The present invention relates to a hot-rolled steel sheet, and more particularly to a hot-rolled steel sheet having excellent steel sheet shape and toughness. The present application claims priority based on Japanese Patent Application No. 2018-079352 filed in Japan on April 17, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　In recent years, efforts have been actively made to reduce the weight of the vehicle body by utilizing a high-strength thin steel plate for the purpose of improving the fuel efficiency and collision safety of automobiles. However, when the strength of the steel sheet is increased, the toughness generally deteriorates. In particular, it is important to ensure collision characteristics in hot-rolled steel sheets applied to automobile members. Here, it is generally known that toughness is improved by rolling at a low temperature and applying a high cumulative strain with unrecrystallized austenite. However, in high cumulative strain and low temperature rolling, the rolling load is high, the steel sheet cannot be thinned, and it becomes difficult to finely control the shape of the steel sheet.
[0003]
　On the other hand, in Patent Document 1, the volume ratio of unrecrystallized austenite is increased by setting the reduction rate and the average strain rate at 860 to 960 ° C., where austenite is in the unrecrystallized region, within an appropriate range, and the volume ratio of unrecrystallized austenite is increased by hot rolling. A cold-rolled steel sheet in which the toughness of the cold-rolled steel sheet is improved from the fine-grained structure that has been created has been proposed. However, if the reduction rate of unrecrystallized austenite is increased, the strength of the steel sheet increases, and it becomes difficult to finely control the shape of the steel sheet.
[0004]
　In Patent Document 2, the finishing temperature is raised and the rolling reduction ratio of 1000 ° C. or lower is increased to promote the recrystallization of austenite, and the time until cooling after rolling is shortened to suppress the coarsening of crystal grains. Has been proposed. However, if the rolling reduction ratio is increased, it becomes difficult to predict the deformation resistance during rolling, and it becomes difficult to finely control the shape of the steel sheet due to the increase in the rolling load.
[0005]
　Patent Document 3 proposes a method for producing a fine-grained steel sheet having an excellent shape by utilizing a CVC roll and an extremely small diameter roll. However, when the CVC roll is used, the strain distribution is adjusted in the width direction in order to stabilize the shape, and a uniform structure in the width direction cannot be obtained. Further, when the extremely small diameter roll is used, the contact time of the steel sheet is shortened, so that the strain rate is increased and the rolling anisotropy is strengthened.
Prior art literature
Patent documents
[0006]
Patent Document 1: Japanese Patent No. 3858146
Patent Document 2: Japanese Patent No. 50686888
Patent Document 3: Japanese Patent No. 3418738
Outline of the invention
Problems to be solved by the invention
[0007]
　In recent years, there has been an increasing demand for increasing the strength of steel sheets and reducing the thickness of steel sheets in order to achieve both safety and fuel efficiency of automobiles. That is, a thin hot-rolled steel sheet with excellent collision characteristics and toughness is required.
　The present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent toughness, and excellent steel sheet shape.
Means to solve problems
[0008]
　Conventionally, various efforts have been made to increase the cumulative reduction rate of unrecrystallized austenite and to miniaturize the structure in order to improve the toughness of steel. On the other hand, with these methods, the rolling load is very high and the steel sheet cannot be thinned. The present inventors have diligently studied a method for forming a fine-grained structure of austenite required for toughness in a rolling stand that is continuous at high speed such as finish rolling without increasing the rolling load. As a result, it was found that the hot deformation resistance does not increase in a specific temperature and strain rate range, and a fine-grained austenite structure can be obtained. Specifically, it was confirmed that the steel plate structure can be miniaturized without increasing the rolling load by controlling the contact time between the steel plate and the roll and the inlet temperature of the plate material (steel plate) during rolling.
[0009]
　The present invention has been made based on the above findings, and the gist of the present invention is as follows.
[1] In mass%,
C: 0.10% or more, 0.50% or less,
Si: 0.10% or more, 3.00% or less,
Mn: 0.5% or more, 3.0% or less,
P : 0.10% or less,
S: 0.0100% or less,
Al: 1.00% or less,
N: 0.010% or less,
Ti: 0% or more, 0.20% or less,
Nb: 0% or more, 0 .100% or less,
Ca: 0% or more, 0.0060% or less,
Mo: 0% or more, 0.50% or less,
Cr: 0% or more, 1.00% or less, and the
　balance is Fe and impurities There,
　the average particle size of the old austenite structure is at 0.1μm or 3.0μm or less,
　and the thickness of the plate width central portion, toward the plate width central portion along the sheet width direction from the plate width end 10mm A hot-rolled steel plate having a plate crown amount of 80 μm or less, which is the difference from the plate thickness at the separated portions.
[2] In mass%,
Ti: 0.02% or more, 0.20% or less,
Nb: 0.010% or more, 0.100% or less,
Ca: 0.0005% or more, 0.0060% or less,
Mo: 0.02% or more, 0.50% or less,
Cr: 0.02% or more, 1.00% or less
containing one or more types The hot-rolled steel sheet according to [1].
The invention's effect
[0010]
　According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having an excellent product shape, high strength and excellent toughness. According to this hot-rolled steel sheet, the absorbed energy at the time of high-speed deformation is high, the collision characteristics are improved as an automobile part, the weight of the car body such as an automobile can be reduced, and the size of the press-molded part can be increased, resulting in improved fuel efficiency. , The manufacturing cost can be reduced.
Mode for carrying out the invention
[0011]
　In order to improve the toughness of steel, various efforts have been made to increase the cumulative reduction rate of unrecrystallized austenite and to miniaturize the structure. On the other hand, with these methods, the rolling load is very high and the steel sheet cannot be thinned. The present inventors have diligently studied a method for forming a fine-grained structure of austenite required for toughness in a rolling stand that is continuous at high speed such as finish rolling without increasing the rolling load. As a result, it was found that the hot deformation resistance does not increase in a specific temperature and strain rate range, and a fine-grained austenite structure can be obtained. Specifically, it was confirmed that the steel sheet structure can be miniaturized without increasing the rolling load by controlling the contact time between the steel sheet and the rolling roll of the final stand and the temperature at the rolling entrance side.
[0012]
　Hereinafter, the hot-rolled steel sheet according to the embodiment of the present invention will be described. The hot-rolled steel sheet according to the present embodiment can be obtained by controlling heat transfer and recrystallization during hot finish rolling. By adjusting the temperature at which the steel sheet penetrates into the final stand of finish rolling and the contact time between the steel sheet and the rolling roll of the final stand, the temperature drop due to heat removal from the surface of the steel sheet and the recrystallization temperature are balanced. As a result, an increase in deformation resistance due to rolling is suppressed, and the temperature required for forming a fine recrystallized structure is secured. By recrystallization during hot rolling, the increase in rolling load is suppressed, and while obtaining high toughness, the plate thickness at the center of the plate width and the plate width from the end of the plate width to the center of the plate width It is possible to control the amount of plate crown, which is the difference from the plate thickness at locations separated by 10 mm. Specifically, the hot-rolled steel sheet according to the present embodiment has a predetermined chemical composition, has a structure in which the average particle size of the former austenite grains is 0.1 μm or more and 3.0 μm or less, and has a central portion of the plate width. The difference between the plate thickness (the central part in the width direction of the steel plate) and the plate thickness at a location 10 mm away from the plate width end (the end in the width direction of the steel plate) toward the center of the plate width in the plate width direction. A certain plate crown amount is 80 μm or less.
[0013]
　Hereinafter, the individual constituent elements of the present invention will be described in detail. First, the reason for limiting the chemical composition (chemical composition) of the hot-rolled steel sheet according to the present embodiment will be described. % With respect to the component content means mass%.
[0014]

　C is an important element for improving the strength of the steel sheet. In order to obtain the desired strength, the lower limit of the C content needs to be 0.10% or more. The lower limit of the C content is preferably 0.25% or more. However, if the C content exceeds 0.50%, the toughness of the steel sheet deteriorates. Therefore, the upper limit of the C content is 0.50% or less.
[0015]

　Si is an element having an effect of improving the strength of a steel sheet. In order to obtain this effect, the lower limit of the Si content is set to 0.10% or more. The lower limit of the Si content is preferably 0.50% or more. On the other hand, if the Si content exceeds 3.00%, the toughness of the steel sheet deteriorates. Therefore, the upper limit of the Si content is set to 3.00% or less. The upper limit of the Si content is preferably 2.50% or less.
[0016]

　Mn is an element effective for improving the strength of a steel sheet by improving hardenability and strengthening solid solution. In order to obtain this effect, the lower limit of the Mn content is set to 0.5% or more. The lower limit of the Mn content is preferably 1.0% or more. On the other hand, when the Mn content exceeds 3.0%, MnS harmful to the isotropic toughness is generated. Therefore, the upper limit of the Mn content is set to 3.0% or less. The upper limit of the Mn content is preferably 2.0% or less.
[0017]

　P is an impurity, and the lower the P content, the more desirable. That is, when the P content exceeds 0.100%, the workability and weldability are significantly lowered, and the fatigue characteristics are also lowered. Therefore, the upper limit of the P content is limited to 0.100% or less. The upper limit of the P content is preferably 0.050% or less.
[0018]

　S is an impurity, and the lower the S content, the more desirable. When the S content exceeds 0.010%, inclusions such as MnS, which are harmful to the isotropic toughness, are prominently produced. Therefore, the upper limit of the S content is limited to 0.010% or less. When particularly severe low temperature toughness is required, the upper limit of the S content is preferably 0.006% or less.
[0019]

　Al is an element required for deoxidation in the steelmaking process. However, when the Al content exceeds 1.00%, alumina precipitated in clusters is generated, and the toughness deteriorates. Therefore, the upper limit of the Al content is set to 1.00% or less. The upper limit of the Al content is preferably 0.50% or less.
[0020]

　N is an impurity. When the N content is more than 0.010%, coarse Ti nitride is formed at a high temperature, and the toughness of the steel sheet deteriorates. Therefore, the upper limit of the N content is set to 0.010% or less. The upper limit of the N content is preferably 0.006% or less.
[0021]
　The hot-rolled steel sheet according to the present embodiment basically contains the above chemical components and the balance is composed of Fe and impurities. Here, the impurity means a component mixed with a raw material such as ore and scrap, and other factors when the steel material is industrially manufactured. However, although not essential for satisfying the required characteristics, Ti, Nb, Ca, Mo, and Cr may be contained in the following ranges in order to reduce manufacturing variations and further improve the strength. However, since Ti, Nb, Ca, Mo, and Cr are not essential to satisfy the required characteristics, the lower limit of their content is 0%.
[0022]

　Ti is an element effective for suppressing recrystallization and grain growth of austenite. By containing 0.02% or more of Ti, the effect of suppressing recrystallization and grain growth can be obtained. The lower limit of the Ti content is preferably 0.08% or more. On the other hand, when the Ti content is more than 0.20%, inclusions due to TiN are generated and the toughness of the steel sheet is deteriorated. Therefore, the upper limit of the Ti content is set to 0.20% or less. The upper limit of the Ti content is preferably 0.16% or less.
[0023]

　Nb is an element effective for suppressing recrystallization and grain growth of austenite. When this effect is obtained, the lower limit of the Nb content is preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the effect is saturated. Therefore, even when Nb is contained, the upper limit of the Nb content is set to 0.100% or less. A more preferable upper limit of the Nb content is 0.060% or less.
[0024]

　Ca is an element having an effect of dispersing a large number of fine oxides during deoxidation of molten steel and making the structure of the steel sheet finer. Further, Ca is an element that fixes S in steel as spherical CaS, suppresses the formation of stretching inclusions such as MnS, and improves the anisotropy of toughness. When these effects are obtained, the lower limit of the Ca content is preferably 0.0005% or more. On the other hand, even if the Ca content exceeds 0.0060%, the effect is saturated. Therefore, even when Ca is contained, the upper limit of the Ca content is set to 0.0060% or less. A more preferable upper limit of the Ca content is 0.0040% or less.
[0025]

　Mo is an element effective for strengthening precipitation of ferrite. When this effect is obtained, the Mo content is preferably 0.02% or more. A more preferable lower limit of the Mo content is 0.10% or more. On the other hand, if the Mo content is excessive, the slab is more susceptible to cracking, making it difficult to handle the slab. Therefore, even when Mo is contained, the upper limit of the Mo content is set to 0.50% or less. A more preferable upper limit of the Mo content is 0.30% or less.
[0026]

　Cr is an element effective for improving the strength of the steel sheet. When this effect is obtained, the lower limit of the Cr content is preferably 0.02% or more. The lower limit of the Cr content is more preferably 0.10% or more. On the other hand, if the Cr content becomes excessive, the ductility decreases. Therefore, even when Cr is contained, the upper limit of the Cr content is set to 1.00% or less. A more preferable upper limit of the Cr content is 0.80% or less.
[0027]
　Next, the structure of the hot-rolled steel sheet according to the present embodiment will be described.
　The hot-rolled steel sheet according to the present embodiment has a structure in which the former austenite is finely recrystallized. Since the toughness of the hot-rolled steel sheet largely depends on the average crystal grain size of the former austenite, the deformed structure, that is, the steel sheet structure does not matter. Generally, a single phase is preferable in order to improve toughness. For example, in high-strength steel, a martensite single phase is preferable, but the present embodiment is not limited to the martensite single phase. In this embodiment, the hot-rolled steel sheet may have bainite. Further, in the present embodiment, the average particle size of bainite contained in the hot-rolled steel sheet may be 1.0 μm or less.
[0028]
　In order to improve toughness, it has been conventionally known to make the old austenite structure finer. As a means for this, it is common to increase the cumulative reduction rate of unrecrystallized austenite. However, when the rolling reduction ratio is increased, the rolling load increases, and the plate thickness at the center of the plate width of the hot-rolled steel sheet and the plate at a position separated by 10 mm from the end of the plate width toward the center of the plate width. The amount of plate crown, which is the difference from the thickness, becomes large, and there are problems such as poor shape, poor contact during press molding of steel sheet, and variation in surface pressure. As a result of studying the relationship between rolling behavior and structure, by controlling the penetration temperature of the steel sheet into the final stand of finish rolling and the contact time between the rolling roll of the final stand and the steel sheet, the temperature drop due to the rolling roll and austenite The time required for recrystallization of the steel sheet can be balanced, and rolling can be performed without increasing the rolling deformation resistance, that is, the rolling load. As a result, the plate thickness of the central portion of the plate width of the steel plate in which the old austenite structure is a fine grain structure and the plate thickness of the portion separated from the plate width end portion by 10 mm toward the plate width central portion along the plate width direction. It was found that the difference in the amount of plate crown can also be suppressed.
[0029]
If the average particle size of old austenite is less than 0.1 μm, the work hardening characteristics of the hot-rolled steel sheet will be lost. Cracks are likely to occur when the coil is made into a coil or when the coil is unwound. On the other hand, when the average particle size of the old austenite exceeds 3.0 μm, the low temperature toughness becomes inferior in the high-strength steel sheet. The preferred range of the average particle size of the old austenite is 0.5 μm or more and 2.0 μm or less.
[0030]
　In the hot-rolled steel sheet of the present embodiment, the average particle size of the old austenite can be determined by image processing using a microstructure photograph taken by a scanning electron microscope (SEM).
[0031]
　More specifically, the average particle size of the old austenite is determined as follows.
　When the plate width of the hot-rolled steel sheet is W, the cross section parallel to the rolling direction and perpendicular to the plate surface is 1/4 W (width) or 3/4 W (width) from one end in the width direction of the hot-rolled steel sheet. A sample is taken so as to be an observation surface, the cross section is mirror-polished, and then corrosion is performed with picric acid to reveal the grain boundaries of the former austenite crystal grains. Then, using a scanning electron microscope (SEM), a region of 400 μm in the rolling direction × 400 μm in the thickness direction of the steel sheet is observed at a depth of 1/4 of the sheet thickness from the surface of the steel sheet.
　The average particle size of the old austenite is obtained by analyzing the obtained image using an image analysis device. The average particle size of the old austenite is calculated as a circle-equivalent diameter.
[0032]
　Next, the shape of the hot-rolled steel sheet according to the present embodiment will be described.
　The hot-rolled steel sheet according to this embodiment is excellent in shape. That is, as described above, even in the case of a fine-grained steel sheet whose shape is deteriorated by the conventional method, the amount of plate crown after hot rolling is small. By making the amount of the sheet crown small by hot rolling, not only the superiority as a hot-rolled steel sheet, but also the cold-rolled steel sheet and the heat-treated steel sheet which are further processed from this can be obtained as a steel sheet having excellent shape and toughness.
[0033]
　 The
　thickness of the hot-rolled steel sheet after hot rolling at the center of the plate width and the location 10 mm away from the end of the plate width toward the center of the plate width. When the plate crown amount, which is the difference from the plate thickness, is more than 80 μm, the plate thickness difference in the plate width direction of the steel plate is large, and when a hot-rolled steel plate is used as a material, poor contact during press molding and deviation of the surface pressure occur. Large and inferior in moldability. When a large part or high workability is required, it is preferably 60 μm or less. The plate crown amount is the average value obtained by measuring the plate thickness at the center of the plate width at 10 points, and the plate thickness at the locations separated from the end of the plate width by 10 mm toward the center of the plate width along the plate width direction. It is the difference from the average value obtained by arbitrarily measuring 10 points.
[0034]
　
　The plate width of the hot-rolled steel plate according to the present embodiment is not particularly limited, but is preferably 800 to 1200 mm.
[0035]
　
　The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but is preferably 1.0 to 4.0 mm.
[0036]
　The effect can be obtained by having the above-mentioned chemical composition, structure, and shape of the hot-rolled steel sheet according to the present embodiment. In particular, according to the manufacturing method shown below, the hot-rolled steel sheet according to the present embodiment can be stably obtained, which is preferable.
[0037]
　Specifically, the method for producing a hot-rolled steel sheet according to the present embodiment basically preferably includes the following steps (a) to (d).
(A) A heating step of heating a slab having the above component composition to 1100 ° C. or higher and lower than 1350 ° C.
(B) A step of finish rolling the slab after the heating step, in which the steel sheet penetration temperature at the final stand is 850 ° C. or higher and 1050 ° C. or lower, and the contact time between the steel sheet and the rolling roll is 0.005 seconds or longer and 0. .Rolling process in 020 seconds or less.
(C) A cooling step in which cooling is started less than 0.8 seconds after the finish rolling is completed, and the average cooling rate from the finish rolling end temperature to 750 ° C. is 100 ° C./sec or more.
(D) A winding step in which winding is performed after the cooling step.
[0038]
　Further, in the method for producing a hot-rolled steel sheet of the present embodiment, any one of the following steps (e) to (h) may be further performed after the above steps (a) to (d).
(E) A step of pickling and cold-rolling the hot-rolled steel sheet produced in (a) to (d).
(F) A step of pickling, cold-rolling, annealing, and then tempering and rolling the hot-rolled steel sheet produced in (a) to (d).
(G) A step of pickling, cold rolling, annealing, plating, and then temper rolling the hot-rolled steel sheet produced in (a) to (d).
(H) A step of pickling the hot-rolled steel sheets produced in the above (a) to (d), plating, and then temper rolling.
　Hereinafter, each step will be described.
[0039]
The
　slab is heated before hot rolling. When heating a slab having the same chemical composition as the hot-rolled steel sheet according to the present embodiment obtained by continuous casting or the like, the temperature before heating is not limited. Like the equipment directly connected from casting to hot rolling, it may be heated from 1000 ° C., or the slab may be cut out and heated from room temperature. If the heating temperature is less than 1100 ° C., the homogenization of the slab will be insufficient. In this case, the strength and workability of the resulting steel sheet are reduced. On the other hand, when the heating temperature is 1350 ° C. or higher, the initial austenite particle size becomes large, so that the structure of the finally obtained steel sheet tends to be mixed. It also leads to an increase in manufacturing cost and a decrease in productivity. Therefore, the heating temperature is preferably 1100 ° C. or higher and lower than 1350 ° C.
[0040]
The
　rolling process includes a rough rolling process and a finish rolling process, but the rough rolling process is not particularly limited.
　On the other hand, in the finish rolling process, it is important to control the penetration temperature of the steel sheet at the final stand and the contact time between the steel sheet and the roll. The steel sheet penetration temperature at the final stand is necessary to ensure the recrystallization of austenite, and the contact time between the steel sheet and the rolling roll is necessary to balance the temperature drop due to heat removal and the processing time. In the present embodiment, recrystallization can be promoted and the rolling load can be suppressed by controlling the penetration temperature of the steel sheet at the final stand and the contact time between the rolling roll of the final stand and the steel sheet.
[0041]
　Specifically, the penetration temperature of the steel sheet in the final stand is set to 850 ° C. or higher and 1050 ° C. or lower. If the temperature is lower than 850 ° C., the temperature drops when the steel sheet and the rolling roll come into contact with each other, and the temperature required for recrystallization cannot be secured. Moreover, since the rolling load is high, the shape of the steel plate becomes inferior. On the other hand, above 1050 ° C., the recrystallized austenite particle size becomes coarse and the toughness becomes inferior. In order to achieve both better shape and toughness, the temperature is preferably 900 ° C. or higher and 960 ° C. or lower. The penetration temperature of the steel sheet in the final stand is the surface temperature of the steel sheet immediately before being bitten into the rolling roll of the final stand.
[0042]
　Next, the contact time between the rolling roll of the final stand and the steel plate will be described. The recrystallization behavior during rolling can generally be organized by the relationship between strain rate and temperature. However, in the hot rolling process, it is necessary to consider the temperature drop due to the heat removal from the roll and the processing heat generation due to high-speed machining. Therefore, even in the strain rate range where recrystallization occurs, the rolling load and deformation resistance that determine the shape change dynamically, so the contact time between the rolling roll of the final stand and the steel sheet is important.
[0043]
　In a hot rolling facility for manufacturing a general automobile steel sheet, the contact time between the rolling roll of the final stand and the steel sheet is about 0.001 to 0.003 seconds, which is very short. Further, in order to prevent the rolling load from becoming excessive when the steel sheet is work-hardened during contact with the rolling roll and does not recrystallize, the rolling reduction rate of the final stand is generally kept low. When the rolling reduction of the final stand is low, the contact length between the rolling roll of the final stand and the plate is shortened, so that the contact time is shortened. On the other hand, in the present embodiment, the contact time between the steel sheet and the rolling roll of the final stand is 0.005 seconds or more and 0.020 seconds or less. If the contact time between the rolling roll of the final stand and the steel sheet is less than 0.005 seconds, the time required for recrystallization during hot rolling cannot be secured, so that the old austenite structure becomes flat and coarse. On the other hand, if the contact time exceeds 0.020 seconds, the amount of heat removed by the roll contact increases, the recrystallization temperature cannot be secured, and the temperature difference in the width direction of the steel sheet becomes large, so that the amount of the plate crown increases. In order to achieve both better shape and toughness, the contact time between the rolling roll of the final stand and the steel sheet is preferably 0.007 seconds or more and 0.010 seconds or less.
[0044]
　The contact time between the rolling roll of the final stand and the steel sheet can be obtained based on the rolling ratio, the rolling roll diameter, the rolling speed, the steel sheet thickness on the rolling in side, and the steel sheet thickness on the rolling out side. The thickness of the steel sheet after finish rolling and the diameter of the finish rolling roll are not particularly limited, but the rolling reduction of the final stand is about 25 to 50%, the diameter of the finish rolling roll is about 450 to 800 mm, and the strain rate at the final stand is 12.5 to. It is desirable that the thickness of the steel sheet is about 100 / s and the thickness of the steel sheet for automobiles is 1.0 to 6.0 mm. From the above manufacturing conditions, the plate passing speed is set to a speed that satisfies the contact time of the present invention. In the present embodiment, except for the rolling roll of the final stand, the rolling reduction in the other rolling rolls is less than 40% at the maximum in order to suppress the shape deterioration in the pre-finish rolling stage. Except for the rolling rolls of the final stand, the rolling reduction ratio in other rolling rolls is preferably 39% or less. The normal strain rate is obtained from the true strain amount, which is a physical quantity.
[0045]
After the
　finish rolling is completed, cooling is started in less than 0.8 seconds after passing through the final stand of the finish rolling in order to keep the recrystallized austenite structure created by the finish rolling fine. That is, the time required from the time when the finish rolling passes through the final stand to the time when the cooling starts is set to less than 0.8 seconds. For cooling, the average cooling rate from the end temperature of finish rolling to 750 ° C. is cooled at 100 ° C./s or higher. If the average cooling rate is less than 100 ° C./s, grain growth of austenite occurs even during cooling, and the average particle size of the old austenite grains becomes coarse. Since the cooling rate of less than 750 ° C. has a small effect on the average particle size of the old austenite grains, the cooling rate for obtaining the desired hot-rolled structure can be freely selected.
[0046]
　The upper limit of the average cooling rate up to 750 ° C. does not need to be limited, but the average cooling rate is 600 ° C./s or less in consideration of equipment restrictions and the like and to make the structure distribution in the plate thickness direction uniform. Is preferable. The cooling shutdown temperature is preferably cooled to 550 ° C. or lower in order to maintain the particle size of the old austenite. The average cooling rate between 750 ° C. and 550 ° C. does not affect the average crystal grain size of the former austenite, and is not particularly limited. The average cooling rate in this temperature range may be appropriately set according to the target strength of the steel sheet to be manufactured.
[0047]
　In the present embodiment, a cooling facility is installed after the finish rolling facility, and the cooling facility is cooled while passing the steel sheet after the finish rolling. The cooling equipment is preferably equipment that can cool the steel sheet under the above cooling conditions. As such a cooling facility, for example, a water cooling facility using water as a cooling medium can be exemplified.
[0048]
　Further, the cooling equipment includes equipment having no air-cooled section in the middle and equipment having one or more air-cooled sections in the middle. In this embodiment, any cooling equipment may be used. Even when a cooling facility having an air-cooled section is used, the average cooling rate until reaching 750 ° C. may be 100 ° C./sec or more.
[0049]
　The average cooling rate from the finish rolling end temperature to 750 ° C. is a value obtained by dividing the temperature difference between the finish rolling end temperature and 750 ° C. by the time required from the start of cooling to the arrival at 750 ° C. The start of cooling is the start of injection of the cooling medium onto the steel sheet by the cooling equipment. The finish rolling end temperature is the surface temperature of the steel sheet immediately after passing through the final stand.
[0050]
The
　hot-rolled steel sheet, which is a hot-rolled product, is preferably wound at a temperature of less than 550 ° C. in order to secure a tensile strength of 980 MPa or more.
[0051]
　The hot-rolled steel sheet of the present embodiment may be further cold-rolled or the like. Hereinafter, the process after the winding process will be described.
[0052]

　Next, the hot-rolled steel sheet may be pickled in order to remove the scale on the surface, and then subjected to a cold-rolling step in order to obtain the target steel sheet thickness. The conditions for pickling are not particularly limited. In the present embodiment, the conditions of the cold rolling process need not be particularly limited, but usually, if the rolling reduction during cold rolling is 30% or more and 80% or less, there is no particular problem in workability and plate thickness accuracy. .. If the rolling reduction during cold rolling exceeds 80%, the operation becomes difficult due to cracks at the width end of the steel sheet and increased strength due to work hardening.
[0053]

　The cold-rolled steel sheet after cold- rolling may be subjected to an annealing step. If the maximum annealing temperature exceeds 900 ° C., the austenite particle size produced by hot rolling becomes coarse, so it is preferable to set the maximum annealing temperature to 900 ° C. or lower. On the other hand, if the maximum heating temperature is less than 500 ° C., it takes a long time to create a rolled structure by recrystallization, which is not preferable from the viewpoint of productivity. After annealing, a temper rolling step for the purpose of shape correction and surface roughness adjustment may be further performed. Since the temper rolling step does not leave a rolled structure, the rolling reduction ratio is preferably 1.0% or less.
[0054]
The
　hot-rolled steel sheet or cold-rolled steel sheet may be subjected to treatments such as electroplating, hot-dip plating, and alloyed hot-rolled plating in order to improve the corrosion resistance of the surface. When heat is applied in the plating process, the temperature is preferably 900 ° C. or lower. If the temperature exceeds 900 ° C., the austenite particle size formed in the hot rolling process becomes coarse. After plating, a temper rolling step for the purpose of shape correction and roughness adjustment may be further performed. Since the temper rolling step does not leave a rolled structure, the rolling reduction ratio is preferably 1.0% or less.
Example
[0055]
　Hereinafter, the hot-rolled steel sheet of the present invention will be specifically described with reference to examples. However, the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to the following examples. As long as the gist of the present invention is not deviated and the object of the present invention is achieved, it is possible to carry out the work with appropriate modifications within a range that can be adapted to the gist. Therefore, the present invention may adopt various conditions, all of which are included in the technical features of the present invention.
[0056]
　The steel having the chemical composition shown in Table 1 was melted in a converter and continuously cast to obtain a slab having a thickness of 230 mm. Then, the slab was heated to a temperature of 1150 ° C. to 1250 ° C., rough-rolled, and then finished-rolled, cooled, and wound under the conditions shown in Tables 2A and 2B to produce a hot-rolled steel sheet.
[0057]
　Tables 2A and 2B show the steel grade components used, the finish rolling conditions, and the sheet thickness of the steel sheet. In Tables 2A and 2B, the "penetration temperature" is the surface temperature of the steel plate immediately before rolling at the final stand of the continuous finishing rolling stand, and the "contact time" is the time during which the steel plate and the rolling roll are in contact at the final stand. , "Cooling start time" is the time required from the end of finish rolling of the final stand to the start of cooling, "Average cooling rate" is the average cooling rate from the end temperature of finish rolling to 750 ° C, and "Taking temperature" is cooling. It is the winding temperature after the end. "Plate thickness" and "plate width" are the product dimensions after hot rolling, respectively.
[0058]
[table 1]

[0059]
[Table 2A]

[0060]
[Table 2B]

[0061]
　The old austenite structure of the steel sheet thus obtained is corroded at a depth of 1/4 of the thickness of the steel sheet, and the image by SEM observation is analyzed to obtain the average grain size of the old austenite. Calculated. Specifically, when the plate width of the steel plate is W, the cross section parallel to the rolling direction and perpendicular to the plate surface is the observation surface at a position 1/4 W (width) from one end in the width direction of the steel plate. After collecting the sample as described above and mirror-polishing the cross section, the cross section was corroded with picric acid to reveal the grain boundaries of the former austenite crystal grains. Then, using a scanning electron microscope (SEM), a region of 400 μm in the rolling direction × 400 μm in the thickness direction of the steel sheet was observed at a depth of 1/4 of the thickness of the steel sheet from the surface of the steel sheet. The average particle size of the old austenite was determined by analyzing the obtained image using an image analyzer. The average particle size of the old austenite was determined as the diameter equivalent to a circle. Similarly, the average particle size of bainite was also measured.
[0062]
　Regarding the tensile test of the steel sheet, JIS No. 5 test pieces were collected in the rolling width direction (C direction) of the steel sheet, and the tensile strength: TS (MPa) was evaluated according to JISZ2241: 2011. The tensile strength of 980 MPa or more was accepted.
　The ductile brittle transition temperature is measured by performing a Charpy impact test of the C-direction notch with a 2.5 mm sub-sized V-notch test piece specified in JISZ2242: 2005, and the temperature at which the brittle fracture surface ratio is 50% is the ductile brittle transition. The temperature was set. Further, for a steel plate having a final thickness of less than 2.5 mm, the total thickness was measured. If the ductile brittle transition temperature was -50 ° C or lower, it was judged as acceptable.
　Regarding the amount of the plate crown, the difference between the plate thickness at the center of the plate width of the steel plate and the plate thickness at a portion separated by 10 mm from the end of the plate width toward the center of the plate width along the plate width direction was calculated. Specifically, the plate crown amount is the average value of the plate thickness at the center of the plate width obtained by measuring any 10 points at the center of the plate width, and the plate width along the plate width direction from the end of the plate width. It was obtained from the difference from the average value of the plate thickness obtained by arbitrarily measuring 10 points separated by 10 mm toward the central portion.
[0063]
　As shown in Table 2, the example of the present invention had a tensile strength of 980 MPa or more, a ductile brittle transition temperature of −50 ° C. or less, and was excellent in strength and toughness. In addition, the amount of plate crown was small and the product shape was good. All invention examples contained bainite, the average particle size of which was 1.0 μm or less.
[0064]
　On the other hand, in Test No. 6, the penetration temperature is high, the recrystallized grains of the former austenite are coarsened, and the toughness is inferior.
　In test number 15, the contact time is long, the heat removal due to the roll contact is large, the temperature difference in the width direction of the steel sheet is large, and the deformation resistance difference in the width direction is large, so that the plate crown amount exceeds 80 μm.
　In test number 17, the contact time is short and there is no time to recrystallize during hot rolling, so that the old austenite particle size is coarse and the toughness is inferior.
　In test number 24, the penetration temperature is low, the temperature required for recrystallization cannot be secured, the old austenite grains are coarse, and the rolling load is high, so that the plate crown amount is large. Therefore, the toughness and the amount of plate crown are inferior.
　In test number 28, the time from passing through the final stand to the start of cooling is 0.8 seconds or more, the average particle size is coarse due to the growth of the old austenite grains, and the toughness is inferior.
　In Test No. 32, the cooling rate was less than 100 ° C./sec, and the grains grew after recrystallization, so that the old austenite grains became coarse and the toughness was inferior.
　Test number 33 has a small amount of carbon in the steel and is inferior in tensile strength.
　In test number 36, the penetration temperature is high, the recrystallized grains of the former austenite are coarsened, and the toughness is inferior.
　In test number 38, the contact time is short and there is no time to recrystallize during hot rolling, so that the old austenite particle size is coarse and the toughness is inferior.
　In Test No. 39, the cooling rate was less than 100 ° C./sec, and the grains grew after recrystallization, so that the old austenite grains became coarse and the toughness was inferior.
　In test number 40, in addition to the low heating temperature, the contact time between the rolling roll and the steel sheet is short, and there is no time for recrystallization during hot rolling, so that old austenite grains grow and the toughness is inferior. The average particle size of bainite of test number 40 was 1.3 μm.
　In the test number 41, the contact time is long, the heat removal due to the roll contact is large, the temperature difference in the width direction of the steel sheet is large, and the deformation resistance difference in the width direction is large, so that the plate crown amount exceeds 80 μm.
Industrial applicability
[0065]
　According to the present invention, it is possible to provide a hot-rolled steel sheet having an excellent shape, high absorption energy at the time of high-speed deformation, good collision characteristics as an automobile part, and excellent toughness. According to this hot-rolled steel sheet, the shape of the steel sheet is good, so it has excellent press moldability and stability, it is possible to integrally mold parts and shorten the processing process, the automobile has excellent collision characteristics, and the vehicle body is lighter. It is possible to improve fuel efficiency. Therefore, the industrial value of the present invention is high.

The scope of the claims
[Claim 1]
　By mass%,
C: 0.10% or more, 0.50% or less,
Si: 0.10% or more, 3.00% or less,
Mn: 0.5% or more, 3.0% or less,
P: 0. 100% or less,
S: 0.010% or less,
Al: 1.00% or less,
N: 0.010% or less,
Ti: 0% or more, 0.20% or less,
Nb: 0% or more, 0.100% Below,
Ca: 0% or more, 0.0060% or less,
Mo: 0% or more, 0.50% or less,
Cr: 0% or more, 1.00% or less, the
　balance is Fe and impurities, and the
　structure The average particle size of the old austenite is 0.1 μm or more and 3.0 μm or less, and
　the plate thickness at the center of the plate width is separated from the plate width end by 10 mm toward the plate width center along the plate width direction. A hot-rolled steel plate having a plate crown amount of 80 μm or less, which is a difference from the plate thickness of the above.
[Claim 2]
　By mass%,
Ti: 0.02% or more, 0.20% or less,
Nb: 0.010% or more, 0.100% or less,
Ca: 0.0005% or more, 0.0060% or less,
Mo: 0. The hot-rolled steel sheet according to claim 1, wherein one or more of 02% or more, 0.50% or less,
Cr: 0.02% or more, 1.00% or less
is contained.