JP4803108B2 - High strength hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same. Specifically, the present invention relates to a material for structural members used in automobiles and various industrial machines, in particular, a material for structural members typified by automobile suspension parts and bumper reinforcements, or a material for wheels. The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent in ductility and toughness, and a method for producing the same.

  A so-called hot-rolled steel sheet produced by continuous hot rolling is widely used as a relatively inexpensive structural material in various industrial equipment including automobiles. In particular, the high strength hot-rolled steel sheet has been increasingly applied to undercarriage parts of automobiles because it greatly contributes to improving the fuel efficiency of automobiles.

  Recently, the demand for higher strength for hot-rolled steel sheets has further increased, and the tensile strength of 980 MPa or more, which is ultra-high strength, has been increasing as the weight of parts has been reduced in light of further awareness of environmental issues. Hot rolled steel sheets are desired.

  Conventionally, high strength members for automobile undercarriages have been realized to have high strength using precipitation strengthening of Ti contained therein. For example, Patent Documents 1 to 4 disclose inventions that increase the strength mainly by precipitation strengthening of Ti.

On the other hand, as another method for increasing the strength, there is a method using transformation strengthening due to temperature conditions. For example, Patent Document 5 discloses an invention that uses transformation strengthening by low-temperature winding to achieve an ultrahigh strength of 980 Mpa or more.
Japanese Patent Laid-Open No. 6-200351 JP-A-6-228708 JP-A-8-199298 JP-A-11-193443 JP 2000-282175 A

  However, in the inventions described in Patent Documents 1 to 4, an ultrahigh strength of 980 MPa or more in tensile strength has not been obtained. The reason for this is that when a tensile strength of 980 MPa or more is obtained by increasing the Ti content, coarse Ti carbonitride precipitates, which becomes the starting point of fracture, resulting in a decrease in strength and toughness. This is because the value is significantly reduced.

Further, the invention described in Patent Document 5 has a problem that the toughness is insufficient. Furthermore, there has been a problem that flatness becomes unsatisfactory in the cooling process after hot rolling.
The object of the present invention is to solve the problems of the prior art as described above, a steel sheet having a tensile strength of 980 MPa or more in the direction perpendicular to the rolling and excellent in workability and toughness, for example, a tensile strength of 980 MPa or more and an elongation of 14 .1% or higher, Charpy absorbed energy at −50 ° C. at 88 J / cm ² or higher, Charpy brittle fracture surface ratio at −50 ° C. with 0% steel sheet, while maintaining flatness after hot rolling with certainty It is to be.

  As a result of intensive experiments to solve such problems, the present inventors have obtained knowledge 1 to 4 that the following matters are effective.

(Knowledge 1)
With respect to the composition, an ultrahigh strength steel sheet having excellent toughness can be obtained by increasing the V content rather than the Ti content.

(Knowledge 2)
Regarding the structure, the workability (total elongation) is ensured by setting the area ratio of ferrite to 60% or more, the average particle diameter of ferrite is 5 μm or less, and the cleanliness according to JIS G 0555 is 0.05% or less. Furthermore, the toughness (low temperature toughness) can be improved by setting the total number density of inclusions and precipitates having an average particle size of 5 μm or more to 300 pieces / mm ² or less. Thereby, both workability and toughness can be achieved at a high level.

(Knowledge 3)
Regarding the manufacturing method, in order to improve toughness, it is effective to reduce the mixing of alumina inclusions in the steelmaking stage. Specifically, in order to reduce the inclusion of alumina inclusions, the heating temperature from the liquidus temperature of the molten steel is set to a temperature higher by 5 ° C. or more, and the molten steel casting amount is set to 6 ton / min or less. It is valid. Thereby, the fall of the molten steel temperature at the time of casting and the flow of a molten steel can be suppressed, and it can prevent that an alumina type inclusion is supplemented in a slab. Therefore, the cleanliness can be suppressed to 0.05% or less by this manufacturing method.

Further, in order to reduce the total number density of inclusions and precipitates having an average particle size of 5 μm or more to 300 pieces / mm ² or less, the center segregation reduction treatment is effective. By this treatment, it is possible to prevent Ti from being concentrated in the center portion of the slab thickness, and it is possible to suppress coarse TiN precipitation.

Also, in order to achieve an average ferrite particle size of 5 μm or less with an area ratio of ferrite of 60% or more, hot rolling is started after charging the slab into a heating furnace and heating to 1200 ° C. or more, and Ar ₃ points The hot finish rolling is completed as described above, and then cooling is started within 3 seconds at an average cooling rate of 20 ° C / second or more and 100 ° C / second or less, and then primary cooling is finished at 700 ° C or less and 500 ° C or more. Is effective.

Furthermore, in order to prevent the Ti and V-based precipitates from becoming coarse, it is effective to set the time from the slab heating furnace extraction to the end of the primary cooling within 10 minutes.
That is, in order to produce a high-strength steel sheet with excellent toughness, first, Ti and V that segregate in the center of the slab and prevent segregation of alumina inclusions inside the slab during continuous casting are reduced. It suppresses by doing. Furthermore, by shortening the time from the extraction of the slab heating furnace to the end of the primary cooling, it is effective to prevent generation of coarse Ti and V-based precipitates and to build an optimum metal structure.

  Here, the “center segregation reduction process” means a process of reducing the concentration of components other than Fe at the position where the molten steel is finally solidified. The position where the molten steel finally solidifies is the final solidification position when the molten steel gradually cools and solidifies, then changes from the liquid phase state to the liquid phase and solid phase mixed state, and finally changes to the solid phase. means. The specific center segregation reduction treatment can be exemplified by electromagnetic stirring and / or reduction in an unsolidified portion in the vicinity of the position where the molten steel is finally solidified.

(Knowledge 4)
Regarding the manufacturing method, the flattening of the steel sheet can be prevented by setting the coiling temperature of the hot rolling to 350 ° C. or higher. As a result, there is no need to forcibly correct the flat odor and then take up, which is advantageous in terms of manufacturing cost.

  The present invention has been made based on these findings, and produces a high-strength steel sheet having a tensile strength of 980 MPa or more.

In the present invention, C: 0.08% or more and 0.20% or less, Si: less than 0.2%, Mn: more than 1.0%, 3.0% or less, P: 0.05% or less, S: 0.0. 01% or less, Al: 0.001% or more and 0.5% or less, N: 0.01% or less, V: more than 0.1% and 0.5% or less, Ti: 0.05% or more and less than 0.25% , Nb: 0.005% or more and 0.10% or less, having a steel composition composed of the balance Fe and impurities, and having a steel structure in which the area ratio of ferrite is 60% or more and the area ratio of martensite is 5% or less The average particle size of ferrite is 5 μm or less, the cleanliness d is 0.05% or less, and the total number density of inclusions and precipitates having an average particle size of 5 μm or more is 300 pieces / mm ² or less. Is a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more.

Here, the cleanliness d is calculated by the following equation (1).
d = (n / (p × f)) × 100 (1)
In the formula (1), p represents the total number of lattice points on the glass plate in the field of view in JIS G 0555, f represents the number of fields in JIS G 0555, and n represents all inclusions in f fields of view in JIS G 0555. Indicates the number of grid point centers occupied by. Here, all inclusions mean all of A-based inclusions, B-based inclusions, and C-based inclusions in JIS G 0555.

  In the high-strength hot-rolled steel sheet according to the present invention, instead of a part of Fe, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% It is desirable to contain one or more of the following and B: 0.01% or less.

  In these high-strength hot-rolled steel sheets according to the present invention, instead of a part of Fe, one of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less or It is desirable to contain 2 or more types.

From another point of view, the present invention relates to a molten steel having the above-described steel composition, in which the heating temperature of the molten steel is higher than the liquidus temperature by 5 ° C. or more, and the molten steel casting amount per unit time is 6 tons / min or less. Further, a slab is formed by a continuous casting method in which center segregation reduction treatment is performed before the molten steel is completely solidified. The slab is charged into a heating furnace and heated to a temperature of 1200 ° C. or higher, and Ar is added to the slab extracted from the heating furnace. Hot rolling is performed to complete rolling in a temperature range of ₃ points or more to form a hot-rolled steel sheet. The hot-rolled steel sheet is cooled within 3 seconds after the completion of hot rolling, and is at least 20 ° C./second and 100 It is cooled to a temperature range of 700 ° C. or lower and 500 ° C. or higher at an average cooling rate of 0 ° C./second or less, subjected to primary cooling that completes cooling within 10 minutes after extraction, and then wound at a temperature of 350 ° C. or higher. It is a manufacturing method of a high-strength hot-rolled steel sheet .

  The high-strength hot-rolled steel sheet according to the present invention has high strength, excellent workability, and excellent toughness. Moreover, the flatness after hot rolling is excellent and can be manufactured at low cost. Therefore, it is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts.

  Hereinafter, embodiments of the high-strength hot-rolled steel sheet and the manufacturing method thereof according to the present invention will be specifically described.

(A) Steel composition
C: 0.08% to 0.20% C is an element effective for increasing the strength. If the C content is less than 0.08%, the effect is small. On the other hand, if the C content exceeds 0.20%, flatness and characteristic variations during cooling after hot rolling, such as pearlite, bainite, martensite, etc. A decrease in workability occurs due to an increase in the second phase of the metal structure. Therefore, in the present invention, the C content is limited to 0.08% or more and 0.20% or less. From the same viewpoint, the preferable lower limit is 0.10%, and the preferable upper limit is 0.16%.

Si: Less than 0.2% Si is an element effective for increasing the strength. However, when it contains excessively, the chemical conversion processability fall and the surface defect called island scale flaw will become remarkable. Therefore, the Si content is limited to less than 0.2%. From the same viewpoint, it is preferably 0.1% or less, more preferably 0.05% or less.

Mn: more than 1.0% to 3.0% or less Mn is an element effective for increasing the strength. This contributes to lowering the transformation point and controlling the precipitation state of V precipitates, and also contributes to higher strength by strengthening transformation. The effect cannot be obtained at 1.0% or less. On the other hand, if the content exceeds 3.0%, flatness failure and characteristic variation are caused by cooling in hot rolling. Therefore, the content is limited to more than 1.0% and not more than 3.0%. From the same viewpoint, the preferable lower limit is 1.5%, and the preferable upper limit is 2.5%.

P: 0.05% or less P is an undesirable element that deteriorates toughness, so it is desirable that P be less. However, extreme reduction involves a corresponding cost. Therefore, the P content is limited to 0.05% or less. Preferably it is 0.02% or less, More preferably, it is 0.015% or less.

S: 0.01% or less S is an undesirable element that increases the amount of MnS and decreases toughness. Therefore, the content is limited to 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.004% or less.

Al: 0.001% or more and 0.5% or less Al is contained for deoxidation. The effect is insufficient if it is less than 0.001%, and if it exceeds 0.5%, the weldability is lowered. Therefore, in the present invention, the Al content is limited to 0.001% to 0.5%. A preferable upper limit is 0.1%.

N: 0.01% or less N combines with Ti to form a nitride. If the N content exceeds 0.01%, coarse TiN precipitates and the toughness decreases. Therefore, the N content is limited to 0.01% or less. Preferably it is 0.008% or less, More preferably, it is 0.005% or less. Although the lower limit is not particularly defined, it is preferably 0.0005% or more from the viewpoint of steelmaking cost.

V: more than 0.1% and 0.5% or less V is the most important element in the present invention. It precipitates at a relatively low temperature and greatly contributes to high strength. The effect is insufficient at 0.1% or less. Moreover, when it contains excessively, chemical conversion property will deteriorate, V content is limited to more than 0.1% and 0.50% or less. A preferred lower limit is 0.16% and a preferred upper limit is 0.35%.

Ti: 0.05% or more and less than 0.25% Ti is an important element in the present invention. Precipitation strengthening contributes to increasing the strength of the steel sheet, and also acts as a precipitation nucleus for V precipitates, greatly contributing to achieving higher strength and higher strength by precipitation. The effect is insufficient if it is less than 0.05%. On the other hand, if the content is too high, the effect is saturated, and when strengthening is performed mainly by precipitation strengthening of Ti, coarse Ti carbonitride Etc. precipitate and deteriorate toughness. Therefore, the Ti content is limited to 0.05% or more and less than 0.25%. A preferred lower limit is 0.08% and a preferred upper limit is 0.20%.

Nb: 0.005% or more and 0.10% or less Nb contributes to increasing the strength of the steel sheet by refining and precipitation strengthening, but if contained excessively, the effect is saturated, so its content is 0.10% It was determined as follows. Further, in order to obtain Nb fine particles and precipitation strengthening, it is necessary to add 0.005% or more. Preferably it is 0.01% or more.

Next, the optional additive element will be described.
One or more of Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less and B: 0.01% or less

Cr, Mo, Cu, Ni, and B are all optional addition elements in the present invention, and are elements that increase strength by solid solution strengthening. By containing these elements, there is an effect of further increasing the strength, and even if two or more kinds are contained, the respective effects are not lost. The effect is saturated even if Cr: 1.0%, Mo: 1.0%, Cu: 1.0%, Ni: 1.0% and B: 0.01% are contained, respectively, and the cost Is just bloated. Therefore, when these elements are contained, the upper limit of the content is Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B : 0.01% or less is desirable. Furthermore, the effect is remarkable by containing Cr: 0.05% or more and Mo: 0.05% or more, Cu: 0.05% or more, Ni: 0.05% or more and B: 0.0002% or more. Therefore, it is more desirable to set these as the lower limit.
One or more of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less

  REM, Mg, and Ca are all optional elements in the present invention, and are elements that can spheroidize inclusions such as sulfides and oxides and make them harmless. By containing these elements, it has the effect | action which makes an inclusion spherical and detoxifies, and even if it adds 2 or more types, each effect | action is not lost. Even if the action exceeds REM: 0.1%, Mg: 0.01%, and Ca: 0.01%, the action is saturated and the cost is increased. Therefore, when these elements are contained, it is desirable that the upper limit of the content be REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less. Furthermore, since the effect is remarkably exhibited by containing REM: 0.005% or more, Mg: 0.0005% or more, and Ca: 0.0005% or more, it is more desirable to set these as the lower limit.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.

Compositions other than the above are Fe and impurities.
(B) Metal structure and inclusions In order to obtain a steel sheet having a tensile strength of 980 MPa or more and excellent workability and toughness, the ferrite area ratio is 60% or more, and the average particle diameter of ferrite is 5 μm or less. It has a martensite area ratio of 5% or less, a strength of 980 MPa or more, a cleanliness d according to JIS G 0555 of 0.05% or less, and an inclusion and precipitate having an average particle size of 5 μm or more. It is necessary that the total number density is 300 pieces / mm ² or less.

Here, the cleanliness d is calculated by the following equation (1).
d = (n / (p × f)) × 100 (1)
In the formula (1), p represents the total number of lattice points on the glass plate in the field of view in JIS G 0555, f represents the number of fields in JIS G 0555, and n represents all inclusions in f fields of view in JIS G 0555. Indicates the number of grid point centers occupied by. Here, all inclusions mean all of A-based inclusions, B-based inclusions, and C-based inclusions in JIS G 0555.

  If the ferrite area ratio is less than 60%, desired processability cannot be obtained. Further, if the average particle diameter of the ferrite is more than 5 μm, the toughness is deteriorated. Examples of the metal phase second phase include pearlite, cementite, bainite, retained austenite, and martensite. The preferred second phase of the metal structure is pearlite or cementite, bainite, or retained austenite. The martensite is desirably 5% or less in terms of area ratio. The reason why the martensite area ratio is desirably 5% or less is that it is necessary to suppress the area ratio of hard and brittle martensite in order to ensure workability and toughness. As a means for reducing the martensite area ratio to 5% or less, a restriction is imposed on the coiling temperature, and excessive cooling is prevented by coiling at 350 ° C. or higher, thereby suppressing the formation of martensite structure. Illustrated.

Furthermore, in order to obtain a desired toughness at a tensile strength of 980 MPa or more, the cleanliness degree d according to JIS G 0555 is set to 0.05% or less. If the cleanliness d is more than 0.05%, cracks are likely to occur from the interface between the base material and inclusions, and the base material and precipitates, and the toughness deteriorates. Furthermore, even when the cleanliness d according to JIS G 0555 is 0.05% or less, the total number density of inclusions and precipitates having an average particle diameter of 5 μm or more is more than 300 pieces / mm ² . Satisfactory toughness cannot be obtained. This is because even if the cleanliness is the same, large inclusions and precipitates having an average particle size of 5 μm or more are greatly affected by toughness. Invite.

(C) Manufacturing condition To make the total number density of inclusions and precipitates having a cleanness d according to JIS G 0555 of 0.05% or less and an average particle diameter of 5 μm or less to 300 pieces / mm ² or less When continuously casting molten steel, the molten steel heating temperature from the liquidus temperature of the molten steel is set to 5 ° C or higher, the molten steel casting amount per unit time is 6 tons / min or less, and the center segregation before the slab is completely solidified. It is effective to use a slab subjected to a reduction treatment, and then reheat the slab at 1200 ° C. or higher before hot rolling, and set the time from extraction in the heating furnace to the end of primary cooling within 10 minutes.

In casting when the molten steel heating temperature from the liquidus temperature of the molten steel is less than 5 ° C, or casting when the molten steel casting amount per unit time is over 6 tons / min, the molten steel temperature at the time of casting is too low, or The flow of molten steel becomes too large, and alumina inclusions are captured inside the slab, and the cleanliness d becomes more than 0.05%. In addition, when the center segregation reduction treatment in the slab is not performed, Ti is concentrated in the center portion of the slab thickness, and a large amount of coarse TiN is precipitated, so that the total number density of inclusions and precipitates having an average particle diameter of 5 μm or more is It becomes over 300 pieces / mm ² .

The slab heating temperature needs to be 1200 ° C. or higher. When the slab heating temperature is less than 1200 ° C., Ti and V precipitates precipitated during casting or slab cooling do not dissolve in a sufficient amount, resulting in insufficient precipitation strengthening and ensuring a tensile strength of 980 MPa or more. I can't. Further, coarse Ti and V-based precipitates remain, and the total number density of inclusions and precipitates having an average particle size of 5 μm or more exceeds 300 pieces / mm ² , so that the toughness is deteriorated. Although the upper limit of the heating temperature is not particularly defined, it is preferably set to 1320 ° C. or less from the viewpoint of operation cost. By setting the heating time to 30 minutes or longer, a tensile strength of 980 MPa can be stably obtained.

Furthermore, when the time from the extraction of the slab heating furnace to the end of the primary cooling exceeds 10 minutes, the time in the high temperature region becomes long and the Ti and V-based precipitates become coarse, and inclusions and precipitates with an average particle size of 5 μm or more The total number density is over 300 pieces / mm ² . Toughness deteriorates.

  Furthermore, since the fine Ti and V-based precipitates contributing to precipitation strengthening decrease, a tensile strength of 980 MPa or more cannot be ensured.

Further, in order to obtain a ferrite area ratio of 60% or more and an average grain size of ferrite of 5 μm or less, hot finish rolling is completed at an Ar ₃ point temperature or higher, and then an average cooling rate of 20 ° C./second within 3 seconds. After starting the cooling at 100 ° C./second or lower, it is necessary to finish the primary cooling at 700 ° C. or lower and 500 ° C. or higher. If the rolling is less than Ar ₃ points, the ferrite region is rolled, so that the ferrite causes abnormal grain growth, and the average grain diameter of the ferrite exceeds 5 μm. If primary cooling is not started within 3 seconds after finish rolling, ferrite grains grow too much and the average grain diameter of ferrite becomes more than 5 μm. Similarly, if the primary cooling rate is less than 20 ° C./second, the cooling rate is too slow, and the ferrite grain size becomes over 5 μm. On the other hand, if the cooling rate exceeds 100 ° C./second, the cooling rate is too high and the ferrite area ratio becomes less than 60%. The primary cooling end temperature needs to be 700 ° C. or lower and 500 ° C. or higher. If the primary cooling stop temperature is higher than 700 ° C., the ferrite grows too much, and the average grain size of the ferrite exceeds 5 μm. Conversely, if the primary cooling stop temperature is less than 500 ° C., the primary cooling end temperature is too low, and a ferrite area ratio of 60% or more cannot be secured.

  Then, by winding at 350 ° C. or higher, the second phase of the metal structure can be pearlite, cementite, bainite, or retained austenite, and the martensite area ratio can be 5% or less.

  By controlling the cooling condition and the winding condition by such a manufacturing method, it is possible to prevent flatness of the steel sheet. For this reason, it is not necessary to forcibly correct the flattening that has occurred as in the prior art, and there is an advantage in terms of cost.

  Moreover, in order to make a steel plate characteristic uniform, you may heat with a rough bar heater before hot rolling finish rolling. Here, the coarse bar heater means an apparatus for heating the roughly rolled bar. In general, an induction heating apparatus is used as a heating method, but a gas burner or the like may be used as long as the apparatus can uniformly heat the bar.

  In this way, according to the present embodiment, it has high strength and excellent workability, is further excellent in toughness, is excellent in flatness after hot rolling, and can be manufactured at low cost. Provided is a high-strength hot-rolled steel sheet that is optimal as a material for structural members used in industrial machines, particularly as a material for structural members represented by automobile members and suspension parts.

  Furthermore, the present invention will be described more specifically with reference to examples.

  Steel having the composition shown in Table 1 was melted using a test converter, and then made into a slab using a test continuous casting machine. As the center segregation reduction treatment, the gap between the upper and lower rolls of the final unsolidified portion was narrowed, and reduction was performed at a reduction rate of 1.0%. The slab was hot-rolled using a test hot finish rolling mill to produce a hot-rolled steel sheet having a thickness of 2.3 mm.

  Subsequently, about the obtained hot-rolled steel sheet, scale removal was performed with the pickling equipment for a test. The manufacturing conditions are shown in Table 2.

  And the tension test was done by extract | collecting a JIS5 tension test piece in the rolling right angle direction. Moreover, the Charpy test piece was cut out from the obtained steel plate, and the Charpy impact test was done. The shape of the test piece was a U-notch Charpy test piece defined in JIS Z 2202. The test method investigated the absorbed energy at -50 degreeC temperature according to the method prescribed | regulated to JISZ2242.

Moreover, it cut out about the cross section parallel to the rolling direction of a steel plate, and performed nital etching. And the metal structure was observed using the scanning electron microscope.
The measurement was performed at a magnification of 1000 times for the plate thickness surface layer portion, (1/4) t portion, and (1/2) t portion, and 10 visual fields were measured for each plate thickness position of each test material. Based on the obtained image, the area ratio of each structure and the crystal grain size of ferrite were obtained by arithmetic calculation. The average crystal grain size of ferrite was measured according to JIS G 0552.

  Moreover, after mirror-polishing the obtained steel plate, etching is not performed, and the plate thickness surface layer portion, the plate thickness (1/4) t portion, and the (1/2) t portion are used at a magnification of 2000 using a scanning electron microscope. Carried out. Ten fields of view were measured for each plate thickness position of each specimen, and the number of inclusions and precipitates having an average particle size of 5 μm or more was counted and converted into an area ratio.

  Here, in calculating the average particle diameter, the actual area of the inclusions and precipitates is obtained by image analysis, the area is replaced with a circle, and the diameter of the circle is calculated to obtain the average particle diameter. It was.

The cleanliness d was calculated by the above-described equation (1) based on the method of JIS G 0555.
The results are summarized in Table 3.

In the case of Test Nos. 1 to 15 which are steels of the present invention, the tensile strength was 980 MPa or more, the elongation (total elongation) was 14.1% or more, and the workability was excellent. Further, the Charpy absorbed energy at −50 ° C. was 88 J / cm ² or more, the Charpy brittle fracture surface ratio at −50 ° C. was 0%, and the toughness was good. Furthermore, the flatness of the steel sheet was also good.

When Comparative Example No16 continuously casts molten steel, the molten steel heating temperature from the liquidus temperature of the molten steel was less than 5 ° C. Therefore, the cleanliness deteriorated to 0.062%, the Charpy absorbed energy at -50 ° C was 60 J / cm ² , and the Charpy brittle fracture surface rate at -50 ° C was 30%, and the toughness was deteriorated.

When Comparative Example No17 was cast, the molten steel casting amount per unit time was more than 6 tons / minute. Accordingly, the cleanliness deteriorated to 0.073%, the Charpy absorbed energy at -50 ° C was 55 J / cm ² , and the Charpy brittle fracture surface rate at -50 ° C was 40%.

In Comparative Example No18, the center segregation reduction treatment was not performed before the slab completely solidified. Therefore, the total number density of inclusions and precipitates having an average particle diameter of 5 μm or more became 320 / mm ² and more than 300 / mm ² . Therefore, the Charpy absorbed energy at -50 ° C. was 50 J / cm ² and the Charpy brittle fracture surface ratio at −50 ° C. was 45%.

In Comparative Example No19, the slab heating was as low as 1150 ° C. and 1200 ° C. or less. Therefore, the effect of precipitation strengthening was small, and the tensile strength was 950 MPa and 980 MPa or less. In addition, the total number density of inclusions and precipitates having an average particle diameter of 5 μm or more was 330 / mm ² and more than 300 / mm ² . Therefore, the Charpy absorbed energy at −50 ° C. was 58 J / cm ² , and the Charpy brittle fracture surface ratio at −50 ° C. was 30%.

In Comparative Example No20, the finish hot rolling temperature was 720 ° C. and less than the Ar ₃ point temperature. Ferrite grains grew abnormally, and the average grain diameter of ferrite became 7.2 μm. Therefore, the Charpy absorbed energy at −50 ° C. was 63 J / cm ² and the Charpy brittle fracture surface ratio at −50 ° C. was 10%.

In Comparative Example No21, the cooling start time after finish rolling was 4 seconds. Therefore, ferrite grains grew and the average grain diameter of ferrite became 6.8 μm. Therefore, the Charpy absorbed energy at −50 ° C. was 60 J / cm ² , and the Charpy brittle fracture surface ratio at −50 ° C. was 15%.

In Comparative Example No22, the primary cooling rate was 15 ° C./sec. Therefore, ferrite grains grew and the average grain diameter of ferrite became 7.6 μm. Therefore, the Charpy absorbed energy at −50 ° C. was 62 J / cm ² , and the Charpy brittle fracture surface ratio at −50 ° C. was 10%.

In Comparative Example No23, the primary cooling rate was 120 ° C./sec. Therefore, the cooling rate was too fast and the ferrite area ratio was 55%. Elongation was 8% and workability deteriorated.
Comparative Example No24 had a primary cooling end temperature as high as 720 ° C. Therefore, ferrite grains grew and the average grain diameter of ferrite became 8.0 μm. Therefore, the Charpy absorbed energy at −50 ° C. was 55 J / cm ² , and the Charpy brittle fracture surface ratio at −50 ° C. was 25%.

Comparative Example No25 had a primary cooling end temperature as low as 480 ° C. The ferrite area ratio was 30% due to insufficient ferrite formation. Elongation was 7.2% and workability was deteriorated.
In Comparative Example No. 26, the time from extraction in the heating furnace to the end of primary cooling was 12 minutes. The time until the end of the primary cooling was long and the precipitates became coarse. Therefore, the total number density of inclusions and precipitates having an average particle diameter of 5 μm or more became 340 / mm ² and more than 300 / mm ² . Therefore, the Charpy absorbed energy at −50 ° C. was 52 J / cm ² and the Charpy brittle fracture surface ratio at −50 ° C. was 35%, which deteriorated toughness. Further, since the fine Ti and V-based precipitates decreased, the strength became 940 MPa.

  In Comparative Example No. 27, the winding temperature was 320 ° C. Therefore, the martensite area ratio became 8%, and the flatness of the steel sheet deteriorated.

Claims (4)

In mass%, C: 0.08% or more and 0.20% or less, Si: less than 0.2%, Mn: more than 1.0%, 3.0% or less, P: 0.05% or less, S: 0.0. 01% or less, Al: 0.001% or more and 0.5% or less, N: 0.01% or less, V: more than 0.1% and 0.5% or less, Ti: 0.05% or more and less than 0.25% , Nb: 0.005% or more and 0.10% or less, having a steel composition composed of the balance Fe and impurities, and having a steel structure in which the area ratio of ferrite is 60% or more and the area ratio of martensite is 5% or less The average particle size of the ferrite is 5 μm or less, the cleanliness d is 0.05% or less, and the total number density of inclusions and precipitates having an average particle size of 5 μm or more is 300 pieces / mm ² or less. A high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more.
Here, the cleanliness d is calculated by the following equation (1).
d = (n / (p × f)) × 100 (1)
In the formula (1), p represents the total number of lattice points on the glass plate in the field of view in JIS G 0555, f represents the number of fields in JIS G 0555, and n represents all inclusions in f fields of view in JIS G 0555. Indicates the number of grid point centers occupied by. Here, all inclusions mean all of A-based inclusions, B-based inclusions, and C-based inclusions in JIS G 0555.   Instead of a part of Fe, in mass%, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% The high-strength hot-rolled steel sheet according to claim 1, containing one or more of the following.   2. It replaces with a part of Fe and contains 1 type (s) or 2 or more types of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less by the mass%. Or the high intensity | strength hot-rolled steel plate described in Claim 2. The molten steel having the steel composition described in any one of claims 1 to 3 is set such that the heating temperature of the molten steel is higher than the liquidus temperature by 5 ° C or more, and the amount of molten steel cast per unit time is 6 The slab is formed by a continuous casting method in which center segregation reduction treatment is performed before the molten steel is completely solidified, and the slab is charged into a heating furnace and heated to a temperature of 1200 ° C. or higher. The slab extracted from the above is subjected to hot rolling to complete rolling in a temperature range of ₃ or more points of Ar to form a hot rolled steel sheet, and cooling to the hot rolled steel sheet is started within 3 seconds after completion of the hot rolling. Then, it is cooled to a temperature range of 700 ° C. to 500 ° C. at an average cooling rate of 20 to 100 ° C./second, subjected to primary cooling that completes cooling within 10 minutes after the extraction, and then wound at a temperature of 350 ° C. or higher. Of high-strength hot-rolled steel sheet, characterized by Production method.