JP2001240939A - High strength hot rolled steel sheet excellent in ductility and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet in a class of 380 to 500 MPa having excellent ductility by controlling its composition and hot rolling conditions to specified ranges and to provide its production method. SOLUTION: This high strength hot rolled steel sheet excellent in ductility has a composition containing, by mass, 0.0015 to 0.0055% C, 0.1 to 0.6% Mn, 0.045 to 0.09% P, <=0.01% S, 0.5 to 2.5% Al, 0.001 to 0.008% N, 0.015 to 0.085% Ti, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a 380-500M
It is related to a high-strength hot-rolled steel sheet having a tensile strength of Pa, and by setting the composition, structure, finishing temperature in hot rolling, and cooling conditions after finishing in a specific range to a high-strength heat-rolled steel sheet having excellent ductility. An object of the present invention is to provide a rolled steel sheet and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of vehicles and improve collision safety, steel plates having higher strength than ever have been used as steel plates for vehicles. Steel sheets having various tensile strengths are used for high-strength steel sheets for automobiles, and hot-rolled steel sheets having a tensile strength of 380 to 500 MPa are also frequently used. Demand is expected to increase.

[0003] However, when the strength of a steel sheet is increased, ductility generally decreases, and press formability decreases. Therefore, a steel sheet with high strength and high ductility is required.

Many inventions have been disclosed so far for a method for producing a high-strength hot-rolled steel sheet having high ductility. For example, as disclosed in JP-A-7-74377, a high-strength steel sheet having excellent ductility can be manufactured by using a steel structure mainly composed of ferrite and martensite. However, in order to manufacture this, it is necessary to precisely control the cooling conditions after hot rolling, and the productivity is poor due to the restrictions. Further, this method mainly shows a method for producing a steel sheet having a strength of 540 MPa or more.
It is difficult to produce a high-strength steel sheet of 0 to 500 MPa class.

[0005] Further, as disclosed in Japanese Patent Application No. 8-78102, there is a method of producing a high-strength steel sheet having excellent ductility by making a steel structure mainly composed of ferrite, residual γ, and benite. However, also in this case, the cooling conditions after hot rolling are limited, and the productivity is not always excellent. Also, this method
It mainly shows a method for producing a steel sheet having a strength of 540 MPa or more, and 380 to 500 having a lower strength than this.
It is difficult to manufacture a MPa class steel sheet.

[0006]

Therefore, the present invention relates to
It is an object of the present invention to provide a hot-rolled steel sheet having a strength of 80 to 500 MPa and excellent ductility, and to provide a method of manufacturing the steel sheet.

[0007]

Means for Solving the Problems The present inventors have found that the ductility is improved by setting the composition of steel to a specific range, and the temperature control is particularly difficult in hot rolling when manufacturing this steel sheet. Was found to be unnecessary.

The present invention has been completed based on this finding, and the gist thereof is as follows.

(1) In terms of mass ratio, C: 0.0015 to
0.0055%, Si: 0.005 to 0.3%, Mn:
0.1-0.6%, P: 0.045-0.09%, S:
0.01% or less, Al: 0.5 to 2.5%, N: 0.0
01-0.008%, Ti: 0.015-0.085%
A high-strength hot-rolled steel sheet excellent in ductility, characterized by containing Fe and the balance being Fe and inevitable impurities.

(2) Tensile strength of 380 to 500MP
(a) The high-strength hot-rolled steel sheet having excellent ductility according to (1), wherein tensile strength × ductility is 15000 MPa ·% or more.

(3) The ferrite grain number of the crystal structure of the cross section of the steel sheet at a position of 1/4 thickness inside the steel sheet from the surface of the steel sheet is 8 to 13 as described in the above (1) or (2). High strength hot rolled steel sheet with excellent ductility.

(4) By mass ratio, C: 0.0015 to
0.0055%, Si: 0.005 to 0.3%, Mn:
0.1-0.6%, P: 0.045-0.09%, S:
0.01% or less, Al: 0.5 to 2.5%, N: 0.0
01-0.008%, Ti: 0.015-0.085%
, And the remainder is made of a slab consisting of Fe and unavoidable impurities by a hot rolling mill.
A method for producing a high-strength hot-rolled steel sheet having excellent ductility, which comprises setting the temperature to 20 ° C., finishing the hot rolling, cooling the steel sheet to a temperature of 750 ° C. or less, and winding the steel strip at a temperature of 750 ° C. or less.

(5) The steel strip after hot rolling has a tensile strength of 380 to 500 MPa and a tensile strength × ductility of 1
The method for producing a high-strength hot-rolled steel sheet having excellent ductility according to the above (4), wherein the steel sheet has a 5,000 MPa ·% or more.

(6) The ferrite grain number of the cross-sectional crystal structure of the steel sheet at a position 1/4 thicker than the surface of the steel sheet after hot-rolling finishing is 8 to 13 in the cross-sectional crystal structure. The method for producing a high-strength hot-rolled steel sheet having excellent ductility according to the above (4) or (5), wherein

(7) The ductility according to any one of the above (4) to (6), wherein after the hot rolling finish, cooling is performed at an average cooling rate of 50 ° C./s or lower at a temperature of 750 ° C. or less. Manufacturing method of excellent high strength hot rolled steel sheet.

[0016]

First, the reasons for limiting the chemical components of steel in the present invention will be described.

In order to ensure ductility, C is preferably as low as possible, and the upper limit thereof is 0.0055%. However, 0.0
At 015%, the effect saturates. Therefore, 0.00
15 to 0.0055%.

[0018] Si has a function of preventing a small amount of ductility from being reduced by preventing abnormal crystal grain growth. To exert the effect, 0.005% or more is required.
However, when the content exceeds 0.3%, the effect is saturated.
005 to 0.3%.

Mn is required to be 0.1% or more because it is linked to S in steel and has an effect of preventing a decrease in ductility due to S. However, if the content exceeds 0.6%, the effect is saturated.

P has the effect of increasing the strength by the solid solution strengthening action, and is required to be 0.045% or more in order to secure the strength of 340 MPa or more in the present invention. However, 0.
If it exceeds 09%, the ductility is greatly reduced. Therefore, the upper limit must be 0.09%.

If the content of S is large, the amount of sulfide that lowers the ductility increases. Therefore, the content of S should be as low as possible, and should be 0.01% or less.

Al is a characteristic element in the present invention. Conventionally, when the amount of Al is increased in ultra-low carbon steel, the strength is improved, but the ductility is reduced. However, in steel having a specific composition, the present inventors have found that, when the amount of Al added exceeds a certain amount, not only increases the strength of the steel sheet but also has an effect of improving ductility. Was.

As shown in Table 1, 0.03 to 2.
A steel ingot having a content of 50% and other elements having chemical components within the range of the present invention was produced and thinned to 2.3 mm by hot rolling. The slab was heated at a temperature of 1250 ° C and a finishing temperature of 890 ° C. After hot rolling, the slab was cooled to 600 ° C at 120 ° C / s and immediately wound up. JIS5
A tensile test was performed using the No. 10 tensile test specimen according to the method shown in JIS Z 2241, and tensile strength and elongation at break (hereinafter referred to as “ductility”) were investigated.

[0024]

[Table 1]

FIG. 1 shows the tensile strength (TS) of the steel sheet, FIG. 2 shows the ductility (EL), and TS showing the tensile strength-ductility balance.
La EL values are shown in FIG.

It can be seen from FIGS. 1 to 3 that the addition of 0.5% or more of AL increases the EL, and also increases the TS value, which indicates the tensile strength-ductility balance. In addition, when the ferrite grain size number of the steel sheet shown in this example was measured, all were 9.5 to 10.9, and it turned out that it is sufficiently fine.

As described above, in order to improve ductility, 0.5% or more of Al is required. However, 2.5
%, The effect saturates. Therefore, 0.5
２．５2.5%. TS that shows strength and ductility balance
As shown in FIG. 3, it is preferable to add 0.7% or more of Al in order to sufficiently increase the × EL value, and to add 1.0% or more of Al to obtain a higher TS × EL value. Addition is preferred.

Although the reason why the addition of Al exceeding a specific amount improves the ductility has not been sufficiently elucidated, the partial concentration of strain is eased during the processing of a steel sheet by the addition of Al, so that the starting point of cracks hardly occurs. Conceivable.

N is an unnecessary element for ensuring ductility and needs to be 0.008% or less. However, in order to prevent crystal grains from becoming coarse, N is 0.001%.
The above is necessary.

Ti plays a role in preventing a decrease in ductility of these elements in combination with C and N. To do so, the
015% or more is required. However, if it exceeds 0.085%, the effect is saturated. Therefore, 0.015
0.085%.

The steel sheet of the present invention has a tensile strength of 380-5.
It is characterized by high ductility in a steel sheet of 00 MPa, and the value of the product of tensile strength and ductility (tensile strength × ductility) is 15
It is characterized by exceeding 000 MPa ·%. By giving such a high value to the tensile strength × ductility, the occurrence of cracks in press working can be suppressed, and higher working can be performed.

By reducing the ferrite grain size of the steel sheet having the composition of the present invention, the value of strength × ductility can be improved. When the ferrite grain size number of the cross section of the steel sheet at the position of 1/4 thickness inside the steel sheet thickness becomes larger than No. 8, the steel sheet is strengthened, and the tensile strength x ductility becomes 15000 MPa ·% or more. Was found. In the case of a grain size number smaller than No. 8, it becomes difficult to increase the strength of the steel sheet, and the tensile strength × ductility is 1500.
It becomes lower than 0 MPa ·%.

When the ferrite grain size number is larger than 13, the strength of the steel sheet is increased, but the ductility is greatly reduced.
It is less than 0 MPa ·%.

Therefore, 1 /
The ferrite grain size number of the cross-sectional crystal structure of the steel sheet at the position of the thickness 4 should be 8 to 13. In addition, in the case of 10 to 13 numbers in this particle size number range,
Since the tensile strength × ductility value can be increased, preferably 1
Numbers 0 to 13 are good.

Next, a method of manufacturing the steel sheet according to the present invention will be described.

First, the finishing temperature when hot rolling the slab is 9
It must be below 20 ° C. If the finishing is performed at a temperature higher than this temperature, the ferrite grains become coarse and the strength is reduced. On the other hand, when the finishing temperature is lower than 850 ° C., the crystal grains are in a mixed state, which causes a decrease in ductility. Therefore,
The range was limited to 850 to 920 ° C.

After finishing the hot rolling, it is necessary to cool to a temperature of 750 ° C. or less. Cooling to a temperature above 750 ° C, 75
If the film is wound at a temperature exceeding 0 ° C., the crystal grains become coarse, and it is difficult to ensure strength and ductility. When the average cooling rate during cooling is less than 50 ° C./s, TS
0 MPa ·% or more.
By setting the temperature to 0 ° C./s or more, the ferrite crystal grains can be made fine, and the TS value can be improved to 15500 MPa ·% or more.

As described above, the composition of the steel sheet is limited, and the finish at 850-920 ° C., cooling,
It is a feature of the present invention that a steel sheet having high ductility can be manufactured by performing winding at 0 ° C. or less.

[0039]

EXAMPLES Examples of the present invention will be described below together with comparative examples.

A steel ingot having the chemical composition shown in Table 2 was produced and thinned to a 3.6 mm thin steel sheet by hot rolling. Investigations were conducted by changing the slab heating temperature to 1200 ° C, the finishing temperature to 810 to 950 ° C, and the cooling end temperature after hot rolling to 300 to 750 ° C. Investigations were conducted in which the cooling rate at this time was variously changed to 30 to 120C / s. After cooling, the film was immediately wound at the cooling end temperature. Steel symbols A to I both fall within the scope of the present invention in terms of composition and winding temperature. As for J to S, any one or more of the composition, the finishing temperature, and the winding temperature are out of the range of the present invention, as indicated by underlining in the table.

[0041]

[Table 2]

The ferrite grain size number of the obtained hot-rolled steel sheet was measured, and a material was investigated by performing a tensile test using a JIS No. 5 tensile test piece. The results are shown in Table 3 as “Ferrite grain size number”, “TS”, “EL”, and “TS la EL”. From Table 3, those having components and winding temperatures falling within the range of the present invention have a tensile strength of 380 to 500 MPa, and all have TS la EL of 15000 MPa.
% Or more, which indicates a high tensile strength-ductility balance. On the other hand, any one of composition and winding temperature
If one or more deviates from the scope of the present invention, T
At a value of less than 15000 MPa ·%, the tensile strength-ductility balance is lower than that within the range of the present invention. Alternatively, the tensile strength is less than 380 MPa.
Further, when the composition, the finishing temperature, and the winding temperature are within the range of the present invention, and the cooling rate after finishing is 50 ° C./s or more, the ferrite particle size number is larger than 8, and the TS This is a good value of 15500 MPa ·% or more.

The ferrite grain size number was obtained by etching a steel plate with nital, taking a photograph magnified 1000 times with an optical microscope, and measuring the photograph by a cutting method.

[0044]

[Table 3]

[0045]

As described above, by setting the composition of the steel sheet within the range of the present invention, 380 to 500 MPa having high ductility can be obtained.
It has been found that it is possible to achieve a better strength-ductility balance by specifying the ferrite grain size.
In addition, it has been found that this steel sheet can be manufactured by setting a specific range such as a hot rolling finish temperature, a cooling rate, and a winding temperature.

[Brief description of the drawings]

FIG. 1. Effect of Al on tensile strength (TS) of hot-rolled steel sheet
FIG.

FIG. 2 is a view showing the influence of Al on ductility (EL) of a hot-rolled steel sheet.

FIG. 3 is a diagram showing the effect of Al on the TS × EL value of a hot-rolled steel sheet.

Claims (7)

[Claims] 1. A mass ratio of C: 0.0015 to 0.0
055%, Si: 0.005 to 0.3%, Mn: 0.1
0.6%, P: 0.045-0.09%, S: 0.0
1% or less, Al: 0.5 to 2.5%, N: 0.001 to
A high-strength hot-rolled steel sheet having excellent ductility, comprising 0.008% and Ti: 0.015 to 0.085%, with the balance being Fe and unavoidable impurities. 2. A tensile strength of 380 to 500 MPa,
The high-strength hot-rolled steel sheet excellent in ductility according to claim 1, wherein tensile strength x ductility is 15000 MPa ·% or more. 3. The ferrite grain number of the crystal structure of the cross section of the steel sheet at a position 1/4 thickness inside the steel sheet thickness from the steel sheet surface is 8
The high-strength hot-rolled steel sheet having excellent ductility according to claim 1 or 2, wherein the high-strength hot-rolled steel sheet has a No. 4. A mass ratio of C: 0.0015 to 0.0
055%, Si: 0.005 to 0.3%, Mn: 0.1
0.6%, P: 0.045-0.09%, S: 0.0
1% or less, Al: 0.5 to 2.5%, N: 0.001 to
A slab containing 0.008%, Ti: 0.015 to 0.085%, and the balance being Fe and unavoidable impurities is rolled by a hot rolling mill, and the finishing temperature is adjusted to 850 to 920 ° C.
After hot-rolling finishing, it was cooled to below 750 ° C.
A method for producing a high-strength hot-rolled steel sheet having excellent ductility, wherein the hot-rolled steel sheet is wound at a temperature of 50 ° C. or lower. 5. The steel strip after hot rolling has a tensile strength of 380 to 500 MPa, and a tensile strength × ductility of 1500.
The method for producing a high-strength hot-rolled steel sheet having excellent ductility according to claim 4, wherein the pressure is 0 MPa ·% or more. 6. A steel sheet having a ferrite grain number of 8 to 13 in a cross-sectional crystal structure of the steel sheet at a position 1/4 thicker than the steel sheet surface after the hot rolling finish. 3. A method for producing a high-strength hot-rolled steel sheet having excellent ductility according to item 1. 7. The high-strength heat with excellent ductility according to claim 4, wherein the hot-rolling finish is followed by cooling at an average cooling rate of 50 ° C./s or lower to 750 ° C. or lower. Manufacturing method of rolled steel sheet.