JP3332172B2 - High strength hot rolled steel sheet with excellent strength-stretch flange balance and strength-ductility balance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a strength-stretch flange.
The present invention relates to a high-strength hot-rolled steel sheet excellent in balance of strength and strength-ductility balance and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, as the movement to protect the global environment has increased,
There is a greater need than ever for measures to reduce exhaust gas from automobiles and to reduce fuel consumption for energy saving. One of the leading measures for that is to reduce the weight of the vehicle body. In addition to efforts to improve the safety of the vehicle body, efforts are being made to reduce the strength and thickness of the steel plate used. Among them, hot-rolled steel sheets used for undercarriage parts of automobiles such as lower arms, members, wheels, etc. are subjected to severe forming mainly by stretch flange forming, and products need high component strength as important security parts It is said. Therefore, strength 70 with good balance between stretch flangeability and ductility as excellent press formability.
There is an increasing demand for high strength steel sheets of up to 80 kgf / mm2 class.

In the past, as a high-strength hot-rolled steel sheet having an excellent balance between stretch flangeability and ductility, a steel sheet composed of three phases of ferrite + bainite + martensite has been proposed (Japanese Patent Publication No. 1-35005). The strength obtained therefrom is at most about 65 kgf / mm 2, which is slightly lower for recent needs.

[0004] Further, as those utilizing residual γ, those based on a high carbon bainite structure (JP-A-1-159317) and those based on a ferrite + bainite structure (JP-A-3-180445)
No.).

[0005] However, in the former, the residual γ is left in a large amount of 10% or more, and C is 0.35 to 0.55%.
And the hardening of the weld is remarkably poor in weldability. The hole expansion ratio d / do (d: hole diameter when cracks penetrate through the plate, do: initial hole diameter, punched hole diameter) is 1.32 to 1.4 because of the hard bainite structure. Not enough.

The latter has a low C content of 0.05 to 0.15% and has no problem in weldability, but is based on two phases of ferrite and bainite. Is as low as d / do ≧ 1.4, which is not sufficient.

As a 100% bainite material to which high Si is added (hereinafter referred to as full bainite), JP-A-3-1804
26, but still not enough performance for recent needs.

JP-B-63-37166 describes TS: 80.
8 kgf / mm 2, El: 17.5%, hole expansion ratio: 1.
Although 67 was obtained, El was less than 20% and the ductility was insufficient.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and has been made in consideration of the circumstances.
mm2 strength, strength-stretch flangeability balance and
An object of the present invention is to provide a hot-rolled steel sheet having excellent balance between strength and ductility , and a method for producing such a hot-rolled steel sheet at low cost.

[0010]

According to the present invention, there are provided:
A hot-rolled steel sheet excellent in strength-stretch flangeability balance and strength-ductility balance is C: 0.06 to 0.12 by weight%.
%, Si: 0.5 to 1.6%, Mn: 1.5 to 2.2
%, S: 0.0050% or less, Ti: 0.03 to 0.0
It is characterized by containing 8%, the balance being Fe and unavoidable impurities, and having a structure composed of retained austenite having a volume ratio of 3% or more and the remaining fine bainite.

Also, the strength-stretch flange according to the present invention
The method for producing a high-strength hot-rolled steel sheet having an excellent balance of strength and strength-ductility is as follows: C: 0.06 to 0.12 by weight%.
%, Si: 0.5 to 1.6%, Mn: 1.5 to 2.2
%, S: 0.0050% or less, Ti: 0.03 to 0.0
A steel containing 8%, the balance consisting of Fe and unavoidable impurities is finish-rolled in a temperature range of Ar3 +30 to Ar3 + 110 ° C, and immediately after finish rolling, the steel is immediately cooled at a cooling rate of 140 ° C / sec or more to 300-480 ° C. It is characterized by winding in a temperature range of

In this case, the steel having the above composition is further added with C
r: 0.40 to 0.70% may be contained.
b: 0.02 to 0.06%, V: 0.02 to 0.06%
One or two of these may be contained. Also, C
r: 0.40 to 0.70%, Nb: 0.02 to 0.06%, V: 0.02 to 0.06%
%, One or two of them may be contained.

The inventors of the present application have found that the problem of the prior art is strength.
-Stretch flangeability balance and strength- From the viewpoint of poor ductility balance and poor weldability, steel components and
Research was repeated on the composition, microstructure of the steel, and hot rolling conditions.
In the meantime, the inventors of the present application examined in detail the mechanical properties and structures of steel sheets having various thicknesses of 2.6 mn. as a result,
In a material with a structure of full bainite, strength- ductility (TS-
El) The balance is retained austenite (hereinafter referred to as residual γ)
It is expressed as ) Was found to be stratified by quantity. That is, it has been found that when full bainite is used as a base, the TS-El balance becomes better as the amount of residual γ increases. This leads to the fact that stretch bainability is ensured using full bainite as a base, and ductility (El), which has been a drawback of the conventional bainite material, can be compensated for by residual γ. FIG. 1 shows the strength-stretch flangeability (TS-λ) balance and strength- ductility (TS-E) at that time.
1) The relationship between balance and the amount of residual γ is shown.
From this figure, the TS-El balance becomes better as the amount of residual γ increases, and the target T
It is understood that an S-El balance is obtained.

The present invention having the above-described structure is based on the above findings of the present inventors, based on a low-C steel, to which Si and Ti are added to refine the structure and to retain a low-C bainite single-phase structure. By incorporating γ, strength-stretch flangeability
An object of the present invention is to provide a high-strength hot-rolled steel sheet excellent in balance and strength-ductility balance and a method for producing the same.

Hereinafter, the present invention will be described in detail. The hot-rolled steel sheet according to the present invention has a specific composition, has a structure composed of retained austenite having a volume ratio of 3% or more and fine bainite as a balance, and has a strength of 70 to 80 k.
gf / mm2 class, strength-stretch flangeability (TS ×
λ ≧ 6000 kgf / mm 2 ·%) and strength- ductility (TS × El ≧ 1600 kgf / mm 2 ·%). Here, TS indicates tensile strength, λ indicates hole expansion ratio, and El indicates elongation. Hole expansion was performed by punching out a punched hole 10 mmφ with a clearance of 12% and using a 60 ° conical punch inside the burr. From the ratio of the hole diameter (dB) and the initial hole diameter (do) at the time when the thickness penetration break occurred at the hole edge, the hole expansion ratio λ was determined by the following equation.

Λ = (dB−do) / do × 100 (%) Next, the components of the steel according to the present invention will be described. C is
It is added to secure retained austenite (hereinafter abbreviated as residual γ). However, the amount is 0.06% (% by weight,
If the value is less than the same, the effect cannot be obtained, so the lower limit is set to 0.06%. On the other hand, in order to prevent embrittlement of the welded portion and ensure spot weldability, the upper limit is set to 0.
Specify 12%.

Si is a strengthening element for steel and has an effect of suppressing the precipitation of C as a carbide due to the interaction with C. By increasing the C concentration in austenite, Si is indirectly converted to austenite. Stabilize. However, if the amount is less than 0.5%, the effect cannot be obtained.
The lower limit is defined as 0.5%. On the other hand, the amount is 1.6
%, The effect is not only saturated, but also the spot weldability deteriorates. Therefore, the upper limit is set to 1.6%.

Mn is added to secure hardenability, make the matrix have a bainite structure, secure strength, and stabilize γ to secure residual γ. However, if the amount is less than 1.5%, the effect cannot be obtained, so the lower limit is set to 1.5%. On the other hand, if the amount exceeds 2.2%, not only the effect is saturated, but also the spot weldability deteriorates, so the upper limit is set to 2.2%.

S is an impurity element that combines with Mn in steel to form A-based inclusions (MnS-based inclusions) and lowers stretch flangeability. Therefore, it is desirable to reduce S as much as possible. From the viewpoint of prevention, the upper limit is defined as 0.0050%.

Ti is a component of a relatively low C content, which is an object of the present invention, and is an essential element for ensuring a residual γ of 3% or more in a bainite-based steel sheet. Such an effect of Ti has been found for the first time by the present inventors. Although the detailed mechanism of such an effect is not clear, it is considered that the bainite structure is refined in the Ti-containing material and the number of sites where the residual γ exists is increasing. Such an effect of Ti can be substantially exerted at an amount of 0.03% or more, so the lower limit is 0.03%.
%. On the other hand, even if the amount exceeds 0.08%, the effect of securing the residual γ due to the above-mentioned fine structure is saturated, so the upper limit is set to 0.08%.

As other components, Cr: 0.40 to 0.70% is further added as necessary, or Nb:
0.02 to 0.06%, V: 0.02 to 0.06% 1
Add seed or two. Or Cr: 0.40 to 0.70
% Of Nb: 0.02-0.06
%, V: 0.02 to 0.06%.

Cr is added to improve hardenability. And low C-high Si system like the steel of the present invention contributes to prevention of ferrite formation. Although the effect is manifested from 0.4% or more, even if it exceeds 0.7%, the effect is saturated. Therefore, when Cr is added, the amount is set to 0.40 to 0.7.
It is defined as 0%.

Nb and V form carbon / nitride or form a solid solution to refine the austenite grains during hot rolling and refine the bainite of the base, which is the final structure, to increase the elongation. Improves flangeability and ductility. The effect is manifested from 0.02% or more in each case, and even if it exceeds 0.06%, the effect is saturated.
It is specified in the range of 02 to 0.06%.

In the present invention, based on the above-described component system, a structure containing 3% or more by volume of retained austenite in a matrix of fine bainite is formed. By making such a structure, strength-stretch flangeability balance
And a high-strength steel sheet excellent in strength-ductility balance can be obtained.

Next, the manufacturing conditions of the method of the present invention will be described. The steel having the above-mentioned chemical components is formed into a slab by ingot casting or continuous casting, and then turned into a hot coil. The hot rolling and cooling conditions at that time are determined as follows.

Slab heating temperature: In the present invention, since the target steel contains Ti as an essential component and Nb and V are added as necessary, the slab heating temperature is set to 1200 ° C. or more at which these elements form a solid solution. Stipulate.

Finishing temperature: The finishing temperature of hot rolling is Ar
3 +30 to Ar3 + 110 ° C. If the finishing temperature is too high, austenite grains become coarse and the final structure becomes coarse, so that the workability of the final product deteriorates. Therefore, from the viewpoint of not roughening the final structure, the upper limit of the finishing temperature is set to Ar3 + 110 ° C. On the other hand, the lower limit of the finishing temperature is defined as Ar3 + 30 ° C. in consideration of the stability of the finishing temperature in order to prevent ductility deterioration due to the processed structure.

Cooling start time after finish rolling: Cooling start after finish rolling is performed immediately. The steel of the present invention has relatively high Si,
In addition, since the finishing temperature is low, ferrite is easily formed. In order to obtain fine bainite, which is the base structure of the steel of the present invention, it is necessary to suppress the formation of ferrite by quenching immediately after finish rolling. The shorter the water cooling start time, the better, and preferably within 2 seconds after finish rolling.

Cooling rate: The cooling rate after finish rolling is 14
0 ° C./sec or more. The lower limit of the cooling rate is 140 ° C./sec.
As in the present invention, a steel sheet having a relatively high Si has a minimum cooling rate for obtaining a fine bainite structure of the matrix. If it is less than this, ferrite transformation occurs during cooling, and workability, particularly stretch flangeability, deteriorates.

Winding temperature: The winding temperature is 300 to 480 ° C.
And The lower limit is defined to suppress the formation of martensite, and the upper limit is defined to suppress the formation of pearlite and form a fine bainite structure.

By producing a steel having the above-mentioned components and composition under the above conditions, a structure containing 3% or more by volume of retained austenite in the matrix of fine bainite can be obtained.

[0032]

Embodiments of the present invention will be described below. Fourteen steel grades having the component compositions shown in Table 1 were melted. Steel A ~
G is a steel satisfying the components and compositions defined in the present invention,
Steels H to N are comparative steels.

[0033]

[Table 1]

Using the steels A to N in Table 1, hot rolling and cooling were performed under the conditions specified in the present invention to produce hot rolled steel sheets having a thickness of 2.6 mm. Table 2 shows the conditions at that time. In order to examine the mechanical properties of the obtained steel sheet, a tensile test and a hole expansion test were performed. The stretch flangeability was evaluated by the above-mentioned hole expansion ratio (λ).

The structure was confirmed by etching the cross section of the steel sheet with nital, observing the structure with a microscope, and measuring the retained austenite (γ) by X-ray diffraction.

The weldability was evaluated by spot welding of the test material, and whether or not the nugget portion (the portion melted during spot welding and then solidified) was broken. Table 2 also shows the results. In Table 2, the numbers 1 to 7 of the present invention correspond to the steels A to G of the present invention, and the comparative materials 8 to 14 correspond to the comparative steels H to N. Further, in the column of weldability, a case where there was no breakage in the nugget of the spot welded portion was indicated by ○, and a case where there was no breakage was indicated by ×.

[0037]

[Table 2]

As apparent from Table 2, the number 1 of the present invention
7 are TS: 70 kgf / mm 2 or more, and the strength-stretch flangeability balance TS × λ is 6000 kgf / mm 2.
・% Or more, strength-ductility balance TS × El is 1600k
and in gf / mm2 ·% or more, the strength - stretch flangeability server
It was confirmed that the lance and the strength-ductility balance were excellent. Also, the weldability was excellent.

On the other hand, comparative steel Nos. 8 to 14
Is that TS × El is 1600 kgf / mm 2 ·% or more, T
S × λ: 6000 kgf / mm 2 ·% or more was not satisfied or weldability was deteriorated. For example, in Nos. 8 and 12, ferrite is generated because C and Mn are low, and λ is reduced. No. 10 has a low Si content and a small residual γ content. No. 13 has a large amount of S and a low λ. Number 1
No. 4 has a small amount of Ti and has no residual γ. As described above, in the numbers 8, 10, 12, 13, and 14, the formation of fine bainite in the base and the utilization of the residual γ are not sufficiently performed.
The strength-stretch flange balance and the strength-ductility balance were insufficient. Symbols 9 and 11 represent the amount of C, Si
Since the amount was high, it was confirmed that the weldability was deteriorated.

Next, using the steels A to C having the composition range of the present invention shown in Table 1, various conditions of hot rolling and cooling were variously changed, and a hot-rolled steel sheet having a sheet thickness of 2.6 mm was produced. Table 3 shows the conditions at that time. Here, at least one of the hot rolling / cooling conditions does not satisfy the range specified in the present invention, and the numbers 15 to 20 in Table 3 are all comparative materials. The comparative items Nos. 15 to 20 thus manufactured were evaluated for the same items as in Table 2. The results are shown in Table 3.

[0041]

[Table 3]

As is clear from this table, the comparative material No. 1 which does not satisfy any one of the hot rolling and cooling conditions of the method of the present invention.
5 to 20 are the target values of the present invention, TS × λ ≧ 600.
0, not satisfying at least one of TS × El ≧ 1600 kgf / mm 2 ·%. This is because the structure of the present invention containing residual γ was not obtained in the fine bainite matrix.

That is, since the comparative materials 15 and 16 do not satisfy the condition of the finishing temperature of the present invention, the former does not satisfy TS × λ ≧ 6000 kgf / mm 2 ·% due to the formation of ferrite, and the latter does not satisfy bainite coarsening. TS × El ≧ 1
It does not satisfy 600 kgf / mm 2 ·%.

In the comparative material 17, since the cooling time after the finish rolling was long, ferrite was generated, and λ was low. The comparative material 18 produced a large amount of ferrite due to a low cooling rate,
Again, λ is low. The winding temperatures of the comparative materials 19 and 20 were out of the range of the present invention. The former had a low El due to the formation of a hard structure, and the latter had a lambda λ because bainite was not optimized.
Is low.

[0045]

As described above, according to the present invention, it has a strength of 70 to 80 kgf / mm @ 2 and a strength-elongation.
The present invention provides a hot-rolled steel sheet excellent in balance of heat and flangeability and strength-ductility balance , and a method for producing such a hot-rolled steel sheet. The steel sheet of the present invention can be manufactured without making any special changes to the current hot rolling process, and because it does not use exceptionally expensive materials, it is low-cost, and is industrially very useful. is there.

[Brief description of the drawings]

FIG. 1: Strength-stretch flangeability (TS-λ) balance,
FIG. 5 is a diagram showing a relationship between a balance between strength and ductility (TS-EI) and an amount of residual γ.

────────────────────────────────────────────────── ─── front page continued (51) Int.Cl. ⁷ identifications FI C22C 38/14 C22C 38/14 panel consisting referee Satoru Miura judge Yoshizo Hiratsuka judge Nakamura Asako (56) references Patent Rights 3-61346 (JP, A) JP-A-56-136929 (JP, A) JP-A-56-130456 (JP, A) JP-A-59-229464 (JP, A)

Claims (8)

(57) [Claims] C. 0.01 to 0.12% by weight C, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, the balance being Fe and unavoidable impurities, and a volume ratio of 3%
Strength-elongation furan characterized by having a structure comprising the above retained austenite and the balance of fine bainite
High-strength hot-rolled steel sheet with excellent balance of strength and strength-ductility . 2. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: 0.40 to 0.70%, the balance comprising Fe and unavoidable impurities, and having a structure of 3% or more by volume of retained austenite and the balance of fine bainite; strength-stretch flangeability Balance and strength-
High strength hot rolled steel sheet with excellent ductility balance . 3. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, Nb: 0.02 to 0.06%, V: 0.0
2 to 0.06% of one or two kinds, with the balance being F
made from e and unavoidable impurities, and having a structure consisting of a volume of 3% or more retained austenite and the remainder of the fine bainite strength - stretch flangeability balance
And high strength hot rolled steel sheet with excellent strength-ductility balance . 4. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: 0.40 to 0.70%, and Nb: 0.
02 to 0.06%, V: 1 of 0.02 to 0.06%
Containing at least one species, the balance being Fe and unavoidable impurities, and having a structure of 3% or more by volume of retained austenite and the balance of fine bainite.
A high-strength hot-rolled steel sheet excellent in strength-stretch flange balance and strength-ductility balance . 5. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
Steel containing 0.0050% or less, Ti: 0.03 to 0.08%, and the balance consisting of Fe and inevitable impurities is
Finish rolling in a temperature range of +30 to Ar3 + 110 ° C, and immediately after finish rolling, winding at a cooling rate of 140 ° C / sec or more in a temperature range of 300 to 480 ° C.
A method for producing a high-strength hot-rolled steel sheet excellent in strength-stretch flangeability balance and strength-ductility balance . 6. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: steel containing 0.40 to 0.70%, the balance being Fe and unavoidable impurities, was obtained from Ar3 +30 to Ar3 +110.
Finish rolling in the temperature range between ℃ and 1 after finishing rolling
Intensity and wherein the winding in the temperature range of from 300 to 480 ° C. 40 ° C. / sec or more cooling rate - stretch flangeability Roses
A method for producing a high-strength hot-rolled steel sheet excellent in balance and strength-ductility balance . 7. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, Nb: 0.02 to 0.06%, V: 0.0
2 to 0.06% of one or two kinds, with the balance being F
e and steel unavoidable impurities are represented by Ar3 + 30 to Ar
3 Finish rolling in a temperature range between + 110 ° C., and immediately after finishing rolling, at a cooling rate of 140 ° C./sec or more, 300-48.
Strength-stretching characterized by winding in a temperature range of 0 ° C
A method for producing a high-strength hot-rolled steel sheet having an excellent balance of balance and strength-ductility balance . 8. C: 0.06 to 0.12% by weight, S
i: 0.5 to 1.6%, Mn: 1.5 to 2.2%, S:
0.0050% or less, Ti: 0.03 to 0.08%, C
r: 0.40 to 0.70%, and Nb: 0.
02-0.06%, V: 0.02-0.06% balance Fe
And steel consisting of unavoidable impurities, Ar3 + 30 to Ar3
Finish rolling in a temperature range between + 110 ° C, and after finishing rolling,
Immediately at a cooling rate of 140 ° C / sec or more, 300-480 ° C
Strength-elongation furan characterized by winding in a temperature range of
A method for producing a high-strength hot-rolled steel sheet having an excellent balance of strength and strength-ductility .