JP4547951B2 - Thick high-strength hot-rolled steel sheet excellent in workability and method for producing the same - Google Patents

Description

  The present invention is suitable for various members of building equipment, automobile undercarriage members, track frame members, various structural members, and the like, and is a thick high strength hot rolled steel sheet excellent in workability with a plate thickness of 6 mm or more and a tensile strength of 780 MPa or more. And a manufacturing method thereof.

  From the viewpoint of improving fuel efficiency that leads to environmental protection, there is a strong demand for high-strength and thin materials used for building equipment members, automobile undercarriage members, truck frame members, and the like. On the other hand, when the strength of the material is increased, the processability is generally deteriorated. Thus, the point is how to achieve both high strength and workability, and various hot-rolled steel sheets have been proposed as materials for which this has been attempted.

  For example, Patent Document 1 proposes a steel sheet in which most of the structure is polygonal ferrite and the strength is increased by precipitation strengthening and solid solution strengthening centering on TiC. However, in order to increase the strength with generally well-known precipitates used in this steel sheet, a large amount of Ti is required, and precipitates with large dimensions are likely to be formed, resulting in unstable properties. There is a drawback that it is easy.

  On the other hand, Patent Literature 2 and Patent Literature 3 disclose a technique for stabilizing the characteristics of a steel sheet by strengthening with fine carbides of Ti and Mo. The technologies described in these documents ensure the workability by adopting a ferrite single-phase structure, and ensure the strength by strengthening with fine carbides, and are excellent steel sheets that achieve both high strength and workability. Can be obtained.

By the way, as a steel plate for the above-mentioned use, a steel plate having a thickness of 6 mm or more is also required. However, by applying the techniques described in Patent Document 2 and Patent Document 3, a steel sheet having a thickness of 6 mm or more is manufactured. Even so, the precipitates and ferrite grains are unlikely to be as fine as intended in these references, and the desired strength cannot be obtained stably. That is, when the plate thickness is 6 mm or more, strengthening by fine precipitates intended in these patent documents has not yet been realized.
Japanese Patent Laid-Open No. 6-200351 JP 2002-322539 A JP 2002-322540 A

  The present invention has been made in view of such circumstances, and in a plate thickness of 6 mm or more, ferrite grains and precipitates can be stably refined by adding a small amount of a reinforcing element, and a desired strength can be stably secured. An object of the present invention is to provide a thick high-strength hot-rolled steel sheet excellent in workability having a tensile strength of 780 MPa or more and a method for producing the same.

  In order to elucidate the reason why a desired strength cannot be obtained even when a thick steel plate having a thickness of 6 mm or more is manufactured by applying the techniques disclosed in Patent Document 2 and Patent Document 3 described above. investigated. As a result, in the case of a thick steel plate having a thickness of 6 mm or more, the time required for a part of the steel plate to be wound after leaving the finish hot rolling mill (runout passage time) is longer than in the case of a thin steel plate, It was found that the reason is that ferrite grains and precipitates are coarsened because the cooling rate in runout (runout cooling rate) is slow. In other words, the run-out passing time is approximately 20 seconds or less when the plate thickness is 5 mm or less, but exceeds 20 seconds when the plate thickness is 6 mm or more, and is approximately 60 seconds when the plate thickness is particularly thick. As a result, in the case of a thick material, the cooling rate is slow, and transformation and precipitation are started at a higher temperature than in the case of a thin material, and ferrite grains and precipitates become coarse.

  Therefore, in order to suppress such coarsening of ferrite grains and precipitates, an experiment was carried out to produce a thick steel plate by using a cooling strengthening device and increasing the run-out cooling speed from the usual level. I couldn't. In order to elucidate this cause, the hardness distribution in the plate thickness direction was investigated, and the surface layer portion of the plate thickness reached the desired hardness, but the desired hardness was not reached in the central portion of the plate thickness. It was found that the strength was insufficient. For this reason, the present inventors conducted an experiment in which the runout cooling speed was further increased. As a result, the strength was still insufficient. Similarly, when the hardness distribution in the plate thickness direction was investigated, the center portion of the plate thickness reached the desired hardness this time, but the plate thickness surface layer portion did not reach the desired strength, and the overall strength was insufficient. It turned out to be. In other words, even if the cooling device is simply strengthened to increase the run-out cooling speed, temperature distribution occurs in the plate thickness direction. As a result, the ferrite grain size and the size and distribution of precipitates also affect the plate thickness direction. Therefore, the hardness distribution in the thickness direction is generated, and the strength of the entire steel sheet is insufficient.

  From the above experimental results, the present inventors have found that the difference in hardness between the plate thickness surface layer and the plate thickness center is constant in order to exert the effects of Patent Documents 2 and 3 on a steel plate having a plate thickness of 6 mm or more. I found it important to: And as a result of examining the manufacturing conditions for obtaining such a preferable hardness in the thickness direction, the present inventors cooled with a cooling pattern including air cooling once or more, and this cooling pattern was finished from the finish hot rolling end. It has been found that it is effective to complete within a certain time. As a result, the temperature of the entire plate thickness can be made uniform by recuperating during cooling while strengthening the cooling, so the hardness distribution in the plate thickness direction can be reduced, and as a result, the desired strength Can be secured. Specifically, after hot rolling under predetermined conditions, the plate surface temperature 10 seconds after finishing hot rolling is finished by setting the average cooling rate for 10 seconds after finishing hot rolling to 20 to 50 ° C./sec. Is cooled to 500 to 700 ° C., and runout cooling is performed by a cooling pattern including air cooling for a total of 5 seconds or more for 10 seconds after finishing hot rolling 10 seconds to 20 seconds, and at 500 to 650 ° C. It has been found that by rolling up, a steel sheet having both high strength and workability can be obtained at a thickness of 6 mm or more.

This invention is made | formed based on such knowledge, and provides the following (1)-( 6 ).

( 1 ) By weight%, C: 0.03 to 0.11%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0. 06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, with the balance being Fe and inevitable impurities , The metal structure is a ferrite structure having an area ratio of 95% or more, and carbides containing Ti and Mo having an average particle size of less than 10 nm are precipitated in the ferrite structure, and have a plate thickness of 6 mm or more. A thick high-strength hot-rolled steel sheet excellent in workability having a tensile strength of 780 MPa or more , characterized in that the difference between the surface layer and the thickness Vickers hardness (Hv) is 60 or less .

( 2 ) By weight%, C: 0.03 to 0.11%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0. 06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, Nb ≦ 0.08%, V ≦ 0.15% One or more of them, the balance is Fe and an inevitable impurity component composition, the metal structure is a ferrite structure with an area ratio of 95% or more, and Ti having an average particle size of less than 10 nm in the ferrite structure Carbide containing Mo and further containing one or two of Nb and V is precipitated, has a plate thickness of 6 mm or more, and there is a difference in Vickers hardness (Hv) between the plate thickness surface layer and the plate thickness center. A thick high-strength hot-rolled steel sheet excellent in workability having a tensile strength of 780 MPa or more , characterized by being 60 or less .

( 3 ) A thick, high-strength hot-rolled steel sheet excellent in workability characterized by containing C, Ti, and Mo so as to satisfy the following expression (1) in (1) or ( 2 ).
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in said Formula (1), C, Ti, and Mo represent the mass% of each component.

( 4 )% by weight, C: 0.03 to 0.11%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0. 06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, with the balance being Fe and steel with inevitable impurities , A steel slab is formed by a continuous casting method or an ingot forming method, immediately or once cooled and then heated to 1100 ° C. or higher, and then hot-rolled to a thickness of 6 mm or more, and the finish hot rolling outlet temperature is set to 750 The plate surface temperature 10 seconds after the finish hot rolling is finished is cooled to 500 to 700 ° C. by setting the average cooling rate for 10 seconds after the finish hot rolling is finished to 20 to 50 ° C./sec. Cooling pattern including air cooling for a total of 5 seconds or more in 10 seconds after finishing hot rolling is finished 10 seconds to 20 seconds later It performs run-out cooling by emission, wherein the wound at 500 to 650 ° C., a tensile strength method for producing a good, thick high-strength hot-rolled steel sheet of the above workability 780 MPa.

( 5 ) By weight, C: 0.03 to 0.11%, Si ≦ 0.3%, Mn: 1 to 2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0. 06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, Nb ≦ 0.08%, V ≦ 0.15% Steel containing one or more of them, the balance being Fe and unavoidable impurities, was melted and made into a steel slab by continuous casting or ingot forming method, immediately or once cooled and heated to 1100 ° C or higher After that, by hot rolling to a plate thickness of 6 mm or more, the finish hot rolling outlet temperature is 750 ° C. or more, and the average cooling rate for 10 seconds from the end of finish hot rolling is 20 to 50 ° C./sec, Cooling is performed until the plate surface temperature reaches 500 to 700 ° C. 10 seconds after finishing hot rolling is finished, and finishing hot rolling is finished 10 It is excellent in workability with a tensile strength of 780 MPa or more, characterized in that runout cooling is performed by a cooling pattern including air cooling for a total of 5 seconds or more for 10 seconds after 20 seconds, and winding is performed at 500 to 650 ° C. A method for producing thick, high-strength hot-rolled steel sheets.

( 6 ) In the above ( 4 ) or ( 5 ), a thick, high-strength hot-rolled steel sheet excellent in workability characterized by containing C, Ti, and Mo so as to satisfy the following expression (1): Production method.
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in said Formula (1), C, Ti, and Mo represent the mass% of each component.

  According to the present invention, a ferrite structure in which fine carbides are deposited is formed at a plate thickness of 6 mm or more, and the hardness distribution in the plate thickness direction is made uniform, so that a desired strength can be secured stably. A thick high strength hot-rolled steel sheet having excellent workability with a tensile strength of 780 MPa or more can be obtained.

Hereinafter, the present invention will be specifically described.
The steel sheet of the present invention is a thick steel sheet having a tensile strength of 780 MPa or more, the metal structure is a ferrite structure having an area ratio of 95% or more, and Ti and Mo having an average particle size of less than 10 nm are contained in the ferrite structure. The carbide | carbonized_material which contains contains and has plate thickness of 6 mm or more, and the difference of Vickers hardness (Hv) of plate | board thickness surface layer and plate | board thickness center is 60 or less. The precipitate may contain one or two of Nb and V in addition to Ti and Mo. Hereinafter, these will be described.

[Target tensile strength level]
The present invention is directed to a hot-rolled steel sheet having a tensile strength of 780 MPa or more. Generally, it is difficult to show the appropriate addition amount of the strengthening element of steel in one component range for those having greatly different strength grades. Therefore, in the present invention, the strength range is defined as 780 MPa or more, and the hardness in the thickness direction and the preferable component range for obtaining this are defined. The reason why the pressure is set to 780 MPa or more is that there are many cases where 780 MPa or more is required in applications such as each member of building equipment, automobile undercarriage member, truck frame member, and various structural members targeted by the present invention.

[Thickness of steel sheet]
The present invention is directed to a thick steel plate having a thickness of 6 mm or more required in applications such as members of building equipment, automobile undercarriage members, truck frame members, and various structural members. Conventionally, a thick steel plate having a thickness of 6 mm or more has not achieved a target strength level of less than 780 MPa depending on fine precipitates. The steel sheet of the present invention is wound around a coiler after hot rolling to form a hot-rolled coil. Therefore, although it depends on the ability of the coiler, the sheet thickness is preferably 20 mm or less that can be wound. Furthermore, it is more preferable to set it as 15 mm or less which is the board | plate thickness normally requested | required in said use.

[Metal structure]
-Ferrite structure with an area ratio of 95% or more :
The reason why the matrix has a ferrite structure is that ferrite is most effective in improving workability. However, the matrix is rather good not necessarily ferrite single phase, more than 95% by area ratio due to the cross-sectional surface tissue observation may be a ferrite. More preferably, it is 98% or more.

-Precipitate of carbide containing Ti and Mo having an average particle size of less than 10 nm:
The carbide containing Ti and Mo can reinforce the steel without precipitating finely and degrading workability. Since the diffusion rate of Mo is generally slow, the growth rate of precipitates is increased by precipitating Mo together with Ti or the like. If the average particle size of the fine precipitates is 10 nm or more, it is difficult to obtain a strength of 780 MPa or more. In addition, when strengthening with precipitates of 10 nm or more, the volume of the precipitates must be increased, which inevitably increases the amount of precipitate-forming elements added, resulting in an increase in the cost of the additive elements. Therefore, in this invention, the average particle diameter of a fine precipitate shall be less than 10 nm. In order to keep the addition amount of the precipitate-forming element small and ensure high strength, the average particle size of the precipitate is preferably 8 nm or less, and more preferably 5 nm or less.

-Precipitate containing one or more of Nb or V in addition to Ti and Mo:
In addition to Ti and Mo, the precipitate may be a composite of one or more of Nb and V. In other words, Ti carbide, nitride, carbonitride, Nb carbide, nitride, carbonitride, V carbide, nitride, carbonitride, and Mo carbide precipitate alone and / or in combination. Does not affect the essence of the invention. In order to easily obtain fine precipitates, the Ti / Mo ratio Ti / Mo is preferably in the range of 0.4 ≦ Ti / Mo ≦ 2.5 in terms of atomic ratio, and 0.7 ≦ Ti. The range of /Mo≦1.5 is more preferable. In this range, the carbide containing Ti and Mo becomes extremely fine, and the effect of the present invention is maximized.

[Hardness distribution in the thickness direction]
In the thick hot-rolled steel sheet of the present invention, the difference in the Vickers hardness (Hv) between the sheet thickness surface layer and the sheet thickness center is 60 or less. By reducing the difference in hardness between the plate thickness surface layer and the plate thickness center to 60 or less, a desired strength can be obtained in a thick steel plate having a plate thickness of 6 mm or more.

  In addition, by setting the difference between the surface thickness layer and the Vickers hardness (Hv) at the center of the thickness to be 60 or less, the problem of “warping” during bending can be made much less likely to occur. That is, when a thick product with a plate thickness of 6 mm or more is manufactured without strengthening the cooling in the run-out and without air cooling, temperature distribution occurs in the thickness direction during cooling because of the thick product, and the ferrite grain size and the size of the precipitate・ Because distribution occurs in the plate thickness direction even in the distribution state, a difference in hardness exceeding 60 in Hv in the plate thickness direction occurs. However, bending is applied to such a material having a large hardness distribution in the plate thickness direction. Doing so tends to cause a so-called `` (ridgeline) sled '' that causes the bending ridge line to go out without being straight, and if this is severe, an extra correction process must be provided, which increases costs. End up. The reason why materials with hardness distribution in the plate thickness direction are likely to cause sleigh is not necessarily clear, but sleigh occurs due to the difference in expansion and contraction of the material between the outer and inner edges when the plate is bent. In view of this, it is considered that the amount of elongation / contraction of the material in the ridge line direction when bending is applied is greater when the material has a hardness distribution in the plate thickness direction.

  In the present invention, in order to reduce the hardness distribution in the thickness direction of the thick material having a thickness of 6 mm or more in this way, as will be described later, the cooling is strengthened more than usual and the reheating during the cooling is performed. Cooling is performed with a cooling pattern to make the temperature of the entire plate uniform.

  Regarding the hardness measurement, the Vickers hardness of the cross section of the plate is measured with a load of 200 gf. The hardness of the plate thickness surface layer and the hardness of the plate thickness center are respectively 250 μm and the plate thickness center position from the plate surface. Defined as measured value.

[Ingredient composition]
In order to obtain a hot-rolled steel sheet having the structure as described above, the composition of steel is, in mass%, C: 0.03 to 0.11%, Si ≦ 0.3%, Mn: 1 to 2%. , P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21% It is desirable to adjust as follows.

C: 0.03-0.11%
C forms carbides and is effective for strengthening steel. However, if it is less than 0.03%, the effect is insufficient. On the other hand, if added over 0.11%, pearlite, which is a coarse carbide that does not contribute to the strengthening of steel, is formed and the workability is deteriorated, so the content was made 0.03 to 0.11%.

Si ≦ 0.3%
Si is effective as a solid solution strengthening element. However, if added over 0.3%, C discharge from the ferrite is promoted, and coarse iron carbide tends to be precipitated at the grain boundaries, resulting in deterioration of workability.

Mn: 1-2%
Mn is added at 1% or more to strengthen the steel. However, if added over 2%, segregation occurs and a hard phase is formed, and the workability deteriorates.

P ≦ 0.06%
P is effective for solid solution strengthening. However, if added over 0.06%, segregation and workability deteriorate, so the content was made 0.06% or less.

S ≦ 0.01%
The smaller the S, the better. If it exceeds 0.01%, the workability deteriorates, so the content was made 0.01% or less.

Al ≦ 0.06%
Al is added as a deoxidizer. However, if it exceeds 0.06%, the workability deteriorates, so the content was made 0.06% or less.

N ≦ 0.006%
The smaller N, the better. If it exceeds 0.006%, coarse TiN increases and the workability deteriorates. In addition, TiN is formed, which should contribute to strengthening the steel by forming fine carbide with Mo originally. Since Ti does not work effectively, it was made 0.006% or less.

Mo: 0.15-0.45%
Mo is an important element in the present invention. Addition of 0.15% or more suppresses the pearlite transformation to form a ferrite single-phase structure, and forms fine carbon and nitride containing Ti or Ti, Nb, and V, resulting in high strength and workability. Are compatible. However, if added over 0.45%, a hard phase is formed and the workability of the steel deteriorates, so the content was made 0.15 to 0.45%.

Ti: 0.06-0.21%
Ti is an important element in the present invention. By adding 0.06% or more, steel is effectively strengthened by carbon / nitride precipitation. However, even if added over 0.21%, the amount of coarse carbides that do not contribute to the strengthening of the steel increases and the added components are wasted, so 0.06 to 0.21%.

Nb ≦ 0.08%
Nb contributes to strengthening of steel by precipitation of charcoal and nitride. However, if added over 0.08%, the amount of precipitates becomes excessive and the workability deteriorates, so the content was made 0.08% or less.

V ≦ 0.15%
V contributes to strengthening of steel by precipitation of charcoal and nitride. However, if added over 0.15%, precipitates become excessive and workability deteriorates, so the content was made 0.15% or less.

Incidentally, in addition to the above elements, it is of course inevitable impurities is allowed.

  In the present invention, it is further preferable that the value of (C / 12) / {(Ti / 48) + (Mo / 96)} (hereinafter referred to as P value) satisfies 0.5 to 1.5. This is because when the content of C, Ti, and Mo is controlled so that the atomic ratio of C to (Ti + Mo) in the steel is 0.5 to 1.5, carbides containing Ti and Mo precipitate finely. This is because the steel is effectively strengthened. The P value is more preferably 0.8 to 1.3. In addition, C, Ti, and Mo which prescribe | regulate P value represent the mass% of each component.

[Production method]
The hot-rolled steel sheet of the present invention is a steel slab produced by melting a steel having the above-mentioned composition and formed into a steel slab by a continuous casting method or an ingot forming method, immediately or once cooled and then heated to 1100 ° C. or higher and then hot rolled. The finish hot rolling outlet temperature is set to 750 ° C. or higher, and the average cooling rate for 10 seconds from the finish hot rolling finish is set to 20 to 50 ° C./sec, whereby the plate surface 10 seconds after finish hot rolling finishes. Cool until the temperature reaches 500 to 700 ° C., perform runout cooling with a pattern including air cooling for a total of 5 seconds or more for 10 seconds after finishing hot rolling 10 seconds to 20 seconds, and at 500 to 650 ° C. Manufactured by winding. Hereinafter, the reason for this definition will be described.

-The steel slab is immediately hot-rolled or once cooled and then heated to 1100 ° C or higher and then hot-rolled. After hot-rolling, it contains carbides containing Ti and Mo, or Ti and Mo, and Nb and V In order to precipitate a carbide containing one or two kinds, it is necessary to dissolve TI, Nb, V and Mo in the slab stage before hot rolling, so the steel slab is immediately hot-rolled, or Once cooled, it is heated to 1100 ° C or higher and then hot rolled. In other words, since Ti, Nb, V and Mo are in solid solution after slab production, it is not necessary to heat before hot rolling because the solid solution state is maintained when immediately hot rolling, Once cooled, coarse precipitates are formed, so it is necessary to heat the solution to 1100 ° C. or higher to again dissolve Ti, Nb, V, and Mo.

-Finishing hot rolling outlet temperature: 750 ° C or higher In order to ensure strength and workability, the finishing hot rolling outlet temperature is set to 750 ° C or higher. When the temperature is less than 750 ° C., the thickness of the surface layer becomes ferrite region rolling, and the grain size is expanded and the workability is impaired. Also, the strength is difficult to be secured, and the strength is not uniform in the thickness direction. The problem arises that the member shape is not stable.

-Average cooling rate for 10 seconds after finishing hot rolling finish: 20-50 ° C / sec
-Plate surface temperature 10 seconds after finishing hot rolling finish: 500-700 ° C
-10 seconds after finishing hot rolling 10 seconds to 20 seconds, including a total of 5 seconds or more of air cooling These conditions define the cooling pattern on the run-out table, and are important production of the present invention It is a condition.

  When the average cooling rate for 10 seconds after the finish hot rolling is finished is less than 20 ° C./sec, the transformation starts at a higher temperature especially near the center of the plate thickness and the precipitate becomes coarse. On the other hand, when the temperature is 50 ° C./sec or more, the surface thickness of the sheet thickness is particularly low, and even if air cooling is performed, the reheating until the transformation starts is insufficient and the transformation starts without eliminating the temperature difference. Directional material is not uniform. Therefore, the average cooling rate for 10 seconds from the end of finish hot rolling is set to 20 to 50 ° C./sec.

  If the surface temperature of the plate 10 seconds after finishing hot rolling is less than 500 ° C., the surface portion of the plate becomes a bainitic structure, and precipitation of precipitates becomes insufficient. On the other hand, when it exceeds 700 ° C., the transformation starts at a higher temperature especially near the center of the plate thickness, and the precipitate becomes coarse. Therefore, the plate surface temperature 10 seconds after finishing hot rolling is set to 500 to 700 ° C.

  If the air cooling time included in 10 seconds from 10 seconds to 20 seconds after finishing hot rolling is less than 5 seconds in total, the time for reheating is insufficient, and the temperature near the plate thickness surface layer portion and the plate thickness center The difference is not resolved. Therefore, 10 seconds after the finish hot rolling is completed to 10 seconds after 20 seconds, air cooling for a total of 5 seconds or more is included.

  By executing such a cooling pattern, the temperature distribution in the plate thickness direction can be made uniform, and ferrite grains and precipitates can be finely precipitated throughout the plate thickness, and the Vickers hardness of the plate thickness surface layer and the plate thickness center can be obtained. The difference in length (Hv) can be made 60 or less.

-Winding temperature: 500-650 ° C
In order to obtain a ferrite structure, the coiling temperature is set to 500 to 650 ° C. If the temperature is less than 500 ° C., the surface layer of the plate particularly contains bainite, and the precipitates of Ti, Nb, V and Mo are not sufficiently precipitated, the effect of precipitation strengthening is reduced, and the strength is lowered. However, these precipitates are coarsely deposited and the effect of precipitation strengthening is reduced. More preferably, it is 550-650 degreeC, and this further improves the balance of strength and workability.

  Note that the effect of the present invention is not affected even if the direct rolling is performed immediately after casting or after performing heating for the purpose of supplementary heating. Moreover, even if the sheet bar is heated or kept after rough rolling before or during finish rolling, continuous rolling performed by joining the sheet bar after rough rolling, or heating and continuous rolling of the sheet bar are performed. Even if it carries out simultaneously, the effect of this invention is not impaired. In addition, the hot-rolled steel sheet of the present invention has no difference in characteristics between the scaled state and the pickling material. There is no problem with temper rolling as long as it is a condition normally performed.

Example 1
Steels having the composition shown in Table 1 were melted and hot rolled under the conditions shown in Table 2. The metallographic structure of the obtained steel sheet was confirmed with an optical microscope and a scanning electron microscope (SEM), and the precipitate was observed with a transmission electron microscope (TEM) to determine the average particle size of the precipitate. The elements contained in the precipitates were analyzed by the attached energy dispersive X-ray spectrometer (EDX). Further, a tensile test using a JIS No. 5 test piece was performed to determine the tensile strength. These results are also shown in Table 2. In Table 1, the P value represents a value of (C / 12) / {(Ti / 48) + (Mo / 96)}. In Table 2, SRT is the slab heating temperature, FT is the finish hot rolling outlet temperature, CR is the average cooling rate for 10 seconds from the end of finish hot rolling, and CT is the coiling temperature.

  As shown in Table 2, no. The steel sheets 1 to 6 are ferrite single-phase structures in which carbides of appropriate sizes including Ti and Mo are deposited, and appropriate strength is ensured. The plate thickness is 6 to 15 mm, all of which use a cooling strengthening device and perform necessary air cooling for 5 seconds or more. The cooling strengthening device is not used because it is not necessary for the production of a hot-rolled steel sheet having a thickness of 5 mm or less. However, in the production of a steel sheet having a thickness of 6 mm or more according to the present invention, CR was used in order to make the scope of the present invention. In FIG. The electron micrograph of the steel plate of No. 5 is shown. From this photograph, it can be seen that fine precipitates are uniformly dispersed in the ferrite single phase.

  In contrast, no. In the steel sheets of 7 to 9, the size of the precipitate is 13 nm or more, which is out of the range of the present invention, and the tensile strength is insufficient. Of these, No. 7 is because CR is 17 ° C./sec, which is outside the scope of the present invention, and no cooling strengthening device is used. No. Nos. 8 and 9 have a CR of 20 to 50 ° C./sec using a cooling strengthening device, but the strength is not ensured because sufficient air cooling is not performed. No. Since the bainite phase appeared on the surface layer of the steel plate No. 10, sufficient precipitation did not occur and the strength was insufficient. This is because the cooling enhancement device is used and the necessary air cooling is performed, but the CR is too fast as 54 ° C./sec and the temperature distribution in the plate thickness direction is not canceled, so the transformation starts. The difference in organization was made. In addition, even if it does not perform use of a cooling reinforcement | strengthening apparatus or air cooling, if the plate | board thickness is 5 mm or less, the steel plate which has the target structure | tissue and intensity | strength can be manufactured. 11. No. No. 12 steel plate is an example in which the bainite structure is mixed because the coiling temperature is too low.

  Here, no. FIG. 2 shows the results of measuring the Vickers hardness of the cross section at a load of 200 gf for the steel plates 5 and 7-9. No. In No. 7, the hardness did not reach the required level over the entire thickness, because the cooling rate was insufficient because the cooling was not strengthened, and transformation or precipitation occurred at a high temperature. No. 8 and no. No. 9 uses a cooling strengthening device but does not perform the necessary air cooling, so the hardness is not constant in the plate thickness direction, and the difference in hardness between the plate thickness surface layer and the plate thickness center exceeds 60 Therefore, it does not reach the required hardness as a whole. These No. 7-9, no. No. 5 uses a cooling strengthening device and performs necessary air cooling, so that the thickness distribution of the hardness is uniform and the difference in hardness between the thickness surface layer and the thickness center is 60 or less, and the required hardness is The level is secured.

(Example 2)
Steels having the component compositions shown in Table 3 were melted and hot rolled under the conditions shown in Table 4. All the plate thicknesses are 9 mm, and in each case, a cooling strengthening device is used so that CR is within the range of the present invention, and necessary air cooling is performed for 5 seconds or more. The metallographic structure of the obtained steel sheet was confirmed with an optical microscope and a scanning electron microscope (SEM), and the precipitate was observed with a transmission electron microscope (TEM) to determine the average particle size of the precipitate. The elements contained in the precipitates were analyzed by the attached energy dispersive X-ray spectrometer (EDX). Further, a tensile test using a JIS No. 5 test piece was performed to determine the tensile strength. These results are also shown in Table 4.

  As shown in Table 4, no. The steel plates 13 to 16 are ferrite single-phase structures in which carbides of appropriate sizes including Ti and Mo are deposited, and appropriate strength is ensured.

  In contrast, no. All the steel plates 17 to 24 are examples outside the present invention. Looking at Table 3 and Table 4 together, 17 and No. As for the 18 steel plate, C amount is outside the range of the present invention. No. No. 17 has a C amount of less than the lower limit of the range of the present invention, resulting in insufficient strength. On the other hand, the amount of C exceeds the upper limit of the range of the present invention, and the structure contains pearlite. No. 19 and No. As for 20 steel plates, Mo amount is outside the range of the present invention. No. In No. 19, the amount of Mo is less than the lower limit of the range of the present invention, and the strength is insufficient. In contrast, in No. 20, the amount of Mo exceeds the upper limit of the range of the present invention, and the structure contains martensite. No. 21 and no. In the steel plate of 22, the Ti amount is out of the scope of the present invention. No. No. 21 is insufficient in strength because the Ti amount is less than the lower limit of the range of the present invention. In No. 22, since the amount of Ti is too large, coarse TiC is precipitated and the strength becomes insufficient. No. 23 and no. The 24 steel plate has a P value outside the range of the present invention. In any case, the balance between C, Mo and Ti is poor, and precipitates are coarsely deposited, resulting in insufficient strength.

  According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having a thickness of 6 mm or more and excellent workability with a tensile strength of 780 MPa or more that can stably secure a desired strength. It is suitable as each member, automobile underbody member, truck frame member, and various structural members, and has high industrial value.

The transmission electron micrograph which shows the metal structure of the steel plate which concerns on this invention. The figure which shows the hardness distribution of the plate | board thickness direction of the steel plate which concerns on this invention.

Claims (6)

% By weight, C: 0.03-0.11%, Si ≦ 0.3%, Mn: 1-2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, with the balance being the component composition of Fe and unavoidable impurities , The ferrite structure has an area ratio of 95% or more, and carbides containing Ti and Mo having an average particle size of less than 10 nm are precipitated in the ferrite structure, and has a plate thickness of 6 mm or more. A thick high-strength hot-rolled steel sheet excellent in workability having a tensile strength of 780 MPa or more , characterized in that the difference in Vickers hardness (Hv) at the thickness center is 60 or less . % By weight, C: 0.03-0.11%, Si ≦ 0.3%, Mn: 1-2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, further Nb ≦ 0.08%, V ≦ 0.15% Including the above, the balance is the composition of Fe and inevitable impurities , the metal structure is a ferrite structure with an area ratio of 95% or more, and Ti and Mo having an average particle size of less than 10 nm in the ferrite structure In addition, a carbide containing one or two of Nb and V is precipitated, has a plate thickness of 6 mm or more, and the difference in Vickers hardness (Hv) between the plate thickness surface layer and the plate thickness center is 60 or less. Thick high strength hot rolled steel sheet, wherein a tensile strength superior to the above workability 780MPa that there. The thick, high-strength hot-rolled steel sheet having excellent workability according to claim 1 or 2 , wherein C, Ti, and Mo are contained so as to satisfy the following expression (1).
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in said Formula (1), C, Ti, and Mo represent the mass% of each component. % By weight, C: 0.03-0.11%, Si ≦ 0.3%, Mn: 1-2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, the balance is Fe and steel with inevitable impurities is melted, continuous casting method Alternatively, a steel slab is formed by the ingot forming method, or immediately after being cooled or heated to 1100 ° C. or higher, and then hot rolled to a thickness of 6 mm or more, and the finish hot rolling outlet temperature is set to 750 ° C. or higher. Then, by setting the average cooling rate for 10 seconds from the end of finish hot rolling to 20 to 50 ° C./sec, the plate surface temperature 10 seconds after finish hot rolling ends is cooled to 500 to 700 ° C., and the finish heat According to the cooling pattern including air cooling for a total of 5 seconds or more in 10 seconds after the end of hot rolling 10 seconds to 20 seconds. Performs run-out cooling Te, and wherein the winding at 500 to 650 ° C., a tensile strength method for producing a good, thick high-strength hot-rolled steel sheet of the above workability 780 MPa. % By weight, C: 0.03-0.11%, Si ≦ 0.3%, Mn: 1-2%, P ≦ 0.06%, S ≦ 0.01%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.15 to 0.45%, Ti: 0.06 to 0.21%, further Nb ≦ 0.08%, V ≦ 0.15% Including the above, steel with the balance being Fe and unavoidable impurities is melted and formed into a steel slab by a continuous casting method or an ingot forming method. Immediately or after cooling and heating to 1100 ° C. or higher, 6 mm By hot rolling to the above plate thickness, the finish hot rolling outlet temperature is 750 ° C. or higher, and the average cooling rate for 10 seconds from the end of finish hot rolling is 20-50 ° C./sec. The plate surface temperature 10 seconds after the end of rolling is cooled to 500 to 700 ° C., and 10 seconds after the finish hot rolling ends to 2 Thick material excellent in workability with a tensile strength of 780 MPa or more, characterized in that runout cooling is performed by a cooling pattern including air cooling of 5 seconds or more in 10 seconds after the second, and winding is performed at 500 to 650 ° C. Manufacturing method of high-strength hot-rolled steel sheet. C, Ti, and Mo are contained so that the following (1) Formula may be satisfied, The manufacturing method of the thick high strength hot-rolled steel plate excellent in workability of Claim 4 or Claim 5 characterized by the above-mentioned.
0.5 ≦ (C / 12) / {(Ti / 48) + (Mo / 96)} ≦ 1.5 (1)
However, in said Formula (1), C, Ti, and Mo represent the mass% of each component.