JP4705597B2 - Manufacturing method of high-strength hot-rolled steel sheet with excellent fatigue characteristics and stretch flangeability - Google Patents

Description

  The present invention relates to a method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability.

  High-strength hot-rolled steel sheets are widely used for applications called undercarriage parts such as automobile wheels and lower arms. Typical characteristics of steel sheets that are regarded as important in such applications are fatigue characteristics and stretch flangeability, and development of steel sheets that are excellent in both characteristics is underway. As a method for producing such a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics, the microstructure and the r-value are in-plane by setting the hot-rolling conditions of the steel having a predetermined chemical component within a specific range. A technique for controlling anisotropy has been proposed (for example, Patent Document 1).

  The present inventors also have a technology for obtaining a steel sheet having both characteristics by providing fatigue characteristics to a steel sheet mainly composed of a bainitic ferrite (BF) phase or an acicular ferrite phase having excellent stretch flangeability. Proposed (for example, Patent Documents 2 and 3).

  As seen in these examples, in order to develop a steel sheet excellent in stretch flangeability and fatigue characteristics, a method of imparting fatigue characteristics to a steel sheet excellent in stretch flangeability is often employed. The reason is that the stretch flangeability of a martensitic composite steel sheet having excellent fatigue properties is greatly inferior to that of a bainite composite steel sheet or a bainite steel sheet (for example, Non-Patent Document 1). According to Non-Patent Document 1, since the dual-phase steel plate (DP steel plate) is composed of a soft ferrite phase and a hard martensite phase, its interface tends to become a void generation starting point during stretch flange forming, and stretch. It is considered to be inferior in flangeability.

  However, the superiority of martensitic composite steel sheets, particularly DP steel sheets, over the bainite composite steel sheets is difficult. Therefore, it is considered that a development technique for imparting stretch flangeability to the DP steel sheet is also effective, but there are few such cases, and related techniques are disclosed (for example, Patent Document 4).

  Patent Document 4 discloses a technique for obtaining a steel sheet in which crack growth during hole expansion processing is suppressed by setting the microstructure to a ferrite phase coated with a hard second phase. However, in order to achieve such a very special microstructure form, advanced technology is considered necessary, and it is assumed that it is difficult to manufacture easily and inexpensively.

JP 2000-297349 A JP 2003-171734 A JP 2003-313639 A Japanese Patent Laid-Open No. 2005-154813 Hayashi, “Development and Formability Evaluation Method of High-Hole Expandable Hot Rolled Steel Sheet” Plasticity and Processing, Japan Society for Technology of Plasticity, Vol. 40, No. 457, 87-92

  An object of the present invention is to develop a technique that overcomes these problems and provides stretch flangeability and does not deteriorate the original fatigue characteristics.

  The present invention is based on the knowledge that, by performing heat treatment after performing appropriate processing on the DP steel sheet, the crack resistance at the end stage can be improved from the middle stage of the hole expansion deformation without impairing the original excellent fatigue characteristics. This is a production method for imparting stretch flangeability to a steel sheet having excellent fatigue characteristics, and the gist thereof is as follows.

(1) By mass%, C: 0.03-0.2%, Mn: 0.5-3%, Al: 0.01-0.05%, Si: 2.5% or less, P : 0.02% or less, S: 0.01% or less, N: 0.01% or less, and the steel slab consisting of Fe and unavoidable impurities in the balance, directly after casting, or Ac ₃ point or more 1300 ° C After reheating below, hot rolling is performed using Ar ₃ points to Ar ₃ points + 100 ° C. as the finishing temperature, cooling to 600 to 800 ° C. at an average cooling rate of less than 14 ° C./s, and further 14 ° C./s or more. The steel sheet is cooled from room temperature to a temperature of 450 ° C. or less at an average cooling rate, and the contact angle is 30 to 30 mm by a roll in which the sheet thickness t [mm] and the roll radius r [mm] satisfy the following (formula 1). Bending at 180 ° is performed at least once, followed by holding at 440-550 ° C. for 1-10 s Process for producing a high-strength hot-rolled steel sheet excellent in fatigue properties and stretch flangeability which is characterized in that the heat treatment that.
15 ≦ r / t ≦ 50 (Formula 1)
(2) The method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability according to (1), wherein a tension of 12 to 30 MPa is applied to the steel sheet during bending.

  (3) The above (1) characterized by containing at least one of Cr: 0.1 to 1%, Mo: 0.1 to 1%, and V: 0.1 to 1% by mass%. ) Or (2), a method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability.

  (4) Any one of the above (1) to (3), wherein one or both of Ti: 0.01 to 0.2% and Nb: 0.01 to 0.3% are contained in mass%. A method for producing a high-strength hot-rolled steel sheet excellent in fatigue characteristics and stretch flangeability according to item 1.

  (5) High strength with excellent fatigue characteristics and stretch flangeability according to any one of the above (1) to (4), characterized by containing Cu: 0.6-2% by mass% A method for producing a hot-rolled steel sheet.

  (6) It is excellent in the fatigue characteristics and stretch flangeability described in any one of the above (1) to (5), characterized by containing B: 0.0001 to 0.0020% by mass%. Manufacturing method of high-strength hot-rolled steel sheet.

  The steel sheet obtained by the production method of the present invention is excellent in stretch flangeability in addition to the excellent fatigue characteristics inherent in DP steel sheets, and can be applied to parts and applications that could not be applied conventionally. The industrial contribution is very remarkable.

  The present inventors performed a hole expansion test on a steel plate having a BF phase as a main phase and a DP steel plate, and observed the forming (deformation) process in detail. Drilling was performed before deformation with a press tester, and the state of cracks that occurred and propagated in the hole wall surface during the deformation process of the hole expansion test was followed until a plurality of cracks penetrated the wall surface in the thickness direction.

  As a result, the occurrence of cracks in the BF steel sheet, which has excellent stretch flangeability, precedes the DP steel sheet, and the progress of the crack is also in the middle of the diameter expansion process, that is, until the point where the crack reaches the boundary between the fracture surface and the shear plane. Newly discovered that it exceeds the DP steel plate. And although DP steel plate is superior to BF steel plate in the occurrence of cracks in the initial stage of hole expansion deformation and the progress of cracks to the middle part of hole expansion deformation, It was found that the resistance to crack propagation until the crack penetrates through the plate thickness is low.

Then, it was concluded that such “reversal phenomenon” of crack propagation is caused by a change in crack resistance in the thickness direction of the steel sheet. Therefore, in order to improve stretch flangeability without impairing the fatigue properties of the DP steel sheet, the depth from the surface is about 1/8 to 1/4 of the plate thickness, which corresponds to the middle to the end of the hole expansion deformation process. What is necessary is just to raise the crack resistance of a part.
An experiment conducted to complete the present invention will be described.

  First, in mass%, C: 0.1%, Si: 2.5%, Mn: 1.5%, P: 0.002%, and S: 0.0015%, the balance being Fe, And steel consisting of inevitable impurities was melted, and a steel piece obtained by casting was hot-rolled to obtain a steel plate having a thickness of 3.2 mm. The hot rolling heating temperature (SRT) is 1250 ° C, the finishing temperature (FT) is 850 ° C, the average cooling rate from 650 ° C is 12 ° C / s, the average cooling rate from 650 ° C to 20 ° C is 45 ° C / s And wound up at 20 ° C. That is, the temperature (MT) at which the average cooling rate is changed is 650 ° C., the winding temperature (CT) is 20 ° C., the average cooling rate (CR1) from FT to MT is 12 ° C./s, from MT to CT The average cooling rate (CR2) is 45 ° C./s.

The obtained steel sheet has tensile strength (σ _B ), elongation (δ), and hole expansion limit value (λ) of 820 MPa, 16%, and 34%, respectively. The tensile properties were examined with a JIS No. 5 test piece collected so that the tensile direction was perpendicular to the rolling direction. Further, λ was obtained in accordance with Japan Iron and Steel Federation standard JFS T 1001. Moreover, it confirmed that the microstructure was DP steel plate with the optical microscope.

Next, when the above steel sheet was heat-treated at various temperatures and the mechanical properties were examined, it was found that although λ could be increased within a certain range of conditions, σ _B was decreased. Therefore, processing such as tension, bending, and rolling was added, the amount of processing was changed, and then heat treatment was performed to evaluate tensile properties and hole expandability. As a result, it has been found that by combining bending under specific conditions and heat treatment, λ is improved and σ _B is reduced very little. It was also clarified that the fatigue characteristics are equivalent to those of the steel plate before processing and heat treatment.

Details of the present invention will be described below.
The conditions for the bending process and the subsequent heat treatment are the most important in the present invention, and are determined based on the experimental results described in the examples.

<Bending and heat treatment>
Relationship between plate thickness t and bending roll radius r: When bending deformation is performed by applying a strain from the surface of the steel plate 2 to an appropriate depth by bending with the roll 1 schematically shown in FIG. A steel plate having resistance to crack propagation from the middle stage to the final stage is obtained. The relationship between t and r is a factor that controls the depth from the surface to which strain is applied and the amount of strain. The depth to which strain is applied becomes shallow when r / t is large, and deep when r / t is small. Become. If r / t is less than 15, the depth to which strain is applied is too deep and the amount of strain becomes excessive, so that the hole expandability is lowered. On the other hand, if r / t is more than 50, the depth to which strain is applied is shallow and the amount of strain is insufficient, so the hole expandability is not improved.

  Contact angle between steel plate and roll: The application of bending strain by the roll is effectively exhibited when the contact angle 3 between the steel plate 2 and the roll 1 schematically shown in FIG. 1 is 30 ° or more. On the other hand, in order to make the contact angle between the steel plate and the roll over 180 °, it is not easy to construct a mechanism for continuous processing, so 180 ° is the upper limit.

  Heat treatment following bending: Heat treatment temperature moderates the effect of applied strain and affects the properties of DP structure or martensite phase to some extent, so that the crack progresses from the middle to the end of hole expansion deformation. It is thought that it plays an important role in increasing resistance to. When the heat treatment temperature is less than 440 ° C., the effect of alleviating the influence of strain is not sufficient, and a decrease in ductility cannot be ignored. On the other hand, when it is heated to over 550 ° C., the strength of the steel sheet is lowered and the intended high-strength steel sheet cannot be obtained. In order to obtain the effect of improving the hole expansibility, the holding time must be 1 second or longer. On the other hand, if the holding time exceeds 10 s, the strength and ductility are significantly reduced due to the coarsening of the crystal grains and the change in characteristics of the martensite phase.

  Furthermore, it is preferable to apply tension to the steel sheet during bending.

  Tension applied to the steel sheet: When the tension applied to the steel sheet at the time of bending is 12 MPa or more, the resistance to the progress of cracks from the middle to the end of the hole expansion deformation is further improved. This effect is manifested up to 50 MPa. However, when the tension exceeds 30 MPa, wrinkles occur on the steel sheet surface or roll surface, which may impair the fatigue characteristics of the steel sheet and increase the amount of wear of the roll. Therefore, the upper limit is preferably set to 30 MPa.

  Next, the chemical composition and manufacturing method of a steel plate will be described.

<Chemical composition of steel sheet>
C: C is an essential element for generating a martensite phase, and is important for securing the strength of the steel sheet. Therefore, it was set to 0.03% or more in consideration of obtaining a high-strength steel sheet having a useful σ _B ≧ 440 MPa by being applied to an undercarriage part of an automobile. On the other hand, if it exceeds 0.2%, it is difficult to ensure ductility and there is a risk of deterioration of weldability, so this value was made the upper limit.

  Si: Si is a deoxidizing element, and if it is excessively contained, the surface properties are impaired, so the upper limit was made 2.5%. Si may not be added, but it has the effect of promoting the transfer of C from the former to the latter in the temperature range in which the ferrite phase strengthens and the ferrite phase and austenite phase coexist, so 2.5% is added as the upper limit. It is preferable.

  Mn: Mn is useful for securing the strength, and is an important element that works to relax the cooling conditions for improving the hardenability and obtaining the DP steel sheet, and contains 0.5% or more. On the other hand, if it exceeds 3%, the hardenability is increased. As a result, the strength of the steel sheet becomes too high, resulting in deterioration of ductility and fear of deterioration of weldability.

  P: P is an impurity, and if it exceeds 0.02%, toughness deterioration due to grain boundary embrittlement becomes a problem. The upper limit allowed is 0.02%.

  S: S is an impurity, and it is preferable to suppress it as much as possible because it causes cracking during hot rolling and deterioration of ductility. However, since the reduction process involves an increase in cost, the upper limit is 0.01%. .

  Al: Al can be used as a deoxidizing element and has a function of fixing N. In order to exhibit these effects, 0.01% or more is necessary, but even if it exceeds 0.05%, the effect is saturated, so the upper limit is made 0.05%.

  N: It is desirable to suppress N because it works to impair non-aging properties. However, if considering the formation of nitrides and fixation with Al, 0.01% or less is allowed.

  Cr, Mo, V: Cr, Mo, and V all have the effect of enhancing the hardenability and promote the formation of the DP steel sheet. In order to obtain the effect, it is preferable to add 0.1% or more of any element. However, even if added over 1.0%, the effect is saturated, so 1.0% is made the upper limit.

  Ti, Nb: Ti and Nb can be added for the purpose of utilizing precipitation strengthening due to the formation of carbides and nitrides and making the crystal grains finer. In order to obtain such an effect, addition of at least 0.01% is necessary. On the other hand, even if Ti is contained in an amount exceeding 0.2% and Nb exceeds 0.3%, the effect is saturated.

  Cu: Cu is an element that increases the strength of the steel sheet and improves the fatigue characteristics. In order to obtain this effect, it is preferable to add 0.6% or more. On the other hand, the effect is saturated even if added over 2%.

  B: Since B has the effect of enhancing the hardenability and strengthening the grain boundaries, it is preferable to add 0.0001% or more in consideration of the ability to remove P. However, even if added over 0.0020%, the effect is saturated.

<Hot rolling conditions>
Hot-rolling heating temperature SRT: The heating temperature in the case of reheating and rolling the steel slab may be set according to the equipment specifications and capacity, with Ac ₃ points being the lower limit in order to complete finish rolling in the austenite region. On the other hand, when reheated to a temperature exceeding 1300 ° C., the upper limit is set to 1300 ° C. because there is a risk of deterioration of the material after rolling due to coarsening of crystal grains and deterioration of surface quality due to excessive oxidation.

A method of rolling the steel slab as it is in the process of cooling the steel slab after casting can also be adopted.
When rolling end temperature FT: FT is below Ar ₃ point, a DP structure cannot be obtained. On the other hand, if the FT exceeds Ar ₃ point + 100 ° C., the crystal grains become coarse and cause deterioration of mechanical properties. Therefore, the rolling end temperature is Ar ₃ points to Ar ₃ points + 100 ° C.

  In order to obtain a uniform structure, it is desirable that the rolling rate of hot rolling is 60 to 95%.

  Cooling after the end of rolling: After the end of rolling, primary cooling is performed to form a ferrite phase, followed by two-stage cooling in which secondary cooling is performed from a two-phase region with the austenite phase, and winding is performed. Here, the average cooling rate of primary cooling is less than 14 ° C./s, and the average cooling rate of secondary cooling is 14 ° C./s or more. The primary cooling is performed by selecting any one of air cooling, water cooling, mist cooling, and air blowing, and the secondary cooling is performed by selecting any one of water cooling, mist cooling, and air blowing. The upper limit of the average cooling rate of the secondary cooling is not particularly required for controlling the microstructure, but is desirably 200 ° C./s for the purpose of ensuring the uniformity of the cooling rate.

  The secondary cooling start temperature, that is, MT is 600 to 800 ° C., and the secondary cooling end temperature is the winding temperature CT, which is 450 ° C. or less including room temperature. These are limited as conditions necessary for making the microstructure of steel a DP structure.

  The microstructure of the steel sheet produced by such hot rolling and two-stage cooling is as follows.

<Microstructure>
Area ratio of martensite phase: DP structure in which the main phase (phase with the largest area ratio) is the ferrite phase and the second phase is the martensite phase, and the area ratio of the martensite phase is preferably 3 to 25%. . When the area ratio of martensite is 3% or more, high fatigue characteristics as a DP steel sheet can be enjoyed. On the other hand, when the area ratio of martensite is 25% or less, the yield ratio (yield point / σ _B ) can be lowered, and excellent formability can be obtained.

  Other phases and structures, for example, a bainite structure and a (residual) austenite phase may be included in a range in which the sum thereof does not exceed the area ratio of the martensite phase.

  In mass%, C: 0.1%, Si: 2.5%, Mn: 1.5%, P: 0.002%, and S: 0.0015%, the balance being Fe, and inevitable impurities Steel pieces obtained by melting and casting were hot-rolled to produce steel plates having thicknesses of 2.6 mm, 3.2 mm, and 4.0 mm. The hot rolling conditions were: heating temperature (SRT): 1250 ° C., rolling end temperature (FT): 850 ° C., secondary cooling start temperature (MT): 650 ° C., primary cooling average cooling rate (CR 1): 12 ° C./s, Secondary cooling average cooling rate (CR2): 45 ° C./s, winding temperature (CT): 20 ° C.

  While applying a tension of 15 MPa to the obtained steel, bending was performed with rolls of various radii while changing the contact angle. Further, the steel plate after bending was subjected to a heat treatment in which it was placed in an electric furnace maintained at 450 ° C. for 3 seconds, taken out and air-cooled.

Tensile test pieces were prepared from the heat-treated steel sheet, and the tensile strength (σ _B ) and elongation (δ) were measured. Here, for a steel plate having a tensile strength-elongation product σ _B × δ equal to or greater than that of a hot-rolled material (a material not subjected to bending and heat treatment), a hole-expanded test piece and a fatigue test piece are prepared, The hole expansion ratio (λ) and the fatigue limit (σ _W ) were measured. The method for measuring σ _B , δ, and λ is the same as that described above. Further, σ _W was measured by taking a No. 1 test piece (b = 15 mm, R = 30 mm) defined in JIS Z 2275 by taking a longitudinal direction in a direction perpendicular to the rolling direction and performing a plane bending fatigue test at 25 Hz. × 10 ⁶ times time intensity.

σ _W of a steel sheet with σ _B × δ equal to or higher than that of the material as hot rolled is small in change with respect to σ _W of the material as hot rolled, and the value obtained by dividing the absolute value of the difference by σ _W of the material as hot rolled is 0.03 (3%). On the other hand, the change in λ is large, and the value obtained by dividing the absolute value of the difference between λ after bending and heat treatment (bending heat treatment) and λ of the hot rolled material by λ of the hot rolled material is 0.15 ( 15%) or more conditions were also observed.

  Therefore, as shown in FIGS. 2 to 4, when arranged in terms of r / t and the contact angle, depending on the plate thickness in the range of 15 ≦ r / t ≦ 50 and the contact angle of 30 to 180 ° indicated by a black frame. It was found that λ was improved by 5% or more. The improvement rate is defined as (λ after bending heat treatment -λ as hot rolled) / λ × 100 as hot rolled.

  A steel plate having a plate thickness of 3.2 mm was produced under the same composition and hot rolling conditions as in Example 1, and bending was performed with a roll having a radius of 75 mm while applying a tension of 15 MPa. The contact angle was 90 °. The steel sheet after bending was heated for 1 to 13 seconds in an electric furnace maintained at various temperatures, and subjected to heat treatment for taking out and air cooling.

In the same manner as in Example 1, tensile test pieces, hole expansion test pieces, and fatigue test pieces were produced from the heat-treated steel plates, and σ _B , δ, λ, and σ _W were measured.

As in Example 1, the change rate of σ _W of the steel sheet in which σ _B × δ is the same as or higher than that of the hot-rolled material was within 0.03 (3%). On the other hand, there was a large change in λ, and it was recognized that the improvement rate by the bending heat treatment was 0.20 (20%) or more.

  Therefore, when these were analyzed in detail, as shown in FIG. 5, it was found that λ was improved by 5% or more when the heat treatment temperature and the treatment time satisfy the predetermined conditions (within the black frame). In FIG. 5, “0 seconds” means that only bending is performed and heat treatment is not performed.

A steel sheet having a thickness of 3.2 mm was produced under the same composition and hot rolling conditions as in Example 1, and bending was performed with a roll having a radius of 100 mm and a contact angle of 180 ° while applying various tensions to the obtained steel sheet. Alternatively, bending with a contact angle of 90 ° was performed with a roll having a radius of 125 mm.
The steel plate after bending was heated in an electric furnace maintained at 450 ° C. for 2 seconds, and then taken out and subjected to heat treatment for air cooling.

In the same manner as in Example 1, tensile test pieces, hole expansion test pieces, and fatigue test pieces were produced from the heat-treated steel plates, and σ _B , δ, λ, and σ _W were measured.

Similar to Example 1, the rate of change in σ _W of a steel sheet having σ _B × δ equal to or greater than that of the hot-rolled material was 0.03 (3%) or less. On the other hand, there was a large change in λ, and it was recognized that the improvement rate by the bending heat treatment was 0.20 (20%) or more.

  Therefore, as shown in FIG. 6, when the load tension is in the range of 5 to 50 MPa, each λ shows an improvement of 5% or more, and the effect is particularly remarkable in the range of 12 MPa or more. I understood that there was.

  Steel strips obtained by melting and casting steel having the chemical components shown in Table 1 were hot-rolled under the conditions shown in Table 2 to obtain a strip steel having a thickness of 3.2 mm. The steel strip from which the microstructure was confirmed and the DP structure was obtained was subjected to bending with a roll having a radius of 100 mm and a contact angle of 180 ° while applying a tension of 20 MPa.

  The strip steel after bending was cut into sheets, and they were placed in an electric furnace maintained at 450 ° C. for 1 second, taken out and subjected to heat treatment for air cooling.

Tensile test pieces were prepared from the heat-treated sheet and subjected to a tensile test to measure σ _B and δ. Furthermore, for a sheet subjected to bending heat treatment under the condition that the product σ _B × δ of the two is the same as or higher than σ _B × δ obtained from the material as it is hot-rolled, a hole expanding test piece and a fatigue test piece And λ and σ _W were measured in the same manner as in Example 1. The results are shown in Table 3.

sigma _B × [delta] is equal to or hot-rolled Mom material, the bending of the sigma _W thermomechanical processing material above it, small changes to sigma _W of hot-rolled Mom material, the absolute value of the difference between the hot-rolled Mom material The value divided by σ _W was within 0.03 (3%). On the other hand, the change in λ is large, and as shown in Table 3, if the steel plate satisfies the chemical composition and hot rolling conditions of the present invention, the hole expansion rate is improved by 5% or more by processing and heat treatment. Was recognized.

It is a schematic diagram which shows the bending process by a roll. It is a figure which shows the improvement rate of (lambda) using a roll radius and a contact angle as a coordinate (plate thickness 2.6mm). It is a figure which shows the improvement rate of (lambda) using a roll radius and a contact angle as a coordinate (plate thickness 3.2 mm). It is a figure which shows the improvement rate of (lambda) using a roll radius and a contact angle as a coordinate (plate | board thickness 4.0mm). It is a figure which shows the improvement rate of (lambda) by using heat processing time and heat processing temperature as a coordinate. It is a figure which shows the relationship between tension | tensile_strength and the improvement rate of (lambda).

Explanation of symbols

1 Roll 2 Steel plate 3 Contact angle t Thickness r Roll radius

Claims (6)

% By mass
C: 0.03-0.2%,
Mn: 0.5-3%,
Al: 0.01 to 0.05%
Containing
Si: 2.5% or less,
P: 0.02% or less,
S: 0.01% or less,
N: Restricted to 0.01% or less, a steel slab consisting of Fe and unavoidable impurities in the balance, directly after casting, or after reheating to _{3 to} 1300 ° C. of Ar ₃ points to Ar ₃ points + 100 ° C. Is subjected to hot rolling at a finishing temperature, cooled to 600 to 800 ° C at an average cooling rate of less than 14 ° C / s, and further cooled from room temperature to 450 ° C or less at an average cooling rate of 14 ° C / s or more. The steel sheet is rolled at least once with a contact angle of 30 to 180 ° with a roll having a thickness t [mm] and a roll radius r [mm] satisfying the following (formula 1), and then 440 A method for producing a high-strength hot-rolled steel sheet excellent in fatigue properties and stretch flangeability, characterized by performing a heat treatment that is held at 550 ° C for 1 to 10 seconds.
15 ≦ r / t ≦ 50 (Formula 1)   The method for producing a high-strength hot-rolled steel sheet having excellent fatigue characteristics and stretch flangeability according to claim 1, wherein a tension of 12 to 30 MPa is applied to the steel sheet during bending. % By mass
Cr: 0.1 to 1%,
Mo: 0.1 to 1%,
V: 0.1 to 1%
The manufacturing method of the high strength hot-rolled steel plate excellent in the fatigue characteristics and stretch flangeability of Claim 1 or 2 characterized by including 1 or more types of these. % By mass
Ti: 0.01-0.2%
Nb: 0.01 to 0.3%
One or both of these are contained, The manufacturing method of the high strength hot-rolled steel plate excellent in the fatigue characteristics and stretch flangeability of any one of Claims 1-3 characterized by the above-mentioned. % By mass
Cu: 0.6-2%
The manufacturing method of the high strength hot-rolled steel plate excellent in the fatigue characteristics and stretch flangeability of any one of Claims 1-4 characterized by the above-mentioned. % By mass
B: 0.0001 to 0.0020%
The manufacturing method of the high intensity | strength hot-rolled steel plate excellent in the fatigue characteristics and stretch flangeability of any one of Claims 1-5 characterized by the above-mentioned.

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