JP5040475B2 - Thick-walled hot-rolled steel sheet with excellent workability and excellent strength and toughness after heat treatment and method for producing the same - Google Patents

Description

  The present invention relates to a thick hot-rolled steel sheet and a method for producing the same. The “thick hot-rolled steel sheet” here refers to a hot-rolled steel sheet having a relatively thick plate thickness of 6 mm to 12 mm. The thick hot-rolled steel sheet is suitable as a material for producing structural members such as automobiles and construction machines (hereinafter also referred to as construction machines).

 In recent years, from the viewpoint of protecting the global environment, automobile exhaust gas regulations have been strengthened, and the weight reduction of automobile bodies has been promoted in order to improve fuel efficiency. There is no exception for automobile members, and there is a strong demand for weight reduction of automobile members. Similarly, there is a strong demand for weight reduction for structural members such as construction machines. This is because structural materials such as automobiles and construction machinery are often made of thick, large, heavy-weight materials that have a thickness of 6 mm to 12 mm and a length of 10 m. It is. Therefore, if the steel plate to be used is a high strength steel plate in order to reduce the weight of the member, there is a problem that the workability of the steel plate such as elongation characteristics is lowered, and the difficulty of processing is rapidly increased. Also, because of the problem that fatigue strength does not improve in stress concentrated parts such as holes that are drilled for weight reduction and welds that are unavoidably present, unlike other thin-walled small members, automobiles, construction machinery, etc. In the case of a thick and large member such as the structural member, even if the strength is increased, the tensile strength is up to about 540 MPa.

 In recent years, die quenching, quenching while pressing, has been put into practical use as means for increasing the strength of thin-walled small members. However, when this die quench is applied to a thick-walled large member, in addition to the problem that the equipment itself becomes enormous, because it is thick, it can not secure the desired strength without burning to the center, and further In the as-quenched state, there are problems such as brittle fracture peculiar to thick-walled members. For this reason, die quenching has been considered unsuitable for thick-walled large members.

 However, there is a strong demand for reducing the weight of structural members such as automobiles and construction machines, and there is a strong desire to increase the strength of the members. Therefore, recently, for members that require particularly high strength, after processing into a member shape, heat treatment such as quenching and tempering treatment is performed to increase the strength and further toughness of the member. It has become. Therefore, the hot-rolled steel sheet, which is a raw material, is required to have excellent strength and elongation properties, as well as excellent member strength after heat treatment applied after being processed into a member shape, and member toughness.

  In response to such a demand, for example, Patent Document 1 includes C: 0.10 to 0.37%, further contains appropriate amounts of Si, Mn, P, S, and Al, and further B and N are 14B / 10.8. N: The steel contained is hot-rolled at a coiling temperature of 720 ° C. or less so as to satisfy 0.50 or more, and the average grain size of BN, which is a precipitate in the steel, is 0.1 μm or more, and prior austenite grains after quenching A method for manufacturing a thin steel sheet having a diameter of 2 to 25 μm is described. According to the technique described in Patent Document 1, it is said that a thin steel sheet can be manufactured that has excellent hardenability in a low temperature and short time after processing, excellent toughness after quenching, and has small fluctuations in characteristics due to quenching conditions.

Patent Document 2 includes C: 0.10 to 0.37%, further contains appropriate amounts of Si, Mn, P, S, Al, and Ti, and further contains B and N in an effective B amount: 0.0005% or more. In order to satisfy, the steel contained is hot-rolled at a coiling temperature of 720 ° C. or less, and the average grain size of TiN, which is a precipitate in the steel, is 0.06 to 0.30 μm, and the prior austenite grain size after quenching is 2 to 2 A method for producing a thin steel sheet having an impact toughness after quenching of 25 μm is described. According to the technique described in Patent Document 2, it is said that a thin steel sheet can be manufactured that has excellent hardenability in a low temperature and short time after processing, excellent impact toughness after quenching, and small variation in characteristics due to quenching conditions. .
JP 2002-309344 A JP 2002-309345 A

 However, the techniques described in Patent Documents 1 and 2 are intended for hot-rolled steel sheets having a relatively thin thickness of about 2.4 mm, and the techniques described in Patent Documents 1 and 2 are applied to automobiles and construction machinery. When applied to the production of hot-rolled steel sheets with a relatively large thickness for thick-walled large members such as structural members, the structure changes in the thickness direction, and the strength and ductility at the center in the thickness direction decreases, There are problems that a hot-rolled steel sheet having a uniform structure in the sheet thickness direction and having desired strength and ductility cannot be obtained and desired strength toughness cannot be ensured after heat treatment.

 In addition, in thick hot rolled steel sheets for thick large members such as automobiles and construction equipment such as construction equipment, if a desired uniform structure is to be obtained at the center of the sheet thickness, the steel sheet must be rapidly cooled after hot rolling. Is required. However, if the steel sheet is rapidly cooled after hot rolling, the cooling rate becomes too high at the steel sheet surface layer (particularly in the vicinity of the edge in the steel sheet width direction), causing martensitic transformation, increasing the hardness of the steel sheet surface layer, and partially in the thickness direction. In some cases, a hot-rolled steel sheet having a large hardness difference was obtained. When a material for a member is cut from such a hot-rolled steel sheet (coil), non-uniform deformation occurs (when it is slit in the width direction, it is called a camper), and the dimensional accuracy of the material after cutting decreases. As a result, there is a problem that the dimensional accuracy of the member is lowered.

 The present invention solves such problems of the prior art, and is required for a thick and large member, tensile strength: 440 to 640 MPa, preferably 490 to 590 MPa, elongation: 20% or more (marking distance GL: 50 mm) ), A thickness difference in the thickness direction of 10% or less with respect to the average value, high strength, excellent workability, and further excellent strength toughness after heat treatment and method for producing the same The purpose is to provide. Here, “thickness hot-rolled steel sheet with excellent strength and toughness after heat treatment” means normal water quenching and tempering treatment (approx. 930 ° C heating water quenching-approximately 200 ° C tempering), and tensile strength: 980 MPa or more High strength and elongation: 15% or more (GL: 50mm) high ductility and ductility-brittle fracture surface transition temperature in the Charpy impact test vTrs means hot-rolled steel sheet with high toughness of -60 ℃ or less .

 In addition, the heat processing conditions given to the member using the steel plate of this invention are not limited to the above-mentioned normal water quenching tempering process (about 930 degreeC heating water quenching-about 200 degreeC tempering). For example, desired heat treatment conditions such as about 930 ° C. heated water quenching and about 400 ° C. tempering can be used.

 In order to achieve the above-mentioned object, the inventors of the present invention have factors affecting the strength and workability (ductility) of a hot-rolled steel sheet having a relatively thick plate thickness of 6 mm to 12 mm. We have intensively studied various factors affecting strength and toughness. As a result, C: 0.10 to 0.20% by mass of low carbon steel containing a proper amount of Ti and B combined, and the N content is adjusted to be as low as 0.005% by mass or less, and the structure is uniform over the entire plate thickness. By adopting a single-phase structure of bainitic ferrite, the hardness difference in the thickness direction is within 10% of the average value, the desired high strength and excellent workability, and the structure after heat treatment is completely It has been found that a thick hot-rolled steel sheet having uniform martensite over the plate thickness and excellent in strength and toughness after heat treatment can be obtained. In addition, by adjusting the cooling rate after hot rolling to the surface temperature range of 15 to 50 ° C./s, the structure can be made a uniform bainitic ferrite single-phase structure over the entire plate thickness, It was found that the hardness difference in the thickness direction was within 10% of the average value.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005 %, Ti: 0.01 to 0.15%, B: 0.0010 to 0.0050%, the composition consisting of the balance Fe and inevitable impurities, and the structure consisting of a bainitic ferrite phase with an area ratio of 95% or more, High strength, characterized in that the hardness difference in the thickness direction is within 10% of the average value, tensile strength: 440-640MPa, elongation: 20% or more (marking distance GL: 50mm) A thick hot-rolled steel sheet with a thickness of 6mm to 12mm that has excellent workability and excellent strength and toughness after heat treatment.

  (2) By mass%, C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005 % Or less, Ti: 0.01 to 0.15%, B: 0.0010 to 0.0050%, a steel material having a composition consisting of the balance Fe and inevitable impurities, hot rolling with a finish rolling temperature of 820 to 880 ° C. After forming a hot-rolled steel sheet with a thickness of 6 mm or more and 12 mm or less, the hot-rolled steel sheet is subjected to cooling at a surface cooling rate of 15 to 50 ° C./s until the surface temperature reaches 550 to 650 ° C. The hardness difference in the sheet thickness direction is within 10% of the average value, the tensile strength is 440 to 640 MPa, the elongation is 20% or more (marking point) Distance GL: 50 mm), a method for producing a thick hot-rolled steel sheet that has high strength, excellent workability, and excellent strength and toughness after heat treatment.

  According to the present invention, the tensile strength is 440 to 640 MPa, the elongation is 20% or more, the desired high strength and workability are excellent, and the hardness difference in the thickness direction is within 10% of the average value. A thick hot rolled steel sheet having a uniform thickness distribution in the thickness direction and having a thickness of 6 mm or more and 12 mm or less can be manufactured easily and stably, and has a remarkable industrial effect. Furthermore, since the hot-rolled steel sheet of the present invention is also excellent in strength toughness after heat treatment, according to the hot-rolled steel sheet of the present invention, after the hot-rolled steel sheet is processed into a desired shape, heat treatment is performed, In addition, there is also an effect that a thick large-sized member (product) having a high ductility and a high toughness such as a structural member such as an automobile or a construction machine can be easily and stably manufactured.

The hot-rolled steel sheet of the present invention is limited to a thickness of 6 mm or more and 12 mm or less in order to mainly use it for large structural members such as automobiles and construction machines.
First, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter, mass% is simply referred to as%.
C: 0.10 to 0.20%
C is an element that forms carbides in steel and effectively acts to increase the strength of the steel sheet, and at the time of quenching treatment, promotes martensitic transformation and effectively acts on the strengthening of the structure by the martensite phase. It needs to contain 0.10% or more. If the C content is less than 0.10%, it is difficult to ensure a desired steel plate strength (tensile strength: 440 MPa or more), and it is difficult to secure a desired post-heat treatment strength (tensile strength: 980 MPa or more). On the other hand, if the content exceeds 0.20%, the strength of the steel sheet and the strength after heat treatment become too high, and the workability and toughness are lowered, and the weldability is also lowered. For this reason, C was limited to the range of 0.10 to 0.20%.

Si: 0.01-1.0%
Si is an element having an action of effectively increasing the strength of steel by solid solution strengthening, and in order to obtain such an effect, the content of 0.01% or more is required. On the other hand, a large content exceeding 1.0% causes unevenness called a red scale on the surface to lower the surface properties, lower the elongation characteristics, and lower the fatigue strength. For this reason, Si was limited to the range of 0.01 to 1.0%. In addition, Preferably it is 0.35% or less.

Mn: 0.5-2.0%
Mn is an element that effectively increases the strength of the steel by solid solution strengthening and has the effect of increasing the strength of the steel through the improvement of hardenability. In order to obtain such an effect, 0.5% or more Containing is required. On the other hand, if the content exceeds 2.0%, segregation becomes prominent, making it impossible to form a bainitic ferrite single phase over the entire thickness, and the steel sheet properties and the material after heat treatment are deteriorated. For this reason, Mn was limited to the range of 0.5 to 2.0%. In addition, Preferably it is 1.0 to 2.0%.

P: 0.03% or less P increases the strength of the steel by solid solution strengthening, but causes segregation and decreases the uniformity of the material, and significantly decreases the toughness after heat treatment. For this reason, although it is preferable to reduce as much as possible in this invention, excessive reduction raises material cost. Moreover, when it contains exceeding 0.03% excessively, segregation will become remarkable. For this reason, P was limited to 0.03% or less. In addition, Preferably it is 0.02% or less.

S: 0.01% or less S is present as a sulfide in steel and is preferably reduced as much as possible because it lowers ductility and lowers bending workability, but excessive reduction raises the material cost. Moreover, the content exceeding 0.01% significantly reduces the toughness after heat treatment. For this reason, in the present invention, S is limited to 0.01% or less. In addition, Preferably it is 0.005% or less.

Al: 0.01-0.10%
Al is an element that acts as a deoxidizer, and such an effect becomes significant when the content is 0.01% or more. However, when the content exceeds 0.1%, workability is deteriorated and hardenability is reduced. For this reason, Al was limited to the range of 0.01 to 0.1%. In addition, Preferably it is 0.05% or less.
N: 0.005% or less N forms nitrides such as TiN and AlN in steel to reduce workability, and forms BN during quenching to reduce the amount of solid solution B effective in improving hardenability. Such an adverse effect of N is acceptable if the N content is 0.005% or less. For this reason, in the present invention, N is limited to 0.005% or less.

Ti: 0.01-0.15%
Ti works effectively to make the structure after hot rolling into bainitic ferrite, and forms nitrides in preference to B, and exerts the effect of improving hardenability by solute B. It is an element that works effectively. Such an effect is recognized with a content of 0.01% or more, but a content exceeding 0.15% increases the deformation resistance during hot rolling, extremely increases the rolling load, and decreases the toughness after heat treatment. . For this reason, Ti was limited to the range of 0.01 to 0.15%. In addition, Preferably it is 0.03-0.10%.

B: 0.0010-0.0050%
B is an element that has the effect of suppressing the formation of polygonal ferrite and pearlite during cooling after hot rolling, and further effectively improves the hardenability and toughness during heat treatment. In the case of a thick steel plate having a thickness of 6 mm or more, such an effect becomes remarkable when the content is 0.0010% or more. On the other hand, if the content exceeds 0.0050%, deformation resistance during hot rolling is increased, the rolling load is extremely increased, bainite and martensite are generated after hot rolling, and problems such as sheet cracking occur. For this reason, B was limited to the range of 0.0010 to 0.0050%. In addition, Preferably it is 0.0015 to 0.0040%.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include, for example, Cu: 0.3% or less and Cr: 0.3% or less.
The thick hot-rolled steel sheet of the present invention has the above-described composition, and further has a single-phase structure composed of a bainitic ferrite phase over the entire thickness. The single-phase structure here refers to a structure composed of a bainitic ferrite phase with an area ratio of 95% or more. The bainitic ferrite phase includes acicular ferrite and acicular ferrite. As a structure other than the bainitic ferrite phase, a polygonal ferrite phase, a pearlite phase, a cementite phase, a bainite phase, a martensite phase, or the like having an area ratio of 5% or less is acceptable.

  By adopting a single-phase structure consisting of bainitic ferrite over the entire thickness, as a hot-rolled steel sheet, the desired high strength of tensile strength: 440MPa or more and 640MPa or less, and elongation: 20% or more (GL: 50mm) A thick hot-rolled steel sheet having high ductility, excellent workability such as bending properties, and capable of being processed into a thick large-sized member such as a structural member such as an automobile or a construction machine can be obtained. If the bainitic ferrite phase is less than 95% by area ratio, the desired high strength and high ductility cannot be combined. In addition, when the structural fraction of bainitic ferrite phase is reduced to less than 95%, the uniformity of the structure is reduced, resulting in a camber or the like during cutting, resulting in reduced dimensional accuracy and workability such as bending characteristics. To do. The area ratio of the bainitic ferrite phase was determined at a position of 0.1 mm from the surface, a thickness of 1/4, and a thickness of the central portion, and the total thickness was 95% or more in all the above three locations. It is judged that it has a single phase structure consisting of bainitic ferrite phase.

Below, the preferable manufacturing method of the thick hot-rolled steel plate of this invention is demonstrated.
Molten steel having the above composition is melted by a conventional melting method such as a converter or a vacuum melting furnace, and a steel material such as a slab by a conventional casting method such as a continuous casting method or an ingot-bundling rolling method. However, in the present invention, the manufacturing method of the steel material is not limited to this, and any conventional manufacturing method of the steel material can be suitably applied.

  The steel material having the above composition is hot-rolled to obtain a thick hot rolled steel sheet having a thickness of 6 mm or more and 12 mm or less. Note that if the sheet thickness exceeds 12 mm, the rolling reduction in hot rolling is not sufficient, the structure after rolling tends to be coarse, and martensite tends to occur during cooling. It is preferable to do. The heating temperature for hot rolling is not particularly limited as long as the finish rolling finishing temperature of hot rolling described below can be secured, but it is preferably set to 1000 to 1300 ° C., which is a normal heating temperature. . When the heating temperature is higher than 1300 ° C., the crystal grains become coarse and the hot workability tends to be lowered. On the other hand, when the heating temperature is less than 1000 ° C., the deformation resistance increases excessively, the load on the rolling equipment increases, and as a result, the problem that rolling becomes difficult tends to occur. Moreover, if heating temperature is less than 1000 degreeC, melt | dissolution of TiC which exists in a steel raw material will become inadequate, and it will become difficult to ensure a desired structure | tissue and intensity | strength after hot rolling.

Hot rolling is rolling with a finish rolling temperature of 820 to 880 ° C.
When the finish rolling temperature of finish rolling is 820 ° C or higher, ferrite transformation is suppressed in the subsequent cooling process, and the structure composed of a bainitic ferrite phase with an area ratio of 95% or more (a bainitic ferrite single phase) Organization). If the finishing temperature of finish rolling is less than 820 ° C., ferrite transformation is promoted in the subsequent cooling process, and it becomes difficult to obtain a bainitic ferrite single phase structure. On the other hand, when the finishing temperature of finish rolling exceeds 880 ° C and becomes high, not only ferrite transformation but also transformation to bainitic ferrite is suppressed, and it becomes difficult to obtain a bainitic ferrite single phase structure. , Bainite phase and martensite phase are likely to occur. If a bainite phase or a martensite phase is generated, the strength of the steel plate may be too high, or the steel plate may be cracked when wound around the coil or when the coil is rewound. For these reasons, the finishing temperature of finish rolling is limited to the range of 820 to 880 ° C.

After rolling, the hot-rolled steel sheet is cooled to a temperature range in which the surface temperature is 550 to 650 ° C. at a cooling rate of 15 to 50 ° C./s on the steel sheet surface.
In order to obtain a bainitic ferrite single-phase structure over the entire thickness of the steel sheet, the cooling is adjusted so that the cooling after the rolling is 15 ° C./s or more at the cooling rate of the steel sheet surface. When the surface cooling rate is less than 15 ° C./s, the polygonal ferrite phase tends to precipitate at the center of the plate thickness and the like, and it becomes difficult to obtain a uniform bainitic ferrite single phase structure in the plate thickness direction. On the other hand, if the surface is rapidly cooled at a cooling rate exceeding 50 ° C./s, martensite is generated in the surface layer portion, and a uniform bainitic ferrite single phase structure cannot be obtained in the thickness direction. The hardness difference in the direction becomes remarkable, and it becomes difficult to adjust the hardness difference in the thickness direction within 10% of the arithmetic average hardness (average value) in the thickness direction. In addition, it is preferable that cooling uses water cooling, and adjustment of a cooling rate is adjusted by changing the amount of water injection and water injection time. For this reason, the cooling applied to the hot-rolled steel sheet after the end of rolling was determined by adjusting the cooling rate so that the cooling rate of the steel sheet surface was 15 to 50 ° C./s. In addition, the surface cooling rate mentioned above shall measure the surface temperature, and shall use the value averaged between finish rolling completion temperature and cooling stop temperature.

  The cooling stop temperature described above is a temperature in a temperature range where the surface temperature of the steel sheet is 550 to 650 ° C. If the cooling stop temperature is less than 550 ° C. at the surface temperature, a bainite phase or a martensite phase is generated, and a bainitic ferrite single phase structure cannot be obtained. In addition, the hot-rolled steel sheet is cracked during winding, or the strength becomes too high, and the workability of the steel sheet is lowered. On the other hand, when the cooling stop temperature exceeds 650 ° C. and becomes a high temperature, a polygonal ferrite phase or a pearlite phase is generated and a bainitic ferrite single phase structure cannot be obtained. Further, the steel plate strength may be lower than the desired strength. For this reason, the cooling stop temperature after rolling was limited to a temperature in the temperature range of 550 to 650 ° C.

  After the cooling is stopped, the hot-rolled steel sheet is wound in a coil shape in the temperature range. If the coil winding temperature is less than 550 ° C., a bainite phase or a martensite phase is generated, and a bainitic ferrite single phase structure cannot be obtained. On the other hand, when the temperature is higher than 650 ° C., a polygonal ferrite phase and a pearlite phase are generated, and it becomes impossible to obtain a bainitic ferrite single phase structure, and it becomes impossible to secure a desired steel sheet strength, and in the thickness direction Uniformity decreases. For this reason, coil winding temperature was limited to the temperature of the temperature range of 550-650 degreeC by the surface temperature of a steel plate.

After heating a steel material (steel slab) having the composition shown in Table 1 to the heating temperature shown in Table 2, it is hot-rolled under the finish rolling conditions shown in Table 2, and the hot rolling of the plate thickness shown in Table 2 is performed. A steel plate was used. After finishing rolling, the hot-rolled steel sheet was cooled under the conditions shown in Table 2 and wound into a coil at the winding temperature shown in Table 2.
The obtained hot-rolled steel sheet was subjected to a structure observation, a tensile test, a hardness test, and a bending test to evaluate strength, ductility, uniformity of sheet thickness direction hardness and workability (bending workability). In addition, a test plate is taken from the obtained hot-rolled steel plate, and after pickling the test plate to remove the scale on the surface of the steel plate, heat treatment (quenching-tempering treatment) is performed, structure observation, tensile test, impact test. The strength, ductility and toughness after heat treatment were evaluated. The heat treatment is quenching / tempering treatment. The quenching treatment is heated to 930 ° C and held for 10 minutes, and then quenched into water at 20 ° C. The tempering treatment is heated to 200 ° C and held for 60 minutes, then air-cooled after holding. It was set as processing to do. After cooling, a test piece was taken from the test plate and tested. The test method was as follows.

(1) Microstructure observation From the obtained hot-rolled steel sheet, a specimen for microstructural observation was collected, and the thickness cross section parallel to the rolling direction of the specimen was polished and subjected to nital corrosion. About the position of 1/4 thickness and the central part of the plate thickness, the metal structure was observed with a scanning electron microscope (SEM) (magnification: 3000 times) (number of fields of view: 10 each) and imaged. Using an image analyzer, the tissue fraction (area ratio) of each phase was measured, and the area ratio of the bainitic ferrite phase was obtained by averaging the measured values in 10 observed visual fields. When the area ratio of bainitic ferrite phase (average of measured values in 10 fields of view) obtained at the position of 0.1 mm from the surface, the thickness 1/4 position, and the central thickness position is 95% or more. Was determined to have a structure composed of a bainitic ferrite phase with an area ratio of 95% or more over the entire thickness (a bainitic ferrite single phase structure).

(2) Tensile test JIS No. 5 test piece (GL: 50mm) was taken from the obtained hot-rolled steel plate (or test plate) so that the tensile direction was perpendicular to the rolling direction. A tensile test was performed in accordance with the above, tensile properties (yield strength YS, tensile strength TS, elongation El) were determined, and strength and ductility were evaluated.
(3) Hardness test From the obtained hot-rolled steel plate, a test piece for hardness measurement was collected, the plate thickness section parallel to the rolling direction of the test piece was polished, and the entire thickness from the surface to the plate thickness direction was covered. Vickers hardness HV (load: 9.8 N = 1 kgf) was measured at a pitch of 0.2 mm. The hardness was measured at a position 0.2 mm from the surface as the starting point of hardness measurement. Next, when the location to be measured was within 0.2 mm from the other surface, the location was not measured and the hardness measurement was terminated. About each hot-rolled steel plate, the obtained sheet thickness direction hardness was arithmetically averaged and average hardness (average value) HVmean was calculated | required. Further, the difference between the maximum hardness and the minimum hardness, ΔHV, was calculated to obtain [ΔHV / HVmean] × 100 (%), and the uniformity in the plate thickness direction was evaluated.

(4) Bending test From the obtained hot-rolled steel sheet, a bending test piece (size: plate thickness t x 100 x 200 mm) was taken so that the direction perpendicular to the rolling direction was the longitudinal direction of the test piece, and the bending radius was taken. Is bent 180 degrees so that the longitudinal direction of the specimen becomes the circumferential direction at various bending radii with 0.5, 1.0, 1.5, and 2.0 times the plate thickness, and there is no crack on the outer peripheral side of the bent part The minimum bending radius (mm) was determined. The minimum bending radius was expressed as a thickness ratio with the test piece.

(5) Impact test V-notch test specimens are collected from the obtained test plate in accordance with JIS Z 2242 so that the longitudinal direction of the test specimen is perpendicular to the rolling direction, and the Charpy impact test is performed. Then, the ductile-brittle fracture surface transition temperature vTrs (° C.), which is a temperature at which the ductile fracture surface ratio becomes 50%, was determined and the toughness after the heat treatment was evaluated.

  The obtained results are shown in Table 3.

  In each of the examples of the present invention, the structure is a structure composed of a bainitic ferrite phase that is uniform in the sheet thickness direction and has an area ratio of 95% or more (bainitic ferrite single-phase structure), and tensile strength: 440 MPa or more, elongation: 20% or more, hardness difference ΔHV in the thickness direction is within 10% of average hardness (average value) HVmean, excellent in thickness direction uniformity, and minimum bend radius is 0.5 t This is a thick hot-rolled steel sheet with excellent bending workability, high strength and excellent workability. Moreover, when quenching and tempering treatment is performed, tensile strength: high strength of 980 MPa or more, elongation: high ductility of 15% or more, and high toughness with vTrs of −60 ° C. or less can be secured. On the other hand, in the comparative example outside the scope of the present invention, a structure composed of a bainitic ferrite phase with a uniform structure has not been obtained, and the desired value of the strength or ductility or the strength and ductility cannot be ensured. Hardness difference ΔHV in the thickness direction also increases and uniformity in the plate thickness direction decreases, and one or more of strength, ductility, and toughness after quenching and tempering are less than the desired values described above, The hot-rolled steel sheet lacks any of strength, ductility, and toughness after quenching and tempering.

Claims (2)

% By mass
C: 0.10 to 0.20%, Si: 0.01 to 1.0%,
Mn: 0.5 to 2.0%, P: 0.03% or less,
S: 0.01% or less, Al: 0.01-0.10%,
N: 0.005% or less, Ti: 0.01 to 0.15%,
B: 0.0010-0.0050%
And a composition composed of the balance Fe and inevitable impurities and a structure composed of a bainitic ferrite phase with an area ratio of 95% or more, and the hardness difference in the thickness direction is within 10% of the average value Yes, tensile strength: 440 to 640 MPa, elongation: 20% or more (marking distance GL: 50 mm), high strength, excellent workability, and excellent toughness after heat treatment Thick hot-rolled steel sheet of 6mm to 12mm. % By mass
C: 0.10 to 0.20%, Si: 0.01 to 1.0%,
Mn: 0.5 to 2.0%, P: 0.03% or less,
S: 0.01% or less, Al: 0.01-0.10%,
N: 0.005% or less, Ti: 0.01 to 0.15%,
B: 0.0010-0.0050%
A hot rolled steel sheet having a thickness of 6 mm or more and 12 mm or less by subjecting a steel material having a composition composed of the remaining Fe and unavoidable impurities to hot rolling at a finish rolling temperature of 820 to 880 ° C. The hot rolled steel sheet is subjected to cooling at a surface cooling rate of 15 to 50 ° C./s to a temperature range where the surface temperature becomes 550 to 650 ° C., and is wound in a coil shape in the temperature range, Hardness difference in the thickness direction is within 10% of the average value, tensile strength: 440-640MPa, elongation: 20% or more (marking distance GL: 50mm), high strength and excellent workability And the manufacturing method of the thick hot-rolled steel plate excellent in the strength toughness after heat processing.