JP2005290396A - High strength hot rolled steel sheet having excellent elongation property, stretch flange property, tensile fatigue property and impact resistance, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having a high strength satisfying a TS (tensile strength) of ≥780 MPa, having excellent elongation properties, stretch flange properties and tensile fatigue properties, and further having impact resistance at a strain rate of 10/s for checking the deformation of an automobile cabin. <P>SOLUTION: In Ti-Mo compositely added steel, the steel structure of, by volume occupancy to the whole structure, 40 to 93% ferrite, 4 to 15% retained austenite, 3 to 12% martensite, and the balance bainite is formed, and also, the average diameter of precipitates composed of Ti, Mo and C precipitated in the ferrite is controlled to ≤20 nm, and the average spacing between the precipitates is controlled to ≤60 nm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a high-strength hot-rolled steel sheet excellent in elongation characteristics, stretch flange characteristics, tensile fatigue characteristics, and impact resistance characteristics, and a method for producing the same, at a high strength having a tensile strength (TS) of 780 MPa or more.
This steel plate is suitable for application to parts that require not only good formability but also tensile fatigue properties and impact resistance properties, such as automobile frames and under bodies.

For the above-mentioned applications, so-called TS780 MPa grade steel with a TS of about 780 MPa is difficult to form, so conventionally a so-called TS590 MPa grade steel hot-rolled plate with a TS of about 590 MPa has been used. In addition, when using TS780MPa class steel, the plate thickness is naturally thinner than that of conventional TS590MPa class steel, so when viewed as a member, conventional TS780MPa class steel still has a problem in terms of tensile fatigue properties. It was.
However, in recent years, in order to improve the impact resistance characteristics of automobiles, the strength of automobile steel sheets has been increased, and the use of TS780MPa grade steel has also begun to be examined for parts that require tensile fatigue characteristics. In addition, these parts have not been particularly required to have anti-collision properties in the past, but these parts are also required to have anti-collision properties in recent years in order to prevent deformation of the cabin during a car collision. And came to be. Furthermore, from the standpoint of formability, these parts require stretch and stretch flange characteristics.

As a means for improving the elongation, there is a technique using retained austenite as disclosed in Patent Document 1.
However, although retained austenite is effective in improving the elongation characteristics, there is a problem in stretch flangeability. That is, the stretch flangeability is better as the hardness difference between the main phase and the other phase (second phase) is smaller. However, the retained austenitic steel is harder in the second phase and has a hardness difference from ferrite, which is the main phase. Increases, which degrades the stretch flange.
On the other hand, tempered martensite and bainite single-phase structure steel have a problem that the stretch flange formability is good but the stretch properties are inferior because the hardness difference between the main phase and the second phase is small.

Therefore, in order to achieve both elongation characteristics and stretch flange characteristics, it is necessary to use a composite structure steel having a small hardness difference between the main phase and the second phase.
Patent Document 2 discloses a technology related to a composite structure steel plate in which the hardness difference from martensite of the second phase is reduced by precipitation strengthening of ferrite. This technology is TS: 50 kgf / mm ² (490 MPa). ) ~ 60 kgf / mm ² (590MPa) is merely intended to obtain a steel plate, it can not meet the recent requirements. Further, the steel sheet disclosed about TS: about 780 MPa has an elongation of about 20%, and this point needs further improvement.

  In order to solve these problems, in Patent Document 3, in addition to precipitation strengthened ferrite in which carbide containing Ti and Mo or W is precipitated, residual austenite and bainite three phases or martensite is added thereto. A technique for a steel plate having four phases is disclosed. According to the example of Patent Document 3, the plate thickness: 3.2 mm, the elongation: 30% or more, and the hole expansion ratio: 65% or more, which is an index of the stretch flange characteristic, are achieved, but the tensile fatigue characteristics are not always sufficient. That wasn't true.

In addition, Patent Document 4 discloses a technique for achieving TS × elongation (El) ≧ 246000 MPa ·% and hole expansion rate ≧ 70%.
Patent Document 5 discloses a technique for improving both elongation and stretch flangeability by reducing the grain size of ferrite and the second phase and further using retained austenite. Since the slab heating temperature is low, there is a disadvantage that the rolling load is large, the wear of the roll is severe, and the production costs extra than conventional.
In any of these techniques, no consideration is given to the improvement of fatigue characteristics.

  As a technique for improving fatigue characteristics, Patent Document 6 discloses a technique for improving elongation and fatigue characteristics by controlling the structural fraction of the surface layer and the inner layer. However, in Patent Document 6, no consideration is given to improvement of stretch flange formability.

Next, from the viewpoint of collision-resistant characteristics, the members that are required to suppress the deformation of the automobile cabin in order to ensure the passenger's living space are the same as the members that absorb the collision energy. Since the amount of deformation is small even during the collision time, the ultimate strain rate is small, and the absorbed energy at about 10 / s is important.
As a means for improving the impact resistance, Patent Document 7 discloses a technique related to steel having a structure composed of retained austenite, martensite, and acicular ferrite. In this technique, it has been studied to increase the energy absorption amount of the steel material by improving the dynamic n value at a strain rate of 2000 / s. The absorbed energy at a strain rate of 2000 / s is a characteristic necessary for absorbing the collision energy at the time of automobile collision by deformation of the member itself. Since the collision energy absorbing member undergoes large deformation in a short time, the deformation strain rate reaches 10 ² / s, 10 ³ / s. Therefore, it is necessary to improve the absorption energy and static ratio at 10 ² / s and 10 ³ / s. However, in order to improve the impact resistance of automobiles, it is also important to protect the cabin without deforming the parts in order to secure a passenger living space. As described above, the member used in such a location is small in the amount of deformation even in the same collision time as compared with the collision energy absorbing member, so the ultimate strain rate is small, and the absorbed energy at about 10 / s is important. However, Patent Document 7 does not mention any measures for improving the collision resistance when the strain rate is about 10 / s.

Japanese Unexamined Patent Publication No. 7-62485 Japanese Patent Laid-Open No. 9-263885 JP2003-321738A Japanese Patent Laid-Open No. 11-189842 JP 2001-220648 Japanese Patent Laid-Open No. 11-241141 Japanese Patent Laid-Open No. 11-189842

The present invention advantageously solves the above-mentioned problems, and in order to improve elongation characteristics, stretch flange characteristics and tensile fatigue characteristics in a high strength steel having a TS of 780 MPa or more, and to prevent deformation of an automobile cabin. Another object of the present invention is to propose a high-strength hot-rolled steel sheet having improved impact resistance at a strain rate of 10 / s, together with its advantageous manufacturing method.
The target characteristics (thickness: 2.0 mm) in the present invention are as follows.
・ Tensile strength (TS) ≧ 780 MPa
-Elongation characteristics: Elongation (El) ≥ 27%
-Stretch flange characteristics: Hole expansion rate (λ) ≥ 60%
・ Tensile fatigue properties: Durability ratio of tensile fatigue [Fatigue limit (FL) to TS ratio (FL / TS)] ≧ 0.75
-Collision resistance: Strain rate: True strain at 10 / s Absorbed energy up to 0.1 ≥ 80 MJ / m ³

Now, as a result of intensive studies to solve the above problems, the inventors have controlled the steel structure in the Ti and Mo composite added steel, and precipitates composed of Ti, Mo and C in the ferrite. By strictly controlling the grain size and dispersion state, the elongation characteristics, stretch flange characteristics and tensile fatigue characteristics are significantly improved, and even better impact resistance characteristics, specifically strain rate: 10 / We have obtained the knowledge that high energy of 80 MJ / m ³ or more can be obtained with an absorbed energy up to a true strain of 0.1 at s.
In addition, in order to form such a structure, after finishing rolling, controlled cooling consisting of rapid cooling → air cooling and rapid cooling is performed, and the air cooling start temperature is controlled according to the slab heating temperature, and further after winding on a coil. It was found that it is important to control the cooling rate.

This is probably because the amount of carbides containing Ti present in the slab changes depending on the slab heating temperature. As a result, the size of precipitates precipitated in ferrite during cooling after finish rolling (especially during air cooling) This is probably because the distribution state is greatly affected. Further, it is considered that the cooling rate after coil winding is necessary for generating martensite in addition to the retained austenite.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) In mass%,
C: more than 0.08%, 0.2% or less,
Si: 0.3% or more, 2.0% or less,
Mn: 0.3% or more, 2.0% or less,
P: 0.06% or less,
S: 0.01% or less,
Al: 0.05% or less,
Ti: 0.03% or more, 0.2% or less and
Mo: 0.03% or more, 0.5% or less included, the balance is Fe and inevitable impurities composition, volume occupancy to the whole structure, ferrite: 40% or more, 93% or less, residual austenite: 4% or more, 15 % Or less, martensite: 3% or more, 12% or less, balance: steel structure of bainite, including precipitates composed of Ti, Mo, C in ferrite, the average diameter of such precipitates being 20 nm or less, precipitates A high-strength hot-rolled steel sheet with excellent elongation characteristics, stretch flange characteristics, tensile fatigue characteristics, and impact resistance characteristics, characterized in that the average distance between them is 60 nm or less.

(2) In mass%,
C: more than 0.08%, 0.2% or less,
Si: 0.3% or more, 2.0% or less,
Mn: 0.3% or more, 2.0% or less,
P: 0.06% or less,
S: 0.01% or less,
Al: 0.05% or less,
Ti: 0.03% or more, 0.2% or less and
Mo: 0.03% or more and 0.5% or less of slab containing Fe and inevitable impurities in the balance is heated to a temperature above 1150 ° C, and the finishing temperature is Ar ₃ points or higher (Ar ₃ points + 100 ° C) After finishing rolling under the following conditions, the average cooling rate: 20 ° C / s or more to a temperature of less than ₃ points Ar and (slab heating temperature / 1.9 + 80) ° C or higher and (slab heating temperature / 1.5 + 20) ° C or lower Then, after cooling with air for 3 to 15 seconds, after cooling at an average cooling rate of 20 ° C / s or higher to 300 ° C or higher and 500 ° C or lower, winding, and then to 200 ° C, average cooling rate: 20 ° C / hr A method for producing a high-strength hot-rolled steel sheet having excellent elongation characteristics, stretch flange characteristics, tensile fatigue characteristics, and impact resistance characteristics, characterized by cooling at the above speed.

  According to the present invention, in the Ti and Mo composite added steel, the steel structure is made ferrite + residual austenite + martensite (+ bainite), and precipitates containing Ti, Mo, C in the ferrite are finely dispersed. In high-strength steel with a strength of 780 MPa or more, excellent elongation characteristics, stretch flange characteristics, tensile fatigue characteristics, and impact resistance characteristics can be obtained. As a result, the plate thickness of automobile parts can be reduced and the collision safety of automobiles can be improved. It will be possible and will greatly contribute to the improvement of the performance of automobile bodies.

The present invention will be specifically described below.
First, the reason why the composition of the steel plate and the steel slab is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: more than 0.08%, 0.2% or less C is an element necessary for securing a suitable amount of retained austenite and martensite while precipitating the precipitate in the ferrite. is necessary. However, if the content exceeds 0.2%, the weldability deteriorates, so the upper limit was made 0.2%. Preferably it is 0.10 to 0.16% of range.

Si: 0.3% or more, 2.0% or less
Si is an effective element for improving elongation and hole expansion ratio and generating retained austenite. However, if the content is less than 0.3%, a predetermined amount of retained austenite cannot be obtained.On the other hand, if the content exceeds 2.0%, the surface properties are remarkably deteriorated and the corrosion resistance is also lowered. 2.0% or less. Preferably it is 0.5 to 1.5% of range.

Mn: 0.3% or more, 2.0% or less
Mn is added to increase the strength. However, if the content is less than 0.3%, the effect of addition is poor. On the other hand, excessive addition exceeding 2.0% significantly reduces weldability, so the Mn content is 0.3% or more and 2.0% or less. Preferably it is 0.5 to 1.5% of range.

P: 0.06% or less P is not only segregated at the prior austenite grain boundaries and deteriorates low-temperature toughness, but also segregates in the steel to increase the anisotropy of the steel sheet and reduce workability. Is preferred, but up to 0.06% is acceptable. Preferably it is 0.04% or less.

S: 0.01% or less If S is segregated at the prior austenite grain boundaries or a large amount of MnS is formed, the low temperature toughness is lowered and it becomes difficult to use in cold regions. Although preferred, up to 0.01% is acceptable. Preferably it is 0.004% or less.

Al: 0.05% or less
Al is added as a steel deoxidizer, and is an effective element for improving the cleanliness of steel. In order to acquire this effect, it is preferable to make it contain 0.001% or more, but if it exceeds 0.05%, a large amount of inclusions will be generated and cause flaws in the steel sheet, so the upper limit of Al is made 0.05%.

Ti: 0.03% or more, 0.2% or less
Ti is an extremely important element for precipitation strengthening of ferrite. In order to achieve TS: 780 MPa or more, it is necessary to contain 0.03% or more of Ti, and as the amount of inclusion increases, precipitates increase and the strength increases. However, even if the content exceeds 0.2%, the effect is saturated, so the upper limit was made 0.2%. Preferably it is 0.05 to 0.15% of range.

Mo: 0.03% or more, 0.5% or less
Mo greatly affects the precipitation of carbides. When Mo is not contained, the increase in strength is small. In order to achieve TS ≧ 780 MPa, a Mo content of 0.03% or more is necessary. As the Mo content increases, precipitates increase and the strength increases. However, since the effect is saturated even if it is contained in a large amount exceeding 0.5%, the upper limit was made 0.5%. Preferably it is 0.05 to 0.3% of range.

Next, the reason for limiting the steel structure will be described.
Ferrite: 40% or more, 93% or less When the volume occupancy ratio of the entire structure is less than 40%, the hard second phase becomes excessive, and the stretch flange characteristics deteriorate. On the other hand, if it exceeds 93%, the retained austenite and martensite are too small and the elongation characteristics and impact resistance characteristics are not improved. A more preferable range is 60% or more and 90% or less.

Residual austenite: 4% or more and 15% or less The volume occupancy ratio of the entire structure is such that if the retained austenite is less than 4%, the elongation characteristics deteriorate, whereas if it exceeds 15%, the stretch flange characteristics deteriorate. A more preferable range is 5% or more and less than 12%.

Martensite: 3% or more, 12% or less If the volume ratio of the entire structure is less than 3%, the impact resistance is not improved, while if it exceeds 12%, the stretch flangeability deteriorates. A more preferable range is 5% or more and 10% or less.

  The balance of the steel structure is substantially bainite. In addition, although other pearlite etc. may mix here, if these total amount is less than 3%, it will accept | permit, and it can be said that the remainder of a steel structure is substantially bainite.

The average diameter of precipitates composed of Ti, Mo, and C contained in the ferrite is 20 nm or less, and the average interval between the precipitates is 60 nm or less.
A carbide that does not contain any of Ti, Mo, and C cannot achieve the desired tensile fatigue limit. Further, if the average diameter of the precipitates exceeds 20 nm and the average interval between the precipitates exceeds 60 nm, the desired tensile fatigue limit cannot be obtained, and the stretch flangeability also decreases. The preferred range of the average diameter of the precipitates is 10 nm or less, and the average interval is 40 nm or less.
In the present invention, precipitates composed of Ti, Mo, and C are mainly precipitated in ferrite. The reason for this is considered that the solid solubility limit of C in ferrite is smaller than that of austenite, and supersaturated C is likely to precipitate in ferrite. Actually, as a result of observing a thin film sample made of a steel plate with a transmission electron microscope (TEM), the precipitate was observed in the ferrite.

Next, the manufacturing process of the present invention will be described.
In addition, since the composition of the slab used for manufacture of this invention is the same as that of the steel plate mentioned above, description of the reason for limitation is abbreviate | omitted.
The melting method in the present invention may be an ordinary method and is not particularly limited. It is melted in a converter or electric furnace, ladle refining, degassing treatment, etc., made into a slab by continuous casting method or ingot forming method, and subjected to hot rolling.

Slab heating temperature (SRT): over 1150 ℃
Ti and Mo are mostly present as carbides in the slab. In order to precipitate as desired in ferrite after hot rolling, it is necessary to once dissolve the Ti-based carbide. For this purpose, it is necessary to heat to a temperature exceeding 1150 ° C. (preferably 1200 ° C. or higher). Below 1150 ° C., the amount of Ti and C dissolved is small, and Ti—Mo-based carbides precipitated after hot rolling are reduced, so the average interval between the precipitates is widened, and the stretch flangeability and fatigue characteristics are deteriorated. Note that even if slab heating is performed at temperatures exceeding 1300 ° C., the characteristics hardly change and the cost increases. Therefore, the upper limit of the slab heating temperature is preferably about 1300 ° C.

Finishing rolling temperature: Ar ₃ points or more, (Ar ₃ points + 100 ° C) or less If the rolling temperature is less than Ar ₃ points, rolling is performed in the two-phase region (ferrite + austenite). In this case, strain in the ferrite is released. Since it is difficult, the elongation characteristics deteriorate. Further, precipitates contained in the ferrite are coarsened, and the stretch flange characteristics and fatigue characteristics are deteriorated. On the other hand, if rolling is performed under conditions exceeding (Ar ₃ point + 100 ° C.), the structure becomes coarse and the required amount of retained austenite cannot be obtained. Therefore, the finish rolling temperature is limited to a range of Ar ₃ points or more and (Ar ₃ points + 100 ° C.) or less.

Cooling at an average cooling rate of 20 ° C / s or higher to a temperature of less than ₃ points and (slab heating temperature / 1.9 + 80) ° C or higher and (slab heating temperature / 1.5 + 20) ° C or lower In order to achieve this, it is important to control the cooling rate after finishing rolling and the air cooling temperature range to the above ranges.
If the average cooling rate after finish rolling is less than 20 ° C / s, the precipitates become coarse and the stretch flange characteristics and tensile fatigue characteristics deteriorate.
In addition, the cooling stop temperature range, that is, the air cooling temperature range is also important. This temperature range is less than Ar ₃ and (slab heating temperature / 1.9 + 80) ° C. or more and (slab heating temperature / 1.5 + 20) ° C. or less. As a result, precipitates containing Ti, Mo, and C in the ferrite can be precipitated in a state where the average diameter thereof is 20 nm or less and the average interval thereof is 60 nm or less.

Here, the reason why the above-mentioned cooling stop temperature range, that is, the air cooling temperature range is defined in relation to the slab heating temperature is that the higher the slab heating temperature, the more precipitates in the slab dissolve, This is because it is considered necessary to change the air-cooling temperature range depending on the slab heating temperature because the conditions for generating precipitates during air-cooling change.
The cooling stop temperature range is obtained by conducting various experiments based on this idea. The reason why the cooling stop temperature range is set to less than Ar ₃ is that it is necessary to precipitate carbide in the ferrite.

FIG. 1 summarizes the results of examining the influence of the slab heating temperature and cooling stop temperature on the average diameter and average interval of precipitates containing Ti, Mo, and C precipitated in ferrite. In the figure, ◯ indicates the case where the average diameter of the precipitates is 20 nm or less and the average interval is 60 nm or less, and □ indicates that at least one of the average diameter and the average interval of the precipitates deviates from the appropriate range. Show the case. In this investigation, the cooling stop temperature was set to a temperature lower than Ar ₃ points.
As is clear from the figure, the desired precipitation state of the present invention is obtained because the slab heating temperature is higher than 1150 ° C. and the cooling stop temperature range is (slab heating temperature / 1.9 + 80) ° C. or higher. It can be seen that (slab heating temperature / 1.5 + 20) ° C. or less is satisfied.
As the cooling means in the above cooling process, controlled cooling using water cooling is advantageously adapted.

Air cooling time: 3 to 15 seconds If the air cooling time is less than 3 seconds, 40% or more of ferrite is not formed, and the elongation characteristics deteriorate. On the other hand, if it exceeds 15 seconds, the precipitate becomes coarse and the elongation characteristics and the stretch flange characteristics deteriorate, so the air cooling time was set to 3 to 15 seconds.

Cool to an average cooling rate of 20 ° C / s or higher and wind up to 300 ° C or higher and 500 ° C or lower.
When the average cooling rate until the winding after the air cooling is less than 20 ° C./s, pearlite is generated and the characteristics deteriorate. When the coiling temperature exceeds 500 ° C. or less than 300 ° C., a predetermined amount of retained austenite cannot be obtained. A preferable temperature range is 350 ° C. or higher and 450 ° C. or lower.

After winding, the average cooling rate up to 200 ° C .: 20 ° C./hr or more If the average cooling rate after winding is less than 20 ° C./hr or more, a predetermined amount of martensite is not generated, and improvement in impact resistance cannot be expected. A preferable range is 60 ° C./hr or more.
In addition, what is necessary is just to perform forced cooling in mist atmosphere etc., for example after cooling after winding.

The steel having the composition shown in Table 1 was melted in a converter and made into a slab by continuous casting. The slab was then subjected to hot rolling → cooling → winding → cooling under the conditions shown in Table 2, Thickness: 2.0 mm hot rolled steel sheet.
In Table 2, Ar ₃ was determined from Ar ₃ = 910−203 × √C + 44.7 × Si + 31.5 × Mo (where C, Si, and Mo are the contents (mass%) of each element). Value.
Table 3 shows the results of examining the microstructure, tensile properties, stretch flange properties, tensile fatigue properties, and impact resistance properties of the hot-rolled steel sheets thus obtained.

The steel structure and material properties are measured as follows.
(1) Tensile properties were measured by a method based on JIS Z 2241 using a JIS No. 5 test piece taken with the direction perpendicular to the rolling direction as the longitudinal direction.
(2) The hole expansion test was conducted in accordance with the Japan Iron and Steel Federation Standard JFS-T1001-1996.
(3) The amount of retained austenite is determined by using the Kα line of Mo with an X-ray diffractometer on the surface polished by 0.1 mm by chemical polishing after grinding the hot-rolled sheet to 1/4 position. Measure the integrated strength of the (200), (220), (311) planes and the (200), (211), (220) planes of bcc iron, determine the fraction of retained austenite from these, and determine the volume occupied by retained austenite Rate.
(4) The amount of ferrite was expressed with a 3% nital solution, and the area ratio of the ferrite portion was quantified by image processing, and this was defined as the volume occupancy of ferrite.
(5) The amount of martensite is the area of the white part by image processing, using a structure photograph in which only martensite appears white in a corrosive solution in which 4% picric alcohol and 2% sodium pyrosulfate are mixed 1: 1. The rate was quantified, and this was defined as the martensite volume occupancy.
(6) In addition, when observing the structure of the ferrite and martensite, the remaining structure other than ferrite and martensite is also observed, and the fraction of pearlite and the like in the obtained hot-rolled steel sheet is less than 3%. It was confirmed that the balance other than ferrite and martensite was substantially retained austenite and bainite.
(7) Precipitates were observed with a transmission electron microscope at a magnification of 200,000 times or more. The composition of Ti, Mo, etc. was determined from analysis by an energy dispersive X-ray spectrometer (EDX) equipped with TEM. The diameter of the precipitate was determined by image processing to obtain an average diameter when the precipitate was regarded as a circle. Precipitate spacing is calculated by counting the number of precipitates present in the 300 nm square area on the electron micrograph, measuring the sample film thickness, and counting the sample volume. Was obtained by calculation, and was defined as an average precipitate interval.
(8) The tensile fatigue test was performed under the condition of the stress ratio R0.05, the fatigue limit (FL) at 10 ⁷ repetitions was determined, and the durability ratio (FL / TS) was determined. The stress ratio R is a value defined by minimum cyclic stress / maximum cyclic stress.
(9) Hysteresis speed: For tensile test at 10 / s, using a force table block type material testing machine manufactured by Kashiwamiya Seisakusho, the absorbed energy up to true strain of 0.1 was obtained from the true stress-true strain curve. The direction of tension was the direction perpendicular to the rolling direction.

As shown in Table 3, all the inventive examples have a plate thickness of 0.2 mm, TS of 780 MPa or more, El of 27% or more, hole expansion ratio of 60% or more, durability ratio in fatigue test (FL / TS)) Excellent characteristics of 0.75 or more were obtained, and the absorbed energy up to a true strain of 0.1 at a strain rate of 10 / s was 80 MJ / m ³ or more.

It is the graph which showed the influence which the slab heating temperature and cooling stop temperature exert on the average diameter and average space | interval of the precipitate containing Ti, Mo, and C which precipitate in a ferrite.

Claims (2)

% By mass
C: more than 0.08%, 0.2% or less,
Si: 0.3% or more, 2.0% or less,
Mn: 0.3% or more, 2.0% or less,
P: 0.06% or less,
S: 0.01% or less,
Al: 0.05% or less,
Ti: 0.03% or more, 0.2% or less and
Mo: 0.03% or more, 0.5% or less included, the balance is Fe and inevitable impurities composition, volume occupancy to the whole structure, ferrite: 40% or more, 93% or less, residual austenite: 4% or more, 15 % Or less, martensite: 3% or more, 12% or less, balance: steel structure of bainite, including precipitates composed of Ti, Mo, C in ferrite, the average diameter of such precipitates being 20 nm or less, precipitates A high-strength hot-rolled steel sheet with excellent elongation characteristics, stretch flange characteristics, tensile fatigue characteristics, and impact resistance characteristics, characterized in that the average distance between them is 60 nm or less. % By mass
C: more than 0.08%, 0.2% or less,
Si: 0.3% or more, 2.0% or less,
Mn: 0.3% or more, 2.0% or less,
P: 0.06% or less,
S: 0.01% or less,
Al: 0.05% or less,
Ti: 0.03% or more, 0.2% or less and
Mo: 0.03% or more and 0.5% or less of slab containing Fe and inevitable impurities in the balance is heated to a temperature above 1150 ° C, and the finishing temperature is Ar ₃ points or higher (Ar ₃ points + 100 ° C) After finishing rolling under the following conditions, the average cooling rate: 20 ° C / s or more to a temperature of less than ₃ points Ar and (slab heating temperature / 1.9 + 80) ° C or higher and (slab heating temperature / 1.5 + 20) ° C or lower Then, after cooling with air for 3 to 15 seconds, after cooling at an average cooling rate of 20 ° C / s or higher to 300 ° C or higher and 500 ° C or lower, winding, and then to 200 ° C, average cooling rate: 20 ° C / hr A method for producing a high-strength hot-rolled steel sheet having excellent elongation characteristics, stretch flange characteristics, tensile fatigue characteristics, and impact resistance characteristics, characterized by cooling at the above speed.

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