JP2008138231A - Hot-rolled composite structure steel sheet excellent in hole-expanding property, and method of producing therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled composite structure steel sheet which is suitable a material for a car wheel to be subjected to complicated forming processing, and excellent in a hole-expanding property. <P>SOLUTION: The hot-rolled composite structure steel sheet containing, by mass, 0.03-0.2% C, 0.5-2.0% Si, 0.5-2.0% Mn, ≤0.03% P, ≤0.015% S, 0.01-0.1% Al, 0.001-0.006% N and the balance Fe with inevitable impurities, and consisting of main phase ferrite and second phase martensite, is heat treated after skin-pass rolling. Thus, the hot-rolled composite structure steel sheet excellent in the hole-expanding property, wherein a volume fraction of the second phase is 3-20%, the average grain diameter is ≤ 8 μm, a value obtained by dividing the volume fraction of the second phase with the average grain diameter of the second phase is <3 and the ratio of the average hardness of the second phase to the average hardness of the ferrite is 2-3.5, is obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hot-rolled composite steel sheet excellent in hole expansibility and a method for manufacturing the same, and more particularly to a hot-rolled composite steel sheet suitable for a structural member such as an automobile and a method for manufacturing the same.

  Hot rolled steel sheets with a tensile strength of 440 MPa class are mainly used as wheels for automobiles. However, in recent years, high strength, tensile strength of 540 MPa class with excellent fatigue characteristics and elongation has been achieved to reduce the weight of automobiles. Composite steel sheets have been adopted. On the other hand, complex forming processes have been added to wheel materials, and as a result, high hole expansibility has been demanded, but the hole expansibility of composite steel sheets is not good, and complex forming processes are difficult. Since it is not possible to use a composite structure steel plate for a necessary wheel, an improvement thereof has been demanded.

  As a method for solving the above problems, it has been clarified that it is effective to refine the structure or reduce the ratio of the hardness of the main phase ferrite and the second phase. The C-Si-Mn steel containing Ti is used to define the rolling reduction of the hot rolling and the cooling conditions after the hot rolling to define the average grain of ferrite and the second phase as the main phase. Disclosed is a high-tensile steel plate with excellent burring properties (hole expansibility) by reducing the diameter to 3.5 μm or less and defining the ratio of the hardness of the second phase and the hardness of the ferrite phase within an appropriate range. Has been.

Patent Document 2 discloses a composite structure in which the phase with the largest volume fraction is ferrite and the second phase is martensite, and the value obtained by dividing the volume fraction of the second phase by the average particle diameter of the second phase and The composite structure steel plate which is excellent in burring workability (hole expansibility) is disclosed by defining the values obtained by dividing the average value of the two-phase hardness and the average value of the hardness of the ferrite within appropriate ranges.
JP 2000-192191 A JP 2001-303187 A

  However, the steel sheet described in Patent Document 1 requires finish rolling under severe conditions in order to refine the structure. In order to satisfy this rolling condition over the entire length of the steel sheet, a high-frequency heating device or the like is used. Although it is described that heating is performed between rolling stands, this is not preferable because it causes an increase in manufacturing cost. Furthermore, since the second phase is not a martensite-based structure, it is presumed that the fatigue characteristics are not good.

  Moreover, although the steel sheet described in Patent Document 2 has improved the hole expansion workability as compared with the conventional steel sheet, the balance between the strength and the hole expansion characteristic of this steel sheet is about 52000 MPa ·% in terms of tensile strength × hole expansion ratio. However, sufficient hole expandability that can be used for a wheel that requires complicated molding is not obtained.

  Accordingly, the present invention has an object to provide a hot-rolled composite steel sheet excellent in fatigue characteristics and strength-hole expansibility balance that can solve the above-described problems of the prior art, and a manufacturing method that can stably and inexpensively manufacture the steel sheet. To do.

  In order to solve the above-mentioned problems, the inventors of the present invention are based on a hot rolled composite steel sheet made of a main phase ferrite with good fatigue properties and a second phase martensite, and a method for improving the hole expandability. As a result of earnest research, the hot-rolled composite steel sheet made of ferrite and martensite is lightly pressed and then heat-treated to increase the hardness of the ferrite of the main phase and the hardness of the martensite of the second phase. It has been found that the ratio can be reduced. Further research is conducted, the volume fraction of the second phase is 3 to 20%, the average particle size of the second phase is 8 μm or less, and the ratio of the average hardness of the second phase to the average hardness of the ferrite is 2 to 3.5. And that the value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase is less than 3, it is very effective in improving the fatigue characteristics and the strength-hole expansibility balance. This is a new heading and the present invention.

That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.03-0.2%, Si: 0.5-2.0%, Mn: 0.5% -2.0%, P: 0.03% or less, S : 0.015% or less, Al: 0.025 to 0.1%, N: 0.001 to 0.006%, the balance is made of Fe and inevitable impurities, and the steel structure is composed of the main phase ferrite. It is composed of martensite of the second phase, the volume fraction of the second phase is 3 to 20%, the average particle size of the second phase is 8 μm or less, and the volume fraction of the second phase is the average of the second phase A hot-rolling composite excellent in hole expansibility, wherein the value divided by the particle size is less than 3 and the ratio of the average hardness of the second phase to the average hardness of the ferrite is 2 to 3.5 Texture steel plate.
(2) The hot-rolled composite steel sheet having excellent hole expansibility according to (1), wherein the steel sheet further contains Ca: 0.0005 to 0.005% by mass%.
(3) The steel sheet further includes one or more of Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less in mass%. (1) or a hot-rolled composite structure steel sheet excellent in hole expansibility as described in (2).
(4) After heating the steel material having the steel composition described in any one of (1) to (3) and then rough rolling and finishing the finish rolling at a temperature of Ar ₃ or higher, 30 ° C./second After cooling to a temperature of 780 ° C. or less at the above cooling rate, holding in a temperature range of 650 ° C. to 780 ° C. for 5 seconds to 15 seconds, and then to a temperature of 150 ° C. or less at a cooling rate of 30 ° C./second or more After cooling and winding, temper rolling is performed at room temperature at a rolling reduction of 1 to 5%, and then maintained in a temperature range of 50 ° C. or higher and 150 ° C. or lower for 30 minutes or more, and has excellent hole expandability. A method of manufacturing a hot rolled composite steel sheet.

  According to the present invention, a hot-rolled composite steel sheet excellent in fatigue characteristics and strength-hole expansibility balance can be manufactured at a low cost, and can be used as a wheel material to be subjected to complicated forming processing. As a result, it contributes to an improvement in fuel consumption.

The reasons for limiting the steel composition in the present invention will be specifically described below. The unit of component content is mass% and is abbreviated as%.
C: 0.03-0.2%
C is an important element for improving the strengthening and hardenability of steel, and is an essential element for obtaining a composite structure composed of ferrite and martensite. If the C content is less than 0.03%, the volume fraction of the second phase is less than 3% and the effect cannot be obtained, so the lower limit is limited to 0.03%. A preferred lower limit is 0.04%, and a more preferred lower limit is 0.05%. On the other hand, if it exceeds 0.2%, the volume fraction of the second phase exceeds 20%, the workability deteriorates and the weldability also deteriorates, so the upper limit was limited to 0.2%. A preferred upper limit is 0.18%, and a more preferred upper limit is 0.15%.

Si: 0.5 to 2.0%
Si is an element necessary for obtaining a composite structure composed of ferrite and martensite, and is effective for increasing the strength as a solid solution strengthening element. If the Si content is less than 0.5%, bainite is likely to be formed and the fatigue characteristics deteriorate, so the lower limit was limited to 0.5%. A preferred lower limit is 0.8%, and a more preferred lower limit is 1.0%. On the other hand, if it exceeds 2.0%, the workability deteriorates, so the upper limit was limited to 2.0%. A preferred upper limit is 1.8%, and a more preferred upper limit is 1.5%.

Mn: 0.5% to 2.0%
Mn is an element effective for preventing the hot cracking of the slab and increasing the strength as a solid solution strengthening element. If the Mn content is less than 0.5%, the effect is poor, so the lower limit was made 0.5%. A preferred lower limit is 0.8%, and a more preferred lower limit is 1.0%. On the other hand, if it exceeds 2.0%, the workability is deteriorated, so the upper limit is limited to 2.0%. The preferred upper limit is 1.8%, and the more preferred upper limit is 1.6%.

P: 0.03% or less P is an element effective for increasing the strength, but is generally an element to be reduced as an impurity element. If the P content exceeds 0.03%, the workability and weldability are adversely affected and the fatigue characteristics are also reduced, so the upper limit was limited to 0.03%. A preferred upper limit is 0.02%, and a more preferred upper limit is 0.01%. On the other hand, excessive reduction may increase the cost, and the lower limit of the substantial P content is 0.001%.

S: 0.015% or less S is an impurity element, and an element that is preferably reduced as much as possible. When the S content exceeds 0.015%, workability deteriorates significantly, so the upper limit is limited to 0.015%. A preferable upper limit is 0.01%, and a more preferable upper limit is 0.005%. On the other hand, excessive reduction may increase the cost, and the lower limit of the substantial S content is 0.0005%.

Al: 0.01 to 0.1%
Al is an element necessary for deoxidation of molten steel. When the Al content exceeds 0.1%, non-metallic inclusions increase and workability deteriorates, so the upper limit was limited to 0.1%. If it is less than 0.01%, the deoxidation effect cannot be obtained, so the lower limit was made 0.01%.

N: 0.001 to 0.006%
N is an element effective for age hardening. When the N content exceeds 0.006%, coarse nitrides are generated and the workability is deteriorated, so the upper limit is limited to 0.006%. A preferable upper limit is 0.005%, and a more preferable upper limit is 0.004%. On the other hand, if it is less than 0.001%, an aging effect cannot be obtained, so the lower limit was made 0.001%.

Ca: 0.0005 to 0.005%
Ca is an element detoxified by changing the form of non-metallic inclusions that degrade workability. When the Ca content exceeds 0.005%, the effect is saturated, so the upper limit is limited to 0.005%. On the other hand, since the effect is small at less than 0.0005%, the lower limit was made 0.0005%.

Cr: 1.0% or less, Mo: 1.0% or less, Ni: 1.0% or less Cr, Mo, and Ni are elements that improve the strength of steel by solid solution strengthening. When the content of these elements exceeds 1.0%, the strength is excessively increased and the workability is deteriorated. Therefore, the upper limit is limited to 1.0%. In addition, in order to acquire the said effect, it is preferable to contain 1 type (s) or 2 or more types as Cr: 0.05% or more, Mo: 0.05% or more, Ni: 0.05% or more.

  In addition to the above components, the balance consists of Fe and inevitable impurities. It should be noted that other trace elements within a range that does not impair the functions and effects of the invention are allowed to be included.

  The hot-rolled composite steel sheet of the present invention has the above steel composition, and in order to achieve both fatigue characteristics and strength × hole expansibility balance, the main phase with the largest volume fraction is ferrite, and the second phase is mainly used. The composite structure was martensite. However, inevitable bainite and retained austenite are allowed in the second phase. In order to secure good fatigue characteristics, the volume fraction of bainite or retained austenite is preferably 3% or less.

  As a result of investigating the cause of poor hole expandability of a composite steel sheet composed of a conventional main phase ferrite and a second phase martensite, the present inventors have obtained a soft ferrite in the high strain region of the hole expandability test. It was found that the concentration of strain produced microvoids in the vicinity of the interface between martensite and ferrite, which were connected to cause cracks. Therefore, as a result of various studies on methods for preventing the generation of microvoids and their connection, it has been found that it is effective to harden the ferrite phase. If the ratio of the average hardness of martensite to the average hardness of ferrite, that is, (average hardness of martensite / average hardness of ferrite) is 3.5 or less by hardening the ferrite, the hole expandability is improved. I found out that I could do it. However, it has been found that when the ratio of the average hardness of martensite to the average hardness of ferrite becomes less than 2 when the ferrite becomes too hard, the hole expandability is improved but the elongation is lowered.

  In order to obtain further stable hole expansibility, the present inventors have further studied that the volume fraction of martensite is 20% or less, the average particle diameter of martensite is 8 μm or less, and martensite. It has been found that the value obtained by dividing the volume fraction of the site by the average particle size of martensite needs to be 3.5 or less. It is presumed that the hole expandability deteriorates when the volume ratio of martensite exceeds 20% because the microvoids generated around the martensite are easily connected. When the volume fraction of martensite is less than 3%, the amount of martensite effective for retaining fatigue cracks is small and the fatigue characteristics deteriorate, so the lower limit is limited to 3%. When the average particle size of martensite is larger than 8 μm, the stress concentration at the interface between martensite and ferrite becomes significant, and microvoids are likely to occur. On the other hand, when the value obtained by dividing the volume fraction of martensite by the average particle size of martensite exceeds 3, the hole expandability may deteriorate. This is presumed to be due to a decrease in local ductility.

  The microstructure observation of the composite structure steel plate obtained by the present invention is to cut out a sample for observation from a position excluding the unsteady portion near the edge of the steel plate, and to embed and polish a cross section parallel to the rolling direction for cross section observation. After corrosion, the quarter position of the plate thickness was observed at 400 to 1000 times using an optical microscope. The volume fraction of martensite is defined as the area fraction of martensite (white part) in the structure photograph due to repeller corrosion, and the average particle size is defined as the average equivalent circle particle size, which can be obtained by an image analyzer or the like. Value was adopted. Moreover, the hardness measurement method of a ferrite and a martensite was measured according to the Vickers hardness test method of JISZ2244. However, the test force is 0.049 to 0.098 N, and the holding time is 15 seconds.

Moreover, the hole expansion test was conducted in accordance with Japan Iron and Steel Federation standard JFS-T 100 0 1-1 996. That is, the hole diameter d ₀ = 10 mm is given to the hot-rolled steel sheet with clearance 1 2. An initial hole is formed by punching at 5%. The initial hole burr is the die side (that is, the opposite side of the conical punch), the conical punch (vertical angle 60 °) is inserted into the initial hole, the hole is expanded, and the crack is hot rolled. The hole diameter d 1 at the time of penetrating the steel plate was determined. Using these d ₀ and d values, the hole expansion ratio λ (%) was calculated from the following equation (1).
λ = 1 0 0 × (d - d 0) / d 0 ··· (1)

  Next, the method for producing the hot rolled composite steel sheet of the present invention will be specifically described. Slabs obtained by casting molten steel with components adjusted to the desired component content by continuous casting, ingot casting, etc. are sent directly to hot rolling mills as high-temperature slabs or reheated in a heating furnace. And then hot rolling. The reheating temperature is not particularly specified, but if it is 1300 ° C or higher, the scale-off amount becomes large and the yield is lowered, so it is desirable that the temperature is less than 1300 ° C. Therefore, 1000 ° C. or higher is desirable.

In the hot rolling step, finish rolling is performed after finishing rough rolling, but finish rolling is finished at a temperature of Ar ₃ or higher. If the finish rolling temperature is lower than the Ar ₃ point, strain remains, ductility is lowered, and workability is deteriorated. Therefore, the finish rolling temperature is set to Ar ₃ or higher.

  After finish rolling, the steel sheet is cooled to 780 ° C. or lower at a cooling rate of 30 ° C./second. This is because ferrite and martensite are refined by suppressing the grain growth of austenite. Next, the temperature is maintained at 650 ° C. to 780 ° C. for 5 seconds to 15 seconds. This is done to promote ferrite transformation in the two-phase region. If it is less than 5 seconds, the ferrite transformation is not sufficient. On the other hand, if it exceeds 15 seconds, pearlite is generated.

  Next, it cools and winds up to the temperature of 150 degrees C or less from the temperature range with the cooling rate of 30 degrees C / sec or more. Pearlite or bainite is generated at a cooling rate of less than 30 ° C / second, and pearlite or bainite is generated even when the coiling temperature exceeds 150 ° C. The composite structure of the target main phase ferrite and second-phase martensite Cannot be obtained.

  The present inventors performed temper rolling at room temperature as a means of hardening the ferrite of the hot rolled composite steel sheet composed of ferrite and martensite produced by the above method to improve the hole expansion property, and then heat treatment. I found a way to do it. In order to find an appropriate reduction ratio, after temper rolling at various reduction ratios, the average hardness of the martensite relative to the average hardness of the ferrite of the hot-rolled composite steel sheet subjected to heat treatment held at 100 ° C. for 1 hour. The results of investigating the thickness ratio and tensile strength x hole expansion rate are shown in FIG. It was found that when the rolling reduction was 1% or more, the ratio of the average hardness of martensite to the average hardness of ferrite was 3.5 or less, and the tensile strength × hole expansion ratio was 60,000 MPa ·% or more. If the rolling reduction ratio of the temper rolling is less than 1%, the effect of hardening the ferrite is not obtained due to insufficient strain, and if it exceeds 5%, the tensile strength x hole expansion ratio is good, but the ferrite becomes too hard and the elongation is increased. It turns out that it deteriorates.

  As a result of examining the heat treatment conditions, it was found that if the heat treatment temperature is 50 ° C. or higher and 150 ° C. or lower and maintained for 30 minutes or longer, the hole expandability can be improved. It has been found that if the heat treatment temperature is less than 50 ° C., the effect of hardening the ferrite cannot be obtained, and if the temperature exceeds 150 ° C., it is softened. Further, if the holding time is less than 30 minutes, the effect of hardening the ferrite cannot be obtained. If the holding time is 30 minutes or longer, the hardening of the ferrite can be obtained. However, the effect is saturated even if the holding time exceeds 10 hours, so 10 hours or less is preferable from the viewpoint of productivity.

A to N steels having the compositions shown in Table 1 were melted in a converter and made into slabs by a continuous casting method. These slabs were heated and hot-rolled under various conditions shown in Table 2 and rolled to a plate thickness of 2.8 to 4.5 mm, and then cooled and wound up. Next, heat treatment was performed after temper rolling under the conditions shown in Table 2.

  The structure and mechanical properties of the steel sheet obtained by the above method were investigated. The results are shown in Table 3. The structure was revealed by the above-described method, and the volume fraction of ferrite (Vf), the volume fraction of martensite (Vm), the volume fraction of pearlite and bainite (Vp + b), and the average particle diameter of martensite (dm ) Was measured. The average hardness (Hf) of ferrite and the average hardness (Hm) of martensite were measured in accordance with the method described in JIS Z 2244, and the average value of the results obtained by measuring five points respectively. In addition, the steel plate having a pearlite + bainite volume fraction higher than the martensite volume fraction was measured for the hardness of the second phase including pearlite and bainite, and was taken as the value of Hm.

The mechanical properties of the steel sheet were processed into a No. 5 test piece described in JIS Z 2201 in the direction perpendicular to the rolling direction of the steel sheet, and subjected to a tensile test according to the test method described in JIS Z 2241. Tensile properties (yield point σY, tensile Strength σB, elongation at break EL) were measured. The hole expansion test was performed in accordance with the Japan Iron and Steel Federation standard JFS-T 100 0 1-1 996 and the hole expansion ratio (λ) was calculated. Furthermore, a complete double-bending plane bending fatigue test was performed with a plane bending fatigue test piece having a length of 98 mm, a width of 38 mm, a minimum cross-sectional width of 20 mm, and a notch curvature radius of 30 mm as shown in FIG. It was. The fatigue characteristics of the steel sheet were evaluated by a value (fatigue limit ratio σW / σB) obtained by dividing the fatigue limit (σW) at 10 × 10 ⁷ times by the tensile strength (σB) of the steel sheet. These results are also shown in Table 3.

  In accordance with the present invention, there are nine steel types of Invention Examples 1 to 9, which contain a predetermined amount of steel components, and the microstructure is a composite structure in which the main phase is ferrite and the second phase is mainly martensite. The volume fraction of the second phase is 3 to 20%, the average particle size is 8 μm or less, and the value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase is less than 3, And the ratio of the average hardness of the second phase to the average hardness of ferrite is 3.5 or less. Good characteristics with a ratio of 50% or more and a tensile strength × hole expansion ratio of 60000 MPa ·% or more are obtained.

  On the other hand, the steel plate of Comparative Examples 1-4 whose steel component was outside the scope of the present invention could not obtain the characteristics for the following reason. The steel sheet of Comparative Example 1 had a low volume fraction of martensite in the second phase because the C content was too low, and the fatigue limit ratio was low. Since the steel sheet of Comparative Example 2 has too much C, the value obtained by dividing the volume fraction of martensite in the second phase and the volume fraction of the second phase by the average particle diameter of the second phase is high, and the tensile strength × Hole expansion rate was low. In the steel plate of Comparative Example 3, bainite was generated because the Si content was too low, and the fatigue limit ratio was low. The steel plate of Comparative Example 4 had a low elongation because the N content was too high, and the tensile strength × the hole expansion rate was also low.

  The steel plates of Comparative Examples 5 to 9 whose hot rolling / cooling / winding conditions were out of the scope of the present invention could not obtain the characteristics for the following reasons. In the steel sheet of Comparative Example 5, since the cooling rate after finish rolling was too slow, austenite grains grew, and as a result, the average grain size of martensite, which was the second phase, was too large, and the tensile strength x hole expansion rate was low. It was. The steel plate of Comparative Example 6 had a low ferrite fraction because the residence time at 650 ° C. to 780 ° C. was too short, and because the second phase became a bainite-based structure, tensile strength × hole expansion rate, fatigue limit ratio Both were low. In the steel plate of Comparative Example 7, the residence time at 650 ° C. to 780 ° C. was too long, so that pearlite was generated and the fatigue limit ratio was low. In the steel plate of Comparative Example 8, the cooling rate after being retained at 650 ° C. to 780 ° C. was too slow, so that martensite was not generated, pearlite and bainite were generated, and the fatigue limit ratio was low. The steel sheet of Comparative Example 9 had a too high coiling temperature, so bainite was generated and the fatigue limit ratio was low.

  The characteristics of the steel sheets of Comparative Examples 10 to 14 in which the conditions of temper rolling and heat treatment were outside the scope of the present invention were not obtained for the following reasons. Since the steel plate of Comparative Example 10 was not temper-rolled, the ferrite was not hardened, and the ratio of the martensite hardness to the ferrite hardness was too high, so the tensile strength × hole expansion rate was low. In the steel sheet of Comparative Example 11, since the rolling reduction of the temper rolling was too high, the ferrite became too hard, and the tensile strength × hole expansion ratio was improved, but the elongation was low. In the steel sheet of Comparative Example 12, since the heat treatment temperature was too low, the ferrite was not hardened, and the ratio of the martensite hardness to the ferrite hardness was too high, so that the tensile strength × the hole expansion ratio was low. In the steel plate of Comparative Example 13, the heat treatment temperature was too high, so the ferrite softened and the tensile strength × hole expansibility deteriorated. In the steel plate of Comparative Example 14, the heat treatment time was too short, so the ferrite was not sufficiently cured, and the ratio of the martensite hardness to the ferrite hardness was too high, so the tensile strength × the hole expansion ratio was low.

The figure which shows the relationship between the reduction ratio of temper rolling, the ratio of the average hardness of a martensite with respect to the average hardness of a ferrite, and tensile strength x hole expansion rate. The figure explaining the shape of a fatigue test piece.

Claims (4)

  In mass%, C: 0.03-0.2%, Si: 0.5-2.0%, Mn: 0.5% -2.0%, P: 0.03% or less, S: 0.0. 015% or less, Al: 0.01 to 0.1%, N: 0.001 to 0.006%, the balance is made of Fe and inevitable impurities, the steel structure is the main phase ferrite and the second phase The volume fraction of the second phase is 3 to 20%, the average particle size of the second phase is 8 μm or less, and the volume fraction of the second phase is the average particle size of the second phase. A hot-rolled composite steel sheet excellent in hole expansibility, wherein the divided value is less than 3 and the ratio of the average hardness of the second phase to the average hardness of the ferrite is 2 to 3.5.   The said steel plate contains Ca: 0.0005-0.005% by the mass% further, The hot rolled composite structure steel plate excellent in the hole expansibility of Claim 1 characterized by the above-mentioned.   The steel sheet further contains one or more of Cr: 1.0% or less, Mo: 1.0% or less, and Ni: 1.0% or less in mass%. Item 3. A hot rolled composite steel sheet excellent in hole expansibility according to item 1 or 2. A steel material having the composition according to any one of claims 1 to 3 is heated and then rough-rolled, and finish rolling is finished at a temperature of Ar ₃ or higher, and then 780 ° C at a cooling rate of 30 ° C / second or more. After cooling to the following temperature, holding in a temperature range of 650 ° C. to 780 ° C. for 5 seconds to 15 seconds, cooling to a temperature of 150 ° C. or less at a cooling rate of 30 ° C./second and winding up Temper rolling at a rolling reduction of 1 to 5% at room temperature, and then holding in a temperature range of 50 ° C. to 150 ° C. for 30 minutes or more. Production method.

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