JP6036756B2 - High strength hot rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more, which is suitable as a component for automobiles, and the like, and relates to a high-strength hot-rolled steel sheet excellent in ductility, hole expansibility and punchability, and a method for producing the same.

  Conventionally, many automobile parts are formed into a desired complex shape by pressing, burring, or the like on a steel plate as a material. Therefore, it has been required that the steel plate as a material is excellent in workability such as ductility and stretch flangeability.

  In recent years, from the viewpoint of protecting the global environment, automobile exhaust gas regulations have been strengthened, automobile body weight has been reduced, and passenger safety during a collision has been strongly demanded. For these reasons, the strength of automobile parts has been increased, and high-strength steel sheets are used as materials for automobile parts. Recently, high-strength steel sheets having a tensile strength of 780 MPa or more have been used, and such high-strength steel sheets are also required to have excellent workability.

  In response to such a demand, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa or more that is excellent in low-temperature toughness and hole expansibility. In the technique described in Patent Document 1, in mass%, C: 0.001 to 0.05%, Si: 0.01 to 1.5%, Mn: 0.01 to 2.0%, Al: 0.005 to 0.05%, N: 0.01% or less, Ti: A slab having a composition containing 0.01 to 0.20% is heated to 1200 ° C or higher, final finish rolling is performed at 960 ° C or higher, and cooling is started at a cooling rate of 80 ° C / s or higher within 1.0 s after finishing rolling. , Cooled to a temperature 50 to 200 ° C. lower than the final finish rolling temperature, wound at 450 to 600 ° C., parallel to the rolling surface and parallel to the rolling direction, the X-ray random intensity ratio in the {211} <011> direction is 2.5. A high-strength hot-rolled steel sheet having the following structure is obtained. According to the technique described in Patent Document 1, the finishing rolling temperature is increased to randomize the texture of the steel sheet, and further refinement of the subsequent cooling achieves refinement of crystal grains, and the hole expandability and It is said that low temperature toughness is improved.

  Patent Document 2 describes a steel plate having an excellent balance between elongation and hole expansibility. In the technique described in Patent Document 2, a composition containing C: 0.04 to 0.1%, Si: 0.5 to 2.0%, Mn: 0.8 to 2.0%, Nb: 0.04% or less, Ti: 0.06 to 0.20% by mass%. After heating the steel slab having 1050 to 1300 ° C, hot rolling with a finishing temperature of 850 to 1000 ° C, after hot rolling, cooling to 600 to 750 ° C and air cooling for 1 to 10 seconds The steel sheet is cooled at a cooling rate of 15 ° C / s or more and wound up at 400 ° C or less to form a hot-rolled steel sheet. As a result, it has a structure consisting of ferrite of 90% or more by volume and martensite of 3% or more and less than 10%. Ferrite is reinforced with Nb or Ti carbide, tensile strength is 780 MPa or more, and elongation. It is said that a hot-rolled steel sheet having an excellent balance of hole expandability can be obtained.

  In many cases, automobile parts are formed into a desired shape by subjecting a steel plate, which is a material, to a shearing process or a punching process, followed by various processes. If wrinkles or microcracks are generated on the end face by punching, stress may concentrate on the wrinkles or microcracks in the subsequent processing, and may extend into large cracks. For this reason, it is desirable to minimize the occurrence of wrinkles and microcracks on the end face in the punching process.

In response to such a demand, for example, Patent Document 3 proposes a high-strength hot-rolled steel sheet excellent in punching workability. In the technique described in Patent Document 3, in mass%, C: 0.01 to 0.07%, Si: 0.01 to 2%, Mn: 0.05 to 3%, Al: 0.005 to 0.5%, N: 0.005% or less, Ti: A steel slab having a composition containing 0.03 to 0.2% is heated to 1200 ° C or higher, and then hot finish rolling is finished at a temperature equal to or higher than the Ar ₃ transformation point and wound at 450 to 650 ° C to form a hot-rolled steel sheet. . As a result, the ferrite or bainitic ferrite structure is the phase with the largest area ratio, the hard second phase and cementite have a structure with an area ratio of 3% or less, and a high strength with a tensile strength of 690 MPa or more. It is said that a high-strength hot-rolled steel sheet excellent in punching workability and suitable for automobile parts can be obtained.

JP 2012-136773 A JP 2011-184788 JP 2005-298924 A

  However, according to the techniques described in Patent Documents 1 and 2, excellent hole expandability can be secured, however, Patent Documents 1 and 2 have no mention of punchability and can be used for punching under severe conditions. It is difficult to think that it has excellent punchability that can withstand.

  Further, in the technique described in Patent Document 3, when the clearance at the time of punching is about 17 to 23%, no defect is observed on the punched end face, and it can be said that the end face has good properties. However, if the clearance at the time of punching is further increased, there is a problem that the punching end surface becomes rough and the punching property is lowered.

  In addition, regardless of the clearance at the time of punching, if the steel sheet has excellent punching ability to obtain a good punching end face, the labor for adjusting tools and dies for each punching site is greatly reduced, and punching processing is performed. However, none of Patent Literatures 1 to 3 mentions such a thing.

  The present invention advantageously solves the problems of the prior art, has a high strength of tensile strength: 780 MPa or more, has excellent ductility and hole expansibility, and has excellent punchability and production thereof It aims to provide a method.

  The term “excellent in ductility” as used herein refers to the case where the total elongation El when a tensile test is performed with a JIS No. 5 test piece is 20% or more. Further, “excelling in hole expandability” means a case where the hole expansion ratio λ is 80% or more in a hole expansion test performed in accordance with the provisions of JFS T 1001-1996. Also, “Excellent punchability” means that cracking occurs on the punched end face when punching a hole with a diameter of 10mmφ with a clearance of 25% of the plate thickness in accordance with the provisions of JFS T 1001-1996. The case where it is not recognized shall be said.

  In order to achieve the above-mentioned object, the present inventors have intensively studied various factors affecting workability in a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more. As a result, the steel sheet structure is mainly composed of a ferrite phase with an area ratio of 95% or more, and fine Ti carbide with an average particle size of 10 nm or less is precipitated in the ferrite phase to ensure the desired high strength by precipitation strengthening. For example, it was found that both ductility and hole expansibility improved despite the high strength of 780 MPa or more.

  Furthermore, the present inventors examined various factors that affect the punchability of a hot-rolled steel sheet having the above-described structure and a high strength having a tensile strength of 780 MPa or more. As a result, it has been found that in a steel sheet having a structure mainly composed of a ferrite phase, if a hard layer is present in the structure, the punchability is significantly reduced. As a hard layer, a center segregation layer may exist in the center part of the thickness of a continuously cast steel plate. When there is a center segregation layer, a defect may be generated on the end face during punching, which cracks in parallel to the rolling direction (parallel to the rolling surface). However, further studies by the present inventors have reduced the center segregation, and the hardness at the center position of the plate thickness (hardness of the center segregation layer) is different from the hardness of the portion other than the center position of the plate thickness (center segregation layer). It was found that when the difference ΔHV is 20 HV or less, the above-described defects are not observed on the punched end face.

  However, even without the central segregation layer, in the steel sheet having a structure mainly composed of a ferrite phase, a crack-like microcrack extending in a direction perpendicular to the rolling direction is generated on the punched end face, and the punchability is still sufficiently improved. Found that there may not be.

  In a steel sheet having a structure mainly composed of a ferrite phase, the deformability of the ferrite phase is high, so that the ferrite phase is greatly deformed with the movement of the punch at the time of punching. For this reason, particularly when the stress is concentrated at one point, it has been thought that a crack-like microcrack extending from the location in a direction perpendicular to the rolling direction may occur. Therefore, the present inventors can avoid the stress from concentrating on one point, and if the stress is dispersed in a number of locations, it can be avoided to develop into crack-like microcracks extending in the direction perpendicular to the rolling direction, It came to mind that the punching ability was improved. In addition, although microvoids are formed in many places, it has been confirmed that the microvoids do not develop into crack-like microcracks extending in a direction perpendicular to the rolling direction.

The present inventors have found that cementite dispersed and precipitated in the ferrite phase is effective as the starting point of the microvoid, and the density of cementite (also referred to as precipitation density or dispersion density) is 10/10000 μm ² or more. It was found that it is important to adjust to.

  First, the experimental results on which the present invention is based will be described.

  A continuous cast piece (continuous cast piece) having the composition shown in Table 1 is heated to 1230 ° C. and subjected to hot rolling consisting of rough rolling and finish rolling with a finish rolling finishing temperature of 890 to 910 ° C. A hot-rolled steel strip (hot-rolled steel plate) was used and wound at a temperature of 600 to 630 ° C.

  From the obtained hot-rolled steel strip (hot-rolled steel plate), four test specimens for hole expansion test were taken from any location, and in accordance with the provisions of the Japan Iron and Steel Federation Standard JFS T 1001-1996, clearance: 25 ~ At 30%, a punch hole (10 mmφ) was punched in the center of the test piece, and punchability was evaluated. The punchability was performed by visually observing the punch hole end face and observing the presence or absence of cracks.

As a result, cracks may occur in the punched end face at right angles to the sheet thickness direction and parallel to the rolling direction (parallel to the rolling surface). Therefore, with respect to the test piece that cracked and parallel to the rolling direction at the punched end face, and for the test piece that did not crack, a further test piece for hardness measurement was taken from a position 5 mm away from the punched end face, and from the punched end face, The Vickers hardness HV _0.025 was measured with a Vickers hardness tester (load: 25 gf (test force: 0.25 N)) in accordance with JIS Z 2244 for the cross section in the thickness direction at a position 5 mm away. The measurement positions of hardness were the center position of the plate thickness and the 1/4 position of the plate thickness, 10 points were measured at each position, and the respective average values were obtained.

  The obtained results are shown in Table 2.

For specimens with cracks in the punched end face parallel to the rolling direction, the hardness HV _{1 / 2t} at the center of the plate thickness is higher than the hardness HV _{1 / 4t} at the plate thickness 1/4. Show. That is, the central portion of the plate thickness is locally hardened, and it can be estimated that this local hardness variation is a cause of cracks generated in the punched end face parallel to the rolling direction.

  Next, elemental analysis of the elements was performed along the thickness direction of the test piece using EPMA. As a result, it was found that C and Mn were concentrated in the central part of the plate thickness in the test piece in which cracking occurred. This concentration of components is thought to be due to center segregation during continuous casting.

Next, the difference between the hardness HV _{1 / 2t} at the plate thickness center position and the hardness HV _{1 / 4t} at the plate thickness 1/4 position shown in Table 2, ΔHV _0.025 (= HV _{1 / 2t} −HV _{1 / 4t} ) Is shown in FIG. 1 in relation to the hardness HV _{1 / 4t} at the thickness 1/4 position. From FIG. 1, it can be seen that even if there is a hard portion that is considered to be caused by center segregation, if ΔHV _0.025 is 20 HV or less, the effect on punchability is small, and the occurrence of cracks at the punched end face can be avoided.

For this reason, the inventors reduced the central segregation so that ΔHV _0.025 (= HV _{1 / 2t} −HV _{1 / 4t} ) is 20 HV or less by applying light reduction during continuous casting. It has been found that local hardness fluctuations as described above can be suppressed, and cracks generated at the punched end face parallel to the rolling direction can be prevented.

However, cracking that occurs parallel to the rolling direction can be prevented even if light segregation is performed at the time of casting to reduce center segregation, but a crack-like microcrack that extends in a direction perpendicular to the rolling direction occurs on the punched end surface. I found that there was a case. Therefore, a specimen for observing the structure with an observation surface at a position 5 mm away from the punched end face was collected, polished with the cross section in the thickness direction as the observation surface, and subjected to Nital corrosion, using a scanning electron microscope (magnification: 1000 times). And observed the structure. As a result, it was found that when a crack-like microcrack extending in the direction perpendicular to the rolling direction occurred on the punched end face, the cementite precipitation density in the ferrite phase was as low as less than 10 pieces / 10000 μm ² .

  Then, as shown in Table 3, molten steel having various compositions was melted and cast into a slab by continuous casting, and light slab was applied during continuous casting to obtain a slab. The obtained slab is heated to 1230 ° C. and subjected to hot rolling consisting of rough rolling and finish rolling at a finish rolling finish temperature of 890 to 920 ° C. to obtain a hot rolled steel strip having a thickness of 3.0 mm, 600 Rolled up at ~ 650 ° C. Table 4 shows the hot rolling conditions.

  A punched specimen is collected from the obtained hot-rolled steel strip, punched into a punch hole (10mmφ) with a clearance of 25-30% according to the regulations of JFS T 1001-1996, the punched end face is visually observed, and rolled. The presence or absence of crack-like microcracks extending in the direction perpendicular to the direction was investigated.

In addition, a specimen for observing the structure with an observation surface at a position 5 mm away from the punched end surface was collected, polished with the cross section in the thickness direction as the observation surface, and subjected to Nital corrosion, using a scanning electron microscope (magnification: 1000 times) The number of cementite grains deposited in a 100 μm × 100 μm region was measured at three fields of view, the average value was obtained, and the precipitation density of cementite grains (pieces / 10000 μm ² ) was calculated. . The precipitation density of the obtained cementite grains and the cracking results at the punched end face are given by the following formula: C * (%) = C− (12/48) × Ti + (12/14) × N + (12/32) × S − (12/51) × V− (12/93) × Nb− (12/96) × Mo
(Here, C, Ti, N, S, V, Nb, Mo: content of each element (mass%))
I found that I can organize well with C * defined in. When calculating the above-mentioned C *, it is assumed that elements that are not contained among the elements shown in the formula are calculated as zero. The obtained results are shown in FIG. 2 in relation to C *.

From FIG. 2, when C * is 0.010% or more, the precipitation density of cementite grains can be made 10 (pieces / 10000 μm ² ) or more, and the crack-like minute length extending in the direction perpendicular to the rolling direction on the punched end face. It can be seen that cracking can be avoided.

  Furthermore, when the cross-sectional structure in the vicinity of the end surface including the punched end surface is observed with a scanning electron microscope, in the test piece in which the occurrence of cracked microcracks extending in the direction perpendicular to the rolling direction on the punched end surface can be avoided, Microvoids were formed around the cementite grains. On the other hand, in the test piece in which the crack-like microcracks described above occurred on the punched end face, it was observed that the ferrite phase (matrix phase) was greatly deformed near the end face and the cracks spread from inclusions such as TiN. From these findings, when a proper amount of cementite grains are dispersed and precipitated, a large number of microvoids are formed starting from the cementite grains at the time of punching. It is considered that the occurrence of the occurrence was suppressed.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.030 to 0.090%, Si: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.005% or less, N: 0.006% or less, Al: 0.1% or less , Ti: 0.070 to 0.220%, and the following formula (1)
C * ≧ 0.010 (1)
(Where C * = C− (12/48) × Ti + (12/14) × N + (12/32) × S− (12/51) × V− (12/93) × Nb− (12 / 96) x Mo
C, Ti, N, S, V, Nb, Mo: Content of each element (% by mass))
And having a composition comprising the balance Fe and inevitable impurities and a ferrite phase of 95% or more in area ratio, 1.0 × 10 ²² particles / m ³ or more of fine carbides in the ferrite phase, and cementite particles 10 pieces / 10,000 μm ² or more are deposited, the average particle size of the fine carbide is 10 nm or less, the hardness HV _{1 / 2t} at the center of the plate thickness and the hardness HV _{1 / 4t at the} plate thickness _1/4 or the plate thickness 3/4 difference Delta] HV _0.025 of the hardness HV _{3 / 4t} positions have a tissue is below 20 HV, tensile strength: high-strength hot-rolled steel sheet, which is a 780～822MPa.

  (2) In (1), in addition to the above composition, in addition to mass, one or two selected from V: 0.010-0.400%, Nb: 0.010-0.300%, Mo: 0.010-0.300% A high-strength hot-rolled steel sheet containing the above.

  (3) The high-strength hot-rolled steel sheet according to (1) or (2), further containing B: 0.0001 to 0.0020% by mass% in addition to the above composition.

  (4) The high-strength hot-rolled steel sheet according to any one of (1) to (3), having a plating layer on the surface.

(5) After continuously casting the molten steel into a slab, in the method for producing a hot-rolled steel sheet, the hot-rolled steel sheet is hot-rolled to the slab, and the continuous casting is converted into a slab during the continuous casting. The continuous casting is performed by applying light reduction at a reduction speed in the range of 0.5 to 1.5 m / min until the solid phase ratio at the center of the thickness of the slab is 0.4 to 0.8. , In mass%, C: 0.030 to 0.090%, Si: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.005% or less, N: 0.006% or less, Al: 0.1% or less, Ti : Contains 0.070 to 0.220%, and the following formula (1)
C * ≧ 0.010 (1)
(Where C * = C− (12/48) × Ti + (12/14) × N + (12/32) × S− (12/51) × V− (12/93) × Nb− (12 / 96) x Mo
C, Ti, N, S, V, Nb, Mo: Content of each element (% by mass))
And a slab of the composition comprising the balance Fe and unavoidable impurities, the hot rolling, the slab is heated to 1150-1300 ℃, rough rolling and finish rolling, the finish rolling Is a finish rolling finish temperature of 850 ° C. or higher, winding at a winding temperature of 550 to 700 ° C., and has a ferrite phase of 95% or more in area ratio, and the fine carbide is 1.0 × in the ferrite phase. 10 ²² particles / m ³ or more, cementite particles 10 particles / 10000 μm ² or more are precipitated, the average particle size of the fine carbide is 10 nm or less, the hardness HV _{1 / 2t} at the plate thickness center position and the plate thickness 1/4 The difference between the hardness HV _{1 / 4t} at the position HV _{1 / 4t} or the hardness HV _{3 / 4t} at the position 3/4 thickness is a hot-rolled steel sheet having a structure with a ΔHV _0.025 of 20HV or less and a tensile strength of 780 to 822MPa. The manufacturing method of the high intensity | strength hot-rolled steel plate characterized by doing.
(6) In (5), in addition to the above composition, in addition to mass, one or two selected from V: 0.010-0.400%, Nb: 0.010-0.300%, Mo: 0.010-0.300% The manufacturing method of the high intensity | strength hot-rolled steel plate characterized by containing the above.
(7) The method for producing a high-strength hot-rolled steel sheet according to (5) or (6), further comprising B: 0.0001 to 0.0020% by mass% in addition to the above composition.
(8) The method for producing a high-strength hot-rolled steel sheet according to any one of (5) to (7), further comprising pickling and plating after the hot rolling.
(9) The method for producing a high-strength hot-rolled steel sheet according to (8), wherein an alloying treatment is performed subsequent to the plating treatment.

  According to the present invention, a high-strength hot-rolled steel sheet having a high tensile strength of 780 MPa or more, excellent ductility and hole expansibility, and excellent punching properties can be easily manufactured, and is outstanding in industry. There is an effect. According to the present invention, there is an effect that it is possible to perform punching without generating cracks in the punching end surface even under punching processing conditions that have heretofore been difficult to apply.

3 is a graph showing the effect of a difference ΔHV _0.025 between the average hardness at the center position of the plate thickness and the average hardness at the 1/4 position of the plate thickness on the occurrence of cracks in the punched end face. It is a graph which shows the relationship between the precipitation density of cementite and C * on the occurrence of crack-like microcracks extending in the direction perpendicular to the rolling direction on the punched end face.

  First, the reasons for limiting the composition of the high-strength hot-rolled steel sheet of the present invention will be described. Hereinafter, the mass% is simply expressed as%.

  The high-strength hot-rolled steel sheet of the present invention has C: 0.030 to 0.090%, Si: 0.3% or less, Mn: 0.2 to 2.0%, P: 0.03% or less, S: 0.005% or less, N: 0.006% or less, Al: It contains 0.1% or less, Ti: 0.070 to 0.220%, and has a composition composed of the balance Fe and inevitable impurities.

C: 0.030-0.090%
C is an element that has the effect of forming fine carbides and increasing the strength of the steel sheet by precipitation strengthening (dispersion strengthening). In order to obtain such an effect and ensure a desired high strength, the content of 0.030% or more is required. On the other hand, if the content exceeds 0.090%, coarse carbide precipitates, and high workability such as desired high strength, desired ductility, stretch flange workability, etc. cannot be ensured. For this reason, C was limited to the range of 0.030 to 0.090%. In addition, Preferably it is 0.035 to 0.080%.

Si: 0.3% or less
Si is an element that has the effect of increasing the strength of the steel sheet by solid solution, and is usually positively contained in a high-strength steel sheet by 0.01% or more. However, Si is easily segregated and causes a local increase in the hardness at the center of the plate thickness. For this reason, Si was limited to 0.3% or less. In addition, Preferably it is 0.2% or less. Moreover, since excessive reduction leads to an increase in cost, it is preferable to set it to about 0.01% or more practically.

Mn: 0.2-2.0%
Mn is an element having a function of increasing the strength of the steel sheet by solid solution and reducing the γ → α transformation point and improving the hardenability. When the γ → α transformation point is a low temperature, precipitation of carbides is suppressed before winding, and fine carbides can be precipitated during winding. In order to ensure such an effect, the content of 0.2% or more is required. On the other hand, Mn has a strong segregation tendency, and if it is contained in a large amount exceeding 2.0%, the tendency to form a center segregation layer at the center of the plate thickness becomes strong, and the punchability is lowered. For this reason, Mn was limited to the range of 0.2 to 2.0%. In addition, Preferably it is 0.5 to 1.4%.

P: 0.03% or less
P has the effect of increasing the strength of the steel sheet by solid solution, but has a strong segregation tendency, and a large content exceeding 0.03% causes a reduction in stretch flangeability and punchability. For this reason, in the present invention, P is limited to 0.03% or less. From the viewpoint of steel sheet characteristics, it is desirable to reduce P as much as possible. However, excessive reduction leads to an increase in the refining cost, so practically it is preferably about 0.001% or more.

S: 0.005% or less
S combines with Mn, Ti, etc., and exists in the steel as sulfides, leading to a reduction in elongation and hole expansibility. Therefore, in the present invention, it is preferable to reduce as much as possible, but up to 0.005% is acceptable. Therefore, S is limited to 0.005% or less. In addition, since excessive reduction of S causes the refining cost to rise, it is preferable to make it 0.0001% or more.

N: 0.006% or less
N combines with Ti, which is a carbonitride-forming element, to reduce the amount of Ti for forming Ti carbide that effectively contributes to strength improvement. For this reason, it is desirable to reduce as much as possible in the present invention, but it is acceptable up to 0.006%. Moreover, when it contains exceeding 0.006% and an excess, hole expansibility will fall. For this reason, N was limited to 0.006% or less.

Al: 0.1% or less
Al is an element that acts as a deoxidizer. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, when the content exceeds 0.1%, a large amount of inclusions are generated, the cleanliness is lowered, and workability such as elongation and hole expansibility is lowered. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.06% or less.

Ti: 0.070 to 0.220%
Ti contributes to the formation of fine carbides (Ti carbides) of 10 nm or less, and is an important element in the present invention in order to secure a desired high strength by particle dispersion strengthening (precipitation strengthening). In order to ensure such an effect, the content of 0.070% or more is required. On the other hand, if it is contained in a large amount exceeding 0.220%, the carbide is coarsened, and the desired high strength and desired hole expansibility cannot be ensured. For this reason, Ti was limited to the range of 0.070 to 0.220%. In addition, Preferably it is 0.080 to 0.180%.

  The above-mentioned components are basic components, and in addition to the basic components, one selected from V: 0.010-0.400%, Nb: 0.010-0.300%, Mo: 0.010-0.300% as required. It can contain a seed | species or 2 or more types, and / or B: 0.0001-0.0020%.

One or more selected from V: 0.010-0.400%, Nb: 0.010-0.300%, Mo: 0.010-0.300%
V, Nb, and Mo are all elements that form nano-sized fine carbides and contribute to an increase in steel sheet strength by particle dispersion strengthening (precipitation strengthening), just like Ti. Two or more kinds can be selected and contained. In order to obtain such an effect, it is preferable to contain V: 0.010% or more, Nb: 0.010% or more, Mo: 0.010% or more. On the other hand, even if it contains V: 0.400%, Nb: 0.300%, and Mo: 0.300%, the effect is saturated and an effect commensurate with the content cannot be expected. For this reason, when it contains, it is preferable to limit to the range of V: 0.010-0.400%, Nb: 0.010-0.300%, Mo: 0.010-0.300%, respectively.

B: 0.0001 to 0.0020%
B has an action of suppressing the transformation of γ (austenite) → α (ferrite), and can be contained as necessary in the present invention. B contributes to an increase in strength due to fine precipitation of carbides through suppression of the γ → α transformation. In order to ensure such an effect, the content of 0.0001% or more is required. On the other hand, the content exceeding 0.0020% promotes the formation of hard second phases such as bainite and martensite through an increase in the hardenability of steel, and the workability decreases. For this reason, when it contains, it is preferable to limit B to 0.0001 to 0.0020% of range. In addition, More preferably, it is 0.0005 to 0.0020%.

  The balance other than the components described above consists of Fe and inevitable impurities.

In the present invention, each of the above-described components is within the above-mentioned range, and the following formula (1)
C * ≧ 0.010 (1)
(Where C * = C− (12/48) × Ti + (12/14) × N + (12/32) × S− (12/51) × V− (12/93) × Nb− (12 / 96) x Mo
C, Ti, N, S, V, Nb, Mo: Content of each element (% by mass))
The content is adjusted so as to satisfy.

  In the hot-rolled steel sheet of the present invention, fine carbides of 10 nm or less are precipitated to ensure a desired high strength, and an appropriate amount of cementite is further precipitated to improve punchability. In order to form cementite, it is necessary that Ti, V, Nb, and Mo form fine carbides and then free C exists in the steel. C * represents the amount of C remaining as free C after Ti, V, Nb, and Mo form carbides and Ti forms nitrides and sulfides. Ti forms nitrides and sulfides in addition to carbides.

  When the formula (1) is not satisfied, that is, when C * is less than 0.010, there is not enough carbon C in the steel to form cementite, so that an appropriate amount of cementite cannot be dispersed and precipitated. For this reason, there is no starting point for forming microvoids at the time of punching, and crack-like microcracks extending in a direction perpendicular to the rolling direction are generated, and punchability is lowered.

  The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, O: 0.03% or less and Cu, Cr, Ni, Ca, W, etc. in total of 0.1% or less are acceptable.

  Next, the structure limitation of the hot-rolled steel sheet of the present invention will be described.

The hot-rolled steel sheet of the present invention has a ferrite phase with an area ratio of 95% or more, and fine carbides of 1.0 × 10 ²² pieces / m ³ or more and cementite of 10 pieces / 10000 μm ² or more are precipitated in the ferrite phase. The average particle size of the fine carbide is 10nm or less, the hardness HV _{1 / 2t} at the center of the plate thickness and the hardness HV _{1 / 4t at the} plate thickness _1/4 (or the hardness HV at the plate thickness 3/4) _It has a structure with a reduced central segregation, in which the difference ΔHV _{0.025 from 3 / 4t} ) is 20HV or less.

Ferrite phase: 95% or more in area ratio In the present invention, the steel sheet structure is substantially a ferrite single phase. As used herein, “substantially” refers to a case where the area ratio relative to the entire tissue is 95% or more. Thereby, workability, such as elongation and hole expansibility, improves. If the ferrite phase is less than 95%, desired workability cannot be secured. Examples of the second phase other than the ferrite phase include pearlite, bainite phase, and martensite phase.

Fine carbide: 1.0 × 10 ²² particles / m ³ or more, average particle diameter of 10 nm or less Precipitating and dispersing fine carbides in the ferrite phase increases the steel sheet strength by particle dispersion strengthening (precipitation strengthening). In order to ensure the desired high strength (tensile strength: 780 MPa or more), the average particle size of fine carbides precipitated in the ferrite phase is 10 nm or less, and 1.0 × 10 ²² particles / m ³ or more is dispersed and precipitated. There is a need. If the average particle size of the carbide exceeds 10 nm, or if the precipitation density is less than 1.0 × 10 ²² particles / m ³ , the particle dispersion strengthening (precipitation strengthening) action decreases and the desired high strength (tensile strength: 780 MPa or more) Can not be secured. For carbide precipitation density and average particle size, sample thin film specimens from hot-rolled steel sheets, make thin films by grinding, mechanical polishing, and electrolytic polishing, and observe the structure using a transmission electron microscope (magnification: 300,000 times) Then, it was determined by observing the carbides precipitated in the ferrite phase (parent phase) using the obtained structure photograph.

  The precipitation density was determined by dividing the number of precipitates in the visual field by the volume of the visual field using a transmission electron microscope (magnification: 300,000 times). The sample thickness required to determine the volume of the field of view is, for example, using a transmission electron microscope equipped with an electron energy loss spectroscopy (EELS) device, Egerton, RF: Electron Energy Loss Spectroscopy in the Electron Microscope, 2nd ed. Plenum Press, New York, 1986, 304-305. It can be calculated from the ratio of the elastic scattering intensity and the inelastic scattering intensity of electrons in the field of view. The “average particle size” as used herein refers to the longest portion (maximum length) of precipitates (particles) per 100 precipitates (particles) arbitrarily selected from the precipitate image observed with a transmission electron microscope. )), And the arithmetic average. Note that cementite and carbide were distinguished from each other by an analyzer installed in an electron microscope.

Cementite particles: 10 particles / 10,000μm ² or more By dispersing a large number of hard cementite particles in the ferrite phase (matrix), microvoids are formed in the vicinity of the cementite particles (or starting from cementite) during the punching process, and stress Can be prevented from concentrating on one point, and the occurrence of crack-like microcracks extending in the thickness direction, that is, in the direction perpendicular to the rolling direction can be prevented. In order to obtain such an effect, it is necessary to precipitate and disperse 10 cementite particles / 10,000 μm ² or more. The upper limit of the precipitation dispersion amount is determined by the above-mentioned C * amount, but is preferably 1000/10000 μm ² or less from the viewpoint of improving stretch flangeability. In the hot-rolled steel sheet of the present invention, the particle size of the cementite particles precipitated is about 0.05 to 2 μm on average.

  For the precipitation dispersion amount of cementite particles, a specimen for microstructure observation is taken from an arbitrary position of the hot-rolled steel sheet, and the 1 / 4t or 3 / 4t position of the cross section in the thickness direction in the direction (C direction) perpendicular to the rolling direction is taken. Polishing, Nital corrosion, observing 3 or more fields using a scanning electron microscope (magnification: 1000 times), measuring the number of cementite particles observed in the field (100μm × 100μm), and averaging The number of cementite particles in the steel sheet (average).

The difference between the hardness HV _{1 / 2t} at the center of the plate thickness and the hardness HV _{1 / 4t} at the plate thickness 1/4 position or the hardness HV _{3 / 4t} at the plate thickness 3/4 position ΔHV _0.025 : 20HV or less In the case of rolled steel sheets, the center segregation is reduced, the hardness of the segregation part (hardness at the center position of the plate thickness) is reduced, and the hardness at other peripheral positions other than the center position of the plate thickness (typically, the thickness of 1/4) (The hardness at the position or the thickness at the 3/4 thickness position is selected) to prevent cracks that occur during punching (cracks perpendicular to the sheet thickness direction and parallel to the rolling direction).

The difference ΔHV _0.025 between the hardness HV _{1 / 2t} at the center of the plate thickness and the hardness HV _{1 / 4t} at the plate thickness 1/4 position or the hardness HV _{3 / 4t} at the plate thickness 3/4 position should be 20HV or less. _Furthermore , if the hardness HV _{1 / 2t} at the center position of the plate thickness is reduced, cracks that occur during punching can be prevented. When ΔHV _0.025 exceeds 20 HV, the reduction of center segregation is insufficient, and cracks (cracks perpendicular to the plate thickness direction and parallel to the rolling direction) occur during punching, resulting in a decrease in punchability. The hardness HV _{1 / 2t} at the center of the plate thickness can be effectively reduced by light pressure reduction during continuous casting. Note that there is no problem even if the decrease in ΔHV _0.025 is performed by means other than light pressure reduction during continuous casting.

  The hardness is measured using a Vickers hardness meter (load: 25 gf (0.25 N)) in accordance with the provisions of JIS Z 2244. The width of segregation depends on the plate thickness, but if measured with a Vickers hardness tester with a load of 25 gf (0.25 N), the size of the indentation can be kept within the segregation width, and the hardness of the segregation part can be reduced. It can be reflected. If the segregation-induced line (central segregation line) can be confirmed at the central part of the thickness, the measurement position is the central position of the thickness. If the central segregation line is not visible, the central position of the thickness is shown. (Plate thickness 1/2 position) may be used. For example, 10 points may be measured at the measurement position, and the arithmetic average may be the thickness at the plate thickness center position. In addition, the plate thickness 1/4 position or the plate thickness 3/4 position is measured at the plate thickness 1/4 position and the plate thickness 3/4 position, respectively, and the arithmetic average of these 10 points is calculated as the thickness 1/4. The position or thickness of the plate may be 3/4.

  In addition, you may form a plating layer further on the surface of an above-mentioned hot-rolled steel plate. The type of the plating layer formed on the surface is not particularly limited, and any known plating layer can be applied. Among them, a hot dip galvanized layer and an alloyed hot dip galvanized layer are preferable.

  Next, the manufacturing method of this invention hot rolled sheet steel is demonstrated.

  In the present invention, the molten steel melted to have the above-described composition is continuously cast into a slab. The melting method is not particularly limited, and any generally known melting method such as a converter can be applied.

  In the present invention, continuous casting is continuous casting in which slabs in the middle of continuous casting are lightly reduced by adjusting the amount of reduction so as to match the solidification shrinkage at the final solidification position. Here, “light reduction” means that the slab is squeezed with a roll arranged in the casting direction so that the reduction amount is equal to the solidification shrinkage amount of the slab. By “light reduction”, accumulation of the concentrated molten steel at the thickness center of the slab can be prevented. Specifically, “light reduction” is performed by reducing the solid phase ratio at the thickness center of the slab from 0.4 or less to 0.8 or more, and the solid phase ratio at the thickness center of the slab is 0.4. The case where the slab is squeezed at a squeezing speed in the range of 0.5 to 1.5 mm / min from the time point to 0.8. When the reduction rate is less than 0.5 mm / min in this range, the flow of the concentrated molten steel cannot be sufficiently prevented. On the other hand, when the rolling speed exceeds 1.5 mm / min, the concentrated molten steel is squeezed in the direction opposite to the casting direction, and there is a possibility that negative segregation is generated in the center part of the slab.

  When the solid phase ratio of the slab at the time of reduction is less than 0.4 at the thickness center portion, there are too many unsolidified portions, and under light reduction, it is difficult to prevent the formation of center segregation. On the other hand, if it exceeds 0.8, the solid phase increases, and the discharge of the uncoagulated part due to the reduction becomes difficult particularly under the light pressure. For this reason, it is preferable to apply the light reduction time to a slab having a solid phase ratio of 0.4 to 0.8 at the center of thickness.

  The method for reducing the center segregation may be performed using a method other than the above-described light pressure, for example, electromagnetic stirring.

  In the present invention, the slab obtained by the continuous casting method as described above is hot-rolled to obtain a hot-rolled steel sheet. Hot rolling is a rolling in which a slab is heated to 1150 to 1300 ° C. and subjected to rough rolling and finish rolling.

Heating temperature: 1150-1300 ℃
Heating at 1150 ° C. or higher is required to reduce deformation resistance in hot rolling and to solidify carbides and the like precipitated in the slab. On the other hand, at a high temperature exceeding 1300 ° C., the burden on the heating furnace increases and the yield decreases due to scale loss. For this reason, the heating temperature of the slab was limited to a temperature in the range of 1150 to 1300 ° C. In addition, Preferably it is 1180-1260 degreeC. In addition, when the steel raw material after casting is the temperature of an austenite single phase area, you may carry out direct rolling without heating.

Further, the rough rolling is not particularly limited as long as it can have a predetermined sheet bar shape. The finish rolling may be at or above the Ar ₃ transformation point. In the present invention, the finish rolling end temperature is set to 850 ° C. or higher, and the rolling is cooled after the end of rolling, and the winding temperature is 550 to 700 ° C.

Finishing rolling finish temperature: 850 ° C or more When the finish rolling finish temperature is less than 850 ° C, ferrite is generated, and fine carbides are precipitated, and the strength is increased, so that the deformation resistance is increased, and the sheeting property of the rolling mill is increased. Decreases. In addition, the generated ferrite is processed and stretched to form a band-like structure, resulting in a decrease in ductility. For this reason, the finish rolling finish temperature was limited to 850 ° C. or higher, which is higher than the Ar ₃ transformation point. In addition, Preferably it is 880 degreeC or more.

  The cooling after finish rolling is not particularly limited. Air cooling or blast cooling, water cooling or the like is preferable so that the film can be wound at 550 to 700 ° C. The cooling after rolling is preferably performed at a cooling rate of 10 ° C./s or more on average from the finish rolling finish temperature to the coiling temperature. More preferably, it is 30 ° C./s or more.

Winding temperature: 550 ~ 700 ℃
The coiling temperature is adjusted to a temperature in the range of 550 to 700 ° C. in order to make the carbide precipitation form an appropriate form contributing to an increase in strength. After being wound, first, fine carbides such as Ti carbide are precipitated, and then cementite is formed with the remaining carbon. When the coiling temperature is less than 550 ° C., the precipitation of cementite takes precedence, the amount of fine carbides having a size of 10 nm or less is reduced, and the desired high strength cannot be secured. On the other hand, when the temperature exceeds 700 ° C., the carbide is coarsened and a desired high strength cannot be ensured.

  In the present invention, after hot rolling, pickling, plating, or alloying may be further performed to form a plating layer or alloying plating layer on the surface of the steel sheet. Moreover, the adhesion amount of a plating layer should just be determined suitably according to a use, and does not need to specifically limit it. Conventional methods can be applied to pickling, plating, and rationalization.

  Hereinafter, based on an Example, this invention is demonstrated further.

Molten steel having the composition shown in Table 5 was melted in a converter and made into a slab (thickness: 250 mm) by a continuous casting method. Note that the slab in the middle of continuous casting was subjected to reduction under the conditions shown in Table 6. Next, the obtained slab (slab) is heated to the heating temperature shown in Table 6, subjected to hot rolling consisting of rough rolling and finish rolling at the finish rolling finishing temperature shown in Table 6, and the cooling rate shown in Table 6 Then, it was wound in a coil shape at the winding temperature shown in Table 6 to obtain a hot-rolled steel strip (hot-rolled steel plate: plate thickness 3.2 mm). Some hot-rolled steel sheets are hot-rolled, pickled, and then dipped in a hot-dip galvanizing bath (0.1% Al-Zn bath) at 480 ° C. Formed. The amount of adhesion was 45 g / m ^{2 as the} amount of adhesion per one side. A part of the steel sheet in which the hot dip galvanized layer was formed on the surface of the steel sheet was further subjected to alloying treatment of the plated layer at 520 ° C. to obtain an alloyed hot dip galvanized layer.

Test specimens were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, hardness test, hole expansion test, and punching test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is taken from the obtained hot-rolled steel sheet, a cross section (L cross section) parallel to the rolling direction is polished, it is subjected to nital corrosion, and the structure is revealed, and a scanning electron microscope ( (Magnification: 1000 times) was used to observe and image the tissue, and the obtained tissue photograph was used to determine the type of tissue and the tissue fraction (area ratio). Furthermore, the thickness 1/4 position of the cross section in the thickness direction in the direction perpendicular to the rolling direction (C direction) is polished, corroded with nital, and observed with a scanning electron microscope (magnification: 1000 times) over 3 fields of view. The number of cementite particles observed in each field of view (100 μm × 100 μm) was measured, and the arithmetic average thereof was obtained to obtain the precipitation density of cementite particles (pieces / 10000 μm ² ) of the steel sheet. For the cementite particles observed in each field of view, the longest part (maximum length) of the particles was measured, and the arithmetic average of the maximum length of each particle was defined as the average particle diameter of the cementite particles of the steel sheet. In addition, the average particle diameter of the cementite particles of all the steel plates was in the range of 0.05 to 2 μm.

In addition, a thin film specimen is taken from the obtained hot-rolled steel sheet, made into a thin film by grinding, mechanical polishing, and electrolytic polishing, and the structure is observed using a transmission electron microscope (magnification: 300,000 times), and at least 5 fields of view are observed. Was imaged. Using the obtained structure photograph, the average particle diameter and precipitation density (pieces / m ³ ) of the carbide precipitated in the ferrite phase (parent phase) were determined. The average particle size of the carbide was determined by measuring the maximum length of 100 arbitrarily selected carbides, and calculating the average of the average particle size of the carbide of the hot-rolled steel sheet. The carbide precipitation density was determined by using the EELS attached to the transmission electron microscope, obtaining the sample thickness of the field of view from the intensity ratio of electron elastic scattering and inelastic scattering, and calculating the number of carbides in the field by the area of the field x It was determined by dividing by the sample thickness.
Note that cementite and carbide were distinguished from each other by an analyzer installed in an electron microscope.

(2) Tensile test JIS No. 5 tensile test specimen was taken from the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) was the tensile direction, and in accordance with the provisions of JIS Z 2241 A tensile test was carried out to determine tensile properties (tensile strength TS, elongation El).
(3) Hardness test A test piece for hardness measurement was taken from the obtained hot-rolled steel sheet, the cross-section (L cross-section) parallel to the rolling direction was polished, Nital corrosion occurred, and the structure was revealed. (Load: 25 gf (test force: 0.25 N) was used to measure the Vickers hardness HV _0.025 . The measurement positions were the plate thickness center position, the plate thickness 1/4 position, and the plate thickness 3/4 position. If a segregation-induced line (central segregation line) can be confirmed at the center of the plate thickness, the central segregation line can be seen on the central segregation line. 10 points were measured at each measurement position, and the arithmetic average was taken as the hardness at the plate thickness position, with 5 measurement points each at the 1/4 thickness position and 3/4 position. The arithmetic average of these 10 points was taken as the hardness at the thickness 1/4 position, and the obtained value was used to determine the hardness HV _{1 / 2t} at the thickness center position and the hardness at the thickness 1/4 position. HV _{1 / 4t} The _{difference, ΔHV 0.025 (= HV 1 /} 2t -HV 1 / 4t) was calculated.
(4) Hole expansion test A test piece (size: 130 x 130 mm) is taken from the obtained hot-rolled steel sheet and punched into the test piece in accordance with the provisions of JFS T 1001-1996 (clearance: thickness of the test piece) 12.5%), an initial hole (diameter: 10 mmφ: d ₀ ) was formed. A conical punch with an apex angle of 60 ° is inserted into the initial hole from the punch side, the initial hole is expanded, the hole diameter d when a crack penetrates the plate thickness is measured, and the following formula λ (%) = {(D−d ₀ ) / d ₀ } × 100%
Then, the hole expansion rate λ was calculated.
(5) Punching test As with the hole expansion test, a test piece (size: 130 x 130 mm) is taken from the obtained hot-rolled steel sheet and punched into the test piece in accordance with the provisions of JFS T 1001-1996 ( Clearance: 25% of the test piece plate thickness), holes (diameter 10mmφ) were punched out. The end face of the punched hole was visually observed, and the presence or absence of cracks parallel to the rolling direction and crack-like microcracks extending in the direction perpendicular to the rolling direction was examined to investigate punchability. Similarly, the clearance is set to 12.5% of the test piece thickness. Similarly, the hole (diameter 10mmφ) is punched into the test piece, the end surface of the punched hole is visually observed, and the crack is parallel to the rolling direction. The presence or absence of crack-like microcracks extending in the direction perpendicular to the rolling direction was examined. When the clearance is 12.5% and 25% of the specimen thickness and both are not cracked, “○”, when the clearance is 12.5% of the specimen thickness, only when there is no crack, “△”, both are cracked Was evaluated as “×”.

  The results obtained are shown in Table 7.

  Each of the inventive examples has high tensile strength: 780 MPa or more, high ductility with total elongation El of 20% or more, and high hole expansibility with a hole expansion ratio λ of 80% or more. Excellent, and even if the clearance is 12.5% of the plate thickness, and even if the clearance is 25% of the plate thickness, cracks parallel to the rolling direction or crack-like micro-extensions extending in the direction perpendicular to the rolling direction can be achieved on the punched end face. No cracking was observed, the punchability evaluation was “◯”, and the hot rolled steel sheet was excellent in punchability. On the other hand, in comparative examples that are out of the scope of the present invention, the strength is insufficient, the hole expandability is insufficient, or the punchability evaluation is “Δ” or “×”, and the punchability is reduced. In a comparative example that is outside the scope of the present invention, cracks occur when the clearance is 25.0% of the plate thickness even when the clearance is 12.5% of the plate thickness, even when a good punched end surface is exhibited.

Claims (9)

% By mass
C: 0.030 to 0.090%, Si: 0.3% or less,
Mn: 0.5 to 2.0%, P: 0.03% or less,
S: 0.005% or less, N: 0.006% or less,
Al: 0.1% or less, Ti: 0.070 to 0.220%
And satisfying the following formula (1), the composition comprising the balance Fe and inevitable impurities,
The ferrite phase has an area ratio of 95% or more. In the ferrite phase, 1.0 × 10 ²² particles / m ³ or more of fine carbides and 10 particles / 10000 μm ² or more of cementite particles are precipitated. The diameter is 10 nm or less,
Structure where the difference ΔHV _0.025 between the hardness HV _{1 / 2t} at the center of the thickness and the hardness HV _{1 / 4t at the 1/4} thickness or the hardness HV _{3 / 4t} at the thickness 3/4 is 20HV or less. have a tensile strength: high-strength hot-rolled steel sheet according to claim <br/> be 780～822MPa.
Record
C * ≧ 0.010 (1)
Here, C * = C− (12/48) × Ti + (12/14) × N + (12/32) × S− (12/51) × V− (12/93) × Nb− (12/96 ) X Mo
C, Ti, N, S, V, Nb, Mo: Content of each element (% by mass)   In addition to the above composition, the composition further contains one or more selected from V: 0.010 to 0.400%, Nb: 0.010 to 0.300%, and Mo: 0.010 to 0.300% by mass%. The high-strength hot-rolled steel sheet according to claim 1.   The high-strength hot-rolled steel sheet according to claim 1 or 2, further comprising B: 0.0001 to 0.0020% by mass% in addition to the composition.   The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the surface has a plating layer. After continuously casting the molten steel into a slab, in the method for producing a hot-rolled steel sheet, the hot-rolled steel sheet is subjected to hot rolling on the slab,
During the continuous casting, the continuous casting is reduced to a slab at a reduction speed in the range of 0.5 to 1.5 m / min from the time when the solid phase rate of the center part of the slab becomes 0.4 to 0.8. a continuous casting subjected to a soft reduction that,
The slab is in mass%,
C: 0.030 to 0.090%, Si: 0.3% or less,
Mn: 0.5 to 2.0%, P: 0.03% or less,
S: 0.005% or less, N: 0.006% or less,
Al: 0.1% or less, Ti: 0.070 to 0.220%
And satisfying the following formula (1), a slab of the composition consisting of the balance Fe and inevitable impurities,
The hot rolling is a rolling in which the slab is heated to 1150 to 1300 ° C. and subjected to rough rolling and finish rolling, and the finish rolling has a finish rolling finish temperature of 850 ° C. or more, and a winding temperature: 550 to 700 Rolled up at ℃, and has a ferrite phase of 95% or more in area ratio. Fine ferrite is 1.0 × 10 ²² particles / m ³ or more and cementite particles are precipitated 10 particles / 10000 μm ² or more in the ferrite phase. The average particle size of the fine carbide is 10 nm or less,
Structure where the difference ΔHV _0.025 between the hardness HV _{1 / 2t} at the center of the thickness and the hardness HV _{1 / 4t at the 1/4} thickness or the hardness HV _{3 / 4t} at the thickness 3/4 is 20HV or less. Having a tensile strength: 780 to 822 MPa ,
A method for producing a high-strength hot-rolled steel sheet.
Record
C * ≧ 0.010 (1)
Here, C * = C− (12/48) × Ti + (12/14) × N + (12/32) × S− (12/51) × V− (12/93) × Nb− (12/96 ) X Mo
C, Ti, N, S, V, Nb, Mo: Content of each element (% by mass)   In addition to the above composition, the composition further contains one or more selected from V: 0.010 to 0.400%, Nb: 0.010 to 0.300%, and Mo: 0.010 to 0.300% by mass%. The manufacturing method of the high intensity | strength hot-rolled steel plate of Claim 5.   The method for producing a high-strength hot-rolled steel sheet according to claim 5 or 6, further comprising B: 0.0001 to 0.0020% by mass% in addition to the composition.   The method for producing a high-strength hot-rolled steel sheet according to any one of claims 5 to 7, further comprising pickling and plating after the hot rolling.   The method for producing a high-strength hot-rolled steel sheet according to claim 8, wherein an alloying treatment is performed subsequent to the plating treatment.

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