective to increase the cooling rate in the cooling process. If the average cooling rate is less than 40°C / second, coarse ferrite will be formed, and the above-mentioned F-P fine dispersion structure will not be obtained in the final hot-rolled steel sheet product. 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°C or higher and 680°C or lower. If the cooling stop temperature is below 590°C, bainite, martensite, and retained austenite will be excessively formed, and the above-mentioned fine F-P dispersion structure cannot be obtained. As a result, the desired pre-heat treatment strength cannot be obtained. For this reason, the cooling stop temperature is 590°C or higher, preferably 600°C or higher. On the other hand, if the cooling stop temperature exceeds 680°C, coarse ferrite will be formed, and the above-mentioned fine F-P dispersion structure cannot be obtained in the final hot-rolled steel sheet product. For this reason, 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] After the winding and 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 conventional methods should be followed. For example, in the hot rolling process, it is preferable that the number of stands (passes) for rough rolling is 2 to 5, the number of stands (passes) for finish rolling is 5 to 7, and the total reduction ratio is 90% or more.

[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 250 mm 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 to 0.5% and pickling to produce hot-rolled steel sheets with a thickness of 1.6 to 7.2 mm.

[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 sheet, a JIS No. 5 tensile test specimen was 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 determined according to the following criteria. [Criteria] Pass: Satisfies both the yield strength of 350 MPa or more and 550 MPa or less, and the tensile strength of 440 MPa or more and 600 MPa or less. Fail: Does not satisfy at least one of the following criteria: 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. Then, the Vickers hardness was measured at the 1 / 4 position of the plate thickness in the L-section of the heat-treated test specimen by a Vickers hardness test in accordance with JIS Z 2244-1:2020. The test force was 1 kgf, and the holding time of the test force was 15 seconds. The Vickers hardness test was performed five times for each hot-rolled steel sheet, and the average value of the Vickers hardness measured over the five tests was taken as the hardness after heat treatment of the hot-rolled steel sheet. The pass / fail status of the hardness after heat treatment was determined according to the following criteria. [Heat Treatment Conditions] ・Heating conditions: Average heating rate from room temperature to 950°C: 100°C / second ・Holding conditions: Holding temperature: 950°C, Holding time at holding temperature: 10 seconds ・Quenching conditions: Oil cooling (cooling by immersion in oil set to 70°C) ・Tempering conditions: Tempering temperature: 200°C, Tempering time (holding time at tempering temperature): 30 minutes, Cooling after tempering: Air cooling [Judgment Criteria] Pass: Vickers hardness of 360 or more and 530 or less Fail: Vickers hardness of less than 360 or greater than 530

[0073]

[0074]

[0075]

[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. The composition is such that, in mass%, C: 0.17% to 0.30%, Si: 0.80% or less, Mn: 0.80% to 2.50%, P: 0.04% or less, S: 0.0060% or less, Al: 0.01% to 0.08%, N: 0.0150% or less, Ti: 0.005% to 0.070%, Ca: 0.0001% to 0.0060%, Cr: 0.03% to 1.50%, and B: 0.0002% to 0.0050%, with the remainder being Fe and unavoidable impurities, the area ratio of ferrite: 40% to 80%, the area ratio of pearlite: 20% to 60%, and the average length of the ferrite in the thickness direction: 12 μm or less. A hot-rolled steel sheet having a structure in which the standard deviation of the length of the ferrite in the thickness direction is 8 μm or less, the average value of the length of the pearlite in the thickness direction is 8 μm or less, and the standard deviation of the length of the pearlite in the thickness direction is 5 μm or less.

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. A method for manufacturing a hot-rolled steel sheet, comprising: a heating step of heating a steel material having the component composition described in claim 1 or 2; a hot-rolling step of subjecting the steel material after the heating step to rough-rolling and finish-rolling to obtain a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet after the hot-rolling step; and a winding step of winding the hot-rolled steel sheet after the cooling step, wherein the heating temperature in the heating step is 1200°C or less; the time from the end of the rough-rolling in the hot-rolling step to the start of the finish-rolling step is 10 seconds or more and 120 seconds or less; the starting temperature for the finish-rolling in the hot-rolling step is 950°C or less; the completion temperature for the finish-rolling in the hot-rolling step is 790°C or more and 870°C or less; the time from the end of the finish-rolling in the hot-rolling step to the start of cooling in the cooling step is 3.0 seconds or less; the average cooling rate in the cooling step is 40°C / second or more and the cooling stop temperature in the cooling step is 590°C or more and 680°C or less.

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